Optimal Pre-jetted Carbs - Reality or Myth?

[DavidVizard-GFN]

On the Money Carb Jetting – can you buy it already done?

The reality of building a performance 4 barrel carbureted V8 is that the last operation is inevitably the set up. To make sure all your time and financial investment delivers as intended the carb and ignition will need to be dialed in. The time honored way is to go to the drag strip and flog until no further increase in trap speed or reduction in ET is seen. That takes time and money as it costs for the trip to the strip, track charges, tire wear, fuel etc and may take a couple or more visits to get it done. Don’t get me wrong, this may be fun especially if all the calibrations are near but it does absorb time and money. If everything was real close you could have been going faster sooner. Then there is the chassis dyno. This definitely costs money but get with a dyno operator who knows what they are doing and the results can be very satisfying. It’s more a case of arriving at the track and hauling rear end right now.

Track or Dyno – It’s Money Spent

Track or dyno - either way you can spend between a couple of hundred bucks to as much as 600 or more to get that tune. Now I am not suggesting here that the final carb and ignition tune up can be bypassed completely. Dialing the ignition in should be no big deal. Start with it a little on the retarded side then work on the carb jetting first. Do not do things around the other way. The flame speed changes with the mixture ratio so the optimal timing will be slightly different for any jetting more than about 4 numbers or more off optimum. Once the carb has been optimally jetted the ignition can be progressively advanced until the best setting is found.

Although all the forgoing may be telling you little you do not already suspect the bottom line here is that in this complex world there are many hot rodders who may have expertise and skills in many disciplines but optimally calibrating a 4150 Series Holley is not one of them. Sure just about every hot rodder over the age of ten knows that if the mixture is lean put in bigger main jets – if rich put in smaller. That is about as basic as it can get and all it will do at best is barely get you by. If we are building performance then the issue is reduced to building the best that your budget will allow.

Understand this. I have spent many years as an engineer redesigning high performance carbs for factory teams. The Holley carb not only represents a heck of a carburetion deal for the money but also, when correctly set up, a highly effective means of mixture calibration and delivery. That is why it has survived this far into the fuel injection age. For as simple as it is a single 4 barrel carb can play a key role in the delivery of some pretty spectacular output figures as a modern Cup Car engine well demonstrates. Starting at the top of the carb let’s look at the calibrations that need to be optimized on your Holley to make it run as good as it should.

A major factor in overall performance is the size of the main venturi and the booster it is paired with. This combination has to develop a good signal and that means not too much and not too little. Next few factors can be lumped under one heading – the mixture ratio and curve. This, under the various conditions encountered while driving, can vary substantially. It is ultimately a sizing combination of a main jet, power valve restriction channel, and air corrector jet and an emulsion well hole pattern that dictates the ability of the carb to do the right thing under all conditions. Now Holley knows all this and they are very capable of shipping you a carb that runs just fine. But we are hot rodders and we want more than just fine - we want the best our money can buy. So as to minimize returns what Holley ships out of the door to a regular customer is largely on the conservative side. And by conservative I mean a smaller cfm than maybe required for the best output before drivability is compromised. If you go on line and use their excellent carb selection program you will end up usually with a choice of about a half dozen or so carbs. Which ever you choose it will probably work just fine. So we are now back to the ‘just fine’ category and all this begs the question as to whether or not a Holley carb specialist can do a better job.

Getting the Job Done – Fast.

T&L engines in Stanfield NC dyno’s as many as 4 engines per dyno per day. These are custom crate motors and each is a little different. Since the customer is buying from T&L to get something substantially more than just a ‘crate motor’ T&L has to deliver and the dyno sheet has to show just what they deliver. For them time spent calibrating a carb with jets other than what came in it is money out of their pocket.

Since I get my block machining and balancing done at T&L I am in and out on a regular basis. It was not long before I realized that most of the engines are sold with the carb and that the calibrations were untouched from however they came out of the box. I knew that T&L were not letting the engines go with mediocre calibrations as they use Innovate Technology’s data acquisition O2 sensor mixture analyzers which readout the mixture through out an entire dyno pull. Their target for a passing calibration is 12.8 to 13.2/1. This is the span over which power is little affected by a change in mixture ratio. In other word going from 12.8 to 13/1 will not show any measurable change in power. (although it has to be said that BSFC will be marginally better toward the 13.2/1 ratio without a power penalty). I also noticed that the carbs in question came from AED Performance in Richmond, Virginia.

