How do you convert .050 duration from 1.5 rockers to 1.6's
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From: Ocala, Florida
How do you convert .050 duration from 1.5 rockers to 1.6's
I'm thinking abuot getting the gm847 cam, but I have the 846 right now (222/230 with 1.5 rockers) and I was wondering what that converts to with 1.6 rockers. I'm thinking about the next XFI 230/236 but if its barley any bigger than my cam now there's no point. And how would the 242/248 XFI cam work compared to the gm847? 292/300 adv duration vs 296/304?
Last edited by speed_demon24; Nov 26, 2006 at 09:58 PM.
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From: Phila., PA
Since .050 duration is measured when the TAPPET is .050" off the base circle a different set of rocker arms won't change that at all. But I know what you're getting at. How much "bigger" will the cam act with higher ratio rockers? Figure 1.6s will effectively "add" about 4* more .050 duration than with the 1.5s. The difference is pretty modest.
Don't compare cams based on "advertised" duration and ESPECIALLY NOT GM CAMS. They measure advertised duration in a way that is rarely comparable to a typical aftermarket cam from Comp, Crane, etc. Even comparing based on standard .050 durations, LSA and ICL doesn't tell the whole story, but it's far better than using advertised durations.
Don't compare cams based on "advertised" duration and ESPECIALLY NOT GM CAMS. They measure advertised duration in a way that is rarely comparable to a typical aftermarket cam from Comp, Crane, etc. Even comparing based on standard .050 durations, LSA and ICL doesn't tell the whole story, but it's far better than using advertised durations.
Last edited by Damon; Nov 28, 2006 at 06:03 AM.
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Posts: 10,745
From: Buffalo, New York
Higher ratio rockers, unless the ratio gets so extreme as to cause geometry problems, are about as close to a "free lunch" as you can get in top-end mods. At low speeds, the cam will "act" pretty much the same as the valve events are not changed. At higher engine speeds, because of more area under the lift curve, there will be better cylinder filling and more power and torque.
However, if you are looking to substantially change where the valve events occur, you need a different cam regardless of the rockers used. What are you trying to accomplish by the change? What is the rest of your combo? How do you use the car? These are the kinds of things that need to be considered when picking a cam. But as stated above, regardless of the cam, higher ratio rockers usually give gains without a downside. OTOH, IF the cam has been truly optimzed for your combo and intended use there might be only a marginal difference with a rocker ratio change
Rich
However, if you are looking to substantially change where the valve events occur, you need a different cam regardless of the rockers used. What are you trying to accomplish by the change? What is the rest of your combo? How do you use the car? These are the kinds of things that need to be considered when picking a cam. But as stated above, regardless of the cam, higher ratio rockers usually give gains without a downside. OTOH, IF the cam has been truly optimzed for your combo and intended use there might be only a marginal difference with a rocker ratio change
Rich
Thread Starter
Registered User
Joined: Mar 2002
Posts: 3,245
From: Ocala, Florida
Higher ratio rockers, unless the ratio gets so extreme as to cause geometry problems, are about as close to a "free lunch" as you can get in top-end mods. At low speeds, the cam will "act" pretty much the same as the valve events are not changed. At higher engine speeds, because of more area under the lift curve, there will be better cylinder filling and more power and torque.
However, if you are looking to substantially change where the valve events occur, you need a different cam regardless of the rockers used. What are you trying to accomplish by the change? What is the rest of your combo? How do you use the car? These are the kinds of things that need to be considered when picking a cam. But as stated above, regardless of the cam, higher ratio rockers usually give gains without a downside. OTOH, IF the cam has been truly optimzed for your combo and intended use there might be only a marginal difference with a rocker ratio change
Rich
However, if you are looking to substantially change where the valve events occur, you need a different cam regardless of the rockers used. What are you trying to accomplish by the change? What is the rest of your combo? How do you use the car? These are the kinds of things that need to be considered when picking a cam. But as stated above, regardless of the cam, higher ratio rockers usually give gains without a downside. OTOH, IF the cam has been truly optimzed for your combo and intended use there might be only a marginal difference with a rocker ratio change
Rich
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