302 engine issues.
Fleet Mechanic
Joined: Apr 2017
Posts: 1,391
Likes: 106
From: COSMOS
I have read many, many times that people think bigger is better so they put BIG Holleys on small engines! One of the most common mistakes in hot rodding. I believe for a stock 302 truck 350CFM is plenty, probably closer to 300 or less. Depends on rpm max.
600 cfm is for bigger cams, crank, headers, etc , high performance engines with steady high rpm capacity. There is a formula like RPM, CI, Volumetric Efficiency
CFM = RPM x CI x VE / 3456. VE for stock engine is roughly 70%
Anyone know max rpm for my 302 w/headers ?
600 cfm is for bigger cams, crank, headers, etc , high performance engines with steady high rpm capacity. There is a formula like RPM, CI, Volumetric Efficiency
CFM = RPM x CI x VE / 3456. VE for stock engine is roughly 70%
Anyone know max rpm for my 302 w/headers ?
Last edited by RTT; Sep 7, 2018 at 01:09 AM. Reason: ADD info
Hotshot
Joined: May 2003
Posts: 11,141
Likes: 25
From: south louisiana
I have read many, many times that people think bigger is better so they put BIG Holleys on small engines! One of the most common mistakes in hot rodding. I believe for a stock 302 truck 350CFM is plenty, probably closer to 300 or less. Depends on rpm max.
600 cfm is for bigger cams, crank, headers, etc , high performance engines with steady high rpm capacity. There is a formula like RPM, CI, Volumetric Efficiency
CFM = RPM x CI x VE / 3456. VE for stock engine is roughly 70%
Anyone know max rpm for my 302 w/headers ?
600 cfm is for bigger cams, crank, headers, etc , high performance engines with steady high rpm capacity. There is a formula like RPM, CI, Volumetric Efficiency
CFM = RPM x CI x VE / 3456. VE for stock engine is roughly 70%
Anyone know max rpm for my 302 w/headers ?
Post Fiend
Joined: Apr 2002
Posts: 19,311
Likes: 97
From: Waterloo, Iowa
Believe it or not, too big of a carburetor will run LEAN under certain conditions. The air flow or velocity is too low because of a mismatch in the venturi. The intake plenum and carb work together as a tuned circuit. Figure out what you want to do, what the intended use is, and where you do most of your driving. How often do you drive above 5k RPM? The most important adjustment we can make to our carbureted engines is choose the correct size carburetor to begin with. With careful selection and tuning you can have good performance and very reasonable economy. Keep in mind modern gasoline does not "color" on the plugs or tailpipe the way leaded gasoline did. If plugs look like they are running "rich" today, it is off the chart and probably diluting the oil.
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Elder User
Joined: Jun 2015
Posts: 664
Likes: 2
From: York Pa.
Thanks again for the interest and help I have received in this thread. As for the Holley 600 cfm 4 barrel carburetor it was gifted to me from my motor head neighbor. So it is big for my engine but we have been having fun trying to get it to run as good as we possibly can. As I mentioned earlier I did just install Hooker headers and 2.25 dual exhaust with Thrush mufflers.
Fleet Mechanic
Joined: Apr 2017
Posts: 1,391
Likes: 106
From: COSMOS
I replied because I was shocked that after 7,8,or more pages of discussion trying to lean the fuel system from cruddy spark plugs no one mentioned that 600 CFM was too big of a carb! I had to reply.
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Elder User
Joined: Jun 2015
Posts: 664
Likes: 2
From: York Pa.
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Elder User
Joined: Jun 2015
Posts: 664
Likes: 2
From: York Pa.
Friday report, So I got my new parts, a secondary diaphragm, a spring kit and a quick top. When we took the carburetor apart we discovered the diaphragm that was in the carburetor was good but replaced it any how since I already bought the new one. I love the new quick top, it makes changing out springs a breeze. My carburetor had the heaviest spring in it, black, and that was the problem why the secondaries were not kicking in. After running and changing numerous springs we ended up with the long yellow spring that gave us a good smooth power surge. I believe the carburetor is finally as good as we can make it with what we have available. I will drive it as is until I can afford to get true dual exhaust with headers and that may be a few months down the road. Thanks again for everybody's help and a happy ending.
Hotshot
Joined: May 2003
Posts: 11,141
Likes: 25
From: south louisiana
The carb in question is a 600 cfm vacuum secondary. Which means that, unless the secondary spring is completely removed, it will never open fully to flow 600 cfm @ 1.5" hg. Which as I've said before, is only a rating. His engine may or may not create that pressure drop at WOT with that carb sitting on top. So, a 600 cfm carb is not "too big" for a 302. I've run a 3310 Holley 750 on an Explorer 5.0 and it worked just fine. It was not "too big" because that engine never created enough vacuum to fully open the secondaries, so it flowed somewhat less. You do not understand carburetors and how they function.
Hotshot
Joined: May 2003
Posts: 11,141
Likes: 25
From: south louisiana
I'm back and figured I would post an older post from 10-20-2017 when times were happier and also as proof that this 600 cfm carb did and can again run good on my truck. Back to my problem, as I said I recently put headers on my truck, changed spark plugs, changed the accelerator pump arm, changed the base gasket to my carb to a thicker 5/16 gasket to protect from potential heat soak from the new headers all at the same time. As I have already stated I now have an off idle hesitation. I just changed my #31 squirter nozzle to a #35 in hopes that it would resolve my hesitation, it didn't. Today I spend time adjusting the accelerator pump arm trying to get just the right amount of pressure between the two arms, .0015 with a feeler gauge right? What I noticed, I have tightened the accelerator pump arm spring down about 10 threads and the two arms still have contact. It seems to me there is something wrong but I don't know what? I do think this is the cause to my hesitation, does anybody else have their accelerator pump arm spring tightened down so much?
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Elder User
Joined: Jun 2015
Posts: 664
Likes: 2
From: York Pa.
Question, my accelerator pump diaphragm is green on one side and black on the other, is the green side suppose to be on the top and black side facing the ground? Just want to be sure mine is in correctly.
Post Fiend
Joined: Apr 2002
Posts: 19,311
Likes: 97
From: Waterloo, Iowa
The reasoning behind using a .015" feeler gauge is simple, the idea is to prevent damaging the accelerator pump diaphragm at full throttle. There must be at least some clearance when the throttle is pegged all the way. But the "squirters" should initiate two strong streams of fuel as soon as the throttle is goosed to prevent bog or hesitation on acceleration.