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Spreadbore due to smaller throttle bore but the question really has too many other variables including type of intake dual plane versus single and type of cam, etc.
From: At my Bar drinking and wrenching in Lafayette Colorado
If both carbs are properly set up, they will run so close to each other that you can't tell the difference. Use the carb you know how to tune. Here are the facts based on my actual dyno testing:
750 Q-Jet Versus 750 VS Holley 4150 – Dyno Testing on a 400
I just finished up another dyno test of a mildly-built 400, and the results follow…
The engine was a stock-block 400 with stock factory cast iron heads. I got compression ratio up to 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 EGR intake was scrapped and an Edelbrock “Performer” intake was used to keep overall engine height near-stock so that there would be no hood interference problems. The owner wanted a very mild cam to maintain engine vacuum and power brakes, so a mild hydraulic roller was used. No other trick parts – the engine was pretty much a modest compression 400 with a mild roller in it.
The real purpose of the dyno run was to get good numbers for the owner and to get a good tune on the engine, but 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…
For the first “out-of-the-box” dyno pull, the 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.
After getting the timing optimized (the inefficient chambers in the iron heads took 41 degrees total timing for best power and torque without detonation – an increase from my initial setting of 37). 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.
The 4 initial pulls we did on the engine showed a “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. 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.
Once this had been sorted out and the Q-Jet was correctly tuned, I wanted to do the final run 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 I 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 numbers 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 ½ 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.
Spreadbore carbs traditionally work better with mild to moderate builds making over 13 inches of vacuum........the tiny primaries create a lot of velocity past the booster and pull fuel instantly......this is carb "signal".
If you can pull the right amount of fuel quickly through the booster......it makes for a snappy engine. The downside of a Q-jet is the metering rods don't play well with big cams and make part throttle running a crap shoot sometimes.....and the tiny float bowl has to be fed very well in high horse applications over a distance.....like WOT throttle 1/4 mile or further. Holley makes a spread bore carb but I feel it is a trade off to Q-jet....it holds more fuel but the boosters are not quite as sensitive........although the Holley works very well, there is no real need for it in a fresh build unless you are running a spread bore manifold.
Now that the manufactures have had time to sort out the billet annular booster, it is the Cats meow for square bore applications. This is why I like the Holley 670 Truck Avenger so much......it is the ONLY Holley spread bore with small annular boosters on the primarys.....and is as snappy as a Q-jet around town........the downside to annular boosters is after you reach peak horsepower and keep revving higher, they go pig fat when the intake sees vacuum again (the intake runs out of airflow). But most people who I would set this unit up for are in the 400 horse or less range and do not run flat out for extended periods.
Anyway......once again Lars is correct.....metering is metering, the engine does not care how it goes about it unless it is poor........
Holley needs to put the Truck Avenger boosters in the miserable Street Avenger series (One of the most difficult carbs I have ever tuned out of the box).......why they do not is a mystery. Edelbrock finally came out with the AVS 2 series which has billet annular boosters last year.
When you get above 450 horse or so , distribution becomes more critical and the square bore wins every time.......the more air you move, the faster it is and has less "time" to figure out where to go and what to do.......
I run a modified 750 HP on my own 406 which makes about 500 horsepower........this is a high end unit and has removable air bleeds to dial in any situation......and is considerable a "staple" in the magazine community. Is it the best choice for running on the street? Maybe......maybe not......bu it works rather well.
It all depends on what you are doing!
Lastly.....if some dude comes up to you at a car show and starts rambling about how this is junk or that is junk......he/she better know at least of what I said above or they are full of s&%t. Walk away and get more beer.
Jebby
Last edited by Jebbysan; Nov 26, 2019 at 06:47 AM.
From: Las Vegas - Just stop perpetuating myths please.
If you want to boost low end performance it would be easier to bolt on better manifolding than to trick out one of those carbs Brian. What I'm trying to suggest is modifying a carburetor is quite a science project as you can read from the advice given already.
But if you want to improve that "signal" in the carb and the cylinder filling at lower RPM a good set of long tube headers is probably the easiest way. And the longer the tubes the better. What gets overlooked in our corvettes is the option to install side mounted exhaust with some great lengths of header pipe. Kinda a sweet advantage other cars don't have.
I'm no dynomometer professional but my sources tell me the exhaust manifolding is much more effective than the intake manifolding for pulse wave tuning. I see you already have a nice dual plane intake already for the improvement would be on the exhaust side.
Just my 2 cents here. I couldn't resist spending it.
Little OT but if some are wondering what difference a 400 may make over a good tq 350 (TPI)
TPI peaks@3200 around 330/340 lbs Lars close to stock 400 is making 350 at 2000. Love them 400s.
Little better head, little more cam you can roast tires and sit in traffic all day.
I dig the Q jet never learned them though. They are truly a smarter carb
Shame noone made one with a larger float bowl isnt that the downside to them for higher hp apps?
From: At my Bar drinking and wrenching in Lafayette Colorado
That's why you always install the high-flow seat for any performance application. Use the big-diameter seat with proper fuel supply and there is never any problem with Q-Jets at high rpm and high flow conditions.
Lars
