Help with choosing exhaust size ! =D


New member
New Zealand, Auckland
Mazda 323
Hi everyone!
I need some help with making a decision on a straight through exhaust size for my FWD mazda 323. Im going to buy an engine 1.8L DOHC NASP 135ps @ 7000rpm and 16.0 kg*m (156.91 N*m) / 4500 rpm put in my car. but before i do.. Im going to get/grind a fast road cam with a duration of 170-175 degrees... and also get some forged pistons and some forged conrods... and i need to make a 31/32'' manifold i think 4-1 or 2-2??.and a straight through exhaust... but first i need to know if i should get a 2''...2.25''.. or 2.5'' straight through.... keep in mind its Naturally aspirated.. or if these sizes I've mentioned are not what you think i should go with..please tell me! =D
and was wondering what kind of manifold as well...

Thanks so much for reading!! Any Opinions will be much appreciated!
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4-1 or 4-2-1 is easy.

4-1 gives more top end BHP
4-2-1 gives more torque

Exhaust diameter is a little trickier. What BHP and lb/ft are you looking for? Is noise an issue?
Thank you for your reply! =D

thanks for the header layout....
well with the exhaust i don't want to go too small or too big... i need a good flow because of the fast road camshaft.. but i don't want to go too big because i may loose power ..and that's the last thing i want to do. .noise isn't too much of an issue... tho it still has to be road legal.. ill have to put silencers in if its too loud is all..
im working on getting 100hp/l therefore 180hp.. but i don't expect too much of an improvement from the exhaust system alone.
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As the engine is NA you need to keep the gas speeds up to aid cylinder scavenging, so don't go silly on pipe size. I would suggest no bigger than 2.5" (I would go with 2.25"), especially on a road car in NZ. No point in going bigger and then having to quieten it down with extra silencers, further slowing down the gases.

As it's a road car I think a 4-2-1 manifold would work best.

Anyone else got any thoughts?
I'd agree & go for extra torque on a road car, extra bhp at the top end won't be as useful & only good for the track imo.

Think I'll look for a 4-2-1 manifold for mine after chrimbo. :amuse:
aw thank you.
.It will be a road car.. but i do want to have the odd race =P.... i know it hasnt got the racing power.. but just against my friends for fun =]... the torque on this engine when standard is max at only 4500rpm .. whereas the 323 with the original engine is max at 5500 which i like more.. will the fast road cam help this?..

and thats a big help..i will go for the 2.25'' exhaust =]
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i wouldnt go bigger than 2 and a half u gotta remember thats still only a small engine and ur not running a turbo so going to big would just be a waiste u will lose so much back presure all u will make is a really noisy car that runs rich and performs badly choosing exhaust size is very important all 4 cylinder engines i have owned only run 2 inch unless iv had engine wirk than i go 2/5 i think the best system i had was genie extractors and 2/5 system but after the rear muffler it went smaller than had a big canon on the back the best sounding system iv ever had and performance was unreal sorry dont think it was 2 and a half was bigger than 2 but i think maybe 2/25 than back to 2 right at the rear than a big cannon also had high flow cat
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