351w intake question
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From: las vegas
351w intake question
I have a 72 f100 that the P/O placed a 351W. It has the 2 barrel intake and carb set up. The carb choke works off the intake through a couple of tubes. After trtying to rebuild the carb I happened to twist one of the tubes. Boy can it hiss. I tried reseating new tubes from the junk yard and they just don't seal. The question I have is this. How can I seal the plate that the tube came out of? I plan on getting the Holley 500 cfm 2 brrl carb with a cable choke. I can only anticipate vaccuum issues from this plate and open tubes. All help and advice will heaven sent.
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From: Colville, Washington
The hot air choke works by using vacuum to draw heated air from the exhaust crossover thru those tubes from the air cleaner(trace the tubes/hoses). The hiss is a vacuum leak, put a vacuum cap on the tube or fitting on the choke and remove the hose from your air cleaner and install another cap. The plate can be replaced with a piece of steel with the right bolt holes to seal it.
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From: las vegas
I have tried other sets of tubes. They don't seal where they go in to the plate. They are pressure fit I guess. I was thinking of putting a 4 bbl but am limited on funds.
To try and understand your cap Idea. If I just place a steel plate I should be ok? I mean since I am not going to be using the same carb?
To try and understand your cap Idea. If I just place a steel plate I should be ok? I mean since I am not going to be using the same carb?
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From: las vegas
after reading again I noticed you said,"heated air from the exhaust crossover". These tubes are a little coil that go into the intake manifold. The coil is actually in three pieces. On is a tube from the plate to the choke. The other is a tube from the plate to the Egr valve. The other end of the plate is just a coil of tubing about an inch long that goes inside the intake manifold.
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From: Colville, Washington
Not sure why the one tube goes to the EGR, makes no sense to me. It should be receiving filtered air that is heated in the exhaust crossover passage(the small coil) in the intake and is drawn into the choke by manifold vacuum.
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From: las vegas
ok guys, now to change gears a bit. I was talking to my buddy this morning. He offered me a 800 cfm Holley with vaccuum secondaries. Is that just too much carb. The price is free, So now I may be able to dish out some dough for a new intake. I am thinking this is a better solution. What do you guys think?
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Cargo Master
Joined: May 2007
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From: British Columbia
I'm consulting the book "Super Tuning and Modifying HOLLEY CARBURETORS" by Dave Emanuel, published by S-A DESIGN, 1988.
In chapter 3, "Selecting the Right Holley Carb", the basic formula for determining carburetor size is given as:
CFM required = (Engine CID x Maximum RPM) / 3456 x volumetric efficiency.
So assuming 100% volumetric efficiency, and transposing our equation:
Maximum RPM = CFM / Engine CID x 3456
= 7877 rpm.
He goes on to say that stock motors, low to high performance, generally range from 75% to 85% volumetric efficiency, respectively. So if we divide the 7877 rpm by these values we get a maximum RPM for the 351 engine of:
75% efficiency: 10,500 RPM.
85% efficiency: 9267 RPM.
In chapter 3, "Selecting the Right Holley Carb", the basic formula for determining carburetor size is given as:
CFM required = (Engine CID x Maximum RPM) / 3456 x volumetric efficiency.
So assuming 100% volumetric efficiency, and transposing our equation:
Maximum RPM = CFM / Engine CID x 3456
= 7877 rpm.
He goes on to say that stock motors, low to high performance, generally range from 75% to 85% volumetric efficiency, respectively. So if we divide the 7877 rpm by these values we get a maximum RPM for the 351 engine of:
75% efficiency: 10,500 RPM.
85% efficiency: 9267 RPM.
Cargo Master
Joined: May 2007
Posts: 2,071
Likes: 36
From: British Columbia
Yep, the price is right! Maybe try it out, and if it's not quite right sell it or trade it and get a different carb. The 800 should fetch a lot more money than a smaller one.
This carb might work OK since it's a vacuum secondary, but the secondaries probably won't ever open. A smaller carb would have better low to mid range power due to the smaller primaries and would still have sufficient flow to make high rpm power when the secondaries opened.
The Holley book has some interesting insight into why some engines were overcarburetted in the 60s. Mostly because for Stock Class racing they had to use some stock parts (imagine!) like carbs. So they'd use a too large carb on the production car which would be right for racing.
In a similar vein, I have a really nice old Sun Tach in my old beast that I got cheap because it only goes to 6000. Not surprisingly, I don't rev my 351W higher than that. Yet they sell a lot of 10000 RPM tachs to people driving V8s on the street.
This carb might work OK since it's a vacuum secondary, but the secondaries probably won't ever open. A smaller carb would have better low to mid range power due to the smaller primaries and would still have sufficient flow to make high rpm power when the secondaries opened.
The Holley book has some interesting insight into why some engines were overcarburetted in the 60s. Mostly because for Stock Class racing they had to use some stock parts (imagine!) like carbs. So they'd use a too large carb on the production car which would be right for racing.
In a similar vein, I have a really nice old Sun Tach in my old beast that I got cheap because it only goes to 6000. Not surprisingly, I don't rev my 351W higher than that. Yet they sell a lot of 10000 RPM tachs to people driving V8s on the street.
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From: las vegas
So if it is too big, what are the tell tale signs I should look for? I Don't know what are the reprocutions of over carbing. As dragot said," Hard to beat free". If it going to cause more problems than it is worth then I might just hold off.