Evolution or Revolution? Darts BB Chevy heads.

[DavidVizard-GFN]

Dart’s big block Chevy heads hit the hot rod scene way back in 1982 but I did not get around to using any of these heads on my big block Chevy builds until 1990. This was because I happened to have a fair bit of experience with the factory aluminum heads and had ready access to such. But the desire to try something new prompted me to go the Dart head route and I found myself building a tunnel ram equipped, 505 incher that pushed the dyno numbers into the low four figure category. After flow testing it was almost a forgone conclusion that these heads had power potential. But that was nearly two decades ago and, in the high performance community, technology hardly stands still. And that, more than most companies, is so at Dart – especially in the big block Chevy department. Well I’m back building big blocks after about a 15 year hiatus and one of my first moves was to revisit Dart’s big block heads. Would I find any design changes that even bordered on revolutionary or had these heads just steadily evolved over the years?

As a company Dart is often in the news with some hi-tech innovation. As a hot rodder, looking from the outside in, you may wonder just how much of this is actually hi-tech know-how and how much may be just smoke and mirror style product promotion hi-tech hype. In this day and age of high powered marketing that would certainly not be an unreasonable question. It’s also one that, by the time we get to the end of this feature, will have more than likely been answered to your full satisfaction.

Claims have been made that less than obvious changes can make 10 hp difference easily. The chamber form either side of the spark plug is a prime example. Is this hi-tech science or just hi-tech hype?[/center]

Time and Design Line.

Dart’s Aluminum big block heads came out of the gate in 1982 looking good but the competition in this field is fierce to say the least. Looking good 25 years ago, or even 2 years ago for that matter, for sure won’t cut it these days. With Dick Maskin’s (Dart’s boss) passion for Pro Stock racing makes it almost inevitable that the companies big block product line would benefit from technology trickle down.

I got to sit in with Dart’s boss Dick Maskin while this 500 inch Pro-Stock engine was being readied for the upcoming weekend’s race. At 1415 hp this was the first time I had witnessed a 1400 plus hp Pro Stock pull on the dyno.

Certainly the effort put in to Pro Stock engine development was sufficiently effective enough to win – to date – three Pro Stock championships (1996, 2000, and 2001). The question at this point is has any of the Pro Stock work resulted in a revolution in Dart’s regular big block Chevy line or have these heads just reached their current performance level by small evolutionary changes. By definition, refinements and small changes are considered evolutionary while big changes, usually using new technology, are considered revolutionary. Just when an applied development/technology crosses the line from one category to the other is a little vague but if I had to pick out something in Dart’s developmental strategies as at least approaching revolutionary the nearest I could get would be their heavy weight adoption of wet flow testing.

Although a certain amount of wet flow technology was present from about day one in the Pro Stock program with much of that technology trickling down into Dart’s production heads it was not until about 2003 that a decision was made to delve really deeply into this area of development. During 2001 Joe Mondello introduced an add-on wet flow set up for Super Flow benches. It was this that prompted Dick Maskin to approach Joe with the idea of building a super sized wet flow bench. The result is as seen below.

Driven by three electric motors of 50 hp each Dart’s wet flow bench is truly a state-of-art piece of equipment. Built in the Mondello Tech Center in Tennessee, the Lloyd Creek constructed bench has computer control and data acquisition capability. The data acquisition side of the bench’s build was handled by Audie Thomas of Audie Technology, a company well known for its flow bench data acquisition equipment. The bench has user defined fuel/air mixture control and also measures wet and dry airflow. The 55 gallon drums (the second is immediately behind the one seen here) act as a separation and collection point for the fuel/air mix that goes through the system. A special near non-combustible liquid which emulates the specific gravity, viscosity, volatility and surface tension of a typical gasoline is used as a test fluid. This fluid has a fluorescing dye added so, when the system is irradiated with ultra violet light, an image can be more clearly seen.

Just having fancy equipment is only half the story though. To be of use this bench needs someone with extensive cylinder head experience and ability. The man responsible for almost all the flow testing wet or dry, is Tony McAfee and he carries better than a decades worth of no-how in this area. He works directly with Dick Maskin’s Pro Stock program and he brings that technology directly to Dart’s production heads via wet and dry flow testing and dyno results. Best yet is that he constantly pursues answers to even the smallest perceived problems toward making more hp.

