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Old 12-21-2007, 01:14 PM
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Peak power/efficiency 350

Does anyone have a dyno chart for a carbed 350 vortec? It is going into a fun daily driver Impala, and I was planning on running an edelbrock performer cam (or comparable), performer intake and a quadrajet.
The reason I ask is I have heard that peak efficiency (read: gas mileage) is achieved right around the torque peak. This summer's project is transmission and rear end, so I want to pick out gear ratios that will put me at peak efficiency at around 65-70mph.

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Old 12-21-2007, 02:32 PM
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Hi Jon
Let me see if understand. Someone says that peak economy is achieved at the torque peak? So, If I build a reasonably mild street engine, with a torque peak at (say) 4300 RPM, and I want to drive 75MPH, I should gear the vehicle to run at 4300 at 75MPH, rather than gearing it to run at closer to 2800 RPM at 70 MPG. Do I understand that correctly?

Jon, I'm sorry, but I just don't believe it. I think whoever told you this is mistaken.

I do think that you want to build a flat (vs. peaky) torque curve. This can be done (in part) by selecting a reasonable camshaft. I also recommend getting EA 3.0 from Performance Trends. This software package (kinda like Desktop Dyno, but I think it's much better. Course, that's just an opinion) allows you to test a variety of options in terms of camshaft timing, compression, head flow, valve size, intake, exhaust, etc. And while I highly expect its accuracy is not perfect, I also highly expect that relatively similar engines producing relatively similar results will show relatively accurate changes as a result of reconfigurations you make through the software. Playing with engine configurations can help to raise, as well as flatten the torque curve. I think the goal for a flexible, easy driving street motor is to maximize the AREA under the torque curve. If you have an automatic, you want the torque converter to stall out at a point on that torque curve where it is beginning to flatten out. And you want to cruise down the freeway with the engine running just fast enough to minimize slip in the converter.

To me, that makes sense. I'm not positive I'm right, but I'm pretty certain I'm really close.

Best regards for a great holiday season.

Pat
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Old 12-21-2007, 05:38 PM
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Dyno

Look at part 2 also
http://www.chevyhiperformance.com/te...ams/index.html
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Old 12-21-2007, 06:50 PM
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Quote:
Originally Posted by PatM
Hi Jon
Let me see if understand. Someone says that peak economy is achieved at the torque peak? So, If I build a reasonably mild street engine, with a torque peak at (say) 4300 RPM, and I want to drive 75MPH, I should gear the vehicle to run at 4300 at 75MPH, rather than gearing it to run at closer to 2800 RPM at 70 MPG. Do I understand that correctly?

Jon, I'm sorry, but I just don't believe it. I think whoever told you this is mistaken.Pat
No, that's correct. Peak fuel efficiency comes from tuning cruise RPM right around the torque peak. The problem with your example is that if you're making an engine with a 4300-rpm torque peak, you're not concerned with highway mileage. Most street engines have torque peaks much lower than that. I lost mileage when I increased my tire size. My cruise RPM was 2100, then it was 1950 after the tire change and I was going further away from the 2500-rpm torque peak.

Its also not exact. MPG is nearly impossible to predict. Fine tuning is always required, but in your 4300-rpm example, cruising at 2500 might return 12 mpg instead of 13. Hardly a huge difference.

The main reason for putting cruise RPM near the torque peak is so that you're neither lugging nor needlessly over-revving the engine. Both cost MPG.
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Old 12-22-2007, 12:58 PM
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Thanks for the advice, I will look into that software. I'd been considering getting a dyno program just to play with anyway. Until I do, though, anybody have a chart on Vortecs?
I don't particularly understand the mpg thing either, but that is what I have been told, and I do know my 914 porsche (when it ran) got better mpg at 68mph than it did at 55... My parents crown vic, on the other hand, does the opposite. I guess I'm just trying to figure out what gears to use on the impala, and what torque converter, etc.

Last edited by Choir Boy Jon; 12-22-2007 at 01:26 PM.
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Old 12-22-2007, 01:56 PM
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Hmmm, just did some searching, and it doesn't look like anybody makes dyno simulators for Mac, so perhaps I'm out of luck that way.
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