Too much carburetor?
Melting Slicks
Joined: Feb 2002
Posts: 2,298
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From: Sodertalje/Sweden
Re: Too much carburetor? (lars)
BB-
Tuning for Beer Tour comes to Europe this summer. I'll be in Oslo and Trondheim in July. I could certainly make a run over to Sweden at the same time. Let me know if there's interest.
Tuning for Beer Tour comes to Europe this summer. I'll be in Oslo and Trondheim in July. I could certainly make a run over to Sweden at the same time. Let me know if there's interest.
Thread Starter
Le Mans Master
Joined: Sep 2003
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From: Everywhere
Re: Too much carburetor? (Grinchia)
WHERE did you find that worshipping smiley??? There's a wet spot on my chair where I laughed so hard!!
It's from the other forum I hang out on when I'm not out here: http://www.corvetteforums.net/6/ubb.x
Also, check out 96LT4's page http://www.96lt4.com/SMILIES1.html
Melting Slicks
Joined: May 2001
Posts: 2,131
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From: May help you? You can sure as hell try!
Re: Too much carburetor? (lars)
Carbs are cfm rated at a given “Delta Hg” pressure. In other words, with a given pressure differential between the top and bottom of the carb, the carb will allow a certain amount of air to pass through. There are all kinds of graphs and formulas for figuring out how many cfm your engine needs. But fact is, at WOT (wide open throttle), your engine does not produce the Delta pressure that the carb is rated at. So the carb flows a bunch less than its rating. You can successfully “over-carburate” an engine and actually get a performance improvement – even at low rpm. We have proven this on the engine dyno over and over. Look at the guys who are running fast: They’re not running 550 or 650 cfm carbs…. I’ve run carbs as big as 1050 on a small block, and they idle just fine with no off-idle stumble once set up right. No… they don’t run rich because they’re big…
Keep in mind through all this that GM installed 750 cfm Q-Jets on engines 250 cid and bigger (the OHC Pontiac was 250 inches with a 750 Q-Jet). I spite of common misconceptions, the Q-Jet is NOT a vacuum secondary carb: It is a mechanical secondary carb with an airvalve to allow the secondaries to come in without the use of a secondary accelerator pump (a secondary pump would cost more).
I have run back-to-back tests of 650 and 750 carbs on engines down to 302 cubic inches (take a look at this monthÂ’s issue of Hot Rod Magazine: There is an article in there by Matt King on computer simulation versus actual power. The engine in the article is a 302 Ford. We ran that engine with a 650 and a 750 BG mech sec carb. The 750 consistently produced 5 more hp across the rpm range, and the results in the article are with the 750 mech sec carb as shown in the photo). Dyno runs typically start the pull at 3000 rpm, where the engine is placed under massive load and the carb is popped right open to WOT. On every run I have made, a 302, 327, 350 and 383 will take a 750 mechanical secondary carb slammed right open to WOT at 3000 rpm without any bog or hesitation, and it will consistently produce better power across the rpm range than a 650 carb.
Keep in mind through all this that GM installed 750 cfm Q-Jets on engines 250 cid and bigger (the OHC Pontiac was 250 inches with a 750 Q-Jet). I spite of common misconceptions, the Q-Jet is NOT a vacuum secondary carb: It is a mechanical secondary carb with an airvalve to allow the secondaries to come in without the use of a secondary accelerator pump (a secondary pump would cost more).
I have run back-to-back tests of 650 and 750 carbs on engines down to 302 cubic inches (take a look at this monthÂ’s issue of Hot Rod Magazine: There is an article in there by Matt King on computer simulation versus actual power. The engine in the article is a 302 Ford. We ran that engine with a 650 and a 750 BG mech sec carb. The 750 consistently produced 5 more hp across the rpm range, and the results in the article are with the 750 mech sec carb as shown in the photo). Dyno runs typically start the pull at 3000 rpm, where the engine is placed under massive load and the carb is popped right open to WOT. On every run I have made, a 302, 327, 350 and 383 will take a 750 mechanical secondary carb slammed right open to WOT at 3000 rpm without any bog or hesitation, and it will consistently produce better power across the rpm range than a 650 carb.