The size of the bench comes into prospective here as we see Tony McAfee putting it to use. As you might expect this shot was staged as normally the lights would be out so as to reveal the fluorescing fluid patterns more easily.

Wet Flow Test Review.

The object of wet flow testing is simply an effort to see and subsequently improve the way the incoming mixture reacts to the shape of the components it encounters. Keep in mind here that fuel rivulets do not readily burn with the air content of the cylinder. The goal is to have all the fuel dispersed as evenly as possible and sufficiently well atomized to burn both rapidly and completely during the combustion phase. This may not sound too difficult to do but the reality is far removed from that. The truth is that the fuel has a greater tendency to coagulate into big droplets or rivulets much more readily than to stay atomized as it was while exiting the carb booster or to be re-atomized from some corner or other. The purpose of the wet flow tests are to isolate trouble spots and, as far as possible, reduce their negative impact or even have the fuel stream re-shredded on some key form within the port/chamber. On the face of it that does not seem to be too difficult a goal to achieve but the fact is it can suck up cubic hours with ease with apparently no end in sight.

Here is a big block Dart Pro 1 CNC head after we ran a test. The purpose of this test was to see how well any fuel stream misses the spark plug. Too much fuel on the plug wets it down and can either cause a misfire of lead to the generation of a low energy spark. A small amount of fuel wash though can help cool the plug. This test showed only a light wash over the plug – probably just enough to cool the electrodes but not enough to compromise the spark. In this shot the fluorescent green on the cylinder walls indicates light fuel wash. Where it is clear indicates significantly heavier fuel wash.

In the tests we see here only the head is being tested. The reason for this is that the heads are sold to customers who could be using these heads with any one of a few dozen styles if intake and carb/ injection combinations. Although the wet flow patterns can change due to the intake used getting the system right at the cylinder head is probably 75% of the battle here. When a dedicated system is being tested, such as Dart Pro Stock setup, then an entire induction system as per a finish built engine will be installed on the bench.

To show what was going on in these big block heads we used two types of tests. The first, with the aid of lighting to make the fuel fluoresce shows principally where the fuel is streaming in heavy or light concentrations. The second, utilizing yet another form of lighting and selected speed photography reveals much about fuel droplet size, speed, direction of motion and dispersion. The two following photos show the mixture flow patterns at 0.200 (two hundred thousandths) valve lift.

Here the head is turned around 180 from the previous shot. At the back of the cylinder the quench pad can be seen. A very vague outline of the intake valve can be seen about in the center of the shot but a clearer image of the valve can be seen from the following photo which was shot from exactly the same position. The spark plug position is dead center up at the top of the picture. Note how the principle flow of fuel (blue) is down to the right of the plug where as the light flow showing as thin green rivulets is all the plug actually sees. In practice, the fuel wash does not get time during an intake stroke to get to the proportions shown here, as this process actually acts as a magnifying glass allowing, more clearly, to see what is going on.



This shot simulates and engine running at a little over 6000 rpm. In the center of this shot we can see the arc of part of the intake valve. The fine dots are droplets of fuel. Notice the fuel droplets are far more closely spaced where they pass through between the valve and seat. From here they burst into the cylinder as a much more evenly dispersed cloud. The stream in the center (shown in blue in the previous shot) is as close to a real world situation as we can get short of heating the cylinder walls to 200 plus degrees. To the right you can see how the fuel droplets are much more densely packed. This indicates that the primary flow of both air and fuel is in the spark plug/cylinder wall quadrant of the intake valve. By correcting for light dispersion on the fuel droplets their size can be determined. Also the faster moving ones show a short streak. By knowing the average size of the droplets within the area concerned the direction and speed of the droplets can be determined.

There is a lot more to wet flow testing than meets the eye. I don’t intend to go into intricate details here other than to say if you want to know a little more go and check out the wet flow story I posted a few weeks back. The reason I am not going to go right down to the nuts and bolts here is that it will undermine my value as a consultant to those companies who may not be as up to speed as they would like to be.


Iron Eagles.

