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Old 03-21-2001, 05:43 PM
mshelly mshelly is offline
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I see frequent recommendations on cam choices in the absence of a stated CR.

I am under the impression that anything over the stock grind (.450" and up) needs adequate CR in order to perform to spec. Even if the motor was unmolested, the HiPo engines from say '68 -'71 could be any where from 8.5-10.5 CR, (maybe more).

Would say the MP .484" cam work well in any of these?
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Old 03-21-2001, 07:43 PM
WS23 WS23 is offline
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mshelly,

You are mostly correct. Remember the 3 C'c of engine building: Camshaft, Compression, & Carburation. For any given cam to perform to its optimum these 3 must be matched. The key here is optimum.

The .484 cam you mention assuming it is the purple shaft 284 duration will work within the compression range you listed. Even the 292 .509 will too. I have seen many a 8 to 1 motor with a .509. Not the best matched, but workable. However neither will be at its optimum at the lower compression levels. In other words, these cams will perform much better with higher compression.

Without boring you with the engine physics, suffice it to say that longer duration cams need higher compression to offset compression bleed down due to long overlaps.

Hope this helps.
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Old 03-21-2001, 09:04 PM
mshelly mshelly is offline
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Thanks WS23,

Let's say the engine as is, on pump gas with a mild cam, yielded a high enough effective CR to prevent running a good amount of advance without pinging. (can run 30 want 38).

Would switching to another cam with the same lift but longer duration, lower the effective CR thereby allowing more advance? If so how would the net result compare to keeping the same cam and lowering the CR say via head gasket or spacer?
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Old 03-21-2001, 09:33 PM
Keith Keith is offline
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mshelly,

A while ago (~ 1 year) somebody posted a link to an article comparing just this situation. I'm sorry I can't give it to you, but maybe you could try to use the search feature to find it. Anyway, I believe the end result is that if your compression is too high to run a mild cam with good advance you are better off lowering the compression than going with a longer duration cam. The reason is that although the effective CR would be the same in the two cases, you would actually have more air/fuel mixture in the lower compression case which would give you more torque. (Due to extra volume of provided by spacer).

Keith
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Old 03-25-2001, 03:19 AM
451Mopar 451Mopar is offline
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Consider this, Compression would normally begin at the bottom of the engines stroke as the piston starts moving upwards towards TDC, but only if both the intake and exhaust valves are closed. If the valves were both closed at BDC, then the engines running compression ratio would be the same as the theoretical compression ratio or 100% of the theoretical compression ratio.
In the real world the intake valve is still open at bottom dead center, so even though the piston is moving up the cylinder, very little pressure is building untill the intake valve closes usually around 45 to 90 degrees after BDC depending on the camshaft duration and the installed centerline.
The shorter the duration, the sooner the cam closes the intake valve and compression pressure starts to build.
If a short duration cam, say 240 duration, 114 LSA, installed at 112 closes the intake valve at 52 degrees after BDC, then recalculating the compression ratio from the cam closing point gives about 80% of the theoretical compression ratio. A large cam of say 300 duration, 108 LSA, installed at 108 would close the intake valve 78 Degrees after bottom dead center resulting in only about 60% of the theoretical compression ratio.
So for the larger cam to maintain about the same cylinder pressure as the smaller cam the compression ratio would have to be increased almost 20%.
This would be like the small cam engine with 8.5:1 compression having the same cylinder pressure as the larger cam engine with 10.2:1 compression.
The problem is this would only be true if the engine is being turned at very slow RPMs like cranking the engine to check cylinder pressures, and it dosen't show the whole picture of an engines octane tolerance since air/fuel ratio and quality, swirl, quench, head heat disipation and oil contamination can change how tolerant the engine is to pinging and also how much ignition lead the engine really needs.

Oil entering the combustion chamber from worn valve guides or valve stem seals or worn piston rings can make the engine ping even if you don't have alot of cylinder pressure. Also, hot spots in the combustion chamber can cause problems. If your running the correct air/fuel ratio, is the spark plug maybe too hot a heat range?
If you have narrowed your problem down to too much cylinder pressure because the intake closes too soon, you can retard the cam a few degrees. Retarding the cam 4-degrees should reduce cylinder pressure about 3%. If your cylinder pressure was 160 PSI, retarding the cam should drop it to 155 to 156 PSI.
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