Exhaust diameter for a turbo MX-5?

[Boags]

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.

I would like to urge EVERYONE to read the thread reffered to by adamjp in the tech archive BEFORE posting their thoughts.

It might save you looking a little bit like a turkey.

a well design 3" system will release more power than a well design 2.5" system....

Yep, about 3hp; as per the information above. Not at all worth the extra expense on a daily driver, IMO.

[Mooro]

I have read that particular \"tech\" article...likewise I heave read many other \"tech\" articles that range from suggesting that a 3\" is the optimum size to saying that the bigger the better for turbo applications.

I spent about 2 mins searching on the web and found multiple \"tech\" articles suggesting various different theories, many of which \"apparently\" document real-life figures and power differences....but obviously EVERYTHING from the web should be critically assessed before stating as fact.

I'm just curious to know what is so special about that \"tech\" article that if anyone was to suggest their own personal opinion they would look like a turkey? If you could just clear that up for me with some definitive evidence that you obviously have which qualifies you to make that statement otherwise that \"tech\" article is like any other which maybe based on solid technical theory and real life experiment or it could just just re-hashed from some other \"tech\" article on the web.

I'm not suggesting adamjp is wrong or his thoughts are flawed, but I would challenge the suggestion that just because he has written this article that it is discussion over, we should bow down and listen, and if we, dare to suggest a slightly differing opinion then we will look like a turkey and we should just keep our thoughts to ourselves...I dunno isn't this a discussion forum?????

I have a pet hatred of people who express things on the web as though it is fact...that is why I always make a point of using statements like \"I think\", \"I would expect\" and \"Personally..\" etc etc. If you read my thread that is what I did...express an opinion for the sake of discussion.

In regards to the 3Hp statement, are you suggesting that in your opinion and 250rwkw MX5 would lose 3HP by downsizing to a 2.5\" exhaust form a 3\" exhaust...???

[Boags]

Sorry to fire you up Mooro. Definately question things read on the internet. Makes you sound more of a turkey if you don't.

Your post did not mention the tech article to challenge it. You ignored it completely.

My understanding is that as opposed to NA, back pressure is not critical with a turbo setup

Could you point me to a source that says that backpressure is critical for the operation of a 4 stroke engine? I'm happy to STFU if you can.

It sounds to me as if you had not read the tech article. If, as you say, you did read it, why did you ignore it completely rather than challenging its credit?

NO ONE (well, not me) is saying a 3" won't produce more power. I'm saying that it isn't worth the expense. In terms of sound, I hate larger than necessary exhausts; they sound shite. I would never put a 3" on an MX5. Just my taste though.

In regards to the 3Hp statement, are you suggesting that in your opinion and 250rwkw MX5 would lose 3HP by downsizing to a 2.5" exhaust form a 3" exhaust...???

250; no. 150; probably. (give or take) Just as a well sorted MX5 with a well designed 2.5" would only gain 3HPish with the addition of a 3" system; a 3" system going to a well designed 2.5" would only lose 3.

When you start talking about 200KW (not HP), obviously you will flow more exhaust and the need for a 3" becomes more apparent.

[Mooro]

Granted my use of the word backpressure was poor but this was not the focus or point of my post, I wasn't trying to dis-credit adamjp's article...the point of my post was to give my opinion on the original posters question regarding turbo exhaust sizing. I'm sure I could easily find several articles sprouting on about the virtues of backpressure when tuning an NA engine...whether they are factual or full or crap is another issue all together

I was just trying to make the generalised point that unlike NA cars where bigger is not always better when it comes to exhaust sizing, ther eis scope for bigger diameter exhausts with FI cars.

The focus of my post was basically to acknowledge that for low boost setups there is probably no point to a 3\" exhaust but for higher boost setups with larger turbos (ie 200rwkws +) I believe there is benefit to be had from a 3\" exhaust. The original poster does not mention their power goals.

How is this magical size of 2.5\" derived as being the optimum size. I would suggest it is not as simple as that and depends on many factors including, engine capacity, turbo size, boost pressure, exhaust design, power goals etc etc.

Can you point me to a source that says you will oly be 3Hp difference between a 2.5\" and 3\" exhaust on a turbo setup. I'm happy to STFU if you can

In all seriousness I'm not here to have an argument and challenge every technical piece of info I come across, just expressing my opinion

[Boags]

I was just trying to make the generalised point that unlike NA cars where bigger is not always better when it comes to exhaust sizing, ther eis scope for bigger diameter exhausts with FI cars.

for low boost setups there is probably no point to a 3" exhaust but for higher boost setups with larger turbos (ie 200rwkws +) I believe there is benefit to be had from a 3" exhaust.

