Exhaust Size vs HP & TQ
Dec 14, 2009, 08:45 AM
#16
Last edited by fre; Dec 14, 2009 at 08:52 AM.
Dec 14, 2009, 08:48 AM
#17
Just some numbers for pressure drops with air flow at 1300F in 10' of straight tubing.
60 lb/min
3" - 10.8 PSI
3.5" - 4.8 PSI
4" - 2.4 PSI
80 lb/min
3" - 18.9 PSI
3.5" - 8.4 PSI
4" - 4.2 PSI
Every PSI on the exducer gets multiplied across the turbine.
60 lb/min
3" - 10.8 PSI
3.5" - 4.8 PSI
4" - 2.4 PSI
80 lb/min
3" - 18.9 PSI
3.5" - 8.4 PSI
4" - 4.2 PSI
Every PSI on the exducer gets multiplied across the turbine.
Dec 14, 2009, 01:59 PM
#19
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^^^ yes an no if i understand them correctly. Bigger is better but at a cost, 4" is louder and you are more likely to get tickets and point blank can be annoying. But the 4" is less restrictive than the 3" or 3.5" so it can make more power with everything else the same. Im thinking a 4" turbo back with a o2 delete would make some nice hp, thats a real 4" turbo back no o2 housing just 4" tubing with a 2 1/4" wastegate dump. Supplies with vibrant 4" inlet/outlet muffler $370. A 3" would be considerable less, also could use a ebay muffler but wont be as free flowing
Dec 14, 2009, 03:33 PM
#21
Here comes some math!
I know that on the intake piping you want as small as you can go without sending velocities above 300 ft/sec at the TB and 300-450 ft/sec everywhere else.
Likely on the exhaust side for turbocharged cars bigger is better like everyone is saying but there are some practical limits such as noise and clearance we have to worry about. For reference the stock exhaust velocities are likely around 250 ft/sec at stock HP levels so we can use that as a starting point. This is assuming that what goes into the engine must come out again.
So if we throw some math at the question then we can aim for the 200-350 ft/sec range and see what HP levels create those velocities to get a general idea of where it makes sense to upgrade:
2.25"- 250-425 crank HP
2.5" - 300-550 crank HP
3.0" - 450-750 crank HP
3.5" - 600-950 crank HP
4.0" - 800-1400 crank HP
So for a 700 crank HP setup (~600 whp Dynojet) I think 3" would still be fine but you might see some decent gains starting at 600 crank HP mark moving to a 3.5" and likely not much more moving from 3.5" -> 4". Using vented dumptube to divert some exhaust flow would have a similar effect to increasing exhaust diameter.
If someone wants to do some before and after dyno tests I think it would be the only way to confirm my theories with what happens in the real world.
Likely on the exhaust side for turbocharged cars bigger is better like everyone is saying but there are some practical limits such as noise and clearance we have to worry about. For reference the stock exhaust velocities are likely around 250 ft/sec at stock HP levels so we can use that as a starting point. This is assuming that what goes into the engine must come out again.
So if we throw some math at the question then we can aim for the 200-350 ft/sec range and see what HP levels create those velocities to get a general idea of where it makes sense to upgrade:
2.25"- 250-425 crank HP
2.5" - 300-550 crank HP
3.0" - 450-750 crank HP
3.5" - 600-950 crank HP
4.0" - 800-1400 crank HP
So for a 700 crank HP setup (~600 whp Dynojet) I think 3" would still be fine but you might see some decent gains starting at 600 crank HP mark moving to a 3.5" and likely not much more moving from 3.5" -> 4". Using vented dumptube to divert some exhaust flow would have a similar effect to increasing exhaust diameter.
If someone wants to do some before and after dyno tests I think it would be the only way to confirm my theories with what happens in the real world.
Last edited by Hiboost; Dec 14, 2009 at 04:13 PM.
Dec 14, 2009, 03:44 PM
#22
I know that on the intake piping you want as small as you can go without sending velocities above 300 ft/sec at the TB and 300-450 ft/sec everywhere else.
Likely on the exhaust side for turbocharged cars bigger is better like everyone is saying but there are some practical limits such as noise and clearance we have to worry about. For reference the stock exhaust velocities are likely around 250 ft/sec at stock HP levels so we can use that as a starting point. This is assuming that what goes into the engine must come out again.
So if we throw some math at the question then we can aim for the 200-350 ft/sec range and see what HP levels create those velocities to get a general idea of where it makes sense to upgrade:
2.25"- 250-425 crank HP
2.5" - 300-550 crank HP
3.0" - 450-750 crank HP
3.5" - 600-950 crank HP
4.0" - 800-1400 crank HP
So for a 700 crank HP setup (~600 whp Dynojet) I think 3" would still be fine but you might see some decent gains starting at 600 crank HP mark moving to a 3.5" and likely not much more moving from 3.5" -> 4". Using vented dumptube to divert some exhaust flow would have a similar effect to increaseing exhaust diameter.
If someone wants to do some before and after dyno tests I think it would be the only way to confirm my theories with what happens in the real world.
Likely on the exhaust side for turbocharged cars bigger is better like everyone is saying but there are some practical limits such as noise and clearance we have to worry about. For reference the stock exhaust velocities are likely around 250 ft/sec at stock HP levels so we can use that as a starting point. This is assuming that what goes into the engine must come out again.
