Author Topic: Increase in Exhaust Diameter will bhp increase ?  (Read 8494 times)

Offline Nik da Greek

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Increase in Exhaust Diameter will bhp increase ?
« Reply #15 on: January 13, 2009, 10:45:13 PM »
I\'ve never understood why car design is so far behind bikes in exhaust tech...the manufacturers seem happy just to bung on a length of tube with a cat in the middle and hope it works. Yamaha patented an "EXUP" (EXhaust Ultimate Powervalve) back in the late eighties which was basically a barrel valve in the exhaust that opened and closed the aperture of the pipe at different revs. This meant they could tune the exhaust harmonics for a far wider range of rpm than just having a straight-through pipe. Just about every spotsbike has something similar now, and has for years, but there\'s no sign of the most rudimentary version on anything short of a supercar. Just lazy penny-pinching from car manufacturers, I suppose

Offline shaunwil

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Increase in Exhaust Diameter will bhp increase ?
« Reply #16 on: January 14, 2009, 08:16:48 AM »
Was that 2 stoke or 4 stroke ? The 2 stock needs a very good designed exhaust to make any power at all !

I\'ve got a copy of :

http://www.bookfinder.com/dir/i/Scientific_Design_of_Exhaust_and_Intake_Systems/0837603099/

Shows stuff from the 20\'s in how they desgined stuff. Not very relevent to a rotor though !

Shaun
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Offline Nik da Greek

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Increase in Exhaust Diameter will bhp increase ?
« Reply #17 on: January 14, 2009, 08:51:53 AM »
Wow, that book looks a bit intense for my puny brain, Shaun! The Exup-style valves are a four-stroke thing, but you\'re right, wave scavenging effect is even more vital in two-strokes, which is why you used to see such gorgeous bits of engineering as hand-welded titanium expansion chambers on GP two-strokes.

Funnily enough, most of the understanding of two-stroke \'spannies came from an engineer working for MZ of all people (sort of the Lada of the bike world for many years) when they were at Zschopau (sp) behind the Iron Curtain in East Germany. Walter Kaaden, his name was IIRC, and he\'d developed the idea of pulse scavenging on the MZ gp racers, which from then on were unbeatable. In a proper spy thriller stylee, he defected to the West, specifically Suzuki I think, with a whole load of diagrams and tech drawings, at a neutral GP whilst his wife and kids were smuggled out of East Germany to meet up with him. MZ couldn\'t compete with the spending and developement power of the Japanese, and the two-stroke era of GPs was a Jap playground from then on in.

You\'re right, nothing much to do with rotaries, but it makes a good story. I always kinda thought of rotaries as a lot like a two stroke, but it goes roundy-roundy rather than uppy-downy, anyway :Giggle Helps me understand stuff in my feeble brain!
« Last Edit: January 14, 2009, 08:52:36 AM by Nik da Greek »

Offline shaunwil

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Increase in Exhaust Diameter will bhp increase ?
« Reply #18 on: January 14, 2009, 12:59:21 PM »
The Rotary Development book on the same webpage as the manuals explains it in serious depth. That really does mess with your head.

I bought the book for the intake information for the FB but wasn\'t a lot of help.

Shaun
1994 White FD with A-spec GT3574R - 358bhp at the hubs at 0.9bar :3gears-lh
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Offline johnm

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Increase in Exhaust Diameter will bhp increase ?
« Reply #19 on: January 14, 2009, 03:33:57 PM »
Found the below link on google and it should answer most of your questions on exhasut theory both N/A and turbo.  Basically the turbine (the hot bit in the exhaust flow) within the turbo requires a pressure drop across it to work - high pressure upstream (from engine exhaust) and low pressure downstream (in the exhaust pipe), the less back pressure downstream the better.  That why bigger down pipes and de-cats make more responsive (less lag), as the turbo will accelerate quicker and make boost sooner.
 
As with all things there can be a down side, the less back pressure downstream, the quicker the turbine will spool up and you can then end up with over boost - either open loop system (no feed as per original Mazda system) does see the extra boost, so does not control or with an after market closed loop system you need to add more gain to slow the boost rate increase, or the next thing is the original waste gate runs out of capacity.  So before going for a huge exhaust, no cat and a free flow silencer, ensure you have a fast acting/robust boost control system with enough capacity to control overboost.
 
http://www.tercelreference.com/tercel_info/turbo_exhaust_theory/turbo_exhaust_theory.html

Offline clive

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Increase in Exhaust Diameter will bhp increase ?
« Reply #20 on: January 14, 2009, 05:00:44 PM »
Quote from: Nik da Greek;574614
Funnily enough, most of the understanding of two-stroke \'spannies came from an engineer working for MZ of all people (sort of the Lada of the bike world for many years) when they were at Zschopau (sp) behind the Iron Curtain in East Germany. Walter Kaaden, his name was IIRC, and he\'d developed the idea of pulse scavenging on the MZ gp racers, which from then on were unbeatable. In a proper spy thriller stylee, he defected to the West, specifically Suzuki I think, with a whole load of diagrams and tech drawings, at a neutral GP whilst his wife and kids were smuggled out of East Germany to meet up with him. MZ couldn\'t compete with the spending and developement power of the Japanese, and the two-stroke era of GPs was a Jap playground from then on in.



