Retarding Cams

[chris_seven]

has anyone any information on how much impact retarding cams has on the horsepower developed.

The setting range for S cams is quoted at 5.0 to 5.4mm at overlap setting.

If I change setting from 5.4mm to 5.0mm will there be much of a change to the top end or will the mid - range improve.

[Flat6pac]

I tend to find the high side because the chain grows considerably at 150 degrees change in temperature. With everything else expanded, the rockers .1 soon becomes .25 or greater when hot.
Everytime you see spec changed its upward, the only back up I can think of is 911S in 74 and the 911 in the same year and the Euro spec on the SC with the low compression pistons. 78 to 81.
Bruce

[HawgRyder]

Cam retard is usually good for top end power.
I can remember a gimmick for small block Chevys that was a spring loaded device on the cam gear...it retarded the cam as the RPM went up...it had screw in bolts to limit the range.
Some people claimed it gave them the edge...but seat of the pants...and track times did not support the claims as far as I could see.
And...it was another mechinical device to go wrong at the worst time.
Bob

[cgarr]

I think it all depends on how the engine breathes because on our 2.7 with DC-40's 4mm gives us good low end but only runs up to 5500 but at 5.5mm we get a lot more top end then on my 3.4 with super cup cams it's just the opposite.

[Walt Fricke]

My only experience is with an SC motor. The standard wisdom that retarding helped top end/HP and advancing helped low end/torque seemed qualitatively accurate when I went from one extreme to the other after a rebuild However, the guys making the most HP with these motors (a dozen or so of us made a date with a chassis dyno one weekend) were somewhere in the middle, as I recall. Which is what Porsche did (per Bruce Anderson) with the 3.2, after moving the SC hither and yon over the years.

Bruce's observation about chain stretch is an interesting one - hadn't occurred to me. I guess you could measure overlap on a hot engine to see how it varied from cold?

Setting cam timing (with stock parts) is just tricky enough that it seems inconvenient to do in the same session/atmospheric conditions when paying for dyno time.

[Eagledriver]

Originally Posted by Flat6pac

I tend to find the high side because the chain grows considerably at 150 degrees change in temperature. With everything else expanded, the rockers .1 soon becomes .25 or greater when hot.
Everytime you see spec changed its upward, the only back up I can think of is 911S in 74 and the 911 in the same year and the Euro spec on the SC with the low compression pistons. 78 to 81.
Bruce

While I'm sure the chain "grows" when the engine is hot, it grows alot less than the rest of the engine. Therefore, the cam timing will advance, excluding the effect of the rockers opening up.

-Andy

[mamut]

That is why this engines carry hydraulic chain tensioners, ain't it??....to keep tension on the chains and to keep a constant on the timing!.

[chris_seven]

As the chain extends due to wear or expansion there is a change in length and hence timing.

The tensioner only keeps the chain from 'rattling' or jumping teeth, I don't believe it has any impact on timing.