How is this magical size of 2.5" derived as being the optimum size. I would suggest it is not as simple as that and depends on many factors including, engine capacity, turbo size, boost pressure, exhaust design, power goals etc etc.

All good points. 2.5" is "optimum" probably because it is the readily available size of exhaust that is best for exhaust flow without sounding shite. You are right that there are many factors, so at the end of the day for most people it's just the closest to the best, rather than exactly the best.

Boags

[adamjp]

Interesting results from enthuzacar....I'd not seen that before.

The key point that I would reiterate about pipe size is simple. Exhaust gas is REALLY hot when it leaves the exhaust port. It is not terribly hot when it leaves the exhaust tip.

I find it really interesting that Exhaust Gas Temperature is a very common topic when talking about turbo-diesel engines, but fairly uncommon in turbo-petrol car engines. A diesel runs port gas temperatures at maximum continuous load below ~750DegC, a petrol engine should be much the same because at continuous temperatures above ~780DegC, the turbocharger will start to melt (hence the ceramic impellers concept). Diesels generally stay at continuous high load for much longer periods than petrol engines by virtue of the vehicle use they are found in. In a no-load situation exhaust port temperature is lucky to be 300DegC.

In a diesel engine the temperature drop across the turbo is around 200degC, slightly more with a petrol engine due to the more frequent cycling of load conditions (the metal can absorb more heat because it is not as hot). Exhaust tip gas temperature is possibly another 150DegC lower due to cooling of the gas as it travels through the system (assuming a 4m exhaust system with normal ventilation). So what started as exhaust gas at 750DegC (max load) is realistically 400DegC by the time it hits the 25DegC air. Normal temperatures idling in your driveway will be lucky to boil water.

What all of this means is that the theoretical 350DegC temperature drop has also reduced the volume of the gas by nearly half. This is a VERY simple application of the Ideal Gas Law, which is not that simple in practice - but the concept holds.

The point is that your volume requirements for the output of one stroke, through one exhaust port are much less once you reach the exhaust tip than they are at the port. For this reason having the same system size from dump pipe to tip is kinda pointless, and more expensive than you need.

But if you want it, fill yer' boots; I'm not going to criticise you for your choice. If money is tight, go the 3in dump, drop back to 2.5inch from the Catalytic converter onwards. Buy good quality, low restriction mufflers/catalytic converters and get mandrel bends if you can afford them.

A mandrel bent, low restriction muffler system in 2.5in will out-flow a 3in system with cheaper, lesser flowing components. And it will fit under the car much easier.

[Mooro]

Hmmm...I like the idea of a 3\" front and dump and then 2.5\" for the rest.

[orx626]

Hmmm...I like the idea of a 3" front and dump and then 2.5" for the rest.

That's what I run on my 626. 13B Turbo @ 9 psi = 260 rwhp @ 6,500 rpm.

3.0" dump pipe and Cat convertor & two 2.5" home made straight through mufflers.

Cheers,
Danny

[Mooro]

^^^^^Impressive! I like that idea more and more.

By the way from what is obvious from the underbody, that car looks like a nice bit of kit

[AB7]

Nice exhaust.

BTW regarding 2.75 exhaust...call AVO. They do offer this size, but if Im not wrong they only comes in mild steel.

[orx626]

^^^^^Impressive! I like that idea more and more.

By the way from what is obvious from the underbody, that car looks like a nice bit of kit

Thanks!

It's a nice bit of kit of you're into retiree runabouts!

From this....



To this....

[Rotary]

That's what I run on my 626. 13B Turbo @ 9 psi = 260 rwhp @ 6,500 rpm.

NICE!!! Job with the Engine Mount

Guessing its welded on the inside??


On The Topic,

2.5inch has many advantages over 3",
3inch has no advantages over 2.5inch for most applications

I Had a 240kw 13B Turbo running great on 2.5inches, and a Rotaries exhaust needs are greater than an equivalent sized Piston Engines

[orx626]

NICE!!! Job with the Engine Mount

Guessing its welded on the inside??

Thanks . It's welded on three sides...the photo doesn't show the bevel butt weld with a small re-enforcing fillet very well.

Cheers,
Danny

[Benny]

The main reason not many Mx-5's run a 3\" system is that unless you like a lot of rattles, 3\" systems don't fit too well, especially going over the rear axle.
I wanted to run a 3\" system on my SP, but was told by experts that it is very hard to fit a proper 3\" system to a road-going MX-5.

Question answered!