n/a exhaust diameter, a discussion about performance
10-01-2016, 01:07 AM
#1
Race Car
Thread Starter
n/a exhaust diameter, a discussion about performance
Typically the trend has been to larger diameter exhaust when looking for more hp.
The factory n/a is about 2" outside diameter for the 2.0 litre n/a engine, 2 1/16" for the 2.5 n/a and 2 1/8" for the 16 valve 2.5. The trend for Porsche is to enlarge the tube with increased cfm.
I'm interested in hearing what people have found and what peoples thoughts are regarding factory diameter compared to smaller, compared to larger.
pros & cons.
And is it a bad idea to go smaller if you plan on increasing the breathing of the head?
My personal experience is larger breathes better at higher rpms. But is there much useful benefit to going smaller.
The factory n/a is about 2" outside diameter for the 2.0 litre n/a engine, 2 1/16" for the 2.5 n/a and 2 1/8" for the 16 valve 2.5. The trend for Porsche is to enlarge the tube with increased cfm.
I'm interested in hearing what people have found and what peoples thoughts are regarding factory diameter compared to smaller, compared to larger.
pros & cons.
And is it a bad idea to go smaller if you plan on increasing the breathing of the head?
My personal experience is larger breathes better at higher rpms. But is there much useful benefit to going smaller.
10-01-2016, 05:24 AM
#2
Rennlist Member
It sounds like you know what you need to know. Exhaust tubing size is, like so many aspects of design, a compromise. Be realistic with how the engine will be used and the answer will become self-evident.
10-01-2016, 06:48 AM
#3
Rennlist Member
The name of the game is tuning for scavenging without causing back pressure across the board. Easiest solution is to choose diameter based on the power you are making, compared to a factory 944 making the same power probably... They already did the engineering.
10-01-2016, 01:14 PM
#4
Rainman
Rennlist Member
Rennlist Member
I don't think the 944 needs a bigger exhaust pipe size.
The same pipe was used for the 944 2.5, 2.7, 944S 2.5, 944S2 3.0, and I believe even the front section of the 968 exhaust.
And if you read through 924srrl27 or whatever his name is's build thread, he has a 2" OD exhaust (1.875" ID) and he's pushing over 200hp through it.
When my car was STOCK with early DME/AFM my car was making more torque than a 944 was rated from the factory...the only non-stock item on the car at that test was my aftermarket cat which also has 2" OD/1.875" ID inlet/outlet. My guess is that the cat is at the right location for a smaller pipe to increase exhaust velocity through the cat and help with scavenging in the mid-range.
From my thread...
My engine was rated by Porsche as 143hp/137tq at the flywheel.
The numbers kept increasing the more runs we did - I think getting some heat in the engine is useful! The lowest number was when the engine was somewhat cold.
The same pipe was used for the 944 2.5, 2.7, 944S 2.5, 944S2 3.0, and I believe even the front section of the 968 exhaust.
And if you read through 924srrl27 or whatever his name is's build thread, he has a 2" OD exhaust (1.875" ID) and he's pushing over 200hp through it.
When my car was STOCK with early DME/AFM my car was making more torque than a 944 was rated from the factory...the only non-stock item on the car at that test was my aftermarket cat which also has 2" OD/1.875" ID inlet/outlet. My guess is that the cat is at the right location for a smaller pipe to increase exhaust velocity through the cat and help with scavenging in the mid-range.
From my thread...
Originally Posted by V2Rocket
If going by the rule of thumb "15% drivetrain loss" and divide the rwhp/rwtq numbers by 0.85, we get 143.05HP and 150.74TQ at the flywheel.
The numbers kept increasing the more runs we did - I think getting some heat in the engine is useful! The lowest number was when the engine was somewhat cold.
10-02-2016, 06:06 PM
#5
Yeah ! Whatever his name here!
Project 924Srr 2.7 Lightweight has a 1.88" ID Diameter bespoke custom exhaust system.
All thinwall 1.2mm Stainless Steel , and only 1 rear box which is a straight through design, perforated aluminium core, steel case and packed with Wool
No Cat, and the Manifold is from a 951, the Y piece standard 2.5 8 valve 924S / 944
This Exhaust was made in Finland by Martelius, and I thought it was a 2" ID bore when I ordered it, but when it arrived it measured 2" OD and 1.88" ID !
Whatever it's doing, it's created with the rest of the bitzer motor
- short stroke 3.0 8v / 2.5 small valve head (Ported Inlets)
a very powerful and low rpm torquey motor...