However, it is true that a smaller engine can only consume a given amount of air at any given speed. Opening the throttle plates suddenly on a small engine running at low RPM, the engine cannot consume the air that is suddenly made available to it so the manifold pressure suddenly rises. This disrupts the vacuum signal and causes the air fuel ratio to go out of whack. To compensate, a double pumper squirts in extra fuel untill the RPM of the motor comes up high enough to use the air that is available. A vacuum secondary system simply does not open the secondary throttle plates until the engine is capable of consuming the air that is available from the secondaries.
I tried a carb swap on my '69 Z28 (302 cid, incidentally) many years ago on the thought that it was overcarbed from the factory (those Detroit engineers don't know nothin', huh? If they print it in a magazine it must be true. ;)). Swapping on a 650 vacuum secondary carb from the original 780 vacuum secondary carb killed the top end and killed the throttle response as well. It felt like I had to step further in to it to get similar performance. I did! Needless to say I gave the 650 back to my buddy and ran with the 780 from there on out. Lesson learned.
Going with a 750 with vacuum secondaries will give you fine performance. The carb will only give you the air flow YOU NEED. A DP would work too but the pump shot would have to be tweaked a bit to get rid of any bog from a sudden WOT accel from off-idle. :thumbs:
Le Mans Master
Joined: Mar 2001
Posts: 6,237
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From: Watkinsville, GA and Glen Cove, NY
Re: Too much carburetor? (Sling Blade)
This is quite simply one of the most informative posts I have seen. makes me wonder if I should have gone bigger on the 502. Thanks Lars for the great info. Bill.
Pro
Joined: Oct 2003
Posts: 572
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From: MOBILE Al
Re: Too much carburetor? (69ttop502)
Heck im kinda worried. Im about to put the recommended 750vs on my holley systemax II. Keisler tranny will be in in another week. I was debating putting the 4.11s back in instead of my 3.36 gears. torn on whether to sell the 750 vacuum secondary carb that i just bought(4160 but i was gonna do the conver. to 4150) and put a mechanical secondary on. what do you guys think
Race Director
Joined: Mar 1999
Posts: 10,258
Likes: 1
From: Calif.
Re: Too much carburetor? (Sling Blade)
Using the old formula (RPM * displacement / 3456 * .85), your 6,000 RPM 355 should be theoretically perfectly happy with a 523 CFM carb.
I have to join those who say your car will run fine with a 650 vaccum secondary carb.
You could put a 750 or even an 800 up there but I think the 650 will provide you with crisper throttle response across the RPM range where the engine will be seeing most of its time and somewhat easier tuning.
Of course if you're just interested in WOT, go for however big a carburetor as you can afford.
I have to join those who say your car will run fine with a 650 vaccum secondary carb.
You could put a 750 or even an 800 up there but I think the 650 will provide you with crisper throttle response across the RPM range where the engine will be seeing most of its time and somewhat easier tuning.
Of course if you're just interested in WOT, go for however big a carburetor as you can afford.
Thread Starter
Le Mans Master
Joined: Sep 2003
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From: Everywhere
Re: Too much carburetor? (Never Finished)
I'm really glad I read this thread. Is there a "Thread Hall-of-Fame". This one should be nominated.
I'm printing this entire thread out, and saving in a notebook with other tech articles. You never know when the search function fails, and/or there is a problem viewing archived threads.
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Safety Car
Joined: Aug 2000
Posts: 3,711
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From: San Antonio TX
Re: Too much carburetor? (catchme76)
Heck im kinda worried. Im about to put the recommended 750vs on my holley systemax II. Keisler tranny will be in in another week. I was debating putting the 4.11s back in instead of my 3.36 gears. torn on whether to sell the 750 vacuum secondary carb that i just bought(4160 but i was gonna do the conver. to 4150) and put a mechanical secondary on. what do you guys think
Instructor
Joined: Jan 2004
Posts: 246
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From: Marshalltown Iowa
Re: Too much carburetor? (The Dude)
Using the old formula (RPM * displacement / 3456 * .85), your 6,000 RPM 355 should be theoretically perfectly happy with a 523 CFM carb.
