What restriction from 2.5" bore at flanges on 3" exhaust?

What restriction from 2.5" bore at flanges on 3" exhaust?

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JLGsDad

Original Poster:

59 posts

168 months

Sunday 31st January 2016
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I'm fitting a 400 bhp+ turbo rotary engine into a Mazda RX8 where, even with the additional 10%-15% exhaust flow of a rotary for a given power level, a 3" exhaust should be fine. The 3" bore Racing Beat RX8 decat and catback I've got are reduced to 2.5" at their flanges to match the standard RX8 bore.

I'm planning on removing the restrictions by getting 3" bore flanges and tubing welded on in place of the 2.5" sections, a shame given the quality of the original work but needs must. However, I'm curious to see how much difference the reduced bores would make - does anyone have any information?

Pumaracing

2,089 posts

206 months

JLGsDad

Original Poster:

59 posts

168 months

Monday 1st February 2016
quotequote all
Pumaracing, thanks for the sizing info.

Your table shows 3" to be on the small side for me, 3.5" probably being better. If I were starting from scratch then I would indeed look first at 3.5" as it should allow earlier spool and more power at the top end. I'm considering going to 3.5" but am held back because of the potential extra cost, noise issues and available space for the downpipe; here:
@ For cost, I already have the [used] decat and catback. Selling them and getting a bespoke system may be expensive (perhaps I should find out, but I'm strapped for time) and I'm concerned about my costs running out of control.
@ I'm concerned about noise for track days, where I have a good idea of the effectiveness of the current decat and catback and a bespoke 3.5" system may be too noisy.
@ A 3.5" downpipe may cause me packaging issues as the downpipe is very close to the inlet manifold and I'm concerned over heat insulation. If I have a 3"downpipe then I fear the extra breathing of a 3.5" system might be limited by the comparative restriction of the downpipe.

Ah well, more things to think over . . . . . .

stevieturbo

17,229 posts

246 months

Monday 1st February 2016
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At only 400hp I wouldnt worry too much. 2.5" will be fine

Of they are easy to change by all means do, but if they arent, dont worry about it.

Pumaracing

2,089 posts

206 months

Tuesday 2nd February 2016
quotequote all
stevieturbo said:
At only 400hp I wouldnt worry too much. 2.5" will be fine

Of they are easy to change by all means do, but if they arent, dont worry about it.
I have to disagree but of course everyone's MMV given this is the intergoogles. Here are a couple more exhaust sizing references.

http://www.exhaustvideos.com/faq/how-to-calculate-...

This one is a tad more conservative than my own guidelines and suggests approx 10% less max power for a given pipe size but it's in the same ballpark.

http://www.mk5cortinaestate.co.uk/calculator5.php

This one doesn't explicitly say whether it's I/D or O/D being referred to but again in the same ballpark as mine.

I would stress to the OP that a given pipe size doesn't just act as a complete bottleneck for bhp and never allow more bhp than my table suggests. What it shows is the pipe sizes that should cause NO restriction and then at power levels above that it becomes progressively more and more of a restriction.

There is plenty more stuff for the OP to browse through on the archive of my old website. Start from the index page here.

https://web.archive.org/web/20110902010921/http://...

stevieturbo

17,229 posts

246 months

Tuesday 2nd February 2016
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Pumaracing said:
I have to disagree but of course everyone's MMV given this is the intergoogles. Here are a couple more exhaust sizing references.

http://www.exhaustvideos.com/faq/how-to-calculate-...

This one is a tad more conservative than my own guidelines and suggests approx 10% less max power for a given pipe size but it's in the same ballpark.

http://www.mk5cortinaestate.co.uk/calculator5.php

This one doesn't explicitly say whether it's I/D or O/D being referred to but again in the same ballpark as mine.

I would stress to the OP that a given pipe size doesn't just act as a complete bottleneck for bhp and never allow more bhp than my table suggests. What it shows is the pipe sizes that should cause NO restriction and then at power levels above that it becomes progressively more and more of a restriction.

There is plenty more stuff for the OP to browse through on the archive of my old website. Start from the index page here.

https://web.archive.org/web/20110902010921/http://...
Everything you're referring to will be n/a I presume ( havent looked )

I've changed from 2.5" to 3" on turbocharged cars making 500hp....couldnt feel a bit of difference.

I've made 750hp on a full 3" system with 2 large boxes to keep the car very quiet.


I'm not saying the 2.5 will offer no restriction at all...but the affect it will have on power vs 3" at only 400hp...would be negligible assuming both are of good quality straight through designs.

Pumaracing

2,089 posts

206 months

Tuesday 2nd February 2016
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The point about turbo versus N/A is valid but as always somewhat complex. For a given power output the mass of exhaust gas that needs to be expelled without restriction is not that dissimilar for both although the low CR turbo will be less efficient and require somewhat more air/fuel mass to generate a given bhp.

However certainly I'd say a small cam duration, low CR turbo engine will be far less sensitive to exhaust restriction than a high CR, long cam duration N/A engine that relies on pulse tuning for much of its power. To some extent turbos are self compensating for restrictions as long as the back pressure in the exhaust system isn't actually preventing the turbo from reaching the target boost level.

Also as I said above the exhaust size doesn't just impose an absolute bhp limit regardless of the engine trying to breath through it. If you put a bigger more powerful engine up front of the same size exhaust system you'll still get more power than the smaller engine would push out but the restriction will be having a more detrimental effect on what that bigger engine could potentially produce.

Here's a back to back test of a 3" vs 3.5" exhaust system and downpipe on a Toyota Supra. The power levels and exhaust sizes are bigger than the OP's but you can pro rate and get some idea of the gains to be had.

http://www.superstreetonline.com/how-to/engine/mod...

100 or so bhp extra at 19 psi boost on a baseline bhp of 527 wheel bhp on the 3" system. That would be similar to a baseline mid 300s wheel bhp on a 2.5" system so not that unlike the OP's engine.

Weighing everything up I wouldn't be surprised if the OP's 2.5" flanges are costing 30 or so bhp, maybe even more but only doing the job will tell.

stevieturbo

17,229 posts

246 months

Tuesday 2nd February 2016
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Gains in that article seem to average around 50-60hp. A good amount for a relatively small change

But they did also state there were no gains until things were pushing over 500hp.

Interesting all the same. I wonder was silencer box design comparable ? as some Jap exhaust can do strange things inside.

Pumaracing

2,089 posts

206 months

Tuesday 2nd February 2016
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Dunno about Jap silencer boxes but if you read that article carefully you'll see just the bigger downpipe made some pretty huge gains on its own. This I think would be comparable to the OP's flange size issue.

stevieturbo

17,229 posts

246 months

Tuesday 2nd February 2016
quotequote all
Pumaracing said:
Dunno about Jap silencer boxes but if you read that article carefully you'll see just the bigger downpipe made some pretty huge gains on its own. This I think would be comparable to the OP's flange size issue.
Turbos will like a big downpipe, gives the hot gases more room to expand etc.

I guess not too dissimilar to DV's discussion on termination boxes ?

Pumaracing

2,089 posts

206 months

Tuesday 2nd February 2016
quotequote all
stevieturbo said:
But they did also state there were no gains until things were pushing over 500hp.
The power graphs in the article showed gains starting from quite a bit lower down than that. Closer to 400 bhp from memory.