So if we throw some math at the question then we can aim for the 200-350 ft/sec range and see what HP levels create those velocities to get a general idea of where it makes sense to upgrade:
2.25"- 250-425 crank HP
2.5" - 300-550 crank HP
3.0" - 450-750 crank HP
3.5" - 600-950 crank HP
4.0" - 800-1400 crank HP
So for a 700 crank HP setup (~600 whp Dynojet) I think 3" would still be fine but you might see some decent gains starting at 600 crank HP mark moving to a 3.5" and likely not much more moving from 3.5" -> 4". Using vented dumptube to divert some exhaust flow would have a similar effect to increaseing exhaust diameter.
If someone wants to do some before and after dyno tests I think it would be the only way to confirm my theories with what happens in the real world.
Dec 14, 2009, 04:20 PM
#23
Of course I ignored taking the Cat and Muffler bottlenecks into account for overall velocity but it gives you the general idea of when a certain sized system makes sense. There is still decent power to be made by upgrading earlier than you need to, but the limits of when a certain size will start to really hurt you are what I tend to look at. Right now my goals are to hit the 600-650 crank HP range so down the road I may consider an external dump tube for racing or upgrade the catback to 3.5" for some decent gains with reduction of backpressure.
Dec 14, 2009, 05:28 PM
#25
bigger is not always better. I have seen the proof a couple of times in back to back swaps. One being my own car. I hand fabricated a 2.5 exhaust on the car. drove it with 2.5 for a couple of years. All excited about 3 inch exhaust upgrade day. leaving muffler shop was quite disappointing. the loss of torque was quite stark. low rpm pulls from from 3000 rpm 5th the difference was night and day. I am going to guess. it lost 50 ft lbs torque. I really wanted to turn right back around and reinstall the 2.5. but it did make more HP above 5000rpm on the 3.0.never really being happy with the 3.0 about a year later I fabricated a 2.75 exhaust for the car. hoping to get back some of the lost torque. but it didnt happen. it picked up maybe a 1/4 of what it lost.
There is some scavenging effect with a turbo exhaust that helps with torque. the problem is 1/2 inch jump in tubing size is a massive jump in area of a circle. so finding the right size tubing to get a scavenging effect is quite difficult as it would require 1/16th jumps in tubing size to center in on the perfect exhaust for the setup. no one has ever taken the trouble. in the end a good 3 inch works great for anything from 400-700whp.
There is some scavenging effect with a turbo exhaust that helps with torque. the problem is 1/2 inch jump in tubing size is a massive jump in area of a circle. so finding the right size tubing to get a scavenging effect is quite difficult as it would require 1/16th jumps in tubing size to center in on the perfect exhaust for the setup. no one has ever taken the trouble. in the end a good 3 inch works great for anything from 400-700whp.
Dec 14, 2009, 05:32 PM
#26
bigger is not always better. I have seen the proof a couple of times in back to back swaps. One being my own car. I hand fabricated a 2.5 exhaust on the car. drove it with 2.5 for a couple of years. All excited about 3 inch exhaust upgrade day. leaving muffler shop was quite disappointing. the loss of torque was quite stark. low rpm pulls from from 3000 rpm 5th the difference was night and day. I am going to guess. it lost 50 ft lbs torque. I really wanted to turn right back around and reinstall the 2.5. but it did make more HP above 5000rpm on the 3.0.never really being happy with the 3.0 about a year later I fabricated a 2.75 exhaust for the car. hoping to get back some of the lost torque. but it didnt happen. it picked up maybe a 1/4 of what it lost.
There is some scavenging effect with a turbo exhaust that helps with torque. the problem is 1/2 inch jump in tubing size is a massive jump in area of a circle. so finding the right size tubing to get a scavenging effect is quite difficult as it would require 1/16th jumps in tubing size to center in on the perfect exhaust for the setup. no one has ever taken the trouble. in the end a good 3 inch works great for anything from 400-700whp.
There is some scavenging effect with a turbo exhaust that helps with torque. the problem is 1/2 inch jump in tubing size is a massive jump in area of a circle. so finding the right size tubing to get a scavenging effect is quite difficult as it would require 1/16th jumps in tubing size to center in on the perfect exhaust for the setup. no one has ever taken the trouble. in the end a good 3 inch works great for anything from 400-700whp.
You may have gained much more up top...making your car feel like it lost midrange, when in fact it remained the same. boost for boost you'll make more power everywhere... PERIOD
Dec 15, 2009, 11:19 PM
#29
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It seems that theres a pretty big advantage of 3.5" over 3". Even though the backpressure gets cut in have from 3.5" to 4" its not as much of a drop as 3.5" from 3".
Dec 15, 2009, 11:56 PM
#30
This was on my mirage turbo with an e316g, I have a 3" o2 eliminator/downpipe with an externally dumped wastegate. With a 3" exhaust (a muffler and resonator) I had my boost controller set to 35psi (I run e85 all the time). I and a friend made a 4" strait out exhaust. Without touching the boost controller I went from 35psi to 40psi. This was on the same day. 2weeks later I put the 3" back on and back to 35psi the car went. This was all due to lower backpressure. The car felt MUCH stronger with the 4".
The car was louder than hell that is why I out the 3" back on. But I should be getting a 4" magnaflow for Christmas so with out looking back I will run the 4".
PS, the off boost was WAY louder on the 4" without a muffler but WOT was about the same as the 3 with the muffler. So I am hoping with the muffler it will be the way to go when off boost.
Both of these were side exit exhausts, and I had about the same ground clearance with th 3" and 4".
The car was louder than hell that is why I out the 3" back on. But I should be getting a 4" magnaflow for Christmas so with out looking back I will run the 4".
PS, the off boost was WAY louder on the 4" without a muffler but WOT was about the same as the 3 with the muffler. So I am hoping with the muffler it will be the way to go when off boost.
Both of these were side exit exhausts, and I had about the same ground clearance with th 3" and 4".