Exhaust diameter for a turbo MX-5?

[PASHN8]

Quick question guys, why are all the Flyin Miata exhaust 2.5\" in diameter?

Wouldn't a 3\" be a better option keeping future upgrades in mind? I'm at the crossroads of fitting up and exhaust to my SE and am a little perplexed as to why 2.5\" exhausts are so popular on turbo MX-5's, given that the majority of the other 1.8litre turbo guys (Familia's, Silvia/180sx etc) run 3\" turbo back?

[adamjp]

It is probably because a 2.5\" system can flow all that a turbo 1800cc needs. Just because they have a 3\" on the Nissan does not mean they need one. IMHO there is a great deal of w@&k about exhaust sizes, particularly in turbo applications.

I wrote some words in the tech section on exhausts. Have a read.

[Fatty]

dino , as adam said 2.5\" is plenty big enough. there are lots of mx5s out there running more rwkw than your or my car will ever see on a 2.5\" system. mine is 2.5\" and i think it'll do me fine for all future upgrades.

[Casey]

Yep - the day I see a WRC car with a 5\" exhaust tip I might think about it.

[kula]

WRC cars have intake restrictors which limit power to around 300hp, torque wont improve from a large exhaust much, but on a 1.8 turbo MX5 running more than 15psi, I'm confident a 3inch pipe will produce more power than a 2.5 inch.

[Casey]

WRC cars have intake restrictors which limit power to around 300hp, torque wont improve from a large exhaust much, but on a 1.8 turbo MX5 running more than 15psi, I'm confident a 3inch pipe will produce more power than a 2.5 inch.

True, the intake restrictor does reduce power. To compensate WRC cars make impressive torque. For example Neil Bates WRC Corolla made more torque then a V8 Super Taxi.

[blackster]

The yanks appear to be experimenting with 3\" systems on the MSM. There reasoning is opposite to what Adam's exhaust theory suggests.

the goal is to free up as much flow as possible via a larger diameter, not get the exhaust gasses flowing out of the exhaust system as quickly as possible. Any opening of exhaust flow, you will be increasing your exhaust flow. This, inturn, reduces your backpressure.

By using a funneling system, you would be increasing pressure, holding the exhaust back, making it harder for your turbo to spool. This is the same reason you use a catback exhaust system on any n/a car, the mandrel bends and larger exhaust piping allows for your exhaust to flow more freely.

Whether this theory is true or not is open to interpretation, they have yet to produce a dyno plot head to head that shows a 3\" system producing more power then a 2.5\" system with the OEM IHI. From what's been observed both here and abroad, is that there is IHI boost creep issues when a complete 3\" inch system is adapted with all other breathing mods.

Our intercooler was fitted with the new pipe work, standard air filter and an air box removed and our cold air system fitted. After this the 3\" exhaust was tried and dyno tests carried out. The car was producing 13psi of boost, at this point the computer cuts the fuel.

Next step was to fabricate a 3\" header pipe through a 3\" cat and reduce down to 2½\" thereafter to suit the 2½\" mid pipe and muffler. This was tried next evening and achieved 10psi of boost.

The front pipe and downpipe diameters on the SE is weird. Firstly the OEM cast iron front pipe appears squished, however the output diameter of the OEM cast iron front pipe is roughly 2.5\", at the front of the OEM downpipe there appears to be a restrictor, then the pipping becomes 2 1/4\".

If the front pipe is 2.5\" and you apply stainless 3\" system (down, mid and rear pipe) from that point onwards backwards are you really getting the full benefits of exhaust flow?

[PASHN8]

Phil, when you say 'front pipe is 2.5\"', I take it you mean the pipe after the cast turbo dump pipe?

I understand what you guys are saying about the 2.5\" gas flow theory and I agree with all that is said. I'm not leaning toward one over the other, I am just going off experience with building a large amount of turbo cars including GSR's, turbo Liberty's, Evo's, 180sx etc etc and i've never heard of anyone fitting a 2.5\" exhaust to a turbo 4-banger, so I found it strange that most of the off the shelf stuff is 2.5\" which baffled me.

I have no problem going with a 2.5\" system if it is the best system for my application with great spool up response on the standard IHI with a goal output similar to Phil's (Blackster).

It is a shame there doesn't seem to be any online back to back tests of a car with a 2.5\" vs 3\".

