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Could be wrong, but I'll give it a try....
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Posted by Aaron Gilbert [Email] (more from Aaron Gilbert) on Sat, 18 May 2002 14:12:18 Share Post by Email
In Reply to: How would loss of backpressure cause more lag? nm, Simon S, Sat, 18 May 2002 12:32:15
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When you have an exhaust that flows more freely, the turbine shaft can spin more easily as a result of having less pressure on the downstream side. Instinctively, this might say that if the shaft can spin more freely, then it can get up to full speed faster. It seems to me, however, that if you are evacuating the exhaust gasses more efficiently, that you will have a lower pressure differential between the gasses in the exhaust manifold and in the downpipe, and you will have less pressure on the manifold side of the turbine exhaust wheel. Less pressure applied to the exhaust wheel means less 'power' available for the wheel to convert into rotational velocity.

I look at it this way: Imagine you have a device that is a tube, say 1" diameter and 2 feet long, with a propellor blade in the center of it, which can spin perpendicular to the long axis of the tube. Now, if you were to cut to tube off such that the propellor was at the very end of the tube, and you blow in the other end, do you think it will spin faster, or slower (same person blowing same strength)? Why? What if instead of a constant diameter tube, you tapered it down to a 1/4" tube behind the propellor? Still the same speed? We have the same air input...

Why do you think it is that a larger turbocharger will produce more lag than a small one? Less exhaust gas restriction.


Of course, I could be just full of some very hot air and have too much compression in the brain. Others will no doubt be able to answer your question more accurately.


Aaron Gilbert
1987 9000T 275k miles

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