Designing Exhaust for sc 400

[Cryser]

Hello Everyone, this is my first post on Lextreme I will be picking up my sc 400 tomorrow and have started researching to design an exhaust for it. While I may end up supercharging the engine later in it's life my current goals are a nice Daily cruiser/GT style car. BFI intake, S&S or PPE Headers, exhaust and a tunable ecu are in the immediate future.

I've been reading up on a lot of the posts on designing a good exhaust system for this engine in the sc 400 chassis for a couple of days and have come up with the few basic facts.

The stock scavenging Y pipe is pretty hard to improve upon
2.5 center piping split to 2.25 piping seems to be the best compromise
I want a TRUE 2.5 straight through resonator in the center
A high flowing Y pipe out the back
Mandrel Bends at the less possible radius applicable.

I plan to do my exhaust in 2 stages:

1st: About 2" after the stock Y pipe

2nd: with the headers/highflow cats replace the stock Y to the above section

My question is for my first stage of doing the exhaust.

My current plan is to cut about 2" from the stock Y pipe and use a virbant performance 2.5 resonator and a 2.5 pipe to the second Y pipe location.

http://www.ebay.com/itm/Vibrant-Per...pe-2-5-In-Out-14-length-/270855732222?vxp=mtr

Replace this Y pipe with a 2.5 -> dual 2.25 Y pipe out to 2 straight through mufflers

What is the best style Y pipe to use for replacing the second style Y pipe? I never liked the "stamped" Magnaflow ones are it just seems they who reduce velocity too much with that huge plenum area before it splits.

I found on ClubLexus someone who had Jbrady help design his exhaust with great results suggesting the following Y pipe:

http://www.mandrelbendingsolutions.com/servlet/the-459/Aluminized,-Steel,-AS,-Y,/Detail

This type of design seems much better then the stamped ones but there still has that kinda dead end.

Would the optimal design be almost a V shaped pipe so there is less or no center area? say something like this:

http://www.mandrel-bends.com/catalo...allel-2-25-inlets-2-50-outlet-304ss-1658.html

Obviously with the above design the bends to get the pipe to the proper exhaust outlets would be more severe but that could be lessened and smoothed out over the length of the pipe to the muffler.

I also found a splayed design that seems to be a compromise of both

http://www.mandrel-bends.com/catalo...layed-2-25-inlets-2-50-outlet-304ss-1706.html

which would you suggest for the rear Y pipe?

Thanks in advance

 

[hiluxfan]

Hi Cryser
Your pipe design sounds good.
However I think you are trying to split hairs with the Y-pipes. If you are building a full race car, and you want that last half a horsepower, then you can start looking in such detail.
For your application I would say just use the ones that fit the best.

 

[JBrady]

Cryser,

You have done pretty well with studying but there are a couple things to add. You have bought a California model SC400 which has the 3rd center cat (1st two cats bolt to exhaust manifolds before Y pipe). This Y does not have the quality merge that I recommend. Cutting off the cat will leave you 2 open pipes rather than one pipe on the non California cars.

Below are images.

1st is the the California stock Y as it meets the 3rd cat.
2nd is detail on how the non Ca Y pipe is formed. While this may look restrictive it is a great performing part.

So, if you cut off your 3rd cat you will have what is shown in the 3rd image below.

Further you asked about the Y pipes to purchase. The splayed design is the way to go to split the system back into duals. You can go 2.00" or 2.25" with either having much more than stock pipe flow. The 2" will be lighter and easier to package.
http://www.mandrel-bends.com/catalo...layed-2-00-inlets-2-50-outlet-304ss-1701.html

If you cut off your cat you will need to join the 2 sides and the parallel merge would be the way to go. While not quite as torquey as the non Ca Y pipe design it will suffice. Go with the 2.00" size.
http://www.mandrel-bends.com/catalo...rallel-2-00-inlets-2-50-outlet-304ss-946.html

Rather than buy the Vibrant center resonator you may want to fit a straight 2.5" pipe two feet long or so. This may be to your liking, weigh less and of course cost less. You can easily add the resonator later if you are not happy with the sound.

Hope this helps.

 

[JBrady]

Also, you may want to purchase V band clamps to make disassembly easy.

 

[t2d2]

Club Lexus regular here, but mostly a Lextreme lurker thus far. You guys seem to really know your exhaust (and performance) stuff, so what better way to "introduce" myself? I had started typing it up as a new thread, but it's similar enough to this one, I might as well piggyback it on for less forum clutter.

I just upgraded the mufflers on my '94 SC400 to M2 Performance axlebacks, and now I'd like to complete the basic exhaust upgrade by deleting the center sub-cat and replacing it with a straight pipe between the two Y-pipes.

From this thread and a few others, it sounds like the best approach is to cut a couple inches after the first Y-pipe merges into one, just before the sub-cat, and slide a 2.5" pipe over that 2.36" center pipe, then cut the second Y-pipe out and replace it with a better one.

How essential is that last part? Is the second stock Y-pipe really that restrictive? It seems weird that a simple split could have that much of an effect.

Magnaflow's #10758 looks like a good option, being 2.5" In and dual 2.25" Out, but the Outs are parallel and wouldn't mate up with the stock pipes to the resonators. So, if going that route, either the Y-pipe would have to be moved forward enough to allow an angled extension in between it and the resonators, or the resonators themselves would have to be replaced with new piping. The latter may not be a bad thing, but I sort of want to try it out with just the sub-cat removed, rather than gut all of it and regret making it too noisy...

[JBrady's splayed Y link above looks the most promising. Any idea if the 2" Outs would slide into the stock pipes or if 2.25" would slide over them, or if either would be close enough in diameter to be welded on directly?]

The other almost-good option I found is Magnaflow's #10733, which resembles the stock Y shape, but it's 2.5" In and Out so it would require modified piping downstream. I don't possess any welding skills and don't know of any exhaust shops in town, although I'm sure I could fine one with a bit of asking around. I figured Midas could handle a simple cut and weld if I'm not modifying the second Y-pipe.

I don't have any plans to go forced induction, so no need for extra capacity for down the road. I just want it to sound and perform like a good V8 should if I'm going to pay the premium at the pump for the pleasure of driving it.