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Old 05-11-2007, 04:32 PM
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Choosing cruise RPM

I'm trying to figure out a good cruise RPM for an upcoming build. When looking at a dyno chart, how does one choose an optimum cruise RPM? Should I shoot for torque peak? VE peak? Just above or below one of those peaks?

School me on picking the most efficient cruise RPM.

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Old 05-11-2007, 04:51 PM
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Call me crazy, but I don't beleive you want it at peak power...what does that leave you when you tromp on it?
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Old 05-11-2007, 06:28 PM
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The way I think about it is this: The vehicle needs power to get up to cruise speed, but once there it doesn't need much power at all to maintain that speed. With nothing else considered (other intended uses like racing, driving habits, uphill driving, etc) I'd guess that you would want the cruise RPM to be as low as possible. So, no, setting cruise RPM within the peak power band isn't necessary.
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Old 05-11-2007, 08:51 PM
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You don't need power at cruise, you need peak thermal efficiency for MPG and engine life. Putting at peak power on this combo would mean I would be spinning at 5300 rpm on the highway.

Lower RPMs aren't healthy either. You put the engine in a range where its lugging and you need to apply more throttle to maintain speed than if it were higher. What you are in effect doing in that situation is increasing peak cylinder pressure but applying it fewer times per minute in order to maintain speed which uses more fuel and stresses the engine more. The answer is PROPER rpm which is dictated by cam selection and RPM range... I just don't know which of the engine operating functions I need to follow. Its either peak torque or peak VE... or something involving both.

This thread might be better in the Engine forum. If a mod happens to agree, could they maybe move it for me? I put it here originally since it deals with the entire driveline, but its really more of an engine question.
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Old 05-11-2007, 10:02 PM
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Most camshafts now list the RPM cruise range ...
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Old 05-12-2007, 02:59 AM
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Well, they list the RPM range, like 1500-5500, but they don't advertise optimum cruise range... at least not that I've found.
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Old 05-12-2007, 08:31 AM
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Quote:
Originally Posted by curtis73
This thread might be better in the Engine forum. If a mod happens to agree, could they maybe move it for me? I put it here originally since it deals with the entire driveline, but its really more of an engine question.
Let's give it a try. Dan
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Old 05-12-2007, 08:44 AM
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Quote:
Originally Posted by curtis73
I'm trying to figure out a good cruise RPM for an upcoming build. When looking at a dyno chart, how does one choose an optimum cruise RPM? Should I shoot for torque peak? VE peak? Just above or below one of those peaks?

School me on picking the most efficient cruise RPM.
Some additional points to consider if running an automatic trans.

Stock non lockup torque converters slip about 10% when fully engaged. Typically begins to fully engage around 1500-1800 RPM.

Aftermarket non lockup torque converters may be designed to not fully engage until higher RPM's. Many street applications are in the 2200-2800 RPM range. Some street applications as high as 3000 RPM.

Cruise efficiency RPM would be best above when the torque converter is "fully" engaged.
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Old 05-12-2007, 03:41 PM
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I always shoot for 80% of peak torque RPM with a carb. 78% with EFI.

Use a solid flat tappet cam with wide lobe centers and you can go lower by 1 or 2%. A hydraulic roller with tight centers needs to go up a couple. 80% works more times than not.....Dan
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Old 05-12-2007, 07:32 PM
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There is no hard and fast rule for "cruising rpm". But if you would provide details of your engine build and what car its going into (rear tire diameter, car weight), can recommend a rear gear ratio and converter stall speed.
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Old 05-14-2007, 08:36 AM
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That is going to depend a LOT on the torque characteristics of your engine and the weight and aerodynamics of the vehicle and what type of "cruising" you plan to do. City, highway or interstate. Lower RPM isn't always better, sometimes heavier vehicles get better cruise MPG's with slightly higher engine speeds. Note I said "better" NOT "good".......

Ideally you would want your "cruise" RPM to be fairly low in the fat part of the torque curve but not so low you lug the engine or are below converter stall speed. The narrower the torque band the closer to peak TQ the ideal cruise RPM is going to be. If your engine is built for low and mid range and has a nice flat curve then you can go pretty low before you start lugging the engine. I plan to set my cruise RPM @ 55 MPH in my truck at about 1500 but my engine will be built for max TQ from idle through 5,000 RPM and has a torque curve that looks like a topographical map of Western Nebraska, high and flat. Peak TQ occurs at about 3500 RPM but the curve doesn't vary 25 LbFt from 2500 ( the lowest RPM I could plot ) through 4,000. I know from experience with similar engines that the curve is still pretty fat below 2500. I can gear this to turn pretty slow at 55 MPH without lugging it. On something built more for HP and high RPM the cruise RPM will increase accordingly.

It's all a compromise and finding the sweet spot can be tough.

Last edited by Hippie; 05-14-2007 at 08:44 AM.
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Old 05-14-2007, 07:36 PM
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Crane, for one, lists the recommended cruise RPM. Here is the info on the cam I have in my 427.

Cam Facts: Excellent mid range torque and HP, fair idle,moderate
performance usage, good w/manifold nitrous system, mild bracket racing,
auto trans w/3500+ converter, 3800-4200 cruise RPM, 10.5 to 12.0
compression ratio advised.Good w/Roots supercharger, 16 lbs. maximum
boost w/8.0 maximum compression ratio advised.

Barry
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Old 05-15-2007, 06:33 AM
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3800-4200 cruise RPM??!!! Thats freekin Nutz!

My Vette runs at 2800 RPM at 60mph and thats bad enough.
Unless you have a little peanut of a motor you want to run around 2000rpm
Most engines can supply PLENTY of power at the rpm, you keep the piston speed down and vibrations are at a nice rumble.

I think the Vette would be awesome at 1500rpm with a lockup converter. The 434 would still have like 300 ft/lbs available so it would be such a cruiser...

Keith
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Old 05-15-2007, 04:33 PM
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With the 4.56 gear and a 28" tall tire, I'm between 3600 and 3800 at highway speed.
I love the way the 433 sounds through the 3 1/2" exhaust at that rpm.

Barry
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Old 05-15-2007, 05:01 PM
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Personally, I think 2000-2500rpm a good cruise rpm for any vehicle.
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