I have to join those who say your car will run fine with a 650 vaccum secondary carb.
I have to join those who say your car will run fine with a 650 vaccum secondary carb.
Thread Starter
Le Mans Master
Joined: Sep 2003
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From: Everywhere
Re: Too much carburetor? (Sling Blade)
One last post before the swap, so that I have a baseline to compare to.
I just went out with G-Tech, and first ran a 0-60mph time of 4.94 seconds.
Then, I did a 1/4 mile run in 13.46 seconds @ 99.8MPH. I will say that during the 1/4 mile run, two things hurt me. First, I was running out of road so I backed off a little early. :lol: Second, I hit several flat spots at WOT around 5K, either because of the carburetor, fuel pump, or timing. I guess that's evident in my mph time. Anyways, that's my times for a stock Q-Jet, 2101 intake, and who knows what timing on an otherwise modified engine.
Once I set the timing, and install the Demon and RPM intake, I'll make another run and see what happens. High 12's possible :confused:
BTW, with 3.70 gears and 3550lbs total weight, how much hp is my engine making?
I just went out with G-Tech, and first ran a 0-60mph time of 4.94 seconds.
Then, I did a 1/4 mile run in 13.46 seconds @ 99.8MPH. I will say that during the 1/4 mile run, two things hurt me. First, I was running out of road so I backed off a little early. :lol: Second, I hit several flat spots at WOT around 5K, either because of the carburetor, fuel pump, or timing. I guess that's evident in my mph time. Anyways, that's my times for a stock Q-Jet, 2101 intake, and who knows what timing on an otherwise modified engine.
Once I set the timing, and install the Demon and RPM intake, I'll make another run and see what happens. High 12's possible :confused:
BTW, with 3.70 gears and 3550lbs total weight, how much hp is my engine making?
Melting Slicks
Joined: Apr 2000
Posts: 3,286
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From: San Jose California
Re: Too much carburetor? (Sling Blade)
I would suggest using the GTech to test with only one gear, a gear that traction will not be an issue. The more shifts you have to make, the more you have to feather the clutch to keep from spinning, the harder it will be to have consistant results. Based on your readings, I would suggest making runs in 2nd gear from 2,000 rpm through your redline. Traction and shifting won't enter in.
Chuck
Chuck
Thread Starter
Le Mans Master
Joined: Sep 2003
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From: Everywhere
Re: Too much carburetor? (Chuck Harmon)
As weird as it seems, I have no problem with tire spin :crazy: I think that has something to do with the worn out rear spring.
I'm running a TH350 with a 2000 stall convertor, so no worry about the clutch.
Even so, that's a good idea. :cheers:
[Modified by Sling Blade, 8:43 PM 2/13/2004]
I'm running a TH350 with a 2000 stall convertor, so no worry about the clutch.
Even so, that's a good idea. :cheers:
[Modified by Sling Blade, 8:43 PM 2/13/2004]
Advanced
Joined: Feb 2004
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From: MAYWOOD MO
Re: Too much carburetor? (lars)
Thanks for all of the great information on this post!
My stock 71 is mostly for cruising and even the occasional long distance trip, how will vacum vs mech carbs affect fuel economy? Does the CFM rating also affect this?
My stock 71 is mostly for cruising and even the occasional long distance trip, how will vacum vs mech carbs affect fuel economy? Does the CFM rating also affect this?
Race Director
Joined: Feb 1999
Posts: 16,978
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From: In Dreams There Is Truth Ohio
Re: Too much carburetor? (Sling Blade)
Ahhhhh.....the age old debate, ways to strip paint and carb sizes...there are no right and wrong answers...for every 10 guys asked the above Q's you will get 10 different answers....the bottom line, regardless the carb you choose, wheather small, big, vacuum, mechanical, it will work, you just have to tailor the carb to your driving style.
