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It's a 914 ...
 
stownsen914's Avatar
 
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Exhaust sizing for a track duty turbo

I did some searches and didn't find much information on the forum about exhaust sizing for a turbo. The application I am planning is a track duty 2.5L 911 twin turbo. I'm aiming for 500 hp ultimately.

Due to the configuration of my car (it's a 914), I'll be building custom headers and exhaust. I plan to make equal length headers and keep the primaries as short as possible, based on reading I've done to this point.

My questions:
1) I'm planning to use 38 mm exhaust ports. Does that mean I should keep the primaries as close to that size as possible (but not smaller)? I believe that would mean 1.625" OD tubing. Or would I be better to go 1.75" to make sure it's not restricted?
2) Post-turbo, many seem to say that essentially bigger is better. The turbine outlet for the turbos I plan to use is 2" (I'm going twin). My intuition would have told me that 2.25" would be plenty, but should I be considering larger than that. I plan to run open exhaust at most tracks. The pipes post turbo will need to be about 4 feet long to get from the turbo to the rear of the car.
2a. Would I see a significant improvement using short dump tubes from the turbos, out each side of the car, instead of the longer pipes to the back? Dump tubes could be about 12".

Thanks!

Scott

Old 02-04-2017, 08:37 PM
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Generally speaking you want the ID of the exhaust primaries to be the same as the ID of the exhaust ports. This is to preserve velocity and reduce lag. Equal length primaries using merge collectors is ideal.
After the turbo you want a cone into a larger ID tailpipe. Four foot long tailpipes will help quiet the exhaust somewhat. If you cone up from 2" to 3" diameter per turbo straight pipe there should not be any choke point.
Would love to see pictures of this setup!
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'78 911SC Widebody, 930 engine, 915 Tranny, K27, SC Cams, RL8 Headers & GT3 Muffler. 350whp @ 0.75bar
Brian B. (256)536-9977 Service@MKExhaust Brian@RarlyL8
Old 02-04-2017, 08:49 PM
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Quote:
Originally Posted by stownsen914 View Post
I did some searches and didn't find much information on the forum about exhaust sizing for a turbo. The application I am planning is a track duty 2.5L 911 twin turbo. I'm aiming for 500 hp ultimately.

Due to the configuration of my car (it's a 914), I'll be building custom headers and exhaust. I plan to make equal length headers and keep the primaries as short as possible, based on reading I've done to this point.

My questions:
1) I'm planning to use 38 mm exhaust ports. Does that mean I should keep the primaries as close to that size as possible (but not smaller)? I believe that would mean 1.625" OD tubing. Or would I be better to go 1.75" to make sure it's not restricted?
2) Post-turbo, many seem to say that essentially bigger is better. The turbine outlet for the turbos I plan to use is 2" (I'm going twin). My intuition would have told me that 2.25" would be plenty, but should I be considering larger than that. I plan to run open exhaust at most tracks. The pipes post turbo will need to be about 4 feet long to get from the turbo to the rear of the car.
2a. Would I see a significant improvement using short dump tubes from the turbos, out each side of the car, instead of the longer pipes to the back? Dump tubes could be about 12".

Thanks!

Scott
1) yes, but go bigger if using lots of bends. You will properly have short length anyway.
2) yes bigger is better to recude backpressure, but 2 x 2,25 is fine for 500hp
2a) No

Post turbo: You need as less back pressure as possible. If you do twin turbo with 2' outlet from the turbine, then 2,25 would be fine, but if very long distance and bends, you could consider cone up to 2.5' and route out from there. 2 x 2,5' makes space for +1200hp, so overkill already. You will not loose or gain anything by having short or long tubes after the turbo if only back pressure is kept low at same level.

Some comments on primaries:
If configured correctly you can build unequal length and come out with same or a better and more efficient turbo header than equal length due to fewer bends and shorter length. Equal length is primarily due to two factors a) tune for optimal venturi effect between inlet runners and end of exhaust primaries which is used primarily for NA. A lot has to be taken into account to extract any "venturi" gains on a turbo engine and for most people it does not make much sense - it basically means you have to build it like an NA on a different atmospheric pressure (i.e. boost vs. back pressure ratios carefully measured, linked cam profiles, huge turbine, simulation in calculation programs such as pipemax etc.). Instead most people just turn up the boost. More important and relevant in regards to turbo engines is the other reason b) to keep exhaust pulses not interfering with each other in collectors and block up turbine housing too soon. A key note is turbine efficiency is still the same whether the pulses come entering separated in an equal spread out pattern (as from the engine) or separated but not in equal spread out pattern - only are the pulses to be separated and not to come in together or too close. Meaning shorter primaries can be unequal to a certain extent without having the pulses coming in too close. Length has to be set in relation to cylinder, stroke and ignition cycle and it is possible thereby to gain more velocity and less back pressure (more power) compared to a "snake-roll" bent equal length setup.

In order to keep the pulses separated you can calculate from cylinder/ignition setup which tubes could be of which length in order for the pulses not to interfere with each other. Most people make them equal length just to be sure, and hence extend tubes and add bends as a penalty to get to equal length. It is easy to make them equal length if you have the space for it, but if building and you find optimal routes to lay down your pipes you can take your cylinder/ignition setup into calculation and make beneficial shortcuts. When you have twin turbos on a 6 cylinder engine it is easier to make those beneficial layouts as the pulses are coming from only 3 cylinders with the cycles/strokes more spread apart as compared to 6 cylinders mating into a single turbo.

