FOT
Interesting thread and an opportunity to learn something...thanks for
initiating it Richard Taylor.
I put our original engine together in '92 and I have checked the
compression the past 3 years to monitor the condition of the engine and
look for a trend. (The rings are still the originals after 5 seasons)
I have recorded 205# average with a trend to 195#, but the last time I
checked it the battery was a little low and it wasnt spinning as fast.
On the TR-666 Isky cam, there is not much overlap so I suspect my
readings are a little higher than a "hotter" cam would be. My calculated
compression is about 10.5:1 and I am running 87mm pistons.
* How much affect does rpm have on the compression check?
* I know that folks open the pistons on the carburetors, but I
always forget to do this. How much affect does this have on the reading?
Joe
P.S.
There will be an informal gathering of the FOT at the TR4 canopies of
Sean/Joe Alexander & Tony/Jack Drews at Road America. Please stop
by...we may have a special guest....(he says with an air of mystery)
There may be four generations of the Alexander's involved and several
birthdays. Hopefully we will also be flying the Vultures flag in order
to ward off any attacks, but if we are attacked they will have to deal
with my mother.
And not the least...G. Petrolati may be bringing an artist's concept of
the new FOT Logo
> ----------
> From: Malaboge@aol.com[SMTP:Malaboge@aol.com]
> Sent: Sunday, September 14, 1997 12:31 PM
> To: rgb@exact.com; fot@autox.team.net
> Subject: Re: Compression check
>
> In a message dated 97-09-13 20:22:51 EDT, you write:
>
> >
> >Hummmm
> >
> >I would be tempted to think that the dynamic compression, i.e. run
> with
> >all the plugs out, throttle open (I wedge the SU pistons open as
> well),
> >starter turning about 1,500 RPM, would be a excellent way to evaluate
> the
> >effective engine's compression (and octane required).
> >
> >For instance, the static compression means little without a cam
> profile
> >and some means to calculate overlap's effect on the engine, BUT a
> >compression test would take this into account. Not quite the same as
> >a running engine, but certainly a much better approximation than any
> >other bench calculation.
> >
> >Is this true? 170 psi the limit for street gas...?
> >
> >Roger
> >
>
> Roger-
>
> Oh, the temptations...
> Included here is a little missive I have previously posted (sorry for
> those
> who have read this before...but that's what the delete button is for)
> <g>
> Your idea of figuring overlap into your theory might work at one
> distinct
> rpm, but unfortunately, not consistantly. The big problem, as noted
> below, is
> reversion due to the charge being pumped back out of the intake. The
> variables affecting this problem are governed by more than cam profile
> (intake length, diameter, port size, chamber design, temperature,
> straightness of intake system, valve size, etc etc) and any
> computation would
> have to take all of these into consideration and if sucessful, would
> yield an
> answer which would be good at only one specified rpm, as the reversion
> phenomenon changes drastically with rpm, and my guess is that is does
> not
> change linearly.
> Unfortunately, depending on the cam, the reversion factor is still at
> work
> until much higher rpms than a starter motor could achieve.
>
> Also, some of the newer designs on a few cars (Jag H.E. and some
> Hondas, I
> believe) are running fairly high compression ratios compared to other
> designs, and are all running on pump gas with no ill effects.
>
>
> The old post follows...
> >>
> Well Guys...its the semi-factual Nick, with another OPINION...
>
> The Premise:
> There is NO relationship between the pounds per square in of
> pressure noted on a compression guage, and the actual compression
> ratio.
>
> The Arguement:
> The camshaft is the major governing body on the reading. Think
> about
> it this way...Lets start with a stock motor with a weenie cam with
> minimal
> overlap. Lets say the intake closes at bottom dead center, so the
> cylinder is
> closed as the piston starts back up the bore and at 150 rpm (starter
> speed-more or less), the resistance to gas flow thru the intake is
> minimal
> anough to allow complete filing of the cylinder to atmospheric
> pressure. So,
> as the piston rises in the bore, you have a maximum amount of gas
> being
> compressed the maximum amount.
> Now lats take that same motor and put a boy-racer cam in that puppy.
> This cam
> is gonna have mondo overlap. Now lets say the intake valve doesn't
> close til
> 45 degrees after bottom dead center (yeah, I know thats extreme, but
> hey, I'm
> tryin to make a point here). Now the intake valve doesn't close til
> the
> piston is almost 1/4 of the way up the bore on the compression stroke.
> Again
> assume no resistance in the intake system...where do you think the gas
> in the
> cylinder is gonna go as the piston starts back up ? Yep, right back
> out of
> the intake. We've all seen major amounts of fuel standoff on our
> racers at
> low rpm's - thats the piston pushin the stuff right back out the
> intake. So,
> now back to our scenario...the cylinder now has only about 3/4 as much
> compressible gas in the bore when the valve finally shuts. What do you
> think
> that's gonna do to the readings on our Pep Boys compression tester ?
> Think about it !
>
> As usual, I will stand firm on this theory - at least until somebody
> with a
> bunch of letters after his name convinces me I'm an idiot
> Nick in Nor Cal
>
>
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