aaronk785
Well-Known Member
I se where some builders use 1.6 rockers on the intake and 1.5 on the exhaust. What is the purpose of this?
Different rocker ratios intake vs exhaust
- Thread starter aaronk785
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To help the exhaust flow more. I've never been convinced. But then,I've never seen dyno numbers, either, so there might be somethin to it.
The intake will always benefit with more lift unless the port itself doesn’t support the added lift.
The exhaust normally doesn’t care.
One of the reasons is the exhaust is now just spent gas, smoke left over from the burn and ideally no fuel. It is hot and looking for a way out. The expanded burnt gas is also being pushed out and with a properly designed exhaust system, it is also being sucked out at the same time. Between these three things and a good exit port, the burnt fuel just rushes out.
Intake is different once your moving heavy fuel and air through the system. Air cleaner, carb, intake, head port and past the valve. These items are very heavy by compare and do not like to make direction changes. I just listed the whole track of each side. Which wants to move fastest, easiest and quickest of the two?
Exhaust! It is what it is.
When you hood the valve open longer you will make more power. If the port will flow more air with a higher lift, it will make more power. And this is why the added rocker ratio is a power maker and not so much for the exhaust.
The exhaust normally doesn’t care.
One of the reasons is the exhaust is now just spent gas, smoke left over from the burn and ideally no fuel. It is hot and looking for a way out. The expanded burnt gas is also being pushed out and with a properly designed exhaust system, it is also being sucked out at the same time. Between these three things and a good exit port, the burnt fuel just rushes out.
Intake is different once your moving heavy fuel and air through the system. Air cleaner, carb, intake, head port and past the valve. These items are very heavy by compare and do not like to make direction changes. I just listed the whole track of each side. Which wants to move fastest, easiest and quickest of the two?
Exhaust! It is what it is.
When you hood the valve open longer you will make more power. If the port will flow more air with a higher lift, it will make more power. And this is why the added rocker ratio is a power maker and not so much for the exhaust.
aaronk785
Well-Known Member
Makes sense. Thanks.
K.O. SWINGER
Meeting in the alley since 1976
Just an effort to make the exhaust side a little more efficient.
This a trend I have noticed in recent years. Many cams are ground with extra exh duration & lift [ relative to the intake ]. I suspect the factory thinking was that with the restrictive factory exh manifolds & small pipes, the exh could use some help.
Add headers, free flowing mufflers & pipes, & extra exh duration not needed. [ Isky Cams Tech Tip: 'Is extra exh duration really needed' is a good read.].
So builders are using dual pattern shelf grinds [ cheap ] & using lower ratio rockers on the exh to cut back on effective duration & lift.
i have been using reverse pattern cams in the last 20 yrs, less exh duration than the intake. Many factory engines come this way.
Add headers, free flowing mufflers & pipes, & extra exh duration not needed. [ Isky Cams Tech Tip: 'Is extra exh duration really needed' is a good read.].
So builders are using dual pattern shelf grinds [ cheap ] & using lower ratio rockers on the exh to cut back on effective duration & lift.
i have been using reverse pattern cams in the last 20 yrs, less exh duration than the intake. Many factory engines come this way.
MOPAROFFICIAL
Oogliboogli
What cid did you see this on and what heads?
MOPAROFFICIAL
Oogliboogli
Likewise, such as a small block chevy.
Yeah, I think you're finally learnin how this is done. Finally. Whoda thunk it?
Pressure in the cylinder is about 100 psi when the exhaust valve opens. That makes things very different from the intake side, not to mention flow is going the other way.
The exhaust valve size is smaller, so to reach .35 times valve diameter for max flow comes at a lower lift. As compression rises, more time for the exhaust event occurs because preasure of combustion decreases much earlier allowing more exhaust duration.
The exhaust valve size is smaller, so to reach .35 times valve diameter for max flow comes at a lower lift. As compression rises, more time for the exhaust event occurs because preasure of combustion decreases much earlier allowing more exhaust duration.
It would always be an interesting dyno test....
Right or wrong cam.
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