At this point you should be pretty much aware of Darts comprehensive flow test program. The question now is just how well (or not as the case may be) does this translate into power for the likes of you and me. The first test here was on a set of Iron Eagles. These are the cheapest in Darts line and can be had for prices starting at a little over $1420 pair from such places as Jegs. They are available in small (308 cc) and large (345 cc) intake port form and can be sprung for either a solid or hydraulic cam in flat or roller form. The big port head also sports a 2.3 inch intake valve rather than the 2.250 valve of the smaller head.

Here’s the chamber of a Dart Iron Eagle big block head. Note the smooth as-cast finish in the chamber and the hardened exhaust seat insert just showing around the exhaust valve.

The initial reason for going with a set of Iron Eagles was that several local engine builders whose main source of income is no nonsense engine builds that have to compete with crate motor value for money, were successfully doing so. A typical 454 to 475 inch, 850 Holley equipped, two plane intake engine, with a 10/1 CR, would pump out some 575 hp and 565 to 590 lbs-ft of torque. For what was essentially basic budget motors these numbers looked pretty good. Based on this I decided to investigate the Iron Eagle potential. First I acquired one of each of the small and big port head casting from Dart to do flow testing on. Also I wanted to see just how close Dart’s in house measured flow numbers were compared to an accurately calibrated bench such as we use for GFN testing. First the small port head. When the numbers were run on this it was evident that these heads flowed a lot more air then factory iron heads. Typically an 049 factory head with a fresh valve job will, on our bench, go about 268/260 cfm at 700 lift for the good and bad port respectively while the exhaust the flow at this same lift is 177 cfm. As can be seen from the chart below the small port head delivered some 354/350 cfm for the good and bad intake port while the exhaust flowed 259 cfm. This represents a healthy increase over a stock head and will obviously be worth 50 hp plus on a typical budget build. For the big port heads the numbers were even higher. But the good port showed significantly better than the bad. Here the intake numbers were 374/352 for good and bad ports while the exhaust, even though the port is slightly bigger than the small intake port heads was still 259.

So how do these numbers compare to Dart’s advertised numbers – pretty well I would have to say. Dart reckon on 363 and 390 at 800 lift and 266 for the exhaust. We had about 1.5% less for the intake and a tad more for the exhaust. This shows that Dart are pretty honest with their flow numbers.

As can be seen from these curves for the small port (308) Iron Eagle head the good (flows into center of cylinder) and bad (flows into cylinder wall) ports flow about the same. The principle difference in as-cast form is the swirl with the so called bad port is better. It is worth noting that a Morse test revealed that the bad cylinders with their higher swirl, actually made a little more output throughout the rpm range than the good port.

Here are the GFN flow bench results for an as-cast big port Iron Eagle head. Just making the intake ports a little bigger did not have a dramatic effect on flow. The change from about 308 cc to 345 and the consequent reshaping involved, brought about a bigger change in swirl characteristics than flow.

At this point the flow data is showing us just why our local custom crate engine builders are getting so much more power from an as-cast Iron Eagle equipped engine compared with any with the stock factory heads. With this proven performance in mind it was decided to go the next step and see what these heads could do when given a basic porting job.

Basic Porting.

First the definition of basic porting as it applies here. What we are going to do with a set of small port Iron Eagles is to clean up the castings so that they more closely resemble the form that Dart developed before the inevitable flaws introduced by the casting process (even though it is of top quality) came into the picture. The amount of metal removal is minimal – about 4-5 cc is it on the intake and even less on the exhaust. Essentially this is an exercise that involves skinnying down the guide bosses and blending casting irregularities. As such a first timer will more than likely get the job done over a weekend. In our case one other small mod was made and that was to install the 2.3 inch diameter intake valves (up from 2.250) from the big port heads into the small port heads used. The reason for going this rout was that the big block Chevy is so undervalved that any increase in valve size and consequent flow increase pays off. Just to give you an idea of how undervalved a big block is we would, even in the modest 482 incher we are dealing with here, have to install an intake of over 2.8 inches to produce the same valve breathing area per cube as a 350 inch small block with a 2.02 inch valve.

Caption: Here our man Dusty Kennett runs the numbers to see what our basic porting job did on the small port Iron Eagles. The results were very encouraging.



