Lowering exh rocker ratio.

[Stan Weiss]

We were running 17 thou lash. On both. Never changed the lash. The lash called for was 12 cold. That usually put us at 17-18. So kept around there, the previous dyno session we went both ways it lost. We set at 15 thou once on the exh and it lost.

Steve,
If you set the exhaust lash to .015" with the 1.73:1 rocker and lost HP then there is a good chance going looser like .020" to .022" might have helped.

Stan

[Steve.k]

Are you talking loosen with 1.8? The .012 was for 1.8 rockers which put us at .017-.018 hot. This alloy block doesn’t grow as much as most. Maybe scott figured it would and thought we should be i. .020+ lash area.Dragboss is doing a video on results so i wont spill beans to much but i did reach my goal and we know theres some left.

[Steve.k]

And no I didn’t compensate lash for different ratio.

[dannobee]

If it gained hp by reducing the exhaust rocker ratio, the exhaust lobes are too big. On our super stocker, we knew the cam was too big when lower rocker ratio did the same thing and then even opened the lash to 0.035". It may have sounded like a skeleton jerkin off, but that's where it went the fastest down the track until we could change to a smaller cam.

[Steve.k]

Thanks everyone. All very interesting stuff. This last Dyno session taught us alot. You take nothing for granted. Things you think would automatically make power can hurt you fast.

[Mummert]

It doesn't seem that strange, I've never really seen any power gain running extra exhaust duration when running headers only, no tailpipes. I know many grinders like to do it but all I usually see is a loss in the mid range and extended over rev, but no real power gain. With tail pipes or long collecter extension I've seen some extra top end, but I think it has more to do with resonance tuning than flow.

[Adger Smith]

I have used a lot of cams with a few degrees shorter duration on the exhaust for limited circle track engines that have manifolds and ex pipe limitations.
I always thought it helped with Torque and helped with the back flow problems at overlap.

[modok]

First guess-dropping the exhaust ratio reduces the overlap area, and slightly advances it too, and it likes that, so, it's saying you have too much overlap or the cam should be advanced. That overlap window is the most sensitive part of the magic usually.

Second guess... Opening the exhaust valve faster is kicking the header harder and sooner and the combo doesn't like that, so maybe a header change might act different.

But it could be some other reason. Maybe just too much exhaust flow generally. Might have to try fixing it three different ways to see which ways to prove which is really true, or more true, I mean, less rocker works better.... great just by itself, but you always wonder if it can be even more bueno.

[Stan Weiss]

Using a guesstimate of what the intake and exhaust lobe lift curves are. You can see the difference in area from IVC to EVC. But you can also see the difference from EVO to BDC at the valve when using 1.8:1 (red) and 1.73:1 (cyan) rocker arm ratios.

Stan

ab-cam-lift-steve-18-173-2.gif

[Steve.k]

Wow. A lot of good info guys. Thanks again.

[piston guy]

I have used a lot of cams with a few degrees shorter duration on the exhaust for limited circle track engines that have manifolds and ex pipe limitations.
I always thought it helped with Torque and helped with the back flow problems at overlap.

The 18* Chevy head liked shorter exhaust duration in a comp eliminator engine I ran. Maropoluos said the head was too good on the exhaust side and was over scavenging , so the shorter duration compensated.

[piston guy]

On my Boss/Cleveland head , "I" felt the added duration allowed more "scavenging" from the exhaust. A Ford engineer ( long ago) when high ports became the rage ,said that all I had to do with the "stock'' port ( way less than ideal flow bias) was increase the exhaust duration by about 8 degrees . Racer Brown (rip) agreed and suggested the lower lift to increase velocity. It worked "in the day" and I still do it today . It might be particular to my combination and Auto trans. A stick would want something different .

[CamKing]

The 18* Chevy head liked shorter exhaust duration in a comp eliminator engine I ran. Maropoluos said the head was too good on the exhaust side and was over scavenging , so the shorter duration compensated.

The problem with the GM 18deg head, wasn't that the exhaust was too good. The issue was the chamber shape and valve angles. Those heads had a big issue with the intake charge, flowing right out the exhaust during overlap. The band-aid was decreasing the exhaust duration, to lessen the overlap.
The fix was to add a "wall" between the valves in the combustion chamber. to redirect the intake charge, away from the exhaust valve.

[Ken_Parkman]

One thing that always made me wonder on this topic is why would the exhaust lift make any difference? Duration is easy to understand, but what difference lift?

The typical l/d on an exhaust is far beyond the point that the curtain area is even close to the throat area, so actual velocity should be little affected. I suppose the available area/time curve would a little more square which may be good, but on an exhaust is it? I'm under the impression much of the mass flow occurs early in the blow down because of the high density and delta P.

There seems to be no need to beat up on the valvetrain with a lift approaching the intake.

[Mummert]

The 18* Chevy head liked shorter exhaust duration in a comp eliminator engine I ran. Maropoluos said the head was too good on the exhaust side and was over scavenging , so the shorter duration compensated.

The problem with the GM 18deg head, wasn't that the exhaust was too good. The issue was the chamber shape and valve angles. Those heads had a big issue with the intake charge, flowing right out the exhaust during overlap. The band-aid was decreasing the exhaust duration, to lessen the overlap.
The fix was to add a "wall" between the valves in the combustion chamber. to redirect the intake charge, away from the exhaust valve.

This problem plagues many sbc heads. Brodix 23* with 2.08 intake especially I chased some of those heads around for a while before we put some break in rockers on the exhaust and picked up BiG power.

That reminds me if the engine has a scavenging issue BSFC and BSAC will be high, VE will be abnormally high. Thats if you you use an air hat. Many people don't use them anymore, (like air efficiency doesn't matter)