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Exhaust diameter.


jad
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Hi

I am looking at buying stainless exhaust components and building an exhaust for my 1uzfe powered S3 109. The standard lexus exhaust diameter before the catalyst (not being used) is slightly larger than 2.5 inches. After the catalyst I think it is about 2.25 inches and right at the very back where the tail pipe finishers (would like to re-use :wub: ) fit it is 2 inches.

The engine as standard has 250Hp but I am expecting it to make more due to not having cats and being megasquirted. From what I have read/calculated from the internet a 2.8 inch single pipe should support 250Hp. So for the silencer where both sides of the engine will be joined I was thinking of using 3 inch pipe.

Before joining the exhaust I was wondering whether or not to reduce the exhaust size of it from >2.5 to about 2.25? I understand having an exhaust too large at this point will reduce bottom end torque and move power/torque further up the rev range which i obviously want to avoid in a big land rover...

Also will I be ok reducing the exhaust down to twin 2 inch pipes after the 3inch silencer? My calcs show that twin 2 inch pipes should support 261Hp.

I have no idea of any of this is correct though! as there seems to be wildly differing information on the internet..... A graph for the calculations I found on the internet is below.

If anyone knows more about exhausts than me (shouldn't be hard, I did an aeronautical rather than automotive degree :rolleyes:) then your opinions would be much appreciated!

Thanks

Jad

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Sorry not so up with the names and numbers for Toyota/Lexus engines.

Have passed query and my idea to a guy who used to work in engine prototyping and destructive testing, he helped me a lot with an intercooler theory of mine.

Will let you know what he comes up with.

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Ta very much

I really should ask at work. will have to e-mail someone in exhausts when i get back in January!

I have recently seen another post saying twin 2.25 to single 2.5 works well and 2.5 is good to 350Hp. Its hard to know what to believe but lots of people saying on this engine equal length sides to the exhaust seems to be more important. Thats never going to happen! I am using the stock manifolds which frankly are a ghastly design but I dont want to spend a fortune getting some made at the moment! pictures of standard here http://www.toymods.org.au/forums/parts-wanted/52555-1uz-exhaust-manifolds-now-have-manifolds-thank-you.html

Jad

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Ok have given a friend a welcome Christmas interruption and he says you are spot on, he did also say that trying to match the lengths of the "Y" would be good but in a real life situation just try to get as close as possible but don't worry as your series 3 will be unbelievably quick what ever you do with the exhaust on this engine!!

I had the idea in my head to look at a stage 1 V8 exhaust that might be able to make life easier for you only needing to modify the down pipe side of things?

Just an idea mind.

Good luck what ever you decide to do.

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Hi DD

Which bit was I spot on with? :huh: manifolds to 2.25 --> single 3 --> twin 2? or manifolds to 2.25 --> single 2.5 --> twin 2.25?

I have thought about using and modifying standard land rover V8 exhaust parts to try and keep the costs down. but think the pipes tend to be a bit small. I wouldnt mind using good quality mild steel pipe. (I knew when i re built my 200 exhaust with jetex it wouldnt last long!)Do you know the diameters of a stage 1 V8 exhaust?

Cheers

Jad

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Hi DD

Which bit was I spot on with? :huh: manifolds to 2.25 --> single 3 --> twin 2? or manifolds to 2.25 --> single 2.5 --> twin 2.25?

I have thought about using and modifying standard land rover V8 exhaust parts to try and keep the costs down. but think the pipes tend to be a bit small. I wouldnt mind using good quality mild steel pipe. (I knew when i re built my 200 exhaust with jetex it wouldnt last long!)Do you know the diameters of a stage 1 V8 exhaust?

Cheers

Jad

That one, but could be better with 2.5 from manifold.

Sorry, no idea about the bore from the S1 V8, I have to walk the fields after lunch and will take my measure as I think I still have the mid box back of my 3.9 V8 Disco exhaust laying on the rolling chassis, will let you know what it is on that if it helps?

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I measured the center box back and it was 2 1/4 inch inside diameter all the way from middle silencer to end of single exit tail pipe on my 1998 3.9 V8 mostly original apart from a new rear section, I remember down pipe "Y" was at least 2", only just scrapped that last week other wise could of measured that for you too :(

might also be worth while looking at the middle box back from a 2.6, yes I know it was a straight six but I replaced the whole system on mine and is was from memory almost 2" from the mid box back, just throwing something else up, any of the smaller independents who sell stuff like that will help you more than say paddocks as they often only see stuff on a screen, independents will happily go and take some measurements and call you back especially if they think there is a sale involved ;)

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