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Old 01-24-2008, 05:52 PM   #1 (permalink)
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SBC Rocker Arm Ratio

I was wondering what effects I might see if I change the intake valve ratio on a SBC. Here's my reasoniong: the stock rocker arm ratio is 1.5:1. If I change the intake rockers to 1.3:1 (using cam break-in rockers), then I will see less intake valve lift. Extraction (exhaust valve) will be unchanged. Since the intake valve is opening less, less fuel/air mixture can enter cylinder at any given rpm. The ECU will adjust mixture to stoich as long as I am running in closed-loop. In order for the engine to produce the same power, given the smaller valve opening, then I will need a larger throttle opening. This reduces pumping losses, slightly improving volumetric efficiency. This in turn might translate into slightly higher mpg. Peak hp of the engine would be reduced, of course, and that is the trade-off.

I welcome informed comments as to whether or not the above might be true.

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Old 01-24-2008, 05:54 PM   #2 (permalink)
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This sounds like a sensible idea. Many modern engines use drive by wire and intake cam timing in order to maximize throttle opening in order to reduce pumping losses, so I would say that this makes a bit of sense in the same way.
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Old 01-24-2008, 07:04 PM   #3 (permalink)
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The guys that could really answer that would be the cam company techs.
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Old 01-24-2008, 07:34 PM   #4 (permalink)
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Quote:
Originally Posted by plym49 View Post
In order for the engine to produce the same power, given the smaller valve opening, then I will need a larger throttle opening. This reduces pumping losses, slightly improving volumetric efficiency.
It may improve volumetric eff at certain loads, but in terms of the pumping losses that result in the largest efficiency drop in gassers, it won't do anything because those are a result of limiting the amount of air to enter the cylinder. This can be done via a throttle, or valve, but as long as the cylinder can't completely fill up with gases efficiency will suffer. Honestly, the only improvement I think you could count on from something like this would be what you could see from limiting your right foot.
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Old 01-24-2008, 09:56 PM   #5 (permalink)
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Well I did just about the same thing to my sbc, not 1.3 rockers, but I did put antipump up lifters under the intake pushrods. Obviously this lowered the valve lift at the lower rpm, and I have a running dispute with another motorhead as to whether the effective duration was shortened.

The compression at the low rpm increased, quite similiar to advancing the cam. If you look at the xfi cam thread it speaks of lower lift, peak torque at a lower rpm, and smoother idle. I do know that going with higher lift and more duration( more overlap) lowers low rpm torque and raises the rpm that the power range and peak hp occurs.
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Old 01-25-2008, 09:18 AM   #6 (permalink)
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Instead of swapping the rockers, I'd swap the cam. With the SBC you have a world of choices, including economy cams. An econo cam would not only change the lift, but also the timing and duration. That way, your whole valve train would be tuned for economy, instead of just changing the intake lift.

I wish I had as many cam choices for my Cavalier. My only options seem to be stock, two racing profiles, or a custom grind. I think I'll just advance the stock cam 5 or 6 degrees, and call it good.
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Old 01-25-2008, 04:47 PM   #7 (permalink)
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To roflwaffle:

I'm not sure that I follow you. You would have to run with a larger throttle opening to produce the same power. Your statement about filling the cylinders with air seems to neglect the effect of the throttle plate. Or am I not understanding you in some other way?
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Old 01-25-2008, 04:48 PM   #8 (permalink)
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To oldschool:

Hmmmm can't you use a dial indicator at the pushrod end to compare duration?

DO you see any change in mileage when driving economically?
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Old 01-25-2008, 04:55 PM   #9 (permalink)
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Quote:
Originally Posted by s2man View Post
Instead of swapping the rockers, I'd swap the cam. With the SBC you have a world of choices, including economy cams. An econo cam would not only change the lift, but also the timing and duration. That way, your whole valve train would be tuned for economy, instead of just changing the intake lift.

I wish I had as many cam choices for my Cavalier. My only options seem to be stock, two racing profiles, or a custom grind. I think I'll just advance the stock cam 5 or 6 degrees, and call it good.
Yes this is true, but the difference is I can swap out all 8 intake rockers in 30 minutes and be done. A cam swap takes longer and costs a bunch more. If/when I go deeper into the motor I would of course swap cams. I could still experiment with the 1.3 rockers. Which of course raises the issue of how to design the most economical conventional SBC. That would be the real home run here, and to do it right would have to incorporate all-electric accessories and stop/start capability.

WHy don't you swap in the powertrain from a Prius into that Cavvy? If you are running the GM V6, I believe that you have a cut-down SBC and you should be able to try the same rocker trick.
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Old 01-25-2008, 04:58 PM   #10 (permalink)
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Quote:
Originally Posted by Frank Lee View Post
The guys that could really answer that would be the cam company techs.
In my experience they only know what makes power. Mainly because that's how they stay in business. <smile> But it is worth a try. I have a friend who runs the dyno at a race engine shop that only builds Chevy's. Maybe I can get him to run some baselines. Then we would have actual numbers!!!! Including fuel consumption in pounds per hour per HP, over the rpm range. But time is money for them and I don't know if the boss there would allow a science project. LOL

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