Rusty Bowtie
General Category => GM Tech Questions and Answers => Topic started by: themoose on January 13, 2014, 08:56:40 AM
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Jef
We discussed how to properly size and tune a carburetor so how about talking about what happens after the fuel is mixed and starts making its journey into the combustion chamber . It seems like every manufacture of intake manifolds claims that their design is the best for one reason or another. I know that choosing the proper intake depends on the application but maybe you could talk to us about the differences in intake manifold design and how they can affect performance and economy.
Moose
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Hey Moose, you're killin' me!
Seriously folks. If you need good low end and crisp throttle response stay with a low rise, dual plane manifold with heads with small intake ports, like 180cc with a small block almost anything, even stick cars, if you are going to lug the engine at all keep the intake small. As your stall speed or launch rpm you can go to a single plane if you are going to buzz the engine to 7k or so. Now comes the hard part. For those that want to run multiple carbs you need to make sure that the intake and carbs are of proven design so you feed all the cylinders well.
Now for those of you who want to run tunnel ram or some other big volume intake on the street, or the track, there is a little physics involved.
At idle the fuel pulled through the idle circuits is completely atomized. If you had a clear intake you would be surprised, there would be no fuel to be seen. That's because at idle most engines pull high manifold vacuum, which is the same as low pressure, to the point where fuel from the idle circuit changes state to a gas.
When the throttle is opened and more atmosphere enters the intake the fuel's boiling point is not reached and the fuel entering the intake can now be seen all over the inside of your clear intake. That's why you need an accelerator pump. The first load of fuel never makes it into the engine. So the enrichment process offered by the accelerator pump basically wets the inside of the intake, the fuel from the boosters and transition circuits provide the fuel for the engine to run. So a big intake is a problem. Sacrificing driveability for looks is the penalty. The other negative, and this is with any wet intake, is the hydrocarbon spike crested on decel. All the fuel coating the intake under low vacuum conditions immediately boils off on closed throttle, causing excessive furl use and lots of extra engine wear, and back firing in the exhaust sometimes. That's why decel valves were used, to close the throttle slowly and add air to the intake to burn some of that fuel but that gives you lousy engine braking with the engine flaring. Port fuel has hardly any HC spike on decel because the chances of delivering a fuel shot that won't be used is small, and even if it happens it only concerns the real estate surrounding the intake valve.
I like Performer intakes, intakes made by any of the cylinder head manufacturers. I had an RPM air gap on my truck most recently, then Tony Mammo at Air Flow Research told me to get a set and buy their new Titan manifold. It's a plastic two piece deal where the center is the plenum that can be interchanged with other plenums they make. It allows the change to be made with the removal of 6 bolts and it uses modern formed silicone style gaskets like the new cars. But it's expensive, over $600k. But it works great.
The "fuel injection" most of the aftermarket companies are selling is throttle body, which is really old technology. It runs okay, but it is a wet intake system with it's inherent drawbacks. Port fuel is the way to go if you are going to run fuel injection.
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Jef......Sorry to be going so hard on you but your not off the hook just yet. We've talked about how to properly size a carburetor, what to look for in an intake manifold ,touched base on lighting the charge at the right moment and even discussed how to properly vent the crankcase so now I guess the next logical topic should be about how to most efficiently remove the byproducts of ignition. How about your comments on how to chose the correct exhaust system components for both all out racing and just cruising down the street and discuss some of the science that is behind a good system design......Oh yeah, I have one specific question about the benefit or lack thereof for using an equalizer tube on a duel exhaust system. I promise that I'll try to go a "LITTLE" easier on you in the future.
Moose
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Jef,
Thanks for all of your technical post. It give us all a bunch to think about while building these cars. I have an Edelbrock Performer on my 33 Chevy. Just looking to have a good running car to cruise with. Back in my younger days I did run a tunnel ram on a street car I had at the time. It was an Edelbrock TR1Y with two Holley 600s (model 1850) on a 350 small block. We did get it to run pretty good but looking back a single four probably would have run better.
Tom