Here is one of our basic porting jobs cut away. Note how there is still evidence of the cast finish along the port wall. That we left deliberately so you could see for yourself how little material was removed to get results. There were two key areas’ that delivered in terms of flow. This was the short side as arrowed here and -------------------------------

------------ the area either side of the guide boss.

For the exhaust side of these heads it was a just a question of blending in the seat insert into the port beneath, streamlining the guide boss and paying attention to the short side turn. Just 15 minutes of work on an exhaust port punched peak flow up from 266 cfm to just a fraction shy of 300 cfm. A gain of this magnitude on the exhaust could mean that a 150 hp shot of nitrous would deliver 15 to 20 hp more (due to reduced pumping losses) than had the port been left stock.

Mods to the exhaust involved the removal of perhaps 2 cc of cast iron. Other than blending the seat insert into the port and streamlining the guide boss the key point was a smooth radius on the short side turn as indicated here.

When tested the bench delivered the results shown below. Our handy work on the intake paid big dividends in the 100 to 350 lift range. Between 400 and 550 the flow actually dropped slightly but picked up from there on. Also of interest is that the swirl in the higher lift ranges dropped. The drop was more than we would have liked to have seen but the fact that the swirl was still strong in the low lift range was, to an extent, a redeeming factor. If the heads have good flow at low lift this swirl action is put into the charge at the time when the cylinder is getting toward the end of it’s induction stroke. What his means is the charge gets a rotational kick so to speak after much of the fuel has picked up heat from the cylinder and also has had at least some time to disperse. Under these conditions the swirl is less likely to centrifuge raw fuel back onto the cylinder walls.

Since we touched on low lift flow here it might be as good a time as any to establish the value of low lift flow. With such an undervalved head as the big block heads are, the one way to compensate is to open the valves faster. The bottom line is that the piston has no idea whether the valve is opened faster only that it has become easier to get a charge past the valve. What low lift does is exactly emulate faster valve opening. Several years ago I did a test where the before heads were run with optimal valve event timing. These heads were then modified and produced flow increases of 18% at 100 lift dropping to zero at 350 lift. With re-optimized cam events (same profiles in each case) the 468 inch test engine picked up 16 hp! At this point I don’t intend to delve into the advantages of low lift flow as it deserves an article about the size of this one to show all the test data I have to back up the fact that low lift flow is good for output. If anyone has failed to see more power from more low lift flow it’s because they probably have not correctly utilized it!

Here are the kind of results you can expect from a basic porting job on a set of small port iron Eagles. Essentially a weekends work will net more flow than a set of out-of-the-box big port heads while all the port velocity needed for good street performance in the lower rpm range is retained. The same procedure applied to the big port heads nets a similar gain.




What we see here are the gains from the basic porting procedures applied to our small port Iron Eagles. Note the gains in the 50 to 350 lift range. From 400 to 550 the mods actually cost a little in the way of flow but from 550 on up more useful gains were seen. In practice the increase velocity and charge momentum generated from the increase in flow at 250 lift would more than counter the loss of flow in the 400 to 550 lift range.




Here’s what our finished heads looked like on the chamber side. As for -------------------



------ the top side you can see that we used beehive springs to control the valves. It should be noted that all Darts big block heads utilize 11/32 stem valves rather than the thicker 3/8 stem valves the factory uses stock. This drops the valve mass by over 20 grams and is a worth while move on an otherwise heavy valve train. With the beehive springs of only 130 lbs on the seat and 320 lbs over the nose this valve train ran flawlessly to a 6700 rpm redline.

Dyno Time.

