Low compression and huge cam

[turbogrill]

Hi,

 

My engineis a SOHC Nissan L28 (rocker arms).

I have found a cam that has a very large adv duration but the 0.050" duration is pretty mild and has a nice lift. I assume that this means that it's nice on the valvetrain but it might bleed of some compression since when the piston is moving up the intake valve has not fully closed yet. Having a low compression to start with will make things worse.

 

How much does this matter on a race car? As soon as the engine approach torque max (high VE) there will still be gases flowing into the engine even after BDC (due to reversion and what not) so having the intake valve open even if the piston is heading up is not bad. Gases are still flowing in.

 

Or is that the ideal case? In reality at some point the piston will still push out some gas out of the slightly open valve and compression is lost? (Close to intake valve is closed).

 

 

[mender]

Published cam specs can be deceiving, especially with a solid lifter cam.

 

SAE numbers typically cite the duration at 0.006", well shy of the actual lash and also below the threshold of appreciable air flow. That's why the 0.050" numbers are so much more useful. With race cams I also look at the specs at higher lifts such as 0.200", tells me a lot about what to expect and how hard the valvetrain will be worked.

 

If you want, you can PM your cam and engine specs and expected usable rev range and I'll have a look.

[turbogrill]

24 minutes ago, mender said:

Published cam specs can be deceiving, especially with a solid lifter cam.

 

SAE numbers typically cite the duration at 0.006", well shy of the actual lash and also below the threshold of appreciable air flow. That's why the 0.050" numbers are so much more useful. With race cams I also look at the specs at higher lifts such as 0.200", tells me a lot about what to expect and how hard the valvetrain will be worked.

 

If you want, you can PM your cam and engine specs and expected usable rev range and I'll have a look.

 

Colt Cams C.575.S is the cam.

The 0.050" duration is spot on for what I want. But the adv duration is much larger than other cams with the same 0.050" (18 degrees!). 

 

Two examples:

ISKY-L490:

0.490" max lift

242 @ 0.050"

290 adv duration

 

C.575.S:

0.570" max lift

244 @ 0.050"

308 adv duration

 

Almost the same 0.050" duration but VERY different adv! 

 

Do you think the C.575.S will "bleed" of significant more compression than the ISKY-L490? Or is there no such thing? Are you saying that the whatever happens in the first few degrees really doesn't matter for compression?

 

Thanks!!! 

 

 

 

 

 

Edited September 25, 2018 by turbogrill

[mender]

OHC cams are a little harder to get a handle on because of the interaction between the cam lobe and the pad on the follower. A SBC cam will show different valve movement for the same specs. Many factors involved. I usually map out the cam directly with it mounted on a head and sitting on head stands on my bench before I write down the usable cam specs.

 

The 108 LSA indicates a cam that needs good induction and exhaust to work well and it will have a narrow power band. The 0.570" lift will need good springs because of the fast ramps needed and high rpm expected (4500+). If you're putting this in an otherwise stock engine you'll be disappointed and will make less usable power than a much milder cam.

[turbogrill]

12 hours ago, mender said:

OHC cams are a little harder to get a handle on because of the interaction between the cam lobe and the pad on the follower. A SBC cam will show different valve movement for the same specs. Many factors involved. I usually map out the cam directly with it mounted on a head and sitting on head stands on my bench before I write down the usable cam specs.

 

The 108 LSA indicates a cam that needs good induction and exhaust to work well and it will have a narrow power band. The 0.570" lift will need good springs because of the fast ramps needed and high rpm expected (4500+). If you're putting this in an otherwise stock engine you'll be disappointed and will make less usable power than a much milder cam.

 

Thanks!

 

I decided to go with a milder cam. But make no mistake, its a 7500 RPM cam so rods will still be flying out of the engine.

 

 

[Bremsen]

22 hours ago, mender said:

If you want, you can PM your cam and engine specs and expected usable rev range and I'll have a look.

 

Is that an open invitation to anyone?  I'd be really curious to have your thoughts on my current build as well as options for the next/spare engine.

[mender]

1 hour ago, Bremsen said:

 

Is that an open invitation to anyone?  I'd be really curious to have your thoughts on my current build as well as options for the next/spare engine.

You bet, send me a PM.