At this point let me make it clear that speed of carb calibration is very important to me because time is at such a premium. I noticed after a while that something way less than 1 in 20 of these AED carbs needed to be re-jetted. On the relatively rare occasion that re-jetting was required it only amounted to about 2 numbers up or down to bring it into tolerance. All this is pretty impressive so I thought I would start my own investigation as too how well AED can serve it’s customers. Here is what I needed to know. Can they firstly supply a really top notch recommendation on what carb should be used. Next can this be very closely calibrated even if the engine is a little off the beaten path in terms of spec. Lastly, can they do this at a price that is acceptable and to the point we actually save money by spending less time at the track or on the dyno simply calibrating. That’s a tall order and a project I started on about two years ago. Since than I have used AED carbs on about a dozen engines and I have to say I am impressed and when it comes to carbs I am far from easy to impress.

Cost.

For the regular hot rodder it matters little how good a product is if it is too pricey to afford. T&L sells custom crate motors with a dyno sheet for, as often as not, less money than a factory crate motor (and with a lot more output). This cannot be done if the carb is even a little too pricey. It turns out that AED’s ‘HO-Series’ which are their entry level carbs, (which in my experience run great) are about the same money as a stock Holley from a discount speed shop. This apparent anomaly actually has a simple answer. AED does not buy carbs from Holley. Instead they buy huge quantities of parts, the bulk of which are raw castings, which, as it happens, Holley sells at very competitive prices.

Bottom left to top right. AED’s wet flow bench was, like so much of their specialized equipment, built in-house. All the parts machining is done on one of several CNC machines as seen here. Every carb that leaves AED’s shop is run on on an engine on the dyno. That is a 100% product inspection!

The custom Holley carbs AED sell are made up of both stock and in-house CNC machined Holley parts plus a large number of in-house manufactured AED parts. By starting from the parts bin they can put together, on their own ‘assembly line’, a parts match that is just what they reckon the customer needs - even at the most basic entry level carb. In addition to this they build carbs in sufficient numbers for ‘economy of quantity’ factors to weigh heavily in favor of a cost effective product. By the same token they are not so big as to loose sight of the individual customer and that has made the company the success it is today.

Function.

Ok we now have at least a handle on how AED gets down to a competitive price. The question now is what do AED actually do in terms of carb build and function that results in an advantage to the end user. First you have to be able to drive the car so AED’s calibration priorities go like this: idle, transition, cruise and finally Wide Open Throttle or, as it is commonly known, WOT. Of the dozen or so engines I have used AED carbs on the idle has been good as has the transition from idle to cruise. Here we come to an important part of a carbs calibration in a world where fuel costs are escalating on a near monthly basis. To get good fuel economy the part throttle mixture needs to be quite a bit leaner than at full throttle. Typically 16 to 17/1 air/fuel ratios are what is used although, with a killer ignition system, a kind of lean burn situation can be had where the mixture can be leaned to as much as 19/1. This cruise mixture is achieved by calibration of the main jet (this should really be called the cruise jet). This means any changes to the main jet also affect the cruise to about an equal or even greater extent. Where WOT calibration should be done is the Power Valve Restriction Channel (PVRC). The main jet should be sized to deliver best fuel economy consistent with good drivability and the PVRC should then be sized to give the 12.8 to 13.2/1 air/fuel ratio for max output. At AED the focus for cruise is the main jet and for WOT the PVRC sizing. By really refining their in-house calibration techniques AED can deliver great hp along with better than average fuel economy. To refine there product AED not only does a lot of dyno and track/road testing but they also built their own wet-flow bench specifically for carb development. This is fully computer controlled and can measure air/fuel ratio’s throughout the operational spectrum. The claimed accuracy/repeatability of this bench is to within less than ½% so even very small changes can be measured.

Camshafts and compression ratio can be major effects on the resulting air/fuel ratio as rpm increases. When we talk about a fuel curve what we really want amounts to about a dead straight line. A fuel curve of 13/1 from bottom to peak rpm is just that – a straight line. Getting it is no easy task. It involves the design of the booster, sizing and placement of the holes in the emulsion well, the air corrector size, and the main jet and PVRC. Getting all this right take real knowledge of carburetor design. Added to that knowing what it takes for a very wide variety of engines requires a lot of experience. Some of the calibration techniques AED does also make the carb less sensitive to a spec change. This it appears is inherent in their calibration procedures. The end result is that a relatively big change can be made in one or two area’s and the carb still delivers very well. An example here is we used the 750 HP with the 850 base plate on a hot 350 that, with the calibrations of the 408 test engine I am about to describe, cranked out some 520 hp. The mixture came in at an average of about 12.6 and the subsequent jetting session to 13/1 only netted us another 5 hp.

After acquiring quite a bit of experience using AED carbs I felt it was time to put their carb sizing and calibration skills to the test with an engine that was somewhat less than main stream. For an engine that falls into such a category we had a big cammed stroker 350 that, with a 4 inch stroker crank and a 30 overbore, topped out at 408 inches. This engine was intended to test the power production capabilities of AFR’s now very popular Eliminator CNC heads. A publication that fails to test a piece of speed equipment properly does disservice to all concerned. The Eliminator heads had already proved they were good and what we were doing here was to establish just how good they were. This meant that the valve train and induction had to be up to par with the already proven performance of these heads. So our 408 inch test engine spec was AFR CNC heads, 10.5/1 CR a Comp Street roller of 248/252 duration at 50, a Holley Strip Dominator, (we are seeing some good dyno numbers from this intake by the way) and 1-3/4 inch Hookers. That info was fed to AED along with a request for two additional carbs each about 75 -100 cfm smaller than the one they recommend. The test was to see how well the carb they recommended was both sized and calibrated for the application.

