Quote:
Originally Posted by Cliff R
Not a lot to explain, but CYLINDER PRESSURE is controlled by the camshaft. How much and when it occurs. The cam compliments the CID and static compression ratio and should be chosen accordingly.
Cams with tighter LSA narrow up power and higher peak torque. I think it was mentioned at least once on this thread that they will make "more power". I agree, peak numbers anyhow, and that fact RAISES cylinder pressure and makes peak number occur at lower RPM's.
Just something to keep in mind choosing camshafts. Short seat timing, tighter LSA, earlier ICL (closing the intake sooner), lobes with improved cylinder filling qualities, etc basically improve VE and make the engine more efficient a bit earlier in the RPM range. Folks look at ALL that stuff with a "static" mind, not "dynamic".
You can take a lower compression engine and very quickly make it act like a much higher compression engine. Case in point. I've had half dozen 455's here to date with relatively "low" compression ratios that ping HARD on pump gas with "normal" timing/fuel curves.
Common denominator in all of them were short seat timing cams on tight LSA's. The worst of the bunch were the two that used the Comp XR276HR cam followed very closely by one using an XE268 flat cam in it. One was 9.5 to 1 the other about 9.3. I could NOT figure it out at first, but kept pulling out timing and adding fuel. Never really cured either one of them, but learned from it.
Then jump ahead about 15 years and I get a call from a custom who bought a custom carb kit with tuning parts. He's on the dyno and his "fresh" 455 topped with professionally ported #96 heads (250cfm), 9.3 compression, etc is pinging hard and they haven't put a lot of timing to it yet. They ended up spinning the rod bearings on the dyno. I suggested a cam swap instead of continuing to blame the carburetor (happens a lot from folks unfamiliar with them when Q-jets are asked to support big power levels).
The dyno charts are below. Combo #1 didn't make chit for power anyhow, and had a "quirky" idle quality the engine builder didn't much like. He noted that the second camshaft idled better with not much more than a "deep/heavy" sound and nearly smooth. Of course this happened because he was able to run more timing, at idle and total as well, plus the engine was just overall a LOT happier with it anyhow, which is pretty obvious by the dyno results........
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Thanks Cliff, I appreciate it, and I understand completley. Wouldn't you agree that both the XR276HR and XE268 both are quite a bit smaller camshafts than the 704 Lunati? Especially when you consider that the Comp cams are measured at .006" vs the Lunati .007-.0078", which would make the Lunati quite a bit bigger in reality, if it was compared at .006". Also the 10° difference at .050" 224/230 of the XR276HR vs 233/241 of the 704 Lunati? I know the 704 does have the narrower lobe seperation, and has been said to supposedly act much more like the wider lobe seperation camshafts in reality, due to the lobe offset and design. I believe the Voodoo closes the Intake valve at around 64°, atleast according to the 276/281 on the Cam Card, but that number would be skewed, if the Voodoo Seat Duration is indeed actually measured up at .007-.0078" instead of .006". That would actually make it close a little later than is published. The card shows it closes the Intake Valve at 42.5°@ 050" I believe. Again, I'm trying to learn here LOL