Restricter Race Motors -make more power than your opposition

[DavidVizard-GFN]

Restricted Race Motors – Make more power than your opposition

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By

David Vizard

Half an hour ago I had no idea I was going to write this feature but a few posts asking questions on restricted motors made up my mind. So with such short notice don’t expect any fancy photo artwork and the like. If I get around to that later then so be it if not –tough it out!

I am not going to claim I am the worlds last word on restricted motors but the limited experience I have had in this area has shown that I at least have a competitive handle on the situation as the engines I have done have run up front.

My first restricted motor was the building of an 850 mini race engine where the rules call for all the air to go through a stock 1¼ inch SU (later revised to a stock 1½ inch SU). Also, for this class we had to use a stock cam (252 seat to seat duration single pattern on a 110 LCA and a little over 300 thousandths valve lift).

What with head porting, retiming the cam, low friction pistons and ring pack (for the day) plus all the compression we could find, open exhaust etc the engine went from 20 hp at the wheels to 44! Sounds good but about 5 years later ace engine builder Dave Mountain was making 54 at the wheels from his championship winning engines – now that’s a real achievement!

My next motor was a 1000 cc Ford Anglia based F3 engine where all the air had to pass through a single 22 mm venturi. This motor was just a fraction shy of 110 hp at about 10,000 rpm. It also cost an arm and a leg to build so the restrictor failed to keep costs down if that was the intent.

After this I built a few engines which would be more along the lines of semi-restricted as there was a fair amount of carburetion but not as much as would be optimal. My British Touring Car Championship engines (pushrod 1600 cc units) are an example here. The 40 DCOE Weber carbs had to be used with a 30 mm choke although a 34 made the most power. With those 30 mm chokes a 9.5/1 CR and a valve lift limited to 0.390 the final engine of the year made 156 hp all from 1600 cc. That was with a wet sump, stock crank, rods and pistons and a lot of other rule restrictions.

After thus came the V8 era and what I learned from previous engines got applied here and seemingly successfully at that.

Carb Restriction & Flow Basics

OK- so much for the background - let’s start with the source of all the problems of a restrictor – the restrictor itself. The whole point of the rules calling for an intake restriction is to a) limit power and b) limit costs. For the moment at least lets forget about limiting costs as ultimately a restrictor does little to limit things in this area.

If a restricted intake is called for he who can, to the greatest extent, defeat the effect of the restriction wins. What I am now going to do is to start on all the techniques to do just that.

First let’s look at the characteristics of drawing air through an orifice that is otherwise too small for the job of supplying an engine with all the air it is demanding.

Let’s consider here a carb being tested on the flow bench. Let us say that the carb we are forced to use flows 500 cfm at 1.5 inches of mercury suction or to give it its common technical description, depression. In terms of depression measured as H2O that’s 20.3 inches or really close to ¾ of a PSI suction. If we want to get more air through that carb but cannot touch it then the way to do it is to turn up the bench so it now creates a depression of say 3 inches of mercury. Since the flow through the carb will increase based on a square law function we find that doubling the suction draws in more air at the rate of the square root of 2 (i.e. 1.414) So doubling the suction netted us an increase in air flow of 41%. Now keep this in mind as we move on.


Restrictions in Series.

Let’s look at the induction system now in terms of individual area’s of restriction from the point at which demand for air starts. That’s namely the cylinder. The first point of restriction is the intake valve followed by the intake port in its entirety (that includes what is in the manifold). At the end of each port is the plenum from which all (or half if it’s a two plane intake) cylinders draw. Because the plenum volume is relatively large the runners think that this is ‘open air’ and it is not really seen as a restriction to any significant degree. The next and last restriction is the carb itself.

Let us assume the piston is zipping down the bore and has created say a depression of 3 inches mercury or about 41 inches of water. The intake valve is the first restriction here so let us assume that as such only 30 inches of the vacuum in the cylinder are communicated to the intake port so now our ‘demand signal’ is down from 40 inches to 30. Also lets say that due to restrictions in the port that only 20 inches of the signal are communicated to the plenum. At this point we can see that the depression in the plenum used to pull air through the carb is now only 20 inches. So our 500 cfm carb is flowing air into the engine at a rate of just on 500 cfm.

