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Re: Oversize Valve Seats

To: WILLIAM B LAWRENCE <ynotink@msn.com>
Subject: Re: Oversize Valve Seats
From: Dave & Marlene <rusd@velocitus.net>
Date: Fri, 06 May 2005 20:59:09 -0600
Bill,
If you must disagree - OK.
Note that I said exhaust around 70 to 80% & of the intake. Not exhaust
the same as intake. I never said that they should be equal.

The intake can be so large that velocity & thus inertia drops to the
point where there is no cylinder filling after the piston reaches BDC &
starts back up & the intake valve is still open. As area is increased
velocity is decreased. Quite a few engine developers have discovered
that too large intakes, with their resultant loss of velocity & inertia
are detrimental to power output.

On the other hand, there is not such a disadvantage to reducing exhaust
port velocity. If larger exhaust valves "never help" then why bother to
use low restriction exhaust systems?

Denis Welch seems to think that larger exhaust valves  benefit the
Healey Four. As I said before it is a matter of balancing the complete
intake & exhaust flows with the particular cam & intended usage of the
engine.

Regards,
Dave Russell
BN2

WILLIAM B LAWRENCE wrote:
 > I have to disagree. Larger inlet valves are advantageous because a
 > larger port opening makes filling the cylinder easier considering the
 >  fact that you have only the ambient air pressure outside the
 > cylinder to induce the air to flow.
 >
 > The exhaust gases, on the other hand, are forced out of the cylinder
 > by the motion of the piston. Since the exhaust is being forced out
 > the port doesn't need to be as large. That's why the inlet valve is
 > usually larger (sometimes considerable larger) than the exhaust
 > valve. Whoever enlarged the exhaust ports while leaving the inlets
 > alone demonstrated a basic misunderstanding of physical principles.
 >
 > The exhaust is going to leave anyway because it physically cannot
 > stay. The intake charge, however, needs to be romanced to enter the
 > cylinder, Larger ports with reduced resistance is the way it's done.
 >
 > Bill Lawrence
 >
 >
 > On 5/6/05 11:19 AM, "Dave & Marlene" <rusd@velocitus.net> wrote:
 >> Hi Mike,
 >>
 >> It depends on the overall engine design. The in & out need to be
 >> somewhat balanced. What can't get out can't get in & vs. Usually
 >> the exhaust needs to flow about 70% to 80% of what the intake
 >> flows. Some folks believe that the original exhaust valves/ports
 >> are disproportionately restrictive. I have certainly not found any
 >> disadvantage to this modification.
 >>
 >> Dave Russell Bn2
 >>
 >> Mike MacLean wrote:
 >>
 >>> To all that answered my question  about what size exhaust valve
 >>> to use in my 4 cylinder head because of the oversize seats, 3000
 >>> valves fit perfectly.  Some suggested just that, but I always
 >>> thought if you wanted more power you used bigger intake valves.
 >>> What kind of a performance gain do you get from bigger exhaust
 >>> valves only? Mike MacLean 56 BN2 60 AN5




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