We just finished up the dyno testing of a mildly-built Pontiac 400, and the results follow…
The engine was a 1976 Firebird 400 – the mighty 185-horse with an actual compression ratio of 7.1:1. I basically converted it into a 1967 335-horse (gross rating) GTO configuration with the “670” closed chamber, large-valve heads, which ended up producing an actual compression ratio of 10.08:1 with flat-top pistons and the stock deck height. The bottom-end was kept stock with some good rod bolts, and the heads were given a good 3-angle valve job. The restrictive ’76 EGR intake was scrapped and an Edelbrock “Performer” intake was used to keep overall engine height near-stock so that the ’76 Formula Ram Air System could still be used. This intake is no better than a stock ’67 Pontiac Q-Jet intake, and may actually flow a little less, but it was the quick and easy way to get this thing put together. The owner wanted a very mild cam to maintain engine vacuum and power brakes, so a mild hydraulic roller with specs similar to a Ram Air IV was used. No other trick parts – the engine was pretty much a 1967 GTO 400 with a mild roller in it. Here’s the engine loaded in the back of the truck and heading to the dyno shop:
We had a little fun here on the Forum earlier with several people guessing torque and power numbers. Most people, including myself, were guessing pretty high. The problem is with the stock casting “670” heads, which simply don’t flow very well without some extensive port work, so we were hoping for “net” power numbers that would at least exceed the factory “gross” 335 rating. There was no porting work done to the heads – they were in the stock, as-cast condition. The winner of the dyno guess contest is a split win: Lars (me) guessed 400 ft/lbs torque, and we hit a max actual of 398.9. facn8me had the closest horsepower guess at 370: we hit a best actual of 346.5. Good job on the guessing, guys – lots of fun seeing the numbers!
In addition to getting a good tune on the engine, we also wanted to get a back-to-back comparison of a well-tuned Q-Jet to a properly set up Holley of similar size. Those results came in as expected…
I was so busy getting the engine set up and ready to pull that I didn’t get photos of the engine fully set up on the stand, but here is the engine coming off the dyno right after the final dyno pull with the Holley still on the engine:
For the first “out-of-the-box” dyno pull, the stock ’76 Q-Jet was used (17056263). This carb is factory jetted at 70/42/DB, which is awfully lean. I re-jetted the carb to 76/42/DB with a 3/8” float level and .640 secondary rod height. Other carb tuning and parameters were set up exactly as outlined in my Q-Jet Tuning Paper under the “Quickie Performance Setup” section.
Total timing (stock ’76 HEI distributor) was set to 37 degrees, and we were using 91-octane pump gas with 10% ethanol – right out of the gas station down the street. Total timing was coming in before 2000 rpm – a very quick curve.
Once oil temp stabilized, we did 3 back-to-back initial pulls running the engine right up through 5800 rpm. The pulls all verified the repeatability of the data, producing virtually identical numbers. Those numbers said the engine wanted more timing, so we bumped total timing up to 41 degrees and pulled it 2 more times to assure that the data was backed up. The change in timing made a big difference across the entire rpm range, and we had no indication of any detonation, even at wide open throttle under load at 2000 rpm. The first chart below shows the comparison of the first runs at 37 degrees timing to the change going to 41 degrees. The peak numbers are highlighted in yellow, and although they are not huge, the total “area under the curve” changed across the entire rpm range, making the engine much more responsive. With the same settings on the dyno program as on the 37-degrees runs, the dyno couldn’t hold the engine under 2200 rpm with the new timing setting, resulting in the “curve blip” at the start of the 41-degree run (the red lines are the 41-degree timing lines):
Next, we wanted to address the “dip” in the torque curve (with resulting “sag” in the power curve) between 3500 and 4000. This is right where the Q-Jet secondary airvalve is starting to open up, and the air/fuel numbers were showing a lean condition right at the tip-in of the secondaries. The stock “DB” secondary rods in the carb have the “short” power tips, which delay fuel enrichment. We installed a pair of Edelbrock “CL” rods which have about the same diameter (just a small tad richer), but they have the “long” power tips. We also re-adjusted the bottom-end of the dyno brake to allow it to hold the engine below 2000 rpm at WOT. As you can see, richening up the secondary tip-in point really flattened out the torque curve and made the power curve a near straight-line shot towards the top – this car is going to feel like a rocket as the rpm comes up, with no sag or let-up in the power…(the black lines are the "long" CL secondary rods:
The final run was done just to prove one of my pet-peeve points: Q-Jets are garbage, and any Holley will outperform a Q-Jet… blah, blah, blah… As I state in all my seminars and papers, a properly tuned
Q-Jet will perform almost identical to a properly tuned
Holley, except that the Q-Jet tends to produce better torque and throttle response in the low and mid range in a street driven vehicle than the Holley. Now that we had our Q-Jet pretty well dialed in, it was time for a switch to the Holley.
The Holley we selected was a 3310-1, which is a 750 vacuum secondary with the secondary metering block (not the cheap plate). The carb was set up and tuned to spec with 72 primary jets, 80 secondary, and the “tall yellow” secondary spring was selected to assure that the secondaries would actually open. Choke system was removed to give it all the airflow advantage it could get. Float levels, shooters, and all tuning parameters were all set up correctly and verified and “blessed” by noted NASCAR engine builder Steve Yacki (who was also our dyno operator this day). The Holley was given 2 pulls on the dyno, and we verified that the air/fuel ratio on the Holley was virtually identical to the Q-Jet: The A/F number matched exactly at many data points, and were never different by more than 0.5:1 at any time through the entire rpm range – the 2 carbs were metering air and fuel at exactly the same ratio, so there was no “fudging” any numbers on these pulls.
The Q-Jet bettered the Holley by 15 ft/lbs of torque on the bottom end, and pulled more than 20 ft/lbs at many data points, with a 10 to 20 horsepower gain over the Holley at many points through the range. The Holley produced a couple of peak numbers at limited points, but not enough to give it any advantage over the Q-Jet. The mass airflow numbers through the Holley were also lower than the airflow numbers through the Q-Jet at high rpm, and this can be seen by the drop-off in the Holley performance at the top of the curve. We even did one Holley run with me forcing the secondaries fully open for the top Ĺ of the rpm range, but this killed all the top-end numbers completely: The carb liked the “tall yellow” spring.
The numbers between the two carbs are fairly close, but if these 2 engines had been installed in 2 identical street cars, the Q-Jet car would have come across the finish line ahead of the Holley car by a significant margin.
This is not to say that either carb is “better.” But the fact is, that if you set these carbs up correctly, and know how to tune them, they can be made to run very well. There is no point in replacing a badly-tuned Q-Jet with a badly-tuned Holley or vice versa. A well-tuned Holley will run better than a badly-tuned Q-Jet, and this is what is usually the case in the “Holley is better than that crap Q-Jet” argument. If you have a good Q-Jet and know how to set it up, run it on your street car. If you don’t have a carb, or don’t know how to tune a Q-Jet, a Holley will run fine, but you better be able to tune it, too (black is Q-Jet, green is Holley):
A good dyno day with good numbers on a mild street engine build-up. The stock iron heads and intake are really choking this engine down on its potential, but it will still feel very good in a 4-speed street car: It looks like a completely stock ’76 smogger engine, complete with EGR, but it’s going to be a real surprise to some people on the street…