cam and compression questions

[v-8power]

I was just curious why some high lift or big duration cams need higher compression to run.

Also about what duration do you need to start running higher compression.

Thanks.

 

[PetesPonies]

The more overlap a cam profile has, the more cylinder pressure you are going to bleed off. So not having enough cylinder pressure to start with is a problem. Thus having higher compression helps the situation a lot. Also, the size of the cam is also dependent on what size engine it is in. The more cubic inches, the less the effect the cam has. So a cam that might be large for a 289 or 302 is not too large for a 351. So ging a generic answer to how much duration is the point where more compression is needed is not an exact science. And . . how much compression are you considering stock or low? 9:1 ?? I have run durations in the 220s at .050 easily with 9-9.5 CRs with a 302. If I were to be in the 230s, I would want compression to be about 10:1.

 

[v-8power]

The more overlap a cam profile has, the more cylinder pressure you are going to bleed off. So not having enough cylinder pressure to start with is a problem. Thus having higher compression helps the situation a lot. Also, the size of the cam is also dependent on what size engine it is in. The more cubic inches, the less the effect the cam has. So a cam that might be large for a 289 or 302 is not too large for a 351. So ging a generic answer to how much duration is the point where more compression is needed is not an exact science. And . . how much compression are you considering stock or low? 9:1 ?? I have run durations in the 220s at .050 easily with 9-9.5 CRs with a 302. If I were to be in the 230s, I would want compression to be about 10:1.

Thanks. I was just curious about this. Im not building any engines for awhile. lol

 

[baddad457]

The more overlap a cam profile has, the more cylinder pressure you are going to bleed off. So not having enough cylinder pressure to start with is a problem. Thus having higher compression helps the situation a lot. Also, the size of the cam is also dependent on what size engine it is in. The more cubic inches, the less the effect the cam has. So a cam that might be large for a 289 or 302 is not too large for a 351. So ging a generic answer to how much duration is the point where more compression is needed is not an exact science. And . . how much compression are you considering stock or low? 9:1 ?? I have run durations in the 220s at .050 easily with 9-9.5 CRs with a 302. If I were to be in the 230s, I would want compression to be about 10:1.

Excellant explanation, but I'll add that the amount of overlap in todays cams aimed at EFI applications is less than those intended for carbs. But I've found that the EFI cams with their wide LSA's allow the carb to function better than those intended for carbs. There's the theory that this limits the topend, but I haven't found that to be the case.

 

[v-8power]

Thanks Baddad.

So if and when i get a different cam i should get the higher degree lobe separation?

 

[baddad457]

Thanks Baddad.

So if and when i get a different cam i should get the higher degree lobe separation?

I would, definately. But just remember to tailor the comp ratio to avoid having to run more expensive fuels (octane) than you want to. My 331 likes 91+ premium only, with a 10.5 to 1 ratio. The 5.0 in my Ranger when I sold it was fine for most of the year on 87 and 9 to 1, but unless the timing was backed off in hot weather, would need 89 in summer. The 331 has a Z303 (112 LSA) and the Ranger's 5.0 had a stock F4TE roller (116 LSA) The Ranger worked so well with that cam and carb that it needed no choke whatsoever, down to 20*F.

 

[PetesPonies]

And added bonus of cams with a lot of overlap is that because cylinder pressure is lowered, you have less detonation problems. I have a hot 306 that runs 12.8s. I run 87 octane, never once had a single ping.