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Mike9311
Joined: 14 Dec 2004
Posts: 1798
Location: Chicago-ish
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 Posted: Thu Apr 15, 2010 4:49 am Post subject: |
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| ideola wrote: | | Mike9311 wrote: | | ok, now we opened up a can of worms and sorta hijacked the thread. |
Marti and I are good for that  so you're welcome to join in the fun  |
Cool, I love a good technical discussion! Just wait until I ask about any long rod possibilities with a 931 motor I want to start building but I need to start a new thread!
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1980 931 since 1989
1981 Ideola 931 Club Sport
1982 931 Entwicklungsfahrzeug
1979 924 NA ohne 650 mit 471
1982 931 Red Resurrection - 951 IC
1982 931 parts car / resurrection?
1980 924 NA (R&D lightweight)
1982 931 wana-be GTR race car |
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Martijnus
Joined: 29 Dec 2006
Posts: 2019
Location: Netherlands
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| Posted: Thu Apr 15, 2010 5:01 am Post subject: |
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| ideola wrote: | | Mike9311 wrote: | | ok, now we opened up a can of worms and sorta hijacked the thread. |
Marti and I are good for that so you're welcome to join in the fun |
I'm a bit afraid to start deep discussions lately  but I'll keep doing it.
Nice theory mike, it indeed corresponds with my ideas. it's funny to see how people tend to think that bigger is better...especially on exhausts
_________________
"Rule: Turbo's make torque, and torque makes fun." (C. Bell)
924 "50-jahre", 1981.
MSII/extra, LPG, ITB's, 5lug.
To be turbo'ed in a while.
Killed her at the Nurburgring, Porscheless at the moment  |
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ideola
Joined: 01 Oct 2004
Posts: 15550
Location: Spring Lake MI
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Mike9311
Joined: 14 Dec 2004
Posts: 1798
Location: Chicago-ish
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| Posted: Thu Apr 15, 2010 6:32 am Post subject: |
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That is a good thread...You must have been saving that one for quite a while ecs944 knows his stuff. I wish I was there during that conversation
Anyway, back to the muffler. Try another smaller muffler and see what happens.
_________________
1980 931 since 1989
1981 Ideola 931 Club Sport
1982 931 Entwicklungsfahrzeug
1979 924 NA ohne 650 mit 471
1982 931 Red Resurrection - 951 IC
1982 931 parts car / resurrection?
1980 924 NA (R&D lightweight)
1982 931 wana-be GTR race car |
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ideola
Joined: 01 Oct 2004
Posts: 15550
Location: Spring Lake MI
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| Posted: Thu Apr 15, 2010 7:36 am Post subject: |
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| Mike9311 wrote: | | ecs944 knows his stuff |
Pffffffftttttttttttttt! 
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erstwhile owner of just about every 924 variant ever made |
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Mike9311
Joined: 14 Dec 2004
Posts: 1798
Location: Chicago-ish
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| Posted: Thu Apr 15, 2010 8:07 am Post subject: |
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| ideola wrote: | | Mike9311 wrote: | | ecs944 knows his stuff |
Pffffffftttttttttttttt! |
ok, I am a sudo noobie that just mispoke??? I don't even know who he is! 
I was questioning his longer rod length causing more stress on the piston wall??? Its the opposite. Maybe I misread. And no one brought up the whys of the n ratio. There was just the what would happen with differing n ratio's. Now we can tie it back to the whole intake/exhaust resonance during overlap and how a longer rod would benefit the 5in muffler at high RPM's...just kidding
Seriously, the whole reason I even mentioned it was because I want to start putting together a good strong short block (place an order with you for rods/pistons) and as always I don't want to leave well enough alone.
_________________
1980 931 since 1989
1981 Ideola 931 Club Sport
1982 931 Entwicklungsfahrzeug
1979 924 NA ohne 650 mit 471
1982 931 Red Resurrection - 951 IC
1982 931 parts car / resurrection?
1980 924 NA (R&D lightweight)
1982 931 wana-be GTR race car |
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ideola
Joined: 01 Oct 2004
Posts: 15550
Location: Spring Lake MI
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| Posted: Thu Apr 15, 2010 9:26 am Post subject: |
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Mike, no worries. That guy was a blow hard and a scam artist who was very good at copying and pasting from other sources. So anything you read by him should be taken with a heavy dose of salt.