Our test engine was a 482 inch unit. The extra inches from the original 454 were achieved from an 1/8th overbore on a previously sonic tested block showing sufficient wall to go the distance. Pistons were hypereutectic from KB. For the valve train we had Comp Cams grind us one to our specs specifically to suit the rest of the engine combination. This was a single pattern solid street roller with advertised 286 degree duration along with 248 degrees at 50 duration on a 107 lobe centerline angle. This cam with 1.7/1 rockers delivered, after lashing, an observed valve lift of 0.640 (640 thousandths). The reason for going with a solid street roller is that it has a long life as it is easy on the valve train plus we did not need the low noise qualities of the hydraulic equivalent as the exhaust noise would always be greater than what ever small amount of noise these solid street rollers have anyway. Note the use of a Holley Dominator carb. This may be a little more money than say a regular 4150 series 850 but you need to be aware that big block Chevys really like Dominators. It’s worth power and drivability – there is no down side if you can afford the extra couple of hundred bucks a Dominator costs. After some testing of different manifolds the intake we elected to use, by a small margin, was the Weiand Team G item. All that was done to this intake was to port match it to the Dart heads. Throw in a set of 1-7/8 inch hooker headers and we were in business.

Here’s our 482 inch test engine. It looked good with regular tinware but adding the Moroso pan and valve covers really turned it into a looker. Just for the record this whole engine owed us less than $7200 and that included stuff you don’t see such as ARP rod and head bolts and the like.

Peaks of 676 hp and 623 lbs-ft for this pump gas burning 10/1 CR 482 inch streetable big block are good. Achieving 1.25 lbs-ft per cube from a 10/1 compression engine falls into the ‘good’ category. This motor achieved 1.29 lbs-ft per cube with can be considered excellent.

Check Point.

About now it’s time to summarize where we have got to in this analysis of Darts big block cylinder head offerings. At this point in time we can see that the Iron Eagles present a far cheaper option to make decently big hp numbers at a price typically about $700 less than most of the competitions aluminum heads. Of course these heads will never be as light but that’s life. On the other hand if you want the motor for marine applications, especially in salt water, these heads might be actually more desirable than aluminum ones. All-in-all our experience to date has shown that these Iron Eagles do what they are supposed to do – make plenty of power!

Aluminum Dart Big Block Heads.



Essentially we have seen how effective the Iron Eagles can be in terms of power production per dollar spent. Now it’s time to focus on darts Aluminum heads. All the preamble concerning wet flow and potential performance gains really begs the question as to how much has this paid off. In fact, when we get asked a question on Dart’s big block heads as often as not it concerns the potential gains from wet flow testing. Whereas we were able to run some before and after tests on Darts small block heads at T&L Engine Development the opertunity to do so on big block heads did not arise. Fortunatly Scott Shafiroff was able to save the day for us here and supplied the numbers we needed. So, before looking at any flow numbers we will go straight to Scotts dyno tests and put an end to the suspense. The following graph just about says it all – but we are going to talk about it anyway.

Here are the numbers from a Shafiroff sportsman 540 engine. This is a reletivly low budget unit equipped with a Dart intake and an 1150 Holley Dominator. From these tests it can be seen that attention paid to the heads wet flow charactoristics can pay off with some really worthwhile gains. Here we see a set of as-cast heads tested back to back. The wet flow developed Platinum Pro 1’s produced a peak power and torque increase of 20 hp and 14 lbs-ft. However at some points the difference was as much as 31 hp and 23 lbs-ft.

So the merit of wet flow testing seems to be more than reasonably proven. Now we know what we are getting in terms of power potential lets look at the flow numbers.

As Cast and Ported Platinums.

As we have seen from the graph of the Shafiroff 540 the as-cast 345 heads have enough airflow to hit the 850 hp mark. Indeed a 572 with the appropriate cam will crack the 900 hp barrier. But good though they may be there is still plenty of potetial locked up in these Dart castings. Essentially there is little to no difference in the form of the aluminum heads compared to the iron ones. Out of the box flow for the 308 or 345 cc heads is much the same as for the Iron Eagles shown in the earlier flow tests. Where the biggest difference lies is that doing a basic port job on aluminum heads is a far faster operation than on iron. The only tedious part is blending the chambers into the top of the seat inserts. This takes an old valve to protect the seats and a steady hand from there on out.

Here are the as-cast chambers and ports (good intake on the right and bad on the left). Smoothing evertying out and adding the finishing touches to streamlining the guide bosses is worth 25 to 35 hp on a nominally 700 horse big block.




The intrusion into the port on the roof caused by the rocker stud boss cuts flow about about 0.600 lift. Cleaning this up and streamling the guide bosses is an easy 20 cfm. The exhaust is also responsive to guide boss mods as well as attention to blending in the seat to the rest of the port and rounding the short side turn.

