Rocker ratio
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Rocker ratio

  1. #1
    Senior Member Constant840's Avatar
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    Default Rocker ratio

    I was talknig to guy at the river this past weekend. He was breaking in a new motor and metioned that he used a 1.8 in the intake and a 1.7 on the exhaust?
    Any one ever hear of this.

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    normally its the other way around(bigger on the exhaust) Gives you more lift.

    Dan

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    Senior Member Constant840's Avatar
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    He could of said that... but "aggressive on the intake" is stuck in my head. So is "sacrafice some on the top end".
    I guess he could have had a nice buzz going and just mixed up his words.

    I have never heard of this. Is it a way of compensating for a less aggressive cam or something?

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    Owner/Crew Chief 1/4 Miler's Avatar
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    Did the fellow mention, or did you see, what heads he had on the motor?

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    Senior Member Constant840's Avatar
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    I didn't look that close. I would think some iron square port. I doubt anything special.

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    Senior Member Futs's Avatar
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    Im running 1.8 on my intake and 1.7 on the exhaust. Seems to work for mine. Thats the way my cam guy designed it. I always thought it was the other way around too. Im running AFR heads.

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    Depending on cam and head flow numbers.

    I worked with Dan Crower on 4 "Engine Master" engines my 409" smblk Ford, his 366 SmBlk Chevy and his 470" Chevy all had less rocker ratio on the ex. and all picked up the Avg trq and HP numbers from 2,500-6,500 rpms.

    If the cam has enough ex. duration and the port flow's in excess of approx 70% of the intake flow it will work just fine and may gain you power. Keep in mind that in excess of 60% of the ex. gases are evacuated out of the cylnd. during "blown down"(from the time the valve cracks open at the end of the power stroke, until it starts the ex. stroke)

    The 565" runs 1.8 ratio of both intake and ex. If I had the $$ I'd get a set of 1.7's from T&D for the exhaust and try them on the dyno back to back. More than likely it would make more power under the curve. The big question would be; would that hold true with the 200 hp Nitrous shot. ?

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    031 RIVER4FUN's Avatar
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    That's what I'm running in my 565. Clay Smith roller, AFR 335 heads, Morel lifters

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    I talked to Dan today about a couple of things.. ( he's really wanting to put a bigger bump stick in my 565 among a few other things that might have to wait for the winter).

    I asked him about this rocker ratio deal, on his 366"er he went from the stock 1.5 to 1.3 on the exhaust and it picked up 20 hp doing it.

    He said the the exhaust event is nearly 100% complete by about .300 lift on the exhaust valve. Between .300-.600+ there is not much being done and that he would guess that on the 565" I could put a cam in it with a max of .450" exhaust lift with the same dur. @ .050 of 285* and it would probably make the same power. It would make more if I adjusted the valve springs to accommodate for the lower lift.

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    Senior Member Constant840's Avatar
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    ^^^^^

    That is interesting.
    This guy has a Bahner that runs way harder than it should... I'm trying to steal some tricks to apply to mine.

    Kind of funny actually. We got to talking about sleeper type boats as his Bahner is nothing to look at, has through the transome exhaust, and a bimini flapping in the wind.
    Anyway... he too has a an open bow CP. It has a big clay smith blower motor with a chiller. I mention some guy on the forums running a ford in an open bow CP. He says "oh yea. The orange and white one". I guess he remembered you from the CP regatta a couple years ago.

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    Quote Originally Posted by Constant840 View Post
    I was talknig to guy at the river this past weekend. He was breaking in a new motor and metioned that he used a 1.8 in the intake and a 1.7 on the exhaust?
    Any one ever hear of this.
    Depending on the combination I have done this in several applications when short duration moderate lift is need for an engine combination. In some class rule applications I will grind 2 intake lobes on the cam, one for short one for long runners. Nothing new as they use to do this many years ago in Pro Stock before Chevy had a symetrical head. By using the lobes you can tailor the duration and lift to each of Chevys intake runners.
    Chris Straub
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    Senior Member XClutchboy725's Avatar
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    I would imagine having less duration on the exhaust valve helps prevent reversion too?

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