205bhp / 205 ft lbs
http://924srr27l.co.uk/wp-content/up...924srr27L-.pdf
There is approx. 30% more extra cfm going in (Extrude honed Inlet + Cylinder head) and the smaller bore Exhaust
expels the hotter gases quicker ? the Exhaust Manifold is also Plasma Coated www.zircotec.com
R
Project 924Srr 2.7 Lightweight has a 1.88" ID Diameter bespoke custom exhaust system.
All thinwall 1.2mm Stainless Steel , and only 1 rear box which is a straight through design, perforated aluminium core, steel case and packed with Wool
No Cat, and the Manifold is from a 951, the Y piece standard 2.5 8 valve 924S / 944
This Exhaust was made in Finland by Martelius, and I thought it was a 2" ID bore when I ordered it, but when it arrived it measured 2" OD and 1.88" ID !
Whatever it's doing, it's created with the rest of the bitzer motor
- short stroke 3.0 8v / 2.5 small valve head (Ported Inlets)
a very powerful and low rpm torquey motor...
205bhp / 205 ft lbs
http://924srr27l.co.uk/wp-content/up...924srr27L-.pdf
There is approx. 30% more extra cfm going in (Extrude honed Inlet + Cylinder head) and the smaller bore Exhaust
expels the hotter gases quicker ? the Exhaust Manifold is also Plasma Coated www.zircotec.com
R
Last edited by 924srr27l; 10-02-2016 at 06:32 PM. Reason: spec
10-03-2016, 01:59 PM
#6
Drifting
Noah,
My 2.5l race car with some of your "old " engine parts makes the best horsepower with a 2.5 inch custom straight stainless exaust system coupled to a Stahl header with a 2.5 inch collector, that was proven on the dyno as compared to the previous 2.5 and 3 inch hybrid exaust i had on last season !
This is for a 2.5 N/A , a 2.7 or turbo engine will differ.
You saw the dyno sheet , of course it has to remain our secret ... lol ... so do the engine parts !
If you have any doubts, you can text me !
Even when i attach a Dynomax muffler for test days, it still works very well
My 2.5l race car with some of your "old " engine parts makes the best horsepower with a 2.5 inch custom straight stainless exaust system coupled to a Stahl header with a 2.5 inch collector, that was proven on the dyno as compared to the previous 2.5 and 3 inch hybrid exaust i had on last season !
This is for a 2.5 N/A , a 2.7 or turbo engine will differ.
You saw the dyno sheet , of course it has to remain our secret ... lol ... so do the engine parts !
If you have any doubts, you can text me !
Even when i attach a Dynomax muffler for test days, it still works very well
10-03-2016, 02:51 PM
#7
That reminds me, on the rolling road day we tried a Decibel Insert because the downside of a (1) silencer system is it's very Loud & Boomy for street use especially at 3000rpm..
However, adding this lost 18bhp !
Trending Topics
10-03-2016, 03:09 PM
#8
Drifting
Forgot to mention ... i had to McGyver a friggin ugly turndown pipe in order to pass sound at Mont tremblant last time i idid a DE with the car, even with the Dynomax muffler.Cost me over $50.00 in assorted exaust parts and i had to give two bozos at a shop $20.00 for them to let me use their welder while they watched me as they were eating their lunches !
Where did you get those inserts ?
I could use two of them for the droning in my 3.6 911, the Fabspeed headers are nice but the drone is horrible between 2-3 K rpm !
Where did you get those inserts ?
I could use two of them for the droning in my 3.6 911, the Fabspeed headers are nice but the drone is horrible between 2-3 K rpm !
10-03-2016, 03:33 PM
#9
Forgot to mention ... i had to McGyver a friggin ugly turndown pipe in order to pass sound at Mont tremblant last time i idid a DE with the car, even with the Dynomax muffler.Cost me over $50.00 in assorted exaust parts and i had to give two bozos at a shop $20.00 for them to let me use their welder while they watched me as they were eating their lunches !
Where did you get those inserts ?
I could use two of them for the droning in my 3.6 911, the Fabspeed headers are nice but the drone is horrible between 2-3 K rpm !
Where did you get those inserts ?
I could use two of them for the droning in my 3.6 911, the Fabspeed headers are nice but the drone is horrible between 2-3 K rpm !
They should be available from the Bay!
http://www.ebay.com/itm/48MM-Outlet-...-/141474728090
I've given up and wear Ear defenders instead...
