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Old 12-06-2010, 08:22 PM
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Peak volumetric efficiency & Peak BMEP?

I have a 350ci with a Peak volumetric efficiency = 91.2%@ 4500 RPMs and Peak BMEP = 189.0 @ 4000 RPMs.

Can someone explain what Peak volumetric efficiency and Peak BMEP means? Also, are numbers I posted above for my 350ci good, bad ,ok? Numbers came from dyno simulator software. Thanks in advance.

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Old 12-07-2010, 12:48 AM
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http://www.google.ca/#hl=en&source=h...9e0ba9ccbcf7c4

First link in the list!

http://www.epi-eng.com/piston_engine..._yardstick.htm

Google is your friend.
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Old 12-07-2010, 07:20 AM
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VE has to do with the efficiency of the engine at inhaling and trapping an intake charge in the cylinder. It has to do with cam timing events and intake flow mostly, but there are other factors.

Basically, a 100% VE means that each cylinder fills with the same volume, mass, and pressure as the ambient air around it before the intake valve closes. As you can imagine in a running engine, that is difficult to do. The smallest restriction will cause lower VE. Most street engines from the factory run with approximately 75-85% VE. Race engines with careful tuning can actually run greater than 100% VE with some tuning tricks. For instance, cam timing events that are designed to take advantage of the inertia of both the intake flow and exhaust flow. The exhaust velocity is high during the middle part of the exhaust stroke. Then as the piston slows down the rapidly exiting exhaust pulls a little bit of vacuum. Tuning the cam overlap so that the intake opens at the right point will use that vacuum to start sucking in new charge before the piston even starts to drop. Then carefully tuning the point ABDC that the intake valve closes takes advantage of the intake's inertia. After the piston reaches the bottom, the inertia of the intake velocity keeps pushing a little more intake charge in. The secret is to know when to close the intake valve to trap the most air inside. ...and of course that cam timing will only be optimal at one RPM point. Above that point and you could stretch out the intake closing a few degrees, below it and you need to close it a little sooner.

That is the basic premise behind cams and their RPM ranges. That is why VVT systems retard the cams just a bit at higher RPMs to better tune them to closing a tad later to trap more at higher RPMs.

The other way to get greater than 100% VE is with intake tuning. Have you ever slapped your hand over a long piece of pipe and it made an almost musical tone? The same things are happening in an intake runner. When the intake valve closes, the rushing column of air in the runner stops and makes a pressure wave just like when you slap that pipe. Tuning the intake runners with length and diameter can make it so that the next intake valve opening even occurs when that pressure wave has travelled back to the intake valve. If you open the valve right when that pressure wave is next to it, it will help with a little burst of pressure. This also only works at certain RPM since the fixed length of the runner is tuned to one narrow frequency range. This is also the motivation behind variable length intake runners on newer cars, and also why low rpm torquey engines (like a TPI chevy) have long skinny runners, and higher RPM engines have shorter runners.

BMEP is better outlined in that link above
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Old 12-07-2010, 07:47 AM
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Quote:
Originally Posted by Hanks56Chevy
Also, are numbers I posted above for my 350ci good, bad ,ok? .
The more important numbers are the actual HP and TQ curves. The BMEP and VE are kinda the wizards behind the curtain that make the HP and TQ behave the way they do.

HP and TQ are analagous to the answers of a math problem, VE and BMEP are the formula. I fully understand wanting to grasp the formula behind how you get the answer, though. Think of it this way, if you run a dyno sim and you want to know why your combination is, for instance, weak on HP or the TQ peak isn't what you anticipated, looking at the VE and BMEP might help a savvy tuner identify why. VE of 90+ is about on target for a mild to hot street motor.

That's one of the brilliant things about the dyno tuning software, helping you work out all the kinks. It may not be perfectly accurate on the actual numbers, but its close. The greater power of that software comes from the ability to look at all of the predicted parameters and help pick parts before you just go trial and error with a $300 intake
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Old 12-07-2010, 10:38 AM
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Quote:
Originally Posted by curtis73
The more important numbers are the actual HP and TQ curves. The BMEP and VE are kinda the wizards behind the curtain that make the HP and TQ behave the way they do.

HP and TQ are analagous to the answers of a math problem, VE and BMEP are the formula. I fully understand wanting to grasp the formula behind how you get the answer, though. Think of it this way, if you run a dyno sim and you want to know why your combination is, for instance, weak on HP or the TQ peak isn't what you anticipated, looking at the VE and BMEP might help a savvy tuner identify why. VE of 90+ is about on target for a mild to hot street motor.

That's one of the brilliant things about the dyno tuning software, helping you work out all the kinks. It may not be perfectly accurate on the actual numbers, but its close. The greater power of that software comes from the ability to look at all of the predicted parameters and help pick parts before you just go trial and error with a $300 intake

I like the way you worded that, and I agree with you with a VE of 101.5%. Imagine how much chevrolet would have loved to have had one of these programs back in 1955. They had to do all that math out on paper!! Not even a pocket calculator back then!

Another good thing sims can do is help you make the most from the parts you already have, like for example: picking a cam and headers that will compliment your stroke, rod length, heads and intake that you already have. That intake and heads are going to run at certain rpm better than others, so getting a cam and headers to match that rpm could be one way to increase the VE of what you already have, saving a few bucks!
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Old 12-08-2010, 07:53 PM
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Quote:
Originally Posted by curtis73
The more important numbers are the actual HP and TQ curves. The BMEP and VE are kinda the wizards behind the curtain that make the HP and TQ behave the way they do.

HP and TQ are analagous to the answers of a math problem, VE and BMEP are the formula. I fully understand wanting to grasp the formula behind how you get the answer, though. Think of it this way, if you run a dyno sim and you want to know why your combination is, for instance, weak on HP or the TQ peak isn't what you anticipated, looking at the VE and BMEP might help a savvy tuner identify why. VE of 90+ is about on target for a mild to hot street motor.

That's one of the brilliant things about the dyno tuning software, helping you work out all the kinks. It may not be perfectly accurate on the actual numbers, but its close. The greater power of that software comes from the ability to look at all of the predicted parameters and help pick parts before you just go trial and error with a $300 intake

Appreciate the reply. I now have a better understanding of these terms.
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