Hey fellow H-T'ers. I'm trying to get my exhaust system set up. I currently have the old school RS*R exhaust with 2.36" piping. I'm looking at the different headers available that would suit my needs. I have approximately 190-200 whp in my 97 GSR and am deciding on what header would work best with 2.36" exhaust piping. Here are my options:


2" header collector -> 2.25" cat -> 2.36" exhasut. (Seems to be the best)

2.5" header collector -> 2.5" cat -> 2.36" exhaust. (Seems the exhaust would cause a bottleneck)

The JDM 2.5" header is being suggested to me and I'm wondering if it would create a bottleneck in my set-up. I don't want to buy a new exhaust with bigger piping if I don't have to. What do you guys think about these options?

 

I would go with a quality header with a 2.5" collector. Mate that with a 2.5" cat, and install a 2.5 -> 2.36 tapered reducer to remove the bottleneck at the B-pipe flange. Keep in mind that regardless of which header you select, the old RS*R cat-backs are not particularly free-flowing, and aren't ideal for a high-horsepower NA setup.

Also, I'm curious what the additional components of your current setup are, since you stated that it's generating 190+ whp with what I assume (since you're asking about a header) is the stock exhaust manifold, stock cat, and the relatively restrictive RS*R exhaust.

 

Thanks for the advice. the approximate 190 whp came from my short ram intake, type r cams, skunk 2 cam gears, port&polished head with 3 angle valve job, mugen head gasket, soon to be headers and cat, and an RS*R exhaust. I approximate that after tuning, I'll hopefully achieve 190-200 whp.

I didn't think that 200 whp are big N/A HP numbers to warrent a 2.5" exhaust system. I didn't know how restrictive the old rsr exhaust was. Is there a reason for this?

And how would a 2.5 -> 2.36 tapered reducer remove the bottleneck? I thought the gases are still flowing from a large diamer piping to a smaller diameter piping and will still create restrictions and slow down the flow wouldn't it?

 

where is the love for option 2.5" collector with 2.5" cat to 2.5"piping

2.36 for 190-200 is reasonable..im sure you dont need to get it bigger. You could if you want, but it would be the reference for top end

how is this setup

2.25collector (is there one) 2.25 cat > 2.36exhaust of ur. Honestly, if you go 2.0 for 190-200whp is not funny. Why would 2.5collector > 2.5 cat > 2.36 cause a bottleneck... it ain't that much diff. It would cause BN if it bolts on a stock car...at 190 why woudl you worry about it

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by jadebellyt &raquo;</TD></TR><TR><TD CLASS="quote">the approximate 190 whp came from my short ram intake, type r cams, skunk 2 cam gears, port&polished head with 3 angle valve job, mugen head gasket, soon to be headers and cat, and an RS*R exhaust. I approximate that after tuning, I'll hopefully achieve 190-200 whp.</TD></TR></TABLE>

That's an extremely optimistic estimate. I think you realistically might see something in the low 170's on most dynos, however that's straying from the original question.

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by jadebellyt &raquo;</TD></TR><TR><TD CLASS="quote">I didn't think that 200 whp are big N/A HP numbers to warrent a 2.5" exhaust system. I didn't know how restrictive the old rsr exhaust was. Is there a reason for this?</TD></TR></TABLE>

Two hundred whp is a relatively high N/A output, and while it doesn't necessitate a 2.5" exhaust system, most people in that power range will be running one, or at least a 60 mm with a straight-through design. The main reason the original RS*R cat-back can be considered somewhat restrictive isn't the diameter of its piping, but rather the fact that it uses a chambered muffler. While this results in a quiet setup, it also flows less efficiently than a straight-through design.

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by jadebellyt &raquo;</TD></TR><TR><TD CLASS="quote">And how would a 2.5 -&gt; 2.36 tapered reducer remove the bottleneck? I thought the gases are still flowing from a large diamer piping to a smaller diameter piping and will still create restrictions and slow down the flow wouldn't it? </TD></TR></TABLE>

Most exhausts (both stock and aftermarket) have a somewhat severe bottleneck at the B-pipe flange. Removing this restrictive section of small-diameter piping and replacing it with a tapered adapter will create a much smoother transition from the cat to the rest of the exhaust, thus improving exhaust flow and power output.

 

Do not go down in exhaust size from piece to piece. And remember, header collectors and cat-backs are measured on the OD while cats are measued on the ID. So you're looking at something like this:

ID:
1.82" -&gt; 2" -&gt; 2.25"
or
2.38" -&gt; 2.5" -&gt; 2.25"

Both not ideal for 200whp n/a. The first one doesn't create any bottlenecks, but it should be a little restrictive in size for 200whp n/a. The problem is, not many people make a 2.25" OD header collector (2.12" ID) which would be perfect for your setup, and if you go to a 2.5" OD (2.38" ID) then you're stuck with a 2.5" ID cat and then going back to a 2.38" ID cat-back. Which makes a bottleneck, and is probably a little big for your setup.
Overall though, I would be more conserned with the specs for the header and catback than anything. And don't forget about the b-pipe bottleneck that Padawan mentioned.

And don't forget, what lies below the 200whp peak is going to be what makes you fast. Tune tune tune.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by siegen &raquo;</TD></TR><TR><TD CLASS="quote">Do not go down in exhaust size from piece to piece. And remember, header collectors and cat-backs are measured on the OD while cats are measued on the ID. </TD></TR></TABLE>

Interesting. I didn't know that. Well, going along with using the ID sizing, would this be a good combination:

2.38" header (2.5") -&gt; 2.38" custom cat -&gt; tapered adapter -&gt; 2.25" exhaust (2.36")

There is still that bottleneck, but would this be the best I could do without replacing the exhaust?

 

If you can get a 2.38" custom cat that would be good. I don't know where you could get one though. I know magnaflow/carsound only make 2, 2.5, and 3.

Idealy, you would get rid of that cat-back if you are going 2.38" ID header and cat. And not to mention it is chambered, which is going to hold you back the most right there.

 

The chambered design that bad huh? Well, ok, let's say I were to get a whole new exhaust system. With 2.38" header and 2.38" exhaust ID sizes. Which cat would go best with these set ups? If they are normally sold in only 2, 2.5, and 3" sizes, there still wouldn't be a good match! Why is the market like this? Nothing seems to fit well with anything.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by jadebellyt &raquo;</TD></TR><TR><TD CLASS="quote"> The chambered design that bad huh? Well, ok, let's say I were to get a whole new exhaust system. With 2.38" header and 2.38" exhaust ID sizes. Which cat would go best with these set ups? If they are normally sold in only 2, 2.5, and 3" sizes, there still wouldn't be a good match! Why is the market like this? Nothing seems to fit well with anything. </TD></TR></TABLE>

Combining a 2.5" OD header collector with a 2.5" ID cat and a 2.5" OD exhaust is not the significant mismatch that others have represented it as. Keep in mind that all cats open up to a larger volume where the catalyst itself is contained anyway, so even a 2.5" OD cat would be larger throughout most of its length. If you do plan to replace your entire system, my recommendation is still to go with a 2.5" header, 2.5" cat (I suggest SMSP's new spun bodied version) and a 2.5" cat-back with a straight-through muffler and a reasonably larger resonator.

 

Thanks for all the advice padawan. And for the clarification of the ID and OD siegan. I think I will go with the 2.5" setup. Now I just have to save up some money for a new setup. Thanks again.