What we got was an AED HP 750 (dry flow 803 cfm @ 1.5 inches Hg) a 750 HP with an 850 base plate (dry flow 901 cfm) and an AED 850 HO (dry flow 934 cfm). The reason for quoting dry flow is that we do not have the type of bench that can wet flow a carb (it’s not quite like wet flow testing heads). For the record the wet flow is about 8% less than the dry flow. This would make the numbers 739, 829, and 858 respectively. These numbers can be compared directly to Holley’s rating numbers. We also know going in that the combo would run better with a spacer. For this a 4 hole to open two inch spacer was used. This put the flow of the 850 HO carb up to 946 dry cfm which equates to 870 cfm wet.

Dyno Time.

Here it is best to look at graphs but a few round numbers will set the scene. First the AED HP 750 (803 cfm dry flow) carb. This went onto our air hungry 408 and resulted in a respectable 518 lbs-ft and 575 hp. Not to bad for a starter. The air/fuel ratio was 13.2/1 and held to close limits throughout the pulls we made on it. Next came the AED HP 750 with the 850 base plate. This pumped the output up to 529 lbs-ft and 579 hp. The air/fuel ratio ran right on the rich limit at 12.8/1. Next came the 850 HO which also delivered a 12.8/1 air/fuel ratio. This carb gave our 408 the lungs for 532 lbs-ft and 586 hp. Next the spacer was added which, it will be remembered, bumped the flow buy some 12 cfm. With this we saw the mixture lean slightly to an even 13/1 average and the power climb to 541 lbs-ft and 595 hp.

At this point we see that AED's recommendations and calibrations were very good and this follows what we have seen on all previous tests. To see what was gained by making sure a carb of adequate but not excessive CFM was used it is best we look at the graphs of the power and torque curves. First the torque curves. These are shown below.

From these curves we can see that over the range of 3500 to 6800 the biggest carb plus the spacer got the job done the best. Down at the 2900 rpm mark the smallest of our test carbs began to show a small advantage over the bigger ones which indicates that if drivability down at these rpm is a requirement the 850 HO and spacer was as much carb cfm as we should go. As far off idle drivability goes this is hard to establish for certain on the dyno but I like to think the thousands of pulls I have done with Holley carbs would at least give me a fair indication of what was working here and what was not. No matter what rate the throttle was opened – slow or fast against no load or moderate load there was no hesitation with any of these carbs. This indicated the pump jets were also well sized as did the lack of over enrichment as indicated by the Innovate O2 mixture data logger.

The power curves resulting from the preceding torque curves are as per below.

As for power we can see that the AED 850 HO plus the two inch spacer paid off over the entire rpm band width that would be used on the track – 5000 to 6800. Peak power difference from the smallest flow set up to the biggest was a worthwhile 19 hp. However the power difference at peak rpm was up from 522 to 584. That is a 62 hp advantage to the bigger and more optimally, in terms of cfm, specced carb.

Other than set the idle speed and mixture we did nothing to the calibration of our AED test carbs. Since our tests have covered such a large number of carb/engine combos and the only results seen have been outstanding and, other than some big smiles from big numbers, drama free, GFN has to award AED a 5 star rating on all the AED carbs we have tested to date.

The 850 HO carb used for the final numbers would have cost you $570 from AED’s shop in Richmond. That is a bargain price to pay for a carb which has a way better than 95% chance of being right on the money right out of the box. Does the 5 star rating mean this is a perfect product? Not really. When we humans make anything there is always a possibility, no matter how slim, that it may not work as intended. This five star rating was earned on the premise that this was about as good as it gets, or to put it another way, the chances of your AED carb not working as intended are really slim.

So that AED don’t end up with an overloaded switchboard they have asked us to include the following carb spec request sheet as shown below for anyone wanting to purchase a carb. Just fill this in and email it to them and you will get a carb part # and price within 24 hours.

Spec sheet will be posted by 1st Jan

[Ron Golden]

David,
Am I correct in assuming the sag in the TQ curve from about 3750-4250 rpm is caused by the incoming air velocity being too low to effectively generate a good booster signal? In other words, over carburation at that rpm?

As usual, a very good article.

Ron

[DavidVizard-GFN]

Dear Ron,
No - that dip is a common feature of the intake length and cam timing used. We can juggle some of it out with the exhaust but there is a loss elsewhere.

DV