R
10-03-2016, 08:09 PM
#10
Race Car
Thread Starter
Noah,
My 2.5l race car with some of your "old " engine parts makes the best horsepower with a 2.5 inch custom straight stainless exaust system coupled to a Stahl header with a 2.5 inch collector, that was proven on the dyno as compared to the previous 2.5 and 3 inch hybrid exaust i had on last season !
This is for a 2.5 N/A , a 2.7 or turbo engine will differ.
You saw the dyno sheet , of course it has to remain our secret ... lol ... so do the engine parts !
If you have any doubts, you can text me !
Even when i attach a Dynomax muffler for test days, it still works very well
My 2.5l race car with some of your "old " engine parts makes the best horsepower with a 2.5 inch custom straight stainless exaust system coupled to a Stahl header with a 2.5 inch collector, that was proven on the dyno as compared to the previous 2.5 and 3 inch hybrid exaust i had on last season !
This is for a 2.5 N/A , a 2.7 or turbo engine will differ.
You saw the dyno sheet , of course it has to remain our secret ... lol ... so do the engine parts !
If you have any doubts, you can text me !
Even when i attach a Dynomax muffler for test days, it still works very well
---------
of course it's interesting to hear (or especially see dynos) of everone's results.
10-08-2016, 03:10 PM
#11
Race Car
Thread Starter
Not quite true brother. I have at my disposal both & can provide photos if you require proof. the 2.0liter is smaller than the 8v and the 16v is bigger (1/16" increment increases). I am dazzled by 27L's dyno numbers. He essentially installed the same as pipe size as a 924 2.0L stock n/a comes with, mated to a higher flow fart can and a MUCH higher volume engine. That's unusual.
The benefit of a smaller system is lighter and *sometimes* improved acoustics. When I have asked Porsche race engine engineer JM about performance results of some of his cams, he has stated "... WITH RACE EXHAUST". And you know that doesn't mean stock. Probably doesn't mean smaller than stock either.
In the past with my v8 Dodge Ram I have gone from catless and large diameter freeflow to stock system with cat. I noticed it pulls harder from the green light but once up to hiway speed it seems to hit a "wall"... it was a move into the wrong direction for performance.
It would be very cool to take a stock car and feel how the car drives with smaller exhaust and compare to stock & then compare to 2.25 and 2.5". How does it feel for you in the type of driving that you do?
The benefit of a smaller system is lighter and *sometimes* improved acoustics. When I have asked Porsche race engine engineer JM about performance results of some of his cams, he has stated "... WITH RACE EXHAUST". And you know that doesn't mean stock. Probably doesn't mean smaller than stock either.
In the past with my v8 Dodge Ram I have gone from catless and large diameter freeflow to stock system with cat. I noticed it pulls harder from the green light but once up to hiway speed it seems to hit a "wall"... it was a move into the wrong direction for performance.
It would be very cool to take a stock car and feel how the car drives with smaller exhaust and compare to stock & then compare to 2.25 and 2.5". How does it feel for you in the type of driving that you do?
10-08-2016, 03:19 PM
#12
Rainman
Rennlist Member
Rennlist Member
Best comparison I can give is...
Stoplight "Drag" racing another 944 NA.
My car was 100% stock (DME/AFM) with the following two exceptions:
- aftermarket cat that I still have now with the 2" OD in/out
- 951 transmission (looong gears).
The other 944 was 100% stock.
We did probably 10 runs in open territory, from a dig/red light every time...and every single time my car would pull on the other one.
I attribute it mostly to the longer gearing (turbo box is still in 2nd gear when an NA box is halfway through 3rd) but maybe the exhaust-given torque helped some too.
I will admit that over the years I've never driven another NA that feels/sounds like mine does, but I can only attribute it to my car being fresher in the suspension department and non-stock cat vs all the other old 944s.
.
.
.
Noah, one thing to keep in mind is how OEM's do it.
For them (US ones anyways) COST is everything. Naturally a smaller pipe is cheaper than a bigger pipe, less material used and a smaller pipe is easier to bend into shape, etc.
We live in a "golden age" of horsepower right now, that the 1960's Muscle Car era could have only fantasized about. You can get a Ford Fusion with a 325hp V6 turbo for like $30k right now...for example.
If you go down to the Ford dealer and look at their hi-po stuff like that Fusion, the F-150 ecoboost, the Mustang 2.3T, etc...you'll probably be surprised at how small their exhausts are. An F-150 3.5TT makes what, 350hp/400tq? And at the largest point (tail pipe) has a 2.75" single pipe.
