Exhaust rocker ratios
Ive always wondered why dropping the exhaust rocker ratio shorter than the intake makes more power, at least it has been said to make more power on this board several times.
1) Is it due to the intake to exhaust flow capabilities of Pontiac heads? 2) Does it only help if you have a symmetrical cam lobe profile? My cam has more exhaust duration, but less lift.( not by much ) Thanks in advance for the insights |
I've kept the EXH at 1.5:1, desiring 1.45:1, to promote lobe life, and easier valve opening under PSI. Probably a method for others to help valvetrain rpm and reliability.
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If the exhaust rocker ratio is smaller on the exhaust and it makes more power - isn't that an indication that the exhaust lobe is too big?
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Example:
Andy / Torquewar, a 578 HP engine hyd grind . 400 lobe 249 / 233 @ .050 110 LSA / 107 1.65s rocker on intake and 1.5s exh. Tried 1.6s on ex for a test and lost 22 hp. ---------------------------------------------------- A few tips from Mike Jones - jonescams.com Quote:
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Net result at the valve will depend if the lobe series are designed the same way. Here are specs for two hydraulic roller lobes. I kept seat duration the same for both. Applied 1.6 ratio rocker to the .375 lobe , 1.5 rocker to the .400 lobe. Valve lift traces very much overlap, the lighter trace is the 1.6 / .375 combo. https://i.imgur.com/1ACLVgD.png Valve lift for both below. Lobes designed with one program and valve lift graphed with another. https://i.imgur.com/wfE9cqI.png |
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Extreme examples are 2 bbl or restrictor plate racing that often have less area on the exhaust side, heads may flow enough to make 500 hp, but the induction is only allowing 400, using a exhaust profile that will support 500hp on the 400 hp engine is going to send some potential power out the exhaust. Methanol engine has less heat to expel versus a gasoline and like smaller exhaust profiles also. Round port Pontiac has pretty capable exhaust flow, combining that with the big port exit they don’t seem to need extra lift on the exhaust. I flowed a head last year that went 320 cfm on the intake and 180 on the exhaust side, a much different scenario than a round port. |
On the flip side my 500 IA motor liked more lash n the intake and less on the exhaust 1.65 ratios on both on the shaft system.
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I don't teally know the reason but even in horribly crappy intake to exhaust ratio engines (ie 60% or less) the exhaust side still usually doesn't like the added lift. Exhaust duration (open/closing points) seem to be the key on the exhaust side not lift.
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