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erstwhile owner of just about every 924 variant ever made |
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Mike9311
Joined: 14 Dec 2004
Posts: 1798
Location: Chicago-ish
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| Posted: Thu Apr 15, 2010 9:31 am Post subject: |
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| ideola wrote: | | Mike, no worries. That guy was a blow hard and a scam artist who was very good at copying and pasting from other sources. So anything you read by him should be taken with a heavy dose of salt. |
cool, thanks
_________________
1980 931 since 1989
1981 Ideola 931 Club Sport
1982 931 Entwicklungsfahrzeug
1979 924 NA ohne 650 mit 471
1982 931 Red Resurrection - 951 IC
1982 931 parts car / resurrection?
1980 924 NA (R&D lightweight)
1982 931 wana-be GTR race car |
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Oz Owen
Joined: 25 Feb 2010
Posts: 43
Location: Canberra, Australia
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| Posted: Thu Apr 15, 2010 3:41 pm Post subject: |
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Thanks for the info guys,
Yeah, its basically a Mack truck synthesizer or "fart box" - as you guys describe.
Its the muffler that is 5 inches wide. The whole pipe that is 2.25 or 2.5 inches joins straight to the 5 inch rear muffler.
So its basically a few kilos of pointless metal hanging onto a straight pipe exhaust?
I'll still post back with my findings though as I want to restrict the flow out the end gradually to see what effect it has a lower revs - thats before I hack it off and find it was actually something at the front of the car causing the issue.
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1977 924
1989 Pugot 405 Mi16 |
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daniel
Joined: 18 Jun 2009
Posts: 686
Location: Sydney, Australia
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| Posted: Thu Apr 15, 2010 3:57 pm Post subject: |
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I have 2 complete NA exhaust systems that I will part with cheap. Located in Sydney
Daniel
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Over the top of skyline, total brake failure.... hit the wall at over 200 kp/h at the dipper, so anyone who has to brake for the esses is a pussy.
1977.5 Race Car, CAMS Group S Spec
1989 944 Cabriolet |
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Martijnus
Joined: 29 Dec 2006
Posts: 2019
Location: Netherlands
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| Posted: Thu Apr 15, 2010 7:21 pm Post subject: |
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| Oz Owen wrote: | Thanks for the info guys,
Yeah, its basically a Mack truck synthesizer or "fart box" - as you guys describe.
Its the muffler that is 5 inches wide. The whole pipe that is 2.25 or 2.5 inches joins straight to the 5 inch rear muffler.
So its basically a few kilos of pointless metal hanging onto a straight pipe exhaust?
I'll still post back with my findings though as I want to restrict the flow out the end gradually to see what effect it has a lower revs - thats before I hack it off and find it was actually something at the front of the car causing the issue. |
like I mentioned... restricting the airflow just restricts the airflow. You'll probably end up with the same poor performance at low revs, and a worse performance in high revs, compared to how it's currently.
You'll need to improve the velocity of the gas, not create backpressure.
You won't speed up the gas by putting in a restrictor, that only causes the gas to speed up at the restrictor, like a venturi in a carburator. Thats fun when you want to do something at that point (like inject fuel) but you don't.
The only solution is to install different pipes.
Still, try to restrict the flow and post your findings. It's your chance to challenge the theory
I'm interested.
_________________
"Rule: Turbo's make torque, and torque makes fun." (C. Bell)
924 "50-jahre", 1981.
MSII/extra, LPG, ITB's, 5lug.
To be turbo'ed in a while.
Killed her at the Nurburgring, Porscheless at the moment |
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Mike9311
Joined: 14 Dec 2004
Posts: 1798
Location: Chicago-ish
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| Posted: Thu Apr 15, 2010 11:56 pm Post subject: |
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| Martijnus wrote: | | Oz Owen wrote: | Thanks for the info guys,
Yeah, its basically a Mack truck synthesizer or "fart box" - as you guys describe.
Its the muffler that is 5 inches wide. The whole pipe that is 2.25 or 2.5 inches joins straight to the 5 inch rear muffler.
So its basically a few kilos of pointless metal hanging onto a straight pipe exhaust?
I'll still post back with my findings though as I want to restrict the flow out the end gradually to see what effect it has a lower revs - thats before I hack it off and find it was actually something at the front of the car causing the issue. |
like I mentioned... restricting the airflow just restricts the airflow. You'll probably end up with the same poor performance at low revs, and a worse performance in high revs, compared to how it's currently.
You'll need to improve the velocity of the gas, not create backpressure.
You won't speed up the gas by putting in a restrictor, that only causes the gas to speed up at the restrictor, like a venturi in a carburator. Thats fun when you want to do something at that point (like inject fuel) but you don't.
The only solution is to install different pipes.
