Bill,
The exhaust cycle and intake cycle in a typical four stroke engine are _not_
mutually exclusive. There is a period of valve overlap - when the exhaust valve
is closing the intake valve is partially open. This causes a scavanging
effect. The exhaust gases flowing out help draw the fresh fuel/air mixture in.
Each cylinder in your motor is 187.5 cc's (plus the volume of the combustion
chamber), but because of inefficiencies in the intake and exhaust system you
don't get a full exchange of 187.5 cc's with every intake/exhaust cycle. By
improving the flow of the exhaust system on your engine, you have improved the
efficiency. You have more air flowing through your engine.
Rick
Bill Miller wrote:
> Ok, forgive me for being dense, but explain this to me in detail! Everyone
> who has answered has agreed that by increasing the exhaust gas flow, I have
> decreased the fuel/air mixture (made it lean)
>
> Here's my problems with that theory:
> 1) This is a four stroke engine. Exhaust "cycle" and intake "cycle" are
> mutually exclusive. Exhaust valve open, exhaust gases leave. Intake Valve
> opens, in comes the fuel/air mixture. So what does it matter that the
> exhaust leaves fast or slow (as far as mixture is concerned)?? Granted it
> is less work to push the exhaust gases out the valve, but again, how can
> this affect mixture???
>
> 2) For sake of argument, lets say it does matter. Intake air is going in
> "faster." Ok, this is a 750 cc motor. Each cylinder holds 187.5 cc of a
> fuel/air mixture. I am not increasing volume. I am simply increasing rate.
> still X amount of air, still Y amount of fuel. Y hasn't changed. All I've
> increased is how fast it is going in, not compressing it or anything.
> Again, how did I affect mixture???
>
> See this started out about motorcycles, but has evolved into engine theory!!
> Help me understand what you guys mean!
>
> Bill
>
> -----Original Message-----
> From: owner-spridgets@autox.team.net
> [mailto:owner-spridgets@autox.team.net]On Behalf Of Rick Fisk
> Sent: Sunday, July 09, 2000 9:22 AM
> To: William M. Gilroy
> Cc: WFO Herb; Bill Miller; fooshst; George Procyshyn;
> spridgets@autox.team.net
> Subject: Re: Non LBC Motorcycle question.
>
> Bigger jets to get the air/fuel ratio back where it belongs = more power -
> arr,
> arr <g>
>
> Rick
>
> "William M. Gilroy" wrote:
>
> > WFO Herb wrote:
> > >
> > > By reducing back pressure, exhaust gases flow freer allowing for intake
> to
> > > fill the cylinder quicker yet without increased fuel flow... a.k.a.
> amount.
> > >
> >
> > So would the a solution to this be re-jetting the carbs, and if so would
> that
> > keep the HP increase? Or if done correctly could you wring a bit more
> > power out of the bike by using bigger jets or is a lean burn required?
> >
> > Bill Gilroy
|