Cam choice for low compression motorhome 440

[brian6pac]

The whiplash closes the intake valve at 38* ABDC, the Comp closes the intake at 63* ABDC.
Do you see the difference,

 

[cpearce]

The whiplash closes the intake valve at 38* ABDC, the Comp closes the intake at 63* ABDC.
Do you see the difference,

Measure from two different lifts, so not directly comparable.

 

[brian6pac]

No but the intake valve closing is the start of the compression stroke, You can't make pressure if the valve is open. Most performance cams close the intake valve at around 70*, That's why you need a higher compression piston.

 

[IQ52]

Measure from two different lifts, so not directly comparable.

Correct. One is measured @ .050" lift and the other @ .006".

A careful look at the Whiplash/Thumper type cams listed on the previous page shows that the Thumper cam will close the intake valve sooner @ .050" lift than the Whiplash cam. An earlier suggestion, of the 21-222-4 cam, will close the intake at 31 ABDC @ .050", which is sooner that both the others, and can potentially give an even greater increase of cylinder pressure.

 

[crackedback]

Just a question, not arguing. I've always heard that a wide LSA (112?) is better when using exhaust manifolds. I understand advancing the cam to get the DCR up on a low compression engine but, why the tight LSA? Is it for a choppy idle?

Just because the cam is on a tight lsa, doesn't mean it has to be huge. The comp XE on a tighter LSA would be something I'd consider. You could use it as is and install it at 102 if you wanted. It's not going to be making any steam above about 5K anyways with the heads/intake/manifolds, etc.. I was in no way endorsing the use of either hughes or comp thumpr cam. It was an example of what I've done on multiple ocassions when dealing with low comp engines and camshafts that were "too big" according to most folks. Size it a bit smaller and install it way ahead.

Yeah wide LSA seems to work better for iron exh. If you have a wide LSA, you need to be careful with putting exh lobes out of bounds when advancing the cam to get an earlier intake closing event.

I wouldn't be smacking a 230ish at .050 cam in that engine without some serious thought about drawbacks.

 

[roccodart440]

Just a question, not arguing. I've always heard that a wide LSA (112?) is better when using exhaust manifolds. I understand advancing the cam to get the DCR up on a low compression engine but, why the tight LSA? Is it for a choppy idle?

Tighter LSA will increase your cranking compression. Hence why low compression cams have tight or tightish LSA's and are installed advanced.

In regards to wide LSA cams being good with manifolds, never heard that. Factory cams are typically cut with wider LSA's which help to improve idle quality, broaden the powerband etc. Most factory cars run iron manifolds.