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Great thread...

Exhaust velocity should also be taken into consideration.

I have only a 3"id DP going into my various sized catback (Apexi GT 80-90mm I think).

I also have a 3" electric cutout on the DP that I can open and close.

Its amazing how drastically having the cutout open or closed changes my dyno graph.

Obviously with it open the turbo spools quicker and at about 150 rpm sooner, and also makes more peak HP.

But with it closed I make more low end HP (4000- 5500).

The car feels much faster with it open mainly because the tires don't spin as much when the boost comes on due to the slight loss in power in the 4000-5500 range and also because the car sounds like a jet fighter with it open.

I think the loss in power in the 4000-5500 area is mainly due to the loss of exhaust velocity but I may be wrong.

Anyone have any input on this?

Is having my exhaust cutout open similar to using a larger diameter exhaust?

I will try to post the open and closed dyno graphs on my website later today.

www.fuzionauto.com
 

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Thanks Greg!

I apologize for the filthy appearance of the Supra. It'd been sitting in the garage for about 4 weeks while I found time here and there to fab the exhaust, and I didn't clean it up prior to snapping the photos. :)

Steve Hayes
 
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I have been trying to get someone to make something like this for months. Never could. Exhaust shops won't do it cause it isn't street legal.

You should try one of these to quiet it down some:



Full desription here.

Boost Junkie: If you send me the pipe, which apparently isn't on the car anyway, I could have it prototyped to make more. Lots more.
 

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Derek,

I wouldn't mind sending the dump DP, but it won't fit anything except my car. I have a custom stubby DP that I fabbed (the titanium-looking one in the picture). The stubby connects to the turbine outlet, and then the dump DP connects to the stubby, all using V-bands. The 3.5" stubby that I fabbed is similar in configuration to the original 3" RPS stubby, but it's positioned somewhat differently. Given that, if you still think you could make use of it, I'll send it to you.

Steve Hayes
 

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That cone thing looks kinda interesting. I'd like to try one just to see what it does.

I built an Apex-type slip-in silencer for the dump DP and it absolutely KILLED power. With a 1.5" ID center opening the motor wouldn't make more than about 9 PSI. LOL That's was was with an SP74 that of course had no problem making 30+ PSI with a regular exhaust system. It was as if the 1.5" opening seemed to be a greater flow restriction when the exhaust system is short and the EGT is still very high, which, technically makes some sense, but still I can't really see it making THAT much difference. <shrug>
 
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I think I just came up with another idea.

I can use flexible exhaust tubing, bend it to the exact shape I want, then fill it with that expanding foam stuff. Gonna cap off the turbine to the turbo of course. When it sets, it will be rigid, and a perfect shape to take to a mandrel bending place to make the real ones of actual hard stainless tubing.

Think that will work?
 

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going back to the original thread topic but, how much more HP and spool up would a 4" dp/midpipe/exaust make over a 3" dp/midpipe-4"exaust?

rough guesstimate???
 

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Derek,
The flexible tubing/foam thing sounds like a good idea.


Sleeknsilver,
At the time of this comparison I had the SP74 with "old-school" Q-trim exhaust wheel in a .81 housing. Full boost in 4th gear on the dyno appears to happen at about 5200 RPM, so measuring power at 5000 should be a pretty good indicator of spool. At 5000 RPM with the 3" downpipe and original front flange on the GT Spec (about 2.85" ID on this flange) my car made 485 RWHP.

At 5000 RPM with the 3.5" downpipe and 3.5" front flange on the GT Spec, it made 520 RWHP. This was on two different Dynojets.

FWIW, seat-of-the-pants feeling also indicated that the spool-up was a little bit better with the 3.5" DP.

Strangely, on that turbo the car didn't make any more peak power than with the 3" DP. However, now that I've switched to a 74GT-R turbo power seems to have picked up noticeably, but I haven't been back to the dyno with the new turbo.


Steve Hayes
 
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