ScionFred

iTrader: (1)

Just some hopefully useful info about our stock exhaust system and optimal replacement pipe sizing for hp and tq gains.

OEM

S-pipe: 2.25" tapering to 2.00"
Scuba Tank: 2.00" in, 1.8" out
Mid-pipe: 1.8" in/out
Axle-back: 1.8" in/2" out

Considerations when upgrading for increased HP & TQ

According to Flowmaster, 2" ID single exhaust is good for 100bhp. 2.25" is good for 160bhp and 2.5" is good for 250bhp.

According to another credible source, 2" is good for 144bhp, 2.25" is good for 185bhp and 2.5" is good for 232bhp.

Every expert source I've found says that when changing oem exhaust for increased performance the exhaust pipe diameter should be increased between .25 and .5" for N/A street applications.

A good rule of thumb is to step up one pipe size when using crush bends instead of mandrel bends.

Using simple math we can deduce the following about the 2AZ-FE:

6250rpm x 147CI = 918750 /2 = 459375 * 0.000578703704 = 266 cfm

So now we know that at 100% volumetric efficiency our engines consume 266cfm of intake air. We also know that hot exhaust gasses occupy more space due to expansion. A known good ratio to convert intake air volume to exhaust gas volume is 1.467. So then:

266cfm * 1.467 = 390cfm
Now that we know our maximum exhaust gas volume we can make an educated decision regarding optimal pipe diameter for our exhaust system. Since round exhaust pipe flows ~115cfm per sq inch of area we can easily calculate the following:

1.80" = 240cfm
2.00" = 318cfm
2.25" = 408cfm
2.50" = 509cfm

Naturally there are other factors to consider such as pipe bends (mandrel or crush), mufflers, etc. but it should be fairly obvious that the oem exhaust sizing was designed to sacrfice maximum hp/tq in exchange for quietness and low rpm tq. Increasing the exhaust diameter will increase both tq and hp but at the expense of moving the tq curve a little higher in the rpm range. IMHO, 2.25" exhaust diameter would be as close to optimal for a N/A IHE 2AZ-FE as we can get without a lot of expermentation and dyno testing.

Turbocharging is a little different because exhaust gas velocity and scavenging are controlled by the manifold, turbine housing and wheel. The optimal exhaust size after that would be a short section of straight 3" pipe if you could stand the noise. 2.5" is a little quieter than 3", all else being equal, and is good for up to 300bhp.

Feel free to post any comments, corrections or additional info.

FromTheOld

Lots of theory.....but I'd have to agree with it all. 2.5 inch exhausts are good for our N/A cars in particular not because of the gains, but the ability to maintain the same exhaust if we decide to go turbo. Although for turbo, the best exhaust is no exhaust

TrevorS

iTrader: (1)

This I pretty much agree with, if you choose an oversize exhaust, it makes far more sense to think in terms of eventual turbo than with staying N/A. However, if you don't plan on spending the bucks to convert your xB2 to turbo ($4K is it?), then the pipe expansion itself should be approached very carefully!

ScionFred

iTrader: (1)

Yes, it is... it's a lot easier to simply use the formula of 2.2 cfm exhaust gas flow per bhp although IMO it tends to err on the side of larger pipe size than actually necessary.

rhythmkid31

isn't there a certain diameter that a N/A motor shouldn't exceed to maintain back pressure?

ScionFred

iTrader: (1)

Sort of, but it depends on numerous variables such as displacement, HP, rpm, desired powerband, etc. If you're looking for max hp and tq you should choose the size that supports your max engine airflow but no larger. In our case 2.25" should work very well. Smaller will improve low rpm scavenging and tq while limiting max hp and tq. Bigger will cost a little low rpm tq and is generally louder.

ScionFred

iTrader: (1)

I'm not sure what size turbo you have but I'm running a .63 A/R turbine with a stage III wheel and a .60 A/R T04E comp with a 50 trim wheel. My downpipe is a custom made 2.5" with recirculated WG dump.

My exhaust system is a hodge-podge because I attempted to use the CXR 3" system. It was way too loud so I had a big 7" round resonator installed where the scuba was. It was still too loud and the CXR mid-pipe hung down a bit low so I had my exhaust tapered down to 2.5" after the resonator and a custom 2.5" mid-pipe made to mate to the TRD axle-back. If I could do it over I'd go with 2.5" from the DP back and a 6" resonator. The 7" fits but just barely.

