# Anybody clued up on airhead pistons?

Discussion in 'Airheads' started by Rob Farmer, Oct 5, 2010.

1. ### supershaftbecause I can

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Now you are making sense. In that last quoted sentence, Obert forgets to mention his whole point: air consumption and hp OVER A PERIOD OF TIME are directly proportional. That has been my whole point as well. That doesn't change the fact that the most air an engine draws in is at peak torque. With no time frame peak torque IS peak power. Any torque peak limited to below 5252 rpm results in the peak torque and peak hp being at exactly the same rpm regardless. A lot of engines don't rev over 5000 rpm. More air is more air. More air over time is more air over time. I thought it was clear that I was talking about more air. Now you are talking about more air AND more air over time. That's fine as long as you understand the difference.

I am hoping this conversation helps or sparks an interest in understanding.
2. ### supershaftbecause I can

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A mile has nothing to do with hp? Hp is a calculation and 5252 is at its center. 5252 has no significance at all unless you want to measure and talk about hp. And like Obert, there you go talking about power again within a time frame without mentioning the time frame. Speed is relative.
3. ### lkchrisAlbuquerque

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Not true.

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1. The pressure drop through components such as an air filter is a function of air flow per unit time. If we say something doesn't have enough airflow for, e.g., 40's, it really means that the pressure drop through that component becomes great enough at these larger airflows (BTW, just saying flow means per unit time) to effect performance. (There are also nonsteady compressible effects but the math becomes very complicated.

2. The statement that if an engine doesn't turn over 5000 rpm peak torque and peak horsepower must be at the same rpm is completely false. Go here for some examples from flatheads: http://www.vanpeltsales.com/FH_web/flathead_specs-85early.htm

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No. Horsepower is a unit of power. Power is the rate at which work can be done.

In general, P=T*N, where P is power, T is torque and N is angular speed. The number that concerns you, 5252, comes from the result of multiplying T is ft-lbf by N in RPM, which has results of ft*lbf*rev/min. We need a conversion,

We need rad(ians) because that's the way it works in physics, and 1 HP = 550*ft*lbf/sec.

Multiplying that all out gives you your 1/5252. Miles have nothing to do with it.

Or, in metric:

T (in N*m)*N (in rev/min)*(2*PI*rad/rev)*(1 min/60 sec) gives you P in (Watts). Or, in the familiar form:

P (in Watts)=T (in N*m) * RPM/9.55

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Explain.
7. ### supershaftbecause I can

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Thanks, I don't know where I came up with the mile from. Nevertheless, 5252 has everything to do with dyno charts. And, more to the point, when peak hp is at a higher rpm than peak torque, it is because the engine can manage to suck less air faster. It's a classic case where less is more when you factor in time.

It reminds me of when a 220 hp engine doesn't make enough power to replace a 160hp engine. The 220 engine is cranking out 220hp at 2100 rpm and the 160hp engine is cranking out 160hp at 1100 rpm. The 160hp engine is cranking out 200ftlbs more torque than the 220hp engine. Guess which one is sucking more air and making more power?
8. ### supershaftbecause I can

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Who says that just saying flow means per unit of time? You still haven't explained how an engine draws more air than peak volumetric efficiency when you don't consider a unit of time? Why would they call it peak volumetric efficiency if something else is really peak volumetric efficiency? Is it because we have been using the word 'flow' instead of 'draw'.

Why has every other dyno chart I have ever seen have torque and hp converge at 5252 rpm? On every chart I have ever seen up until now the peak hp and peak torque are always at the same rpm until the engine revs over 5252rpm. It has to be or the graphs won't converge there. What gives?
9. ### supershaftbecause I can

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Now I had time to check out those weird dyno charts. What I said still holds water. Check out those charts. The left and right side have different values. I have never seen charts like those before. They are like two completely different graphs in one. What's the point? If both the torque curves and the hp curves had the same gradients on the graph you would see that I am correct. I didn't catch that at first either. Very strange!

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Yeah, Explain.
Cuz that's wrong.
Robert
11. ### chasbmwLong timer

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Peak torque on many airhead engines is at around 3-4K rpm......I somehow don't think that an airhead engines flows less air at 3500 rpm than at 6000rpm

Charles
12. ### chasbmwLong timer

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To go back to the original query, when Jim Cray did the first tuning work on my 82 euro RT engine, he fitted low compression pistons, then machined the cylinders to produce a proper squish band at around 9.5 CR. it was a very good engine and was perhaps part of the most cost efficient tuning package.

He also drilled 4 X 1.5inch holes in the top of the square air box and reduced the length of the intake snorkels by 50%. The K&N air filter dealt with the rainwater issue.

Charles
13. ### Rob FarmerLong timer

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