3 inch AMS tbe, rerouted wastegate dump into downpipe gt35r @ up to 550whp/420wtq @ 33-35psi. I also know two people in town on 3inch exhaust systems (one of them being my old B&B) making 100 more whp than me. If you really need more than that, then prepare for a huge maintanence budget

 

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.

 

So you guys are saying buy the 4inch?

 

^^^ 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

 

Not really,

4" kills ground clearance and is loud as hell.

It's been well proven a good free flowing 3" can make well over 600HP. A 3.5" might make a few more HP, but it comes at the cost of higher weight and more noise.

 

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.

 

Thank god , some sanity to this question. I am thinking we read the same materials

 

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.

 

Something to consider as well is the fuel that is being used. Combustion of ethanol produces more exhaust gas than combustion of gasoline. So, the HP cutoffs for the next bigger ideal exhaust size may be a bit different.

 

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.

 

Unless you have a dyno chart to prove this... that's total and absolute BS. That's not how a turbine operates and you know it.

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

 

Is there any wayto convert that backpressure to hp&tq lost or gained from each step up in size?

 

Not really.

Just gives you an idea of where the breaks are and the relative difference.


Going from 3" to 3.5" for example halves the back pressure. Going from 3.5" to 4" cuts it in half again.

 

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".

 

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".