Just as important is your choice of turbos in regards to back pressure created at the turbine wheel and indirectly from compressor inducer size. What turbos are you running?

light weight 914 with centered 2.5L twin turbo engine can only be a beast to ride!! pictures please...
Old 02-05-2017, 09:48 AM
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It's a 914 ...
 
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Thank you Brian and Jakob for the input.

To answer the question about what turbos, I'm planning to use twin Holset HE221W's. Possibly not so well known in Porsche circles, but apparently performs well for its size and loves high boost, unlike some other smaller turbos's like T3. Here's a compressor map.




Here are a couple shots of the 914 (ignore the engine lid sitting on top of the roll cage). Currently normally aspirated. The second picture shows a diffuser that I built for the car, which makes packaging of engine/transmission/exhaust a bit challenging!




Last edited by stownsen914; 02-06-2017 at 04:12 AM..
Old 02-05-2017, 05:20 PM
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Daaaayum, there ain't much 914 left there!
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'78 911SC Widebody, 930 engine, 915 Tranny, K27, SC Cams, RL8 Headers & GT3 Muffler. 350whp @ 0.75bar
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Old 02-05-2017, 08:07 PM
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Yes, Holset's are generally more efficient in higher boost settings. It took 1.4 bar before mine woke. At 1 bar, it was weak.
Old 02-05-2017, 08:14 PM
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Dayom Jakob, that was a boat load of information. Very interesting.

OP: Super cool "914."




Quote:
Originally Posted by JakobM View Post
1) yes, but go bigger if using lots of bends. You will properly have short length anyway.
2) yes bigger is better to recude backpressure, but 2 x 2,25 is fine for 500hp
2a) No

Post turbo: You need as less back pressure as possible. If you do twin turbo with 2' outlet from the turbine, then 2,25 would be fine, but if very long distance and bends, you could consider cone up to 2.5' and route out from there. 2 x 2,5' makes space for +1200hp, so overkill already. You will not loose or gain anything by having short or long tubes after the turbo if only back pressure is kept low at same level.

Some comments on primaries:
If configured correctly you can build unequal length and come out with same or a better and more efficient turbo header than equal length due to fewer bends and shorter length. Equal length is primarily due to two factors a) tune for optimal venturi effect between inlet runners and end of exhaust primaries which is used primarily for NA. A lot has to be taken into account to extract any "venturi" gains on a turbo engine and for most people it does not make much sense - it basically means you have to build it like an NA on a different atmospheric pressure (i.e. boost vs. back pressure ratios carefully measured, linked cam profiles, huge turbine, simulation in calculation programs such as pipemax etc.). Instead most people just turn up the boost. More important and relevant in regards to turbo engines is the other reason b) to keep exhaust pulses not interfering with each other in collectors and block up turbine housing too soon. A key note is turbine efficiency is still the same whether the pulses come entering separated in an equal spread out pattern (as from the engine) or separated but not in equal spread out pattern - only are the pulses to be separated and not to come in together or too close. Meaning shorter primaries can be unequal to a certain extent without having the pulses coming in too close. Length has to be set in relation to cylinder, stroke and ignition cycle and it is possible thereby to gain more velocity and less back pressure (more power) compared to a "snake-roll" bent equal length setup.

In order to keep the pulses separated you can calculate from cylinder/ignition setup which tubes could be of which length in order for the pulses not to interfere with each other. Most people make them equal length just to be sure, and hence extend tubes and add bends as a penalty to get to equal length. It is easy to make them equal length if you have the space for it, but if building and you find optimal routes to lay down your pipes you can take your cylinder/ignition setup into calculation and make beneficial shortcuts. When you have twin turbos on a 6 cylinder engine it is easier to make those beneficial layouts as the pulses are coming from only 3 cylinders with the cycles/strokes more spread apart as compared to 6 cylinders mating into a single turbo.

Just as important is your choice of turbos in regards to back pressure created at the turbine wheel and indirectly from compressor inducer size. What turbos are you running?

light weight 914 with centered 2.5L twin turbo engine can only be a beast to ride!! pictures please...
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Old 02-06-2017, 12:01 PM
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Look up Maltese falcon on the 914 world forum, he can answer and assist you in your needs, MSDS systems. He has a 660 hp flat fan twin turbo 914. I have a nice street 3.1 turbo 914 , turbo 930 engined 914 s equal big fun... Richard Clewet also has a nice track turbo 914.
Old 02-10-2017, 07:23 PM
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It's a 914 ...
 
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Thanks for the additional comments, guys!

In perusing the internet for opinions on turbo exhaust sizing (yeah, I know that's dangerous), it seems that many aspire to the "bigger is better" theory when it comes to turbo exhaust, especially the non-Porsche crowd. Is it hogwash? Is there any advantage to having the exhaust larger than the turbine outlet diameter?

Scott
Old 02-15-2017, 08:41 AM
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that thing sure is ugly but I bet its fast and fun

still amazes me of the knowledge here

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Old 02-15-2017, 11:55 AM
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