increasing the DP/MP size...is it worth it?
 

The other day, I was tinkering around, and decided to see just how big my down pipe realy was. Turns out its only 2.85" inside dia. Not realy a true 3", like I thought it was. Now whats cool is that just happen to stumble apon a bunch of exhaust tubing (with mandral bends.) that have a ID of 3.35. I had a couple engineer buddys (Nuke and a mech.) crunch some numbers for me and it turns out it would flow a healthy amount more.

My question is, would the work to fab up a new DP/MP be a worth the gain in HP and spool rate? Not to mention how much faster would it spool and more hp would it make. BTW, The piping is free, and sitting in my garage. As for a catback I'm running a Apexi N1 dual tip.


Anyhelp would be great. CJ

I would think 3-3.5 inch if you are wanting 500whp or so would benefit greatly then a "3 inch or less." Ive heard of people running 4 inch to success, personally I will be doing a 3.5 inch dp that tapers to 4 inch at the end to a 4 inch exhaust.

I have a 4" dp and a full 4" exhaust as well. The bigger, the better. Supra's run up to 6" and thats no BS....!!

My car has full 3.5" or larger from turbine exit to tail pipe on the stock twins ;) No such thing as too big an exhaust for a turbo car so long as the maker/manufacturer knows how to keep it at a decent volume/note and you dont scrape the MOFO over every speed hump :)

This company makes the best exhaust systems for FD's up to 3.5" dia

http://www.smb.net.au/fullsystemsmazdarx7.htm

Ive gone from 3 to 3.5 all the way.Dont know if it will help.

just out of curiosity, which flows more/better, dual N1 or the GT spec N1?



thanks
Glean

GT spec! :) Why do you think its on there?!! lol.....

Sup Big T! I thought it was because it was a hell of a whole lot lighter, being the weight weenie you are:D

Any idea of how much more power the car will make? And how much faster the turbo will spool? CJ

Its not just an increase in power off the bat ist the reduced spool time and more area under the curve. Also, it gives you room for more "growth" your car will be able to make more max whp, without the exhaust holding you back. I am doing the full 4 inch exhaust as well. 4 inch seems to work well on rotaries with big turbos. Plus it sounds killer:)

I have an odd setup. 3.5" directly off my turbo, to 4" just shortly after. Runs 4" all the way back to my catback piping, which is 3" - lol. I want to go 4" all the way, just need to get a 4" cat. I use the apex GT spec =P

Isnt the GT spec, a tad bit bigger then 3 inch? ...

The output pipe is larger, but the pipe it welds to it 3" - I believe it's a 3" inlet with a 4" outlet or somewhere thereabouts.

I have a 3.5" DP, through a mid-pipe resonator all the way to the muffler, which is a single RE-Amemiya 4". If you want your system to create a "vortex" and suck gases out, instead of allowing gases to be pushed through, you have to go single and you have to go from smaller to bigger. Having a 3.5" dp and mid, which then goes to a dual tip end will not permit a vortex to occur. It may flow adequately, but you would have lower EGTs and SUBSTANTIALLY less back pressure with a system that goes from smaller to larger with a single tip. Among other things, that translates into longer engine life from reduced opertating temperatures. Aside from look or sound, why would anyone want to take spinning exhaust gasses in a large single pipe and slam them into two smaller pipes at the end of the line?

CJ, Ramping up the DP to a larger diameter will help in spool and in overall hp. About how much depends on too many thing to give acurate numbers. Hey if you have free piping go for it. Your response will be both better, and crisper on light throttle. I have a 5" on my car from turbo to exhaust tip. It is more than enough.

I just changed mine today to a full 8" from the dp to the exhaust tip! WHOSE THE MAN?!!!

I cant imagine what that would look like, maybe like having a garbage can strapped to your rear bumper?.. Can you post a pic of it Ernie?

My camera is broke, but I meant 10".....:)

hahaha....right....

Oh yea!!! Well I have a full 9" to the tip!!!

Oh wait...nevermind....wrong discussion

yea yea yea, I just had to

hahaha

Stephen

Now I have downpipe envy... If I polish it will it look bigger?...

I think its more desirable to have someone else polish it for you.

Hey CJ,
I'm running a 4" turbo back exhaust, not sure how much HP I gained because I never really dyno'd with anything else.

Jim

I'm going to go ahead and do it this winter I think. Maybe I can scrounge up something larger at work, and go from there. I wish I had a good dyno sheet to put it up against, but last time I was at the dyno the weather station thingy was broken on Ralph Friends dyno causing some inflated hp numbers. According to the dyno it made 407rwhp@11psi. :D (We where testing the GZ LIM on a stock port engine.) Maybe when I get it put back together I'll give Ralph a call and get some real numbers this time. Thanks everyone for the help.

CJ

As long as you're making before and after HP comparisons, why don't you log the before and after EGTs? I think a lot of people would find that interesting, useful info.