Always thought this was a fascinating story, and your right this rider and the drawings of the exhaust system he smuggled away tucked in his his leathers was the single biggest step forward that one of the Japanese bike manufactures ever made (I thought it was Yam)  

Story I read said that this guy could have been champ that year but instead started a GP and litteraly drove around the back of the circuit never to be seen again!

Back to Rotary,

I have been told that Knight Sports always trained for free guys for Mazda\'s race division however they were always taught on N/A rotarys because you need to be good to get the porting, carburation  and exhaust working together properly to obtain power and that Turbo\'s were viewed as "easy power" where as you have to work at it to get power from N/A.
All the best Clive

Offline C J R

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Increase in Exhaust Diameter will bhp increase ?
« Reply #21 on: January 14, 2009, 05:06:07 PM »
Wow, heaps of valuable information all originating from my simple question.
Thanks Guys.
 
Based on the above link "Turbo Exhaust Theory" it seems I\'m contemplating the right course of action to increase the size of exhaust from 75mm to 90mm, as the car is already producing between 450 to 500bhp through the 75mm pipe.
 
The turbo starts to operate at about 3,500 rpm, so I may see an improvement to say 3000rpm with a 90mm exhaust.
 
What\'s also interesting is to avoid bringing the wastegate pipe in further away from the turbo, the above document suggests 12-18" to avoid disrupting the main turbo flow.
 
All I need to do now is find a decent exhaust builder AND I will ensure the car is remapped before use (thanks Clive).
 
Chris

Offline shaunwil

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Increase in Exhaust Diameter will bhp increase ?
« Reply #22 on: January 14, 2009, 06:26:43 PM »
Interesting stuff. Biggest question would be how loud you want to go. Would need some interesting silencer arrangements !

Shaun
1994 White FD with A-spec GT3574R - 358bhp at the hubs at 0.9bar :3gears-lh
2003 Mondeo 2lt 130ps Diesel Estate Daily :driving

Offline C J R

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Increase in Exhaust Diameter will bhp increase ?
« Reply #23 on: January 14, 2009, 08:06:34 PM »
Hi Shaun,
I hope to be around 95 - 100db if possible. Not sure if a mid and a rear silencer would achieve that db ?
 
Chris

Offline shaunwil

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Increase in Exhaust Diameter will bhp increase ?
« Reply #24 on: January 14, 2009, 09:14:07 PM »
Good Question. Probably best off talking to somebody that races the FD to spec a good setup.

Shaun
1994 White FD with A-spec GT3574R - 358bhp at the hubs at 0.9bar :3gears-lh
2003 Mondeo 2lt 130ps Diesel Estate Daily :driving

Offline trotter

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Increase in Exhaust Diameter will bhp increase ?
« Reply #25 on: January 14, 2009, 10:25:53 PM »
Would it be worthwhile making this a stickie somewhere?

Some fairly useful info that peeps might make use of?
Work hard, play hard house :D

Offline suba

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Increase in Exhaust Diameter will bhp increase ?
« Reply #26 on: February 16, 2009, 01:51:20 PM »
I\'m new to Rotaries having just bought an FD3 so I can\'t really comment on how the engine responds best to exhaust design and power resulting from it.  Does the engine need a bit of turbulence at low rpm to get it going off boost?  That\'s got to be a consideration for the chaps on here running a big single...

If you are going to track day the car then you should aim for 95 db on a static noise test.  There is nothing worse than being pulled in and told to limit it to 5,000 rpm.  Most tracks have a 98 db noise limit.

I very much doubt you would be able to make 95 db using a four inch pipe and definately not with a screamer pipe....:mmmm
Single Turbo time!

Offline BlitzBoy

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Increase in Exhaust Diameter will bhp increase ?
« Reply #27 on: February 16, 2009, 01:59:08 PM »
Quote from: suba;584902
Does the engine need a bit of turbulence at low rpm to get it going off boost?  That\'s got to be a consideration for the chaps on here running a big single...



No ,you dont need turbulant air coming out of the exhaust ports to drive the turbine wheel of a turbo, you need fast Hot gasses is what spools up a turbo, so its down to the porting and tuning to help spool
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Offline C J R

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Increase in Exhaust Diameter will bhp increase ?
« Reply #28 on: February 16, 2009, 05:37:26 PM »
I should be able to advise the results of this modification shortly.
The car is in having the Trust 4" exhaust fitted this week and the screemer pipe plumbed in.
The design we are aiming for is to have the screemer pipe lengthened so it enters the main exhaust tubing just before the mid silencer.
 
It was interesting to note that the underside of the car showed scorch marks where the wastegate had opened and it appears flames had exited the screemer pipe, (the exhaust throws out flames, so i guess it was obvious the screemer would be doing the same, I just hadn\'t contemplated it).
 
I need to arrange a remap after the modification and a noise test, but I\'ll post up the results.

Offline shaunwil

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Increase in Exhaust Diameter will bhp increase ?
« Reply #29 on: February 16, 2009, 06:37:19 PM »
Quote from: suba;584902
If you are going to track day the car then you should aim for 95 db on a static noise test.  There is nothing worse than being pulled in and told to limit it to 5,000 rpm.  Most tracks have a 98 db noise limit


lol, With a free flowing 3in system on my single I was 112db at 5500 !! Its all down to the quality of the silencers. I\'m looking for something quieter for track use !

Shaun
1994 White FD with A-spec GT3574R - 358bhp at the hubs at 0.9bar :3gears-lh
2003 Mondeo 2lt 130ps Diesel Estate Daily :driving