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The MSD is not really a performance boost, nor is the Performer which is just one notch above stock. What else you got to justify a 750 Holley?
Too much CFM will just kill throttle response and a vacuum signal on a basically stock engine.
Take the CFM Holley quiz on their web site.
1974 350 Performer manifold intake, MSD distributor runs strong, already have Edelbrock 600 carb. Looking to replace carburetor with Holley 750. Feedback & suggestions please.
Shovel head
What are you hoping to gain by switching carburetors? If (judging by the title of your thread) you are looking for more performance, look elsewhere. That carb will not make a difference unless you're current carb is poorly tuned. A 600 CFM carb is plenty for a basically stock 350.
Two of the most common mistakes in stock Hot Rodding is:
Too big of cam.
Too big of carb.
And Quad-J 750 never seemed as flowing as a Holley 750 for some reason. You'll never hear a QJ as being over carbureted like Holleys are.
But its your car, do what you want. Worth $550 to try it?
Last edited by HeadsU.P.; Aug 8, 2018 at 09:16 AM.
A properly tuned Holley 750 vacuum secondary carb will work on just about any engine. However, don't expect a performance gain on a stock engine over a properly tuned 600 cfm carb of any make on a stock engine. Look to heads, cam, headers, etc... for more performance. A mild cam with headers and good 2.5" dual exhaust would give you a good boost in power even with the 600 in place.
Take that money you where going to use on a carb and put it to heads and a cam. Those are the things holding you back the most. Getting the CR up is also important.
A stock low CR 350 with a stock cam and heads isn't even pushing the 600 to the limit. You could put a 750 vac sec Holley on it and it would run "OK" because you would never be getting in to the secondaries. You would be running on the primary's all the time. Even a $700 set of "summit" brand 2.02 heads would be a big step forward.
From: At my Bar drinking and wrenching in Lafayette Colorado
Originally Posted by Shovelhead08
1974 350 Performer manifold intake, MSD distributor runs strong, already have Edelbrock 600 carb. Looking to replace carburetor with Holley 750. Feedback & suggestions please. Shovel head
Put a properly tuned 1974 Q-Jet back on the car. The Q-Jet will outperform your outdated 600 AFB by a significant margin, and will outperform the 750 VS Holley by about 5 hp in your usable rpm range.
For those of you who haven't seen it, here is the article and test I did on the dyno of a Q-Jet versus a Holley 750 Vacuum Secondary:
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.
On the graph, the black lines are Q-Jet, green lines are Holley:
The reason a Q-Jet will never overfeed an engine is that the secondaries are "demand driven" instead of being 'force-fed'. The internal combustion engine is essentially an 'air pump'. Any engine can wind up to its capacity and will have a maximum possible flow rate thru its system. The Quadra-Jet design allows its secondaries to open only as far as the engine is capable of accepting. Thus, it cannot be choked with excess fuel (if the carb is properly assembled and tuned). This is why Q-Jets were placed on almost all V-8 engines produced by GM from the mid-1960's to the early 1980's. It even found a home on the Pontiac "Sprint" 6 engine! Certainly, the Q-Jet secondaries didn't open all that much on the Sprint engine; but it worked just fine.
Vacuum secondaries attempt to do the same thing, as engine vacuum is varies with engine RPM. But that method isn't strictly driven by engine demand. It's just a bit important to understand the physics behind how these carbs work and why one design has advantages over others.
In the end, ANY decent carb can be tuned to produce approximately the same performance as any other carb--if it has adequate capacity for that engine and given enough time on the dyno. If you ONLY want a performance carb, the Holley design is probably what you need. But, if you also want smooth transition between idle, off-idle, cruise and performance and also good economy when driving normally, the Q-Jet can't be beat as the "whole package".
The folks who buy the Edelbrock carb {old AFB clone) from an auto parts store and throw it on their engine without any tuning, just want simplicity and don't really care about performance or economy. They just want it to "work" (whatever that means). Apparently, there are a lot of them sold. So be it.
What are you hoping to gain by switching carburetors? If (judging by the title of your thread) you are looking for more performance, look elsewhere. That carb will not make a difference unless you're current carb is poorly tuned. A 600 CFM carb is plenty for a basically stock 350.
This. Youll gain nothing
Look into heads, cam, headers, exhaust
Once your motor demands more air/fuel you may need more carb.
Diff between 750 Holley and 750 Q-jet. The primary size. Like intake ports, smaller primaries give you higher air valocity at low throttle, where every car that doesnt have a big number on the door or an ET bracket in soap on the side window spends most of it's time. The higher velocity gives better fuel atomization at throttle settings that are street use. And when sec are part way open, the metering rods have the secondary jets part way open. Holley secondary jets are not variable and pri are just large.
Last edited by derekderek; Aug 8, 2018 at 09:02 AM.
Put a properly tuned 1974 Q-Jet back on the car. The Q-Jet will outperform your outdated 600 AFB by a significant margin, and will outperform the 750 VS Holley by about 5 hp in your usable rpm range.
For those of you who haven't seen it, here is the article and test I did on the dyno of a Q-Jet versus a Holley 750 Vacuum Secondary:
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.
On the graph, the black lines are Q-Jet, green lines are Holley:
Lars, thanks for sharing that. I hadn't seen your test till now, but I've been a proponent that a properly tuned Qjet will make more power under the curve, and should match a Holley at the peak, and potentially get better gas mileage and drive ability. Good to see some hard proof for all the Qjet haters!