[blackster]

Phil, when you say 'front pipe is 2.5"', I take it you mean the pipe after the cast turbo dump pipe?

Yes, the FM and SMB kits attach to the front pipe featured above.

I am just going off experience with building a large amount of turbo cars including GSR's, turbo Liberty's, Evo's, 180sx etc etc and i've never heard of anyone fitting a 2.5" exhaust to a turbo 4-banger, so I found it strange that most of the off the shelf stuff is 2.5" which baffled me.

If your prepared to rework the IHI for better spool, boost above 1 bar and contol the wastegate issue, then i agree with KULA in saying that a complete 3" turbo system would be advantageous over the 2.5" bolt on stuff.

[GS]

It is a shame there doesn't seem to be any online back to back tests of a car with a 2.5" vs 3".

I dont know if you've come across these guys on the net, but this is from a US Miata exhaust specialist called Enthuza:

What diameter pipe should be used with my Miata?
I have dyno tested all of my exhaust and this is what I have found works best for these Miatas: (accounting for sound and horsepower)

M1 Racer and M2 Speeder 2.25" = Normally Aspirated Miatas and 6 psi or less Supercharged Miatas (5-8 hp increase) (stock size is 2" and 2.25" will not affect low end torque). Miatas have a relatively low compression engine which is not affected by backpressure. Through the dyno tests the 2.25" works very well in increasing both horsepower and torque.

M1 Turbo Racer 2.5" = Turbocharged Miatas @ 12 psi and less (19 hp gain over 2.25")

M1 Turbo Racer 3" = Turbocharged Miatas @ 12 psi and higher ( 3 hp gain over 2.5" but a 400 rpm quicker spool up)

From this wesite:

http://www.enthuzacar.com/default.asp

[Okibi]

Good find GS.

Part of any good SE MX-5 exhaust system is upgrading that crappy turbo.

This higher HP MX-5s over here all have 3\" systems I think.

[sabretooth]

I'm running a 2.5\" system on mine. I don't plan to go over 14psi on my turbo, though.

[blackster]

It is a shame there doesn't seem to be any online back to back tests of a car with a 2.5" vs 3".

I dont know if you've come across these guys on the net, but this is from a US Miata exhaust specialist called Enthuza:

What diameter pipe should be used with my Miata?
I have dyno tested all of my exhaust and this is what I have found works best for these Miatas: (accounting for sound and horsepower)

M1 Racer and M2 Speeder 2.25" = Normally Aspirated Miatas and 6 psi or less Supercharged Miatas (5-8 hp increase) (stock size is 2" and 2.25" will not affect low end torque). Miatas have a relatively low compression engine which is not affected by backpressure. Through the dyno tests the 2.25" works very well in increasing both horsepower and torque.

M1 Turbo Racer 2.5" = Turbocharged Miatas @ 12 psi and less (19 hp gain over 2.25")

M1 Turbo Racer 3" = Turbocharged Miatas @ 12 psi and higher ( 3 hp gain over 2.5" but a 400 rpm quicker spool up)

From this wesite:

http://www.enthuzacar.com/default.asp

Enthusa to date has only provided a bolt on 2.5" mid section for the MSM, to run a enthuza muffler on a MSM involves some modification. Dont suggest running a non resonated mid pipe if you care about noise.

Those plot numbers must apply to a regular miata running a garret. Interesting though, those numbers appear to verify whats already been mentioned above...........

[nfs13b]

I run a 3.5\" exhaust on mine, works great and sounds awesome Sorry, probably not relevant

[Mooro]

Personally given the choice I would choose a 3\". My understanding is that as opposed to NA, back pressure is not critical with a turbo setup and basically you just want to get the gases out with as little restriction as possible.

THing to keep in mind is that you need to look at the setup as a whole...not much point having a 3\" exhaust with a 2\" front pipe or a small restrictive CAT etc etc.

Granted on lower HP, lower boost setup there is probably not much gain but I would expect with a decent turbo running say 14psi a well design 3\" system will release more power than a well design 2.5\" system....I think the reason most nissan etc have 3\" is a bit of wank but also they tend to be running bigger turbos and running more boost (14psi+) compared to your average low boost MX5 setup...Bottom line is a 200rwkw MX5 is a rare beast, whereas every second silvia is producing that sort of power...but I would much prefer the MX5