With no more than a serious tidying up job the ports and chambers will look like the following shots. These heads were done over a long weekend, and although done by a vetran head porter, there were no trick techniques involved. Using this porting proceedure on a set of 345 cc heads produced, from 582 inches, over 850 streetable hp along with 740 lbs-ft on a 10.5/1 CR.

The chamber form here is no more than the stock Dart form blended out. The intake port has been 'squared’ at the manifold face, the guide boss streamlined and the port blended out - cc's only increased by 5 -6.

What you see here is the exhaust port rework. Blending the seat into the port while maintaining the biggest radius possible is important to mid and high lift flow. The guide boss streamlining delivered extra flow from about 0.500 lift up.



Shown here are the gains in the flow made from the porting shown previously. Note that the flow increases seen do not really start to show until lift values above 0.300. Intake flow increases were not big by any standards but, because a big block is so undervalved, the increases achieved deliver more additional hp than might otherwise be expected. On the exhaust side some substatial gains were made. If nitrous injection is going to be used reworking the exhaust port will probably be worth an addional 20 hp on a 200 hp nitrous system.

In as-cast form Dart offers these heads in three port volumes, 310, 325 and 345. If the engine is principly a street drive unit of 454 to about 512 inches then the 310 cc ones should most seriously be considerd. Our experience is that this head works well out-of-the-box espcially on near stock displacement short blocks. When those cubes start to approach the 500 mark we have almost always opted to do a basic port job on them. Results have always been excellent with the early heads and even better with the Platinum heads. For street strip where the 'strip’ part has a moderate priority over the street aspect then, on 500 to 540 inch engines, the 325 heads would be a good choice. For 540 inches or more where power is the prime requirementthe 345 cc port heads would be the way to go. Having said that the 345 heads will work OK if the engine is expected to be used on the street but power up to about 2800-3000 rpm will be down some. On 572 inch or more engines it’s pretty much the 345’s all the way.

We have built, or been involved in the build of a number of big block Chevy’s in the moderately priced catagory (from just over $8000 to a little over $11,500). These engines, based on GM blocks, have ranged from 492 to 505 inches. Using the Dart small port heads, we have seen, from streetable pump gas engines, 650 to 700 hp and 645 to 670 lbs-ft depending on the final spec. All these engines idled at sane street rpm levels, that is typically around 700-750 rpm. The example shown here is one we colaberated with T&L Engines (704-888-8111) in Stanfield, NC and was used to test several different cylinder head packages. On basic ported Dart 310’s it made 700 hp and 663 lbs-ft.

At this point in time we will look at Dart’s CNC offerings. One of the first things an even casual glance will show is that the combustion chambers are machined to a finer finish than the ports. While I was up at Dart I asked Tony Mcafee if a finer finish or even an emery roll job on the intake might not net a little more power. He was very positive in his reply here. Basically Tony stated that with a typical single Donminator intake setup the wet flow was marginally compromised by a finer finish and anyone going this route could expect about an 8 hp loss of output. With a tunnel ram’s more direct path from carb booster to valve the situation would be a little different and some small gains might be seen.

The CNC porting work only increases the port volume by about 10 cc over the particular heads non CNC’ed counterpart. From these views it can be seen that the principle area of reshaping is in, and around, the guide boss. The rest of the port closely follows the original as-cast heads form but is a little larger in cross-sectiopnal area.






The CNC heads chamber form benifits from Darts wet flow development program and the form used in the exhaust port really turns on the flow. By delivering over 300 cfm of flow the exhaust is well suited for nitrous applications.

Pulling a set of these heads out of the box was excitment in itself because they simply looked fast. The real question is what will they do on the flow bench. This was a key question because the as-cast port was good and basic porting made the most of that. Since these ports were only marginally bigger it had to be asked what was it worth? Now any time this question crops up concerning heads for a big block chevy, especially one with 500 plus inches we have to remenber that even a small increase in flow pays dividends on what is by any standards an air starved engine. Well the flow bench told the story. If you don’t want to spent a weekend doing even a basic porting job on your own heads then Dart does a nice job for you at a reasonable price. Out tests showed that over about 0.600 lift the CNC heads showed better figures than our basic ported ones. At 0.800 lift the intake was about 10 cfm up and at 0.900 it was better by some 17 cfm. Granted these look like astroniomical lift figures but remember most big blocks are not going to spin that high so although the lift is up the rpm is down. When building a valve train for these heads figure ratio’s of 1.8/1 or more as an aid to high lift and more valve acceleration.