At this point let us say you are idly wandering down a beach and stub your toe into an old oil lamp. You pick it up and rub the sand of it and presto out comes the Genie and gives you three wishes. Being a frustrated restrictor racer you ask for what would otherwise be impossible. A set of heads and an intake manifold that had no flow restriction. Let’s apply that unlikely scenario to out hypothetical engine. This time when the cylinder demand is 40 inches the entire 40 inch depression is seen in the manifold plenum. This now means that the carb is being drawn on with twice the suction. Result – it now flows 705 cfm into the engine. Will the engine make more power – you can bet on it for sure!

Rule # 1

So from the forgoing we can see that to defeat the effect of the restricted carb as much as possible we need to communicate cylinder demand to the plenum as efficiently as possible and that is rule #1. What does this involve? Again let’s go step by step through the induction system.

Understand that the most restrictive area of the induction (and the exhaust for that matter) is the valve. Here is what we need to do with the valve to make the best of a restricted carb situation. First the valve needs to have a high flow efficiency throughout it’s lift range and secondly we need to have as large a valve as possible consistent with each larger increment working. In other words we need to keep on stepping up the valve size until we find the point at which no further gains are seen. Now all of this has to take place in conjunction with the intake ports cross sectional area. Here we will inevitably run into a dilemma where compromises must be made. Keeping in mind what I have just said let’s divert here and look at the reasons for these compromises.

Our goal here is to make torque and hp and to do this we need to pass as much air through the engine as possible. Part of doing this successfully is to make sure that we only allow the air move through in one direction. In other words no reversion. To get as close to this as possible we must be aware of three significant factors. The first is that an intake valve almost always works better as an exhaust valve therefore gasses can flow from the cylinder into the intake manifold better than the other way around. That’s the first factor.

The next factor is a result of factor one. Because an intake flows better as an exhaust it means if we hold the valve open too long after BDC the pressure in the cylinder will easily push out some of the charge we have worked hard to get in – and it does this easier than you may have at first supposed. From this it follows that the point of valve closure is critical. In addition to this the optimal point of closure changes quite dramatically as the intake manifold vacuum changes with rising rpm.

To minimize the negative effects of factor one we want an intake valve, that, due to it’s size/efficiency and rate of opening/closing, changes the rate at which breathing is presented to (on opening) and taken from (on closing) the cylinder as rapidly as possible. The ideal here is an intake which opens to full lift instantly and closes instantly. That’s an impossible situation but it is non-the-less our goal. Now allowing the forgoing here is where our need for careful compromise comes in so on to factor three.

Air has mass and it is much heavier than you might suppose. When I was lecturing at various Universities and colleges I would ask my audience if any one could tell me how heavy a cube of air 100 feet by 100 feet by 100 feet was. Even though this question was posed to thousands of people – most of them Pro engine builders – no one came even close to giving the right answer. Typical answers were from between 100 and about 600 lbs. The real answer is 38 tons! Now I’m sticking that number in front of you for one reason and one alone – and that is to show you that port velocity and charge ramming due to such is not some mythical entity that might exist or is of minor importance. What we need for a port/valve combination is a port with as small a cross sectional area as possible without undue compromise in terms of flow and an intake valve which has as high a discharge co-efficient as possible throughout the lift range. What we are essentially seeking here is a ‘maximum energy’ port. That is mass flow times velocity squared comes to as big a number as possible.

Summing up Factors #1,2 and 3.

At this point we are looking for a cylinder head and an intake runner that has a high flow valve connected to a port that flows as much as possible yet with a high port velocity to minimize port flow reversion. Remember when we talk of an intake port here that includes what ever is in the intake manifold. Velocity probing intake ports to find and fill the low activity area’s can help immensely here.

Camshaft Events.

