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911st 911st is offline
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Join Date: Jan 2004
Location: Sacramento
Posts: 7,269
Just my two cents.

The GHL style system is just one of the popular "shorty style" headers.

In my opinion, on an 89 the biggest bang for the buck is the muffler, turbo, and then inter-cooler.

There is some efficiency available from headers in terms of reduced back pressure and increased power but for most it will be nominal compared to the later HE's.

I am suspect that a significant amount of the HP gain many find from aftermarket "shorties" actually comes from increased boost that is a result there wast-gate design which seem to increase the actual level of boost delivered. I have heard some reports of even triggering the over-boost switch when changing to a new system which is why I suspect this.

In my opinion we should not count the power that might come from a boost increase that is more a result of an increased effective boost as we can get by adjusting our boost levels ourselves.

To put some number to this, on a 300-360hp motor if boost is only bumped from
.8 to .85 bar that could be about 8-10hp.

One tuner quotes 40 hp from the headers alone. I suspect that is when stepping up from the early restrictive heat-exchanger system. I suspect for a later system it might be more like 5-10 from better efficiency and then about 10-20 from the higher boost that comes with the shorties. However, this is just an almost educated guess.

An inter-cooler is probably about 20hp from the lower restriction is makes and the increase in air density that comes from lowering of effective intake tempetures. However, it allows higher boost which could be good for another 30-40hp or so.

The other increase most notice from shorties is a faster boost response. This comes mostly from the reduced amount of volume that comes with a shortie system. Some have concluded that we also get some level of benifit from the coliding of the exhaust events that come from a non-equal length system. The thought be it creates waves of higher peak pressures that can better accelerate the turbo. However it happens, with the shorties boost comes in sooner.

Thus, I suspect that updating to a shorty system might reduce boost time by about 100 to 300rpm and get you another 10-15 HP in true gains.

It might be worth adding that there are also solid gains to be made in HP and boost response from dialing in one's AFR's and ignition values.

I have heard of owners of well tuned 930's that have ben able to get to full boost by 3k rpms with the later stock HE's and a k27-7200. Also learned of one tuner that claimed 600 plus hp with the stock HE's. Thus, there is nothing wrong with your HE's.

Not an expert, just what I believe so far.

Last edited by 911st; 10-30-2009 at 09:00 AM..
Old 10-30-2009, 08:55 AM
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