From: Dave Baker
Newsgroups: rec.autos.tech
Date: 16 Apr 2002 11:53:56 GMT
Subject: Re: Why is exhaust valve smaller than intake valve ?
Message-ID: <20020416075356.11923.00004299@mb-mk.aol.com>

>From: Mike Graham mike@metalmangler.com
>
>On 16 Apr 2002 09:38:17 GMT, pumaracing@aol.com (Dave Baker) wrote:
>
>>The fact that it's under pressure means it flows through a given size
>>opening quicker anyway. The very reason it can be smaller than the
>>intake valve.
>
>  So the bottom line is that the exhaust valve is smaller so that the
>intake valve can be larger, yes?

Pretty much yes. The intake valve relies on the pressure difference across it
to generate flow - at most 15 psi in a normally aspirated engine. Any loss of
intake valve area and hence flow leads to a directly related fall in volumetric
efficiency and power.

The exhaust stroke benefits both from the higher cylinder pressure after
combustion and also positive displacement from the piston. A reduction in valve
area leads primarily just to a pumping loss which acts as a parasite power loss
at the crank. There will also be a minor loss in VE because the chamber has not
been scavenged so completely and can not therefore fill with quite so much
fresh charge. These effects are minor in relation to loss of intake flow
capability though.

Obviously there has to be a break even point. If there was no exhaust valve at
all then the engine can't run. As the exhaust valve is made smaller and the
intake valve made larger the pumping loss increases until it finally balances
out any extra power from more intake flow. This point on most engines is when
the exhaust valve area is around 70% of the intake valve area. Production
engines aren't designed to maximise power. Intake valves are generally smaller
than optimum to improve turbulence, gas speed and hence low rpm torque and fuel
efficiency. Exhaust valves are larger than optimum to reduce pumping loss.
Modify a production engine for race use and the valve area ratio generally
needs to change.

This is pretty trivial engine theory though and not nearly so interesting as
quantum mechanics and relativity. Of slightly more interest is the optimum
valve area ratio in a forced induction engine. Most people never get past the
point of "thinking" (for want of a better word) that because the intake side is
now positively displaced too then the exhaust valve needs to be made larger and
the intake smaller. All this serves to demonstrate is that very few people know
how to think. The solution to the optimum valve area ratio in a forced
induction engine ought also to be easily deducible from a consideration of
pressure differences across the engine but that's another story.


Dave Baker
Puma Race Engines (www.pumaracing.co.uk)