[Scott Stoneburg]

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

[Half-Inch Stud]

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.

[ponjohn]

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?

[pastry_chef]

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.

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A few tips from Mike Jones - jonescams.com

The exhaust valve lift curve has two main jobs. To evacuate the cylinder, and to cause a pressure below the intake valve that is less then the pressure above the intake valve during overlap. Too much valve lift area before BDC will hurt power. Too much area between BDC and beginning of overlap will reduce the pull on the intake during overlap. Not enough area between BDC and the beginning of overlap will cause the pressure under the intake valve to be higher than the pressure above the intake valve.

The opening and closing points may be the same, but one application may need less exhaust lift area during overlap, to keep the velocity up over the valve seat.

[Scott Stoneburg]

Originally Posted by pastry_chef

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

This is the type explanation i was looking for, so in effect wouldnt the same be accomplished by having less lobe lift on the exhaust side.?

[pastry_chef]

Originally Posted by Scott Stoneburg

This is the type explanation i was looking for, so in effect wouldnt the same be accomplished by having less lobe lift on the exhaust side.?

Generally yes.
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.



Valve lift for both below.
Lobes designed with one program and valve lift graphed with another.

[Jay S]

Originally Posted by ponjohn

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?

Not necessarily. An example is Torquewar’s engine that Pastry posted. It has 330+ cfm intake flow on the heads but the HP is nosing over in the mid 5000 rpm range. But that engine has enough head to make 700 hp and turn 7000. Whoever did the cam is using smaller exhaust profile to keep the rpms down on that combo. The bigger rockers on it lost a lot of power because the exh port has adequate flow for those rpms with the 1.5 rockers. Think of it as an area under the curve versus time to evacuate the cylinder at the target rpm.

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.

[Skip Fix]

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.

[slowbird]

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.