cams vs. compression

JohnConnolly · Post by **JohnConnolly** » Sat Jul 14, 2001 4:06 pm

Experience!

Matt Harris · Post by **Matt Harris** » Sun Jul 15, 2001 11:09 pm

Ok cool! Thanks.

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Matt Harris
78 x 90.5
2007cc
1971 Super

howardsoval · Post by **howardsoval** » Mon Jul 16, 2001 7:34 pm

Go to reher- morrison website and red their articles on cam and compression, you will see that they are in agreement with John. You cannot argue with their success. Just a thought.

Matt Harris · Post by **Matt Harris** » Tue Jul 17, 2001 12:27 am

What's the website Howard?

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Matt Harris
78 x 90.5
2007cc
1971 Super

danimal · Post by **danimal** » Tue Jul 17, 2001 1:24 pm

>>>What's the website Howard?<<<

matt, i quoted the reher-morrison website in the 5th post of this thread, where they clearly stated how dynamic compression can change with rpm's and cam profile.

some of the participants in this discussion may never understand that concept, but on the other hand, it's possible that i am overstating the effects of rpm's/cam profile on dynamic c.r.

we really need to see some sophisticated engine math on the subject.

dan
oceanstreetvideo.com

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howardsoval · Post by **howardsoval** » Tue Jul 17, 2001 5:58 pm

www.rehermorrison.com

Matt Harris · Post by **Matt Harris** » Tue Jul 17, 2001 6:26 pm

Hi Dan,

The only thing that I gathered from that article, short as it may be, that would support your statement is this, "The engine with the longer duration camshaft will have less dynamic compression because its intake valve closes later after the piston has traveled farther up the cylinder. On the other hand, if the long-duration cam does a better job of filling the cylinder at high rpm than the short-duration cam, more air and fuel will be trapped in the cylinder and the resulting dynamic compression ratio will be higher."

And their is a big "IF" in front of the sentence that supports your position. It sounds to me like the specific camshaft has allot to do with the dynamic c/r up the rpms range with experience being the major determinant of what static c/r to use. Typically VW engines run much longer duration numbers than V8's as well, due to the isolated carburetors and intake runners running straight to valve and cylinder...more efficient fuel delivery? And this is what allows us to run more c/r.

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Matt Harris
78 x 90.5
2007cc
1971 Super

[This message has been edited by Matt Harris (edited 07-17-2001).]

JohnConnolly · Post by **JohnConnolly** » Tue Jul 17, 2001 8:43 pm

Matt,

all correct except this part.

"And this is what allows us to run more c/r."

I think what you honestly meant was that

"and this is what allows us to run more cam duration" (isolated intakes instead of a plenum)

John
Aircooled.net Inc.

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Matt Harris · Post by **Matt Harris** » Tue Jul 17, 2001 9:04 pm

Yes John, and I was maybe skipping a sentence. Since we can run more duration because of the isolated barrel/manifold runner, we can therefore run more c/r. This is my reasoning.

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Matt Harris
78 x 90.5
2007cc
1971 Super

magoo · Post by **magoo** » Thu Jul 26, 2001 1:09 am

Wow! 98 posts! far better than that measly 53 from the old CLF site! ......

BTW for alittle more light reading might I suggest www.elgincams.com But do be careful on the c/r suggestions,those profiles were used a while ago,or when the fuel was a bit less like dishwater....

chris burns · Post by **chris burns** » Sun Jul 29, 2012 8:11 am

I was wondering about the compression ratio in my street engine and thought I would have a look at this subject. After ploughing through 7 pages of text, the main thing I realised was how much has changed in the past 10 years in VW engine tuning. About 10-12 years ago many people ran 7.7-8.0:1 in whatever engine they had. Nowadays, 10:1 seems to be a minimum in hot street engines. My personal daily driver is set at 11.6:1, but does have a 292 degree at 0.050 inch cam and it runs nice and cool even yesterday (35 degrees celcius). It helps that the car is light and runs close gears. The plan is to up the compression ratio to about 12.5:1 because it still "feels" under squeezed.

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