FWIW, I discovered that the biggest flaw in the CXR exhaust is the muffler. Replacing it with the TRD made a huge reduction in noise level.

Unfortunately the TRD A/B has a 2" flange on it that I had cut off and replaced with a 2.5" flange. IIRC he had to cut about 1" off the TRD to get to 2.5" pipe but it still fits perfectly since he added an inch to the mid-pipe. Here's a sound clip of it:

ScionFred

iTrader: (1)

No worries, we're pretty much still on-topic. I gotta ask something first, how the ____ does your 3" S-pipe bolt up to the little 2" Scuba inlet? There must be massive turbulence at that junction!

If you like the TRD you should keep it. AFAIK no one makes scuba section or mid-pipe replacements that bolt up to the oem pipes anyway so you have to have them made. The CXR system bolts up to the stock header but after that none of the pipe junctions match the oem. Even the Tsudo and Strup cat-backs won't bolt up to different axle-backs.

IMO your best bet would be to find a good shop with fair prices and have them make a custom 2.5" scuba section and mid-pipe that bolt up to your S-pipe and 2.5" re-flanged TRD A/B. You'll want a long 6" round muffler/resonator where the scuba was or it will be loud. 409 SS is good enough, cheaper and easier to work with than 304 SS. The shop I use would probably charge about $400 total for all this, all mandrel bends, 409 ss, muffler included.

Ironically I think my CXR sound clip sounds better than my TRD (MBS) clip but in person it's the opposite. Of course the CXR did sound pretty good after I packed the silencer tip with fiberglass but it all blew out within a day or two. On a scale of 1-10 with 1 being very quiet and 10 being obnoxiously loud, I'd rate the stock system as a 2, the TRD as a 2.5, my current setup a 4 and the CXR gets a 9.

TrevorS

iTrader: (1)

Depending on what HP you're running, I wouldn't count on the stock 1.8" at the scuba exit making a particular difference. No need for 2.5" unless you're making well over 200HP, and believe it or not, the volume requirement of exhaust at peak temperature continually reduces as exhaust temperature drops -- which is what happens in real world exhausts (in our case, particularly after the "scuba").

Regarding a 2.5" scuba back -- Tsudo has been making a 2.5" dual for these cars for several years!

ScionFred

iTrader: (1)

Hey Trevor, long time... in case you didn't already know, Clean XB has a 3" s-pipe mated to the tiny 2" scuba inlet and over 3' of 1.8" pipe before his 2" in/2.5" out TRD axle-back. AFAIK EGT would have to drop pretty _____ quickly to flow freely through that abrupt 3" to 2" reduction and 3+' of 1.8" ID pipe. I'm pretty sure that 1.8" is a bit on the small side for a 250-300bhp turbo setup but open to new ideas. Are you suggesting that his current setup can't or isn't worth improving upon? Thanks for your usual and expected contribution.

BTW, wouldn't adding the equivalent of a 5" single exhaust after the 2"-1.8" scuba actually be counter-productive in terms of maintaining optimal exhaust gas velocity?

FromTheOld

Backpressure is never needed.....why does this myth/rumor still exist??

ScionFred

iTrader: (1)

Good question. I suppose it's because the exhaust restriction necessary to maintain high flow velocity and scavenging usually adds some unwanted back pressure. Your question does bring up a another good point which is that it's better to err on the large side than small side when choosing exhaust pipe diameter for max hp and tq, although someone obviously disagrees.

TrevorS

iTrader: (1)

Unfortunately, nowhere near long enough ! A 2" "scuba" inlet is not tiny for N/A, but if he is running turbo and has a 3" pipe to his "scuba", then something's out of kilter . If he's anywhere near 300HP, it's appropriate to look more closely. I'd say the 3" is over optimistic whereas the post "scuba" ID is worthy of review. If 250 HP is a reasonable expectation for his setup, I'd say a 1.8" exhaust at the "scuba" exit is probably around the limit. However, if 300HP or more is to be expected, then a 1.8" "scuba" exit could be a problem.