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Old 11-10-2005, 12:48 PM   #1
Scooby Newbie
Member#: 60629
Join Date: Apr 2004
Location: Blacksburg
2000 Impreza 2.5RS
Sedona Red

Default Difference in exhaust chambers for EL header sizing

I'm getting supplies together to fabricate an EL header for my MY00 RS, and while I know there's a difference in lengths for the exhaust ports, I haven't been able to find any specific numbers for the EJ25. However, in light of the fact that the Subaru cylinder heads are dimensionally fairly similar, I used the numbers given in this thread:

The difference in exhaust chamber sizes for both the USDM STi and USDM WRX heads are 15cc, while the difference in size is 17cc for the JDM V7 STi Spec C head.

Assuming my EJ25 heads are more similar to the USDM heads than the JDM heads, I'll say that the exhaust chambers for the front 2 cylinders are 15cc smaller than the exhaust chambers for the rear 2 cylinders. Since I haven't chosen whether to go with 1.5" or 1.625" pipe, I figured I would calculate the length difference for both cases (with 1.75" pipe thrown in for good measure). So a 15cc volume difference in exhaust chambers translates to:

0.52" difference -> 1.5" pipe
0.44" difference -> 1.625" pipe
0.38" difference -> 1.75 pipe

I've been searching for quite a few days and those were the closest numbers to what I was looking for, does anyone have numbers that can confirm or refute this?
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Old 11-10-2005, 08:51 PM   #2
Scooby Specialist
Member#: 2181
Join Date: Aug 2000
Chapter/Region: SCIC
Location: So Cal
2008 SpecB


Until that OBX header came on the market I was seriously considering going the DIY route. It's fun to make stuff so I hope I don't put you off.

Since you haven't shown your calculations I'm assuming (dangerous, I know) that your figures are (for example) 0.52" difference in tuned length between front and rear, based off the differences in port volumes?

What I'm going to say is not to worry about it.

1) You don't know what the perfect tuned length is anyway, unless you've got a lot of time invested at the dyno that you haven't told us about.
2) Even if you did know the perfect tuned primary length for your application, the tube lengths having a spread of a half inch will make no noticeable effect on peak power, but may in fact broaden the torque peak.
3) You'd find that the length and diameter of the collector are probably more important than the length of the primaries anyway.

If you were developing this engine to wring every last 0.1hp out of it I'd say worry about it. For a street engine there are more important things to fret about. If it's convenient to make the primaries slightly longer for the rear cylinders then go ahead, but don't lose any sleep over it.

Oh, and please remember to post photos when you're done making this thing!
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