A 240hp, 2.0T Explorer has a pair of tail pipes that are each under 2.0" OD...
They are making this power despite all their emissions compliance, noise compliance, grandma-friendly torque curve, through what we might think are drinking straws for pipes.
One last example...this is the stock exhaust for a 2011+ Ford Mustang GT...5.0L, 412 rated HP (but its really like 430)...
Look at that bend...
Stoplight "Drag" racing another 944 NA.
My car was 100% stock (DME/AFM) with the following two exceptions:
- aftermarket cat that I still have now with the 2" OD in/out
- 951 transmission (looong gears).
The other 944 was 100% stock.
We did probably 10 runs in open territory, from a dig/red light every time...and every single time my car would pull on the other one.
I attribute it mostly to the longer gearing (turbo box is still in 2nd gear when an NA box is halfway through 3rd) but maybe the exhaust-given torque helped some too.
I will admit that over the years I've never driven another NA that feels/sounds like mine does, but I can only attribute it to my car being fresher in the suspension department and non-stock cat vs all the other old 944s.
.
.
.
Noah, one thing to keep in mind is how OEM's do it.
For them (US ones anyways) COST is everything. Naturally a smaller pipe is cheaper than a bigger pipe, less material used and a smaller pipe is easier to bend into shape, etc.
We live in a "golden age" of horsepower right now, that the 1960's Muscle Car era could have only fantasized about. You can get a Ford Fusion with a 325hp V6 turbo for like $30k right now...for example.
If you go down to the Ford dealer and look at their hi-po stuff like that Fusion, the F-150 ecoboost, the Mustang 2.3T, etc...you'll probably be surprised at how small their exhausts are. An F-150 3.5TT makes what, 350hp/400tq? And at the largest point (tail pipe) has a 2.75" single pipe.
A 240hp, 2.0T Explorer has a pair of tail pipes that are each under 2.0" OD...
They are making this power despite all their emissions compliance, noise compliance, grandma-friendly torque curve, through what we might think are drinking straws for pipes.
One last example...this is the stock exhaust for a 2011+ Ford Mustang GT...5.0L, 412 rated HP (but its really like 430)...
Look at that bend...
10-09-2016, 06:00 PM
#13
I am dazzled by 27L's dyno numbers. He essentially installed the same as pipe size as a 924 2.0L stock n/a comes with, mated to a higher flow fart can and a MUCH higher volume engine. That's unusual.
The benefit of a smaller system is lighter and *sometimes* improved acoustics.
The benefit of a smaller system is lighter and *sometimes* improved acoustics.
I think it has a lot to do with all the mods which when combined with
8 hours of mapping have optimised everything from 1000 - 6000rpm
The air flow in and out has all been Increased.
The AFM was checked and given the green light for being in spec (the Info it gives to the ECU)
The throttle body was polished, the Inlet insides also smoothed (Extrude Hone) the claim from memory was approx 20%? more CFM
The Inlet ports were flowed with a 28% CFM Increase, the Valve springs New.
The Valves are the 2.5 smaller versions (not like the 2.7) maybe the velocity is faster through these smaller holes than larger?
The exhaust ports were untouched, but the 951 Tubuar header was plasma coated and this is alledged to scavenge and exit the gases better?
No cat, just straight pipes from the head all the way out to the rear, the (1) straight through silencer is fairly small and is hardly any or no difference to having a straight pipe, so there are no restrictions for the gases at all.
The bore must be critical though because when we tried a decibel insert which fixes inside the rear tail pipe it lost 18bhp!
Also +26bhp was gained from the start to the 16th Rolling road run 8 hours later and with a lot of number crunching and alterations.
The first time 200bhp was achieved the torque was about 190ft lbs maximum, but more changes increased this to 200Ft lbs, then 205bhp was achieved and at the same time an identical torque figure...
That was the point where every last drop of power & Torque was achieved and we called it quits.
Has the knife edged lighter crank / flywheel and lighter forged pistons
done anything to assist the speed air can enter and exit? the engine spins and whips up so much faster than any stock one I've had before (6) even quicker than the 968 I had.
The air pump has been optimised to the max and it would be interesting to change the exhaust to a bigger bore 2 1/4" and see what effect it would have with another rolling road session, but I can't be arsed !
It's good enough as it is, any more power and I will start to get wheelspin and would need bigger and heavier wheels and tyres, and I like the skinny sizes on now as I believe they help with the acceleration and handling, the Wavetrac LSD does do a great job also and this car would be a handful without it.
R