Good flow all the way to 0.900 lift suggests a high lift valve train is a real asset toward making big torque and hp numbers with the Dart CNC heads.

CR Considerations.

All the Dart heads we have considered here have 119 cc combustion chambers as stock. The iron Eagles can be flat milled to 110 cc as can the CNC aluminum heads. The as-cast Pro 1 can be milled to 108 cc. It takes about 0.005 of metal removal to reduce the chamber by 1cc. Just for the record it is far better to get the chamber volume required by milling the heads to a minimum than using a bigger dome with a bigger chamber. The rule then is to make compression with the smallest chamber first then go for piston crown compression. If a stroker crank or a big bore (or better yet both) figures in the equation a much better situation exists because this means less of a dome on the piston. Those tall domes about a half inch high can b e problematical in terms of effective combustion or lack thereof.

Question – Is it possible to bolt together a 900 hp big block with little or no machining - answer – Yes!

As for the amount of compression these heads are good for an above average figure for pump gas. We have run an 11/1 CR with both Iron Eagles and aluminum Pro 1’s. I suspect the Pro 1’s could make it to 11.5/1 but have yet to try it. The bottom line here is that if pump gas is to be used there is a big power advantage to be had by using the highest compression ratio possible. If it’s a race gas motor then a 12.5/1 ratio should be considered an absolute minimum with at least a ratio and a half more as viable under almost all circumstances other than blown or nitrous. If nitrous is to be used then it is better to use a big shot of nitrous and a lower compression than a high compression and less nitrous. With CR much over about 12.5/1 the cylinder can spike some incredibly high pressures and this really marginalizes piston, ring and bearing life.

Conclusions.

With over 18 months experience with both the iron and aluminum Dart heads on better than a half dozen engines we can come to some pretty conclusive conclusions. We have used these heads in a variety of forms, even to the extent of porting a set of small port Pro 1’s and installing 2.350 intake valves for a short cammed 500 incher. In every case we have used them we have not deviated much from Darts basic port and chamber form. In every case results were strong. At this point in time I would have to say that these heads appear to be about fail safe in terms of results. If the cam and compression is in the ball park and a decent intake is use then it will make strong power. Get everything right and the result is outstanding output. Based on performance to date I would have to award the Iron Eagles and Pro 1 a solid 4-1/2 star rating bordering on 4 -3/4. Who knows – a little more experience with these for us to refine what we are doing and they could hit the jackpot at five stars.

Where too From Here?

This somewhat lengthy product report may look like we have investigated Darts big block heads to the limit of human endurance – but this is far from the case. Dart are into big block stuff big time. What we have looked at here is essentially their high performance street and entry level race stuff. Check out their web site at dartheads.com and you will see there are a bunch more variants including Pro Stock style equal spacing heads with almost 2-1/2 intake valves and 500 plus cfm of flow.

Here’s the chamber of an 18 degree Dart head. Compare this to the chamber shown about half way through the article.

We will be looking at these down the road a ways and but before we get that far from a stock configuration I plan on doing a story and flow tests on Dart’s halfway house heads. These have been ‘rolled’ to 18 degrees so they produce a shallower chamber for a better shape when high compression is called for. Basically they flow about 6% more air and can be used with much of the external hardware used with 24 degree heads. The claims are these heads are worth about 40 hp over the Pro 1’s. Having had experience with Darts realistic claims I am looking forward to a build with these heads.

[Stan Weiss]

Just to give you an idea of how undervalved a big block is we would, even in the modest 482 incher we are dealing with here, have to install an intake of over 2.8 inches to produce the same valve breathing area per cube as a 350 inch small block with a 2.02 inch valve.

Hi David,
Its late and maybe I should wait until morning. But by my calculations you would need a 2.37 valve for your 482 to equal the 350 with 2.02.

