C1 - C2: Fuel Injection - Importance of Cold Air

[Russ U.]

An article in Road & Track (September 1956) discusses reductions in gross horsepower caused by various events and devices. The article was based on papers presented by the major auto makers at an annual meeting of the SAE.

One item really surprised me. Hot air into the carb DECREASED bhp by nearly 1% for every 10 degree INCREASE in air temperature. The test engine with carb setup generated 200bhp when the carb was fed 60 degree air. The engine generated only 177bhp when the carb was fed 200 degree air, all other things held constant.

I suspect that cold air impacts FI in about the same way. No surprise that GM added the cold air hoses to the FI 57's so quickly. A 10% increase in bhp would have been huge for performance based engines like the FI 57 et seq Corvette's.

But why in the world did GM limit the cold air hoses to the high HP versions of FI? The low hp version had the same air cleaner setup, but lacked the air hose connecting the air cleaner to the fresh air inlet.




1959 FI Low HP

[Dennis C.]

Anyone ordering the low FI HP version...

...had NO clue about performance, didn't care, didn't understand, AND they paid the same big bucks for almost nothing... Oh, well... it WAS a smooth runner...

[Duke W.]

Re: C1 - C2: Fuel Injection - Importance of Cold A

The one percent/10 degrees HP delta is long known and falls out of Boyles Law.

Carbureted engines could certainly benefit from cold air, but they would also need supplemental air heaters under centain conditions to prevent carburetor icing. Light airplane engines have carburetor air preheaters that and engaged whenever the throttle is closed or substantianlly reduced from cruise power or higher. Without such, carbuetor icing could cause the engine to quit.

That would be a lot of extra plumbing and hardware on a car engine, so cold air systems for carbuetors were limited to a few very high performance engines. In the early emission era inlet air was PREHEATED, which was part of emission control, and you can imagine what that did for HP, but the preheater was sometimes bypassed at WOT.

FI engines are less suspectable to carburetor icing because there is no evaporating fuel between the venturi and manifold that pulls heat out of the air. It can happen in some conditions, but rarely. Like Dennis said, the low horse FI buyers were not concerned with peak power, but high horse buyers were, and the high horse FI engine was what powered Corvettes to several SCCA championships.

One of the biggest advantages of using FI in production/vintage racing Corvette is that it does include a cold air package, versus drawing heated air inside the engine compartment. If the temperature delta between ambient and the engine compartment is 50 degrees that's five percent right there.

Most modern engines draw inlet air from a cold source ahead of the radiator, and when moving down the road they ingest essentially ambient temperature air. Modern emission control systems and EFI have precluded the need for preheated inlet air that was used on most emission controlled carbureted engines.

Duke