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From: Dave Baker
Newsgroups: rec.crafts.metalworking
Subject: Re: Harley motor intake/exhaust ratio question
Date: 22 Jul 1999 01:53:09 GMT

>Hello, folks:
>I'm know this is slightly off topic, but have read numbers of posts in
>the past that referred to motor tuning, so I thought it would be worth a
>I'm looking at various H.P. CNC billet head or porting offerings for a
>Evo Harley motor.  They seem to have a wide range of intake to exhaust
>port and valve sizes, and variance on claimed performance increases.
>Does anyone have particular experience/expertise in Harley performance
>tuning?  Are there any generalizations that apply to ratios between
>intake and exhause valve size?  Any pointers would be appreciated.
>Thanks for reading.
>Pat Law

I'll be covering this in detail in the technical articles on my website as and
when time allows. As yet I haven't got onto the cylinder head and fluid
mechanics sections. The topic is complex and design choices in valve area ratio
and flow need to be compatible with other aspects of the engine design i.e.

If exhaust valve area is small then some compensation can be made in exhaust
cam duration etc. Also compression ratio is important but too complex to go
into here.

As a rule most engines have too much exhaust valve area for optimum power. Look
at the design ratios in ultimate race chevy engines. Much higher inlet/exhaust
area ratio than stock engine.

To give you a simple rule of thumb - only 70% exhaust valve area to inlet valve
area (83.7% diameter) is sufficient if the rest of the engine design
complements this.

In other words - forget about the exhaust valve side of things because the
stock one will be plenty big enough in almost every engine I've ever come
across. Save your money for the inlet flow increases.

Dave Baker at Puma Race Engines (London - England)  - specialist cylinder head
work, flow development and engine blueprinting. Web page at

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