On the subject of wet flowing. Both what you have written here and in your other article and in things I have seen written by Darin Morgan, all of the studies are of the curtain / window area and the combustion chamber. With the ability to CNC ports so easily would there be a reason to CNC the port from clear plastic and study the flow through the port.

Best regards,
Stan

[DavidVizard-GFN]

Stan,

That would be the way to go but it is not quite as easy as it might seem. Polishing a machined port made of plastic is quite a long job. However it becomes much more easy as the form is simplified. This is what we did with an Aston Martin deal when I was doing a spell back in the 90's consulting for one of their satalite companies responsible for much of the development on vintage race engines. The wet flow testing and the consequent changes in the manifold plus the combustion chamber was worth just over 25 hp on a straight six.

As for the valve area discrepance you mentioned I was refering to available area - that's the curtain area as the valve lifts.

DV

[rskrause]

Mr. Vizard: thank you very much for the information. I am looking forward to your write-up of the aluminum heads. I am using a set of 325cc Dart Pro 1 heads on my BBC bracket motor (60 over 454 on methanol with EFI). They were installed "out of the box", with no porting - I had to replace a cracked heads and was in a big hurry to get back out to the track before the season is over while keeping costs to a minimum. Interestingly, they out performed the old heads right out of the box even though I lost a full point of compression (from 13.5 to 12.5:1). Peak hp on the chassis dyno did not change, but there was a consistent 30-40 hp and torque gain in the middle. The car is ~0.1 seconds quicker in the 1/4 mile. The old heads were a very nicely ported set of 325cc rectangle port OEM castings.

In the off-season, I am going to get the heads ported and milled to gain back the lost compression. Do you have any infomation you can share about Dart's CNC porting?

I know you have always emphasized cost-effective modifications in your writing and I really appreciate that. It is much easier to make hp by just throwing money at a problem, but less interesting and for many of us is not an option anyway. A number of years ago a buddy built up a SBC pretty much following the advice you published. That was a great bang for the buck motor. Spent about $2,500 and it made power like a crate motor of nearly three times the price. We were pretty proud of it, though really the credit goes to you.

Richard

[DavidVizard-GFN]

Richard,

Glad the small block build worked out for you.

Just for the record the ratio of compression lost when you swapped to the Dart heads was probably costing you about 15 -20 hp if the cam was of any respectable race duration - more so if it was also on a wide LCA.

As for the porting on the as cast heads I can tell you that we have hand ported a couple of sets of these Pro 1's. With just the most basic of porting they responded very well to the tune of about 35 hp on a 11/1 500 inch motor. As for Darts NC ported BB heads we have flow tested these (will be feature soon in the second half of this story) and have recorded numbers within a percent or so of Darts. We have not done a back to back test but have seen over 850 Hp from a 10.5/1 540 street motor along with really good lbs-ft.

DV

[jerminator96]

I can't wait to read part two, were you planning on giving us your opinion on Dart's "Big Chief" heads? The numbers (that they give us anyway) look amazing.

~Jeremy

[DavidVizard-GFN]

Jeremy,
be sure to check the story again as part 2 is there.
DV

[hsutton]

I have heard from an aquaintence that the CNC 335 & 355 heads use a different casting than the regular 310, 325 & 345 cc heads. Something about the regular castings were prone tro breaking through if the CNC port profiles were attempted. Don't know if it's true or not.

[DavidVizard-GFN]

Originally Posted by hsutton

I have heard from an aquaintence that the CNC 335 & 355 heads use a different casting than the regular 310, 325 & 345 cc heads. Something about the regular castings were prone tro breaking through if the CNC port profiles were attempted. Don't know if it's true or not.

It's quite usual for companies to produce the CNC version of a head from a casting that is much smaller to start with. It's not so much a problem of going thiorugh the casting but one of unmachined 'shadows'.
DV

[nomad]

Thanks for the info on the Iron Eagle BBC heads. I've gotten to the point where I want to do my bracket racing on a budget. The 308 cc heads would go just fine on the 496 I'm planning and gathering parts for. My only concern is that I want to use these with 93 octane pump gas. Would that be a problem?

Bruce