Still, try to restrict the flow and post your findings. It's your chance to challenge the theory
I'm interested. |
I do agree with you and this isn't exactly putting in a restrictor. If the rest of the car is stock then it is likely the original exhaust will work better overall. As far as velocity is concerned, two smaller pipes parallel would be better but at a weight penalty. I actually only want a muffler change because I think it might be rough running that he thinks is there due to how it sounds. His pipe doesn't sound too big for the rest of the system but it could still be an issue on a stock motor at lower revs because of what you have been pointing out: velocity.
Interesting story (well for me anyway), I built an exhaust for a 535i using Borla mufflers. It was a single pipe that I made made larger. I didn't like the resonance I had at highway cruise (here in the States that is slow). I contacted Borla and they sold me a venturi that I then welded in at the entrance of my muffler. This sped up the slow exhaust gasses as they entered my muffler and the resonance all but disappeared.
Another story, I bought an expensive well known aftermarket exhaust for my BMW330Ci and absolutely hated it. They designed to sound good by having two separate pipes all the way back without ever connecting them. This caused the gasses to stall and the car sounded tough because the exhaust would crackle like an old CART (indy) style racer. The car also became a dog at the low end and the high end seemed only mildly improved. Instead of complaining and trying to return it I bought an X-pipe from Burns Stainless (too busy to fab my own at the time. same with the exhaust for that matter). An old hot rodders trick is to spray paint on the exhaust and go beat on your car for a bit, come back and see where the paint peeled off (the hot spot) and install the x-pipe there. I didn't have that luxury due to the spacing of resonators and alike so the next best thing is as close to the header down pipes as possible. The x pipe allows the two pipes to share exhaust pulses, as one is farther down the exhaust it helps to pull the next one along and so on. The car all but quit 'crackling' (sounds like small back fires almost and I will admit cool but still hated it) and this was the awakening for that system. Velocity was back up and the the pulses were now working in my favor. This actually helped my high end too, not just my low end. I was super close to putting my oem system back on and selling the system on ebay before that modification. I will see if I can dig up pictures.
This also gets back to your point about resonance. The exhaust and intake on a NA car do work together during valve overlap. The problem lies with getting things as optimal as possible. Turbos are different, short intakes at each port and a big exhaust work perfectly for them.
My next modification for the x-pipe is a heat shield to trap heat there to further bring up the velocity through the x-pipe. Of course one could over scavenge but I doubt my system will ever do that on that car. Ok, there the can'o'worms as been opened. Funny thing too, my goal was never power or sound it was a lighter exhaust (they claimed more of a loss than it actually was ) and better mileage.
_________________
1980 931 since 1989
1981 Ideola 931 Club Sport
1982 931 Entwicklungsfahrzeug
1979 924 NA ohne 650 mit 471
1982 931 Red Resurrection - 951 IC
1982 931 parts car / resurrection?
1980 924 NA (R&D lightweight)
1982 931 wana-be GTR race car |
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Martijnus
Joined: 29 Dec 2006
Posts: 2019
Location: Netherlands
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| Posted: Fri Apr 16, 2010 8:59 am Post subject: |
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wow apparently I didn't read the part where it says that it's just the muffler which is large.  my bad.
_________________
"Rule: Turbo's make torque, and torque makes fun." (C. Bell)
924 "50-jahre", 1981.
MSII/extra, LPG, ITB's, 5lug.
To be turbo'ed in a while.
Killed her at the Nurburgring, Porscheless at the moment |
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Mike9311
Joined: 14 Dec 2004
Posts: 1798
Location: Chicago-ish
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| Posted: Fri Apr 16, 2010 9:06 am Post subject: |
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| Martijnus wrote: | | wow apparently I didn't read the part where it says that it's just the muffler which is large. my bad. |
That's totally cool
I re-read it a few times myself. That is a serious muffler if it even muffles anything?
_________________
1980 931 since 1989
1981 Ideola 931 Club Sport
1982 931 Entwicklungsfahrzeug
1979 924 NA ohne 650 mit 471
1982 931 Red Resurrection - 951 IC
1982 931 parts car / resurrection?
1980 924 NA (R&D lightweight)
1982 931 wana-be GTR race car |
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| Back to top |
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joejax
Joined: 02 Dec 2009
Posts: 919
Location: Jacksonville,FLA,USA
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| Posted: Fri Apr 16, 2010 11:41 am Post subject: |
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Hey, this is really an interesting topic. I've always thought a free flow exhaust equaled more power.
All the guys running straight pipes seem to have the power to back up the noise, now... that is questionable.
It's also been said, you need to have "back pressure" for it to run right, but I would also assume an engine can be tuned to accept
the lack of "back pressure",but we all know what happens when you assume.
Anyhow, thanks for the great thread.
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I got nothin'
Uhhh...you got any pics? |
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