Now we come to a tricky subject here. However there is one thing going for us. The better we do the job on the intake valve/induction tract the less fussy the cam events are. That is not a ticket to pass go and collect $200 though. Getting valve events optimal still involves a major effort. The first factor here is that the intake valve acceleration too and from the seat needs to be as high as possible. The shorter the cam events are and the faster the valve opens and closes the less of a problem there is toward making good torque numbers over a wider range. If the exhaust is doing a decent job of sucking down the cylinder during overlap the amount of overlap won’t be too different from what we may see for a similar but unrestricted motor with those same heads on. What will happen though is that the intake will need to be closed earlier. This gives us a cam with a tighter LCA than would have otherwise been the case. If the exhaust is restrictive it will drive the exhaust through the intake easier so the amount of overlap must be cut. That makes for a cam with a wider LCA.

The higher the compression used for a restricted motor the better, this being so from severable stand points. First if the cylinder isn’t filled very well you had better squeeze whatever is in there really tight! Secondly the smaller the chamber volume is the higher the speed of exit of the end gasses at the time of overlap at TDC. This makes it harder for exhaust reversion to set in.

Exhaust Port and System.


Minimizing exhaust reversion need to be at least as high a priority as exhaust port flow potential. The higher the compression the less sensitive the exhaust is to outright flow capability. The key here is to get valve events and header/collector dimensions to scavenge the cylinder as well as possible. This is very important if the engine as a whole is to be successful. A race engines induction is run by courtesy of the exhaust in the first place and piston displacement in the second not, as is often supposed, around the other way. If the carb is small then the header diameters also need to reflect this. As for primary lengths these are far less critical in 95% of cases than the collector length. This is fortunate as testing various header primary lengths can be very costly and time consuming whereas collector lengths are just a piece of pipe you can slide back and forth until best results are seen.

Operating Temperatures.

Up until now we have talked about induction performance in terms of CFM induced. The fact of the matter is we are actually trying to induce the greatest weight of charge. The cooler the air is the denser and consequently heavier it is. What I tell people here is that if the cubic feet of air is limited by a small carb then make each cubic foot smaller by cooling it down so more air goes in. An oversimplification yes but even Jacque, my 12 year old daughter, understood it in those terms. When I am building a restricted motor I go to extraordinary lengths to keep the charge cool. Not only does this help the charge weight drawn in but also it has a small effect on increasing the CFM drawn in as air, unlike oil, gets less viscose as the temperature drops. Net result is cold air makes it around corners with less resistance than hot air.

Fuel Volatility

There is also another reason for keeping the charge temperature down If fuel vaporizes in the intake manifold it takes up room that could have been occupied by air passing through the carb. In other words some of the vacuum caused by the cylinder demand is satisfied by fuel vaporizing into a gaseous state. The fact a partial vacuum exists in the intake at high speed means that a lot more of the fuel vaporizes. This you need to counter by using a fuel that has far less front end hydro carbons in it. I have a great story about fuel volatility, race rules and inflexible tech inspectors here – but because this is a rush to finish today deal this may not be the time to air it.

Anyway if you check the fuel rules and find that they prohibit adding anything to the fuel as a sole stipulation then you should be able to blow down a 55 gallon drum to get rid of the more volatile front end stuff. It varies from fuel too fuel and the amount of manifold depression seen but in the past I have blown off as much as 20% of a 55 gallon drum (not too green but it wins races even when up against highly illegal engines/cars and it’s no where near as polutive as a couple of top fuel cars making just one pass). The result is a solid 10 lbs-ft and sometimes as much as 15 every where in the rpm range.

Also you do not want a carb that is too good at atomization of the fuel. This does not mean having globs come from the booster but it does mean building a booster that produces larger than average droplets over most of the size range involved.



Ignition requirements.

The more effectively you apply the points I’m making here the greater the useful rpm becomes. What happens is that the engine sucks harder as the rpm goes up yet the increase in flow does not drop quite as fast. The result is that over the rpm range used the manifold vacuum may go from a moderate level to as much as 9 or more inches of mercury. This will mean having a distributor that advances the timing farther up the rev range to the all out point.

Internal Friction and Component Geometry

An engine can loose a considerable amount of power to internal losses both pumping and friction. The pumping losses on the intake side are almost inevitable but those on the exhaust are not. The exhaust should be at least as efficient as that of an unrestricted motor unless rules call for iron manifolds.(If they do you should be using Randy Brzezinski exhaust manifolds – they are worth a bunch of power).

