Cam timing and reversion solutions

[joespanova]

On my 377 , its time to inspect and freshen. This engine may even grow to a 406.
So I pull off the Victor Ram and notice the worst reversion I've ever seen in my engines. I don't know exactly when or why it started but I've had this engine apart recently ( heads off ) and the ports were nothing like what I see today. Reversion , soot , oil all the way up the ports.
The only change was going to larger carbs ( 750s as opposed to 660s ). The carbs are calibrated by Patrick James , based on his experience with similar combos. Also of note...........those carbs are worth approx 2 MPH over my 660s. Remarkably the car still ran well enough ( 5.90s-6.00s ) at 3000lb.
If the engine was already experiencing SOME reversion , could larger carbs decrease port air speed enough to "aggravate " it.
The current cam is a 109 LCA installed on a 107 ICL.( Bullet ). Based on internet research the cam timing is the most likely culprit. I have another option of using my ( Cam Motion )112 LCA installed about 110.
My guess is the Cam Motion cam will reduce or eliminate the reversion...........
BTW 23 degree std port heads. 14-1.

[joespanova]

I guess I'm the only one with reversion issues?

[CharlieB53]

Have you checked the timing chain for 'slack'? Possibly the retarded intake valve closing could allow an increase in the amount of intake reversion. Check that chain.

[joespanova]

Have you checked the timing chain for 'slack'? Possibly the retarded intake valve closing could allow an increase in the amount of intake reversion. Check that chain.

Based on what I'm seeing / reading...............advance cam and spread lobe centers.

[naukkis79]

Reversion happens only at part throttle, and nothing can prevent it when using long duration cams. You probably have very rich part-throttle AFR which soot everything inside engine.

[digger]

Reversion happens only at part throttle.

That's definitely not true, it's a necessary evil on a high performance engine, just need to keep it in check. The narrower the power band the easier it is

[naukkis79]

That's definitely not true, it's a necessary evil on a high performance engine, just need to keep it in check. The narrower the power band the easier it is

You're right, but reversion from late intake close won't make intake dirty. It's exhaust reversion which makes intake black and it is inevitable at part throttle and if part throttle afr is very rich intake will be very black.

[DaveMcLain]

That's definitely not true, it's a necessary evil on a high performance engine, just need to keep it in check. The narrower the power band the easier it is

You're right, but reversion from late intake close won't make intake dirty. It's exhaust reversion which makes intake black and it is inevitable at part throttle and if part throttle afr is very rich intake will be very black.

I would say that the real question is weather or not it hurts anything...

[joespanova]

That's definitely not true, it's a necessary evil on a high performance engine, just need to keep it in check. The narrower the power band the easier it is

You're right, but reversion from late intake close won't make intake dirty. It's exhaust reversion which makes intake black and it is inevitable at part throttle and if part throttle afr is very rich intake will be very black.

I would say that the real question is weather or not it hurts anything...

I've wondered the same thing................but evidence from Kaase's video illustrates how counter productive reversion is and how blow back into the intake can be very severe. So any effort to reduce that has to be beneficial?
Frankly I'm surprised this topic hasn't drawn more interest.
My guess is still cam timing has to be changed , at least on this engine , with these heads , with this induction . Others , apparently have different results.

[BOOT]

Have you tried shear plates?

[statsystems]

You're right, but reversion from late intake close won't make intake dirty. It's exhaust reversion which makes intake black and it is inevitable at part throttle and if part throttle afr is very rich intake will be very black.

I would say that the real question is weather or not it hurts anything...

I've wondered the same thing................but evidence from Kaase's video illustrates how counter productive reversion is and how blow back into the intake can be very severe. So any effort to reduce that has to be beneficial?
Frankly I'm surprised this topic hasn't drawn more interest.
My guess is still cam timing has to be changed , at least on this engine , with these heads , with this induction . Others , apparently have different results.

How long are the primary tubes on the headers? My guess is they are 36 inches or longer. That's probably 6-8 inches too long.

[ptuomov]

Not that I have any advice to offer, just trying to learn: what’s on the exhaust side as far as the headers etc. go?

[DaveMcLain]

If the engine had only seen wide open throttle running or mostly anyway I wonder if you would have seen any signs of reversion at all? I just wonder if it really is happening all that much when the manifold vacuum is close to zero.

The reason I'm wondering is that I never see much evidence of this on my two barrel circle track engines yet they have more than five inches of depression in the intake at wide open. That's still less than most engines even with a large cam would have with the throttle closed.

[joespanova]

The headers ARE 1.875 Hookers.......as far as the primaries , they are indicated in the catalog as 30-38 inch primaries.............and since I am not using any of the extensions I have to guess , right now , they are close to 30 inches long.

[joespanova]

If the engine had only seen wide open throttle running or mostly anyway I wonder if you would have seen any signs of reversion at all? I just wonder if it really is happening all that much when the manifold vacuum is close to zero.

The reason I'm wondering is that I never see much evidence of this on my two barrel circle track engines yet they have more than five inches of depression in the intake at wide open. That's still less than most engines even with a large cam would have with the throttle closed.

I'm trying to visualize what you are saying here.................if the manifold vacuum is close to zero explain the connection?