Intake Manifold Heat
There is a very good reason that Chrysler designed a manifold heat control device into their intake/exhaust manifold and that Clifford designed water heat into their intake manifold. Many people eliminate manifold heat from their engines in the belief that cooler air is beneficial to an engine. In some ways it is, but cold air should not be an end in itself.
A more powerful, more responsive, and/or more fuel-efficient engine is really what most people are trying to achieve when they modify the carburetion on their engines. However, all street engines (especially inline engines) benefit from a hot spot underneath the carburetor. Its not just my opinion; the authors of Rochester, Holley, and Carter carburetor books all say the same thing. Ive referenced their positions on a page I wrote about intake manifold heat:
http://cars.rasoenterprises.com/Carbs-IntakeHeat.htm
With 4bbl carburetors, the vacuum in the intake manifold can drop too low to vaporize the fuel. At low RPMs, the air flow is insufficient to keep the atomized fuel in suspension when the throttle is cracked wide open. In this case, the fuel drops out of the air stream and puddles on the floor of the intake manifold, which is the reason for the off-idle bog. A hot spot under the carburetor vaporizes the fuel that drops out of the air and minimizes this lean-out of the fuel mixture. You can compensate for a cold intake manifold with a very rich power fuel mixture. However, the hot spot minimizes the need for this extra fuel.
In my case, I ran a 625 CFM Carter AFB on an Offenhauser 4bbl intake with Clifford headers for about 20 years before I upgraded to a 750 CFM Quadrajet and Dutra Duals a few years ago. In my case, I found the headers to cause more problems than the benefit I got from the added power they were supposed to have provided. As 70 Valiant pointed out in the Clifford vs Offy thread, his intake gets very hot after only a few minutes of driving. Well, I never measured how fast and how hot my intake became, I did find that the headers made the engine compartment very hot and my car could not idle for very long in hot weather. In cold weather, the intake manifold took much more than a few minutes to warm up enough to the point where I could easily drive away. However, even with the hot engine compartment, I found that I had an off-idle bog that I just could not cure with any amount of accelerator pump shot.
To control the excessive amount of heat generated by the headers, I ended up using header-wrap. Whether the wrap contributed to corrosion or it was just old age, my headers rusted out after about 20 years. As I planned to keep my car indefinitely and didnt want to bother with replacing headers again, I decided to go with a pair of Dutra Duals. Besides being much more corrosion resistant than headers, I also wanted less heat under the hood. You can read about the Dutra Duals upgrade here:
http://cars.rasoenterprises.com/Exhaust-DutraDuals.htm
I recommend using intake manifold heat for street-driven cars, even if you live in California. The hot spot under the carburetor will ensure that your engine is responsive and fuel-efficient.
There is a very good reason that Chrysler designed a manifold heat control device into their intake/exhaust manifold and that Clifford designed water heat into their intake manifold. Many people eliminate manifold heat from their engines in the belief that cooler air is beneficial to an engine. In some ways it is, but cold air should not be an end in itself.
A more powerful, more responsive, and/or more fuel-efficient engine is really what most people are trying to achieve when they modify the carburetion on their engines. However, all street engines (especially inline engines) benefit from a hot spot underneath the carburetor. Its not just my opinion; the authors of Rochester, Holley, and Carter carburetor books all say the same thing. Ive referenced their positions on a page I wrote about intake manifold heat:
http://cars.rasoenterprises.com/Carbs-IntakeHeat.htm
With 4bbl carburetors, the vacuum in the intake manifold can drop too low to vaporize the fuel. At low RPMs, the air flow is insufficient to keep the atomized fuel in suspension when the throttle is cracked wide open. In this case, the fuel drops out of the air stream and puddles on the floor of the intake manifold, which is the reason for the off-idle bog. A hot spot under the carburetor vaporizes the fuel that drops out of the air and minimizes this lean-out of the fuel mixture. You can compensate for a cold intake manifold with a very rich power fuel mixture. However, the hot spot minimizes the need for this extra fuel.
In my case, I ran a 625 CFM Carter AFB on an Offenhauser 4bbl intake with Clifford headers for about 20 years before I upgraded to a 750 CFM Quadrajet and Dutra Duals a few years ago. In my case, I found the headers to cause more problems than the benefit I got from the added power they were supposed to have provided. As 70 Valiant pointed out in the Clifford vs Offy thread, his intake gets very hot after only a few minutes of driving. Well, I never measured how fast and how hot my intake became, I did find that the headers made the engine compartment very hot and my car could not idle for very long in hot weather. In cold weather, the intake manifold took much more than a few minutes to warm up enough to the point where I could easily drive away. However, even with the hot engine compartment, I found that I had an off-idle bog that I just could not cure with any amount of accelerator pump shot.
To control the excessive amount of heat generated by the headers, I ended up using header-wrap. Whether the wrap contributed to corrosion or it was just old age, my headers rusted out after about 20 years. As I planned to keep my car indefinitely and didnt want to bother with replacing headers again, I decided to go with a pair of Dutra Duals. Besides being much more corrosion resistant than headers, I also wanted less heat under the hood. You can read about the Dutra Duals upgrade here:
http://cars.rasoenterprises.com/Exhaust-DutraDuals.htm
I recommend using intake manifold heat for street-driven cars, even if you live in California. The hot spot under the carburetor will ensure that your engine is responsive and fuel-efficient.