A big issue here is piston assembly to cylinder wall friction. Every effort possible should be made to reduce this to a minimum as it is a direct pay off.

Last on my list is rod/stroke ratio’s. It never cease to amaze me how often people think there is one solution for all situations. How often have you seen it said that a long rod is best period - or a short rod makes more low end. The comments on rod issues abound. The problem is that it’s not that simple. If there was no friction involved the best rod stroke ratio for output on a restricted motor would be had from a very short rod – but guess what? An engine with no friction does not exist. The longer the rod is the less the rotating assembly looses to friction from the piston being thrust into the cylinder wall from rod angularity. If the rod was infinitely long there would be no side thrust loads only simple friction drag in the vertical mode.

Let’s go back to the short rod. Why would this be best if there were no friction? With a short rod the piston moves up the bore more slowly from around BDC. What this means is there is less propensity to push any charge back out of the cylinder from a valve that may be still open. This enhances the trapping efficiency of the system. Result more torque everywhere. But the big enemy of a restricted motor is friction, it amounts to a far greater percentage of the indicated output and a short rod is no help there. So we have to look at a compromise. The better the piston and ring pack is in terms of friction the shorter the optimum rod length becomes.

No let’s throw in the CR as a factor here. The higher the CR is the earlier in the cycle the bulk of work on the piston occurs. This means cylinder pressure drops off faster and by the time peak rod angularity is reached the pressure causing side thrust has dropped much lower. If an engine has a whopping 17/1 CR it is not as demanding on rod length in terms of countering friction as would be the case for a low compression engine. Build a blower motor and you had better have some long rods for a 6/1 engine with 35 psi boost.

OK that’s it from me for today on restricted motors. I hope it gets you restrictor guys through the weekend.

DV

[80' 427]

I want to thank you very much david. I have been a pest pming you and asking question in the enigine tech forum. I find these restricted engines very fun from the point of making more from less.

[Dirt Racer]

DV

Thank you for writing this article. I have looked just about every where for articles on restricted motors and had little luck. One question on the fuel, you stated you would blow down a 55 gal drum to get rid of the front ends. How exactly did you do that.

[Stan Weiss]

David,
I must say that when you told me about writing this article this morning I did not expect to see it here today. I believe it took you less time to write it than it did for me to read it. Of course I did take time to calculate the weight of a million cubic feet of air and came up with a different answer until I use 60 degrees with 29.92 BP. On the intake port what average velocity would you be looking to see through the MCSA?
Stan

[Stan Weiss]

David,
Here is graphic which shows how the piston moves up the bore more slowly around BDC with a shorter rod.
Stan

[80' 427]

So to be clear. Even though we are in the hp range to need 1.75 headers the 1.625 headers would be a better choice to aid cylinder filling.

I should leave our rochester with poor atomization because it be slower to vaporize and take up runner volume.

We are moving in the right direction with moving to 2.05 intake valve, but the new longer rods (6.125) will help friction but not will not help with cylinder filling.

[Racebrewer]

Hi Dave,

Great article. I build two-stroke 'spec' kart engines and we have to run spec high octane fuels that get a post race check on a Digatron gage. There is no need for the high octanes in this class. You talked about burning down the gasoline. Is this letting the aromatics evaporate out? I thought we needed these for high RPM operation. Am I wrong there? Do you know how this process would affect a Digatron reading?

Thanks,
John

[DavidVizard-GFN]

80'427

I am surprised you are in the HP range of a 1.75 inch header. Just how much power is your engine currently making?

RaceBrewer,

I have little idea what the blow down proceedure will do too a Digitron reading. Blowing off those front ends will cause the density to increaase slightly but I have never checked by how much.
Just how much needs to be blown off is subject to experiment.

I did this process by drilling a lot of small holes in a length of 1/4 diameter copper tube. I then connected this to a regulated air line, stuck it in the 55 gallon drum and blew air through it (takes a while).
DV

[80' 427]

We are making 269.9 rwhp (approx 375hp) with a poorly cammed 6" rod 383.

[80' 427]

We are using e-10 89 octane (10% ethonal) it makes the same or a little more power on it than 110 or 92. The hope is it will add a little more oxygen in our limited engine.