Lowering exh rocker ratio.

[Steve.k]

If you lower the exh rocker ratio on engine and it picks up hp what does this tell you? Exh port flows to well? Its velocity is down? Or your cam is not correct for heads? Or all of the above?

[Mummert]

It tells you the engine could have been over scavenging, remember the end of the exhaust stroke i.e. start of the intake stroke (or overlap) is where the important things happen.
Engines with short exhaust systems can get pretty happy lowering the rocker ratio.

[tuffxf]

First thing id question is control of the exhaust valve,
Is the lifter following the lobe , what's going on when the valve is shutting, you sure the valvetrain is happy?

[MadBill]

The preceding post applies, but only if the loss is confined to near/at/over peak power.

[Walter R. Malik]

All the reasons can get rather involved it could be some combination of all ot just a couple.
One reason not mentioned much is the the exhaust valve is opening to fast and reducing the velocity of the exit exhaust at the seat. However, that is on the opening side ... other things are happening on the closing side as well; especially at overlap.
Faster moving air has less static pressure and can help evacuate behind that velocity.

[rebelrouser]

Do you have flow numbers for the head to post?

[Steve.k]

I was dynoing my 434 with bt3.6 head. I had made changes of 1.8 rockers and .250 more plenum on tunnel ram. The engine started out fine on torco 112 oxy fuel but we switched to c12 amd was unable to get more oxy torco. From previous Dyno we seen very little increase in the c12 to oxy. In end we were down 20hp couldn’t figure out why so we started to change things. We decided to change rockers back to 1.73 so we started with exh first. It picked up13hp. Then intake and we lost. Again. So change back. After all that we were still down on power and changed fuel from c12 to vp113. Gained most of it back but it never did match the torco 112 oxy. It was just a stumble why we found the rocker gain.

[Steve.k]

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Flow sheet

[Walter R. Malik]

I was dynoing my 434 with bt3.6 head. I had made changes of 1.8 rockers and .250 more plenum on tunnel ram. The engine started out fine on torco 112 oxy fuel but we switched to c12 amd was unable to get more oxy torco. From previous Dyno we seen very little increase in the c12 to oxy. In end we were down 20hp couldn’t figure out why so we started to change things. We decided to change rockers back to 1.73 so we started with exh first. It picked up13hp. Then intake and we lost. Again. So change back. After all that we were still down on power and changed fuel from c12 to vp113. Gained most of it back but it never did match the torco 112 oxy. It was just a stumble why we found the rocker gain.

Several rocker arm manufacturers have 1.65/1 rockers for a Cleveland/Big block Ford engine.
Maybe help some more.

[piston guy]

Steve,
I run conventional Boss 302 heads and have for well over 40 years been running shorter lift on the exhaust (.060-.100 less) The late ''Racer Brown" (rip) put me on this idea as he said "you are not flowing the same medium , and a flow bench can't simulate that". I have three and a half sets of rockers I test with. 1.65,1.73,1.8 and a half set of 1.9s. If I add rocker and pick up , I could use more cam , and if I lose , I might have too much and go to the shorter rocker. Worst case is I might have to have another cam ground but it is a way to know I if I do.

[Steve.k]

Thats very interesting for sure. You would not believe how many guys tell me my cams to small. After seeing this rocker issue maybe its not. That’s interesting about lift. Wow

[CamKing]

your cam is not correct for heads?

If the power went up, with the lower exhaust rockers. Either the exhaust duration was too high, or the exhaust lift was too high, or both.
It could also mean the Exhaust centerline was to tight.

[Stan Weiss]

Mike,
Wouldn't running a couple more dyno tests with the 1.73:1 rockers, one with the exhaust lash .005" tighter and one with the exhaust lash .005" loser help show which direction Steve still needed to go?

Stan

[Steve.k]

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.

[BradH]

Are you adjusting the lash to go along w/ the change in rocker ratio? I.e., same lash at valve with higher ratio is actually increasing the seat duration slightly because the lifter's starting to move lower on the flank.