j9fd3s

iTrader: (3)

i recall with my old T2 that going from the 2.5" downpipe to the 3" downpipe was a night and day difference, and that was on a stock based turbo (V trim compressor and housing, stock hotside).

we seem to keep the 3" DP with like a T4, but at what point is 3.5 better? and how do we figure it out?

seems like a good question for the peanut gallery

if a scenario is needed, i did buy a manifold for my FC, so i will go 60-1 at some point, and since i have to make a downpipe anyways, cost would be the same. on this car, power isn't important, it'll make more than i need, but response is important.

RGHTBrainDesign

It's all useless unless you follow it up with a 3.5" or 4" exhaust.

I did a bunch of fluid mechanics CFDs for downpipe and wastegate design for my engineering class. One critical factor here is that sudden expansion creates reversion. You need to expand relatively slowly. Boundary layers...ahh.

Would this be something like a ported S4/S5 Turbine Housing?

I think we're looking at a certain power figure here. I would bet around 550-600whp Reciprocating Engine Equivalences would be the stepping point. So a 60-1 would be 3.5", no doubt. I would say 750-800whp Reciprocating Engine Equivalences to be the stepping point for a 4" downpipe and full exhaust.

peejay

Exhaust can get smaller towards the back of the car since the gases are cooling and therefore contracting. It's mainly the first 2-4 feet of pipe after the turbo that are super critical, after that you can go down in size and cut down on noise without hurting anything.

RGHTBrainDesign

Maybe at the entry power levels for each sizing, but definitely not at the piping maximum flow numbers.

If I'm putting down 600whp out of a rotary, that's using a 900-1000whp turbo. That sucker needs 4" all the way.

I really like this page's estimates on sizing:


Diameter > Pipe Area (in^2) > Total CFM > Max Hp (Crank) Per Pipe
3" 6.49 747 339
3 1/4" 7.67 882 401
3 1/2" 8.95 1029 468

So for 3.5" Diameter Pipe, it's maximum is 468 Reciprocating Engine CRANK Hp, which is like 350ish Rotary.

Either way, if you can fit it, there's no downsides to running a larger exhaust on a turbocharged application except for noise. The name of the game is EMAP to MAP Ratio, and you want that exhaust pressure to be as low as possible so you get the most efficient use of the system as a whole.

BLUE TII

iTrader: (9)

Exhaust is very turbulent after the turbine and at its hottest (post turbine temp), but with a constant velocity (no pulsation).

Therefore it is most beneficial to have a larger downpipe diameter first and foremost.

This is what SMB found when developing the exhaust for the Bathurst 12hr endurance race factory RX-7 SP cars. 3.5" downpipe to 3" exhaust on stock turbos.

We were the exhaust manufacturer's who supplied Mazda Australia with the exhaust used on the legendary RX-7 SP both for road cars and race cars (certificate and letter from Mazda), now the time is up and we can supply the general public with what we have learnt. The stainless steel system below is what has been learnt over time and is very similar to the race system used on the 12 hour Porsche beating cars, the dump pipe in this system has been increased in diameter to 3 1/2" to extract the most from the turbo's, for the rear muffler you have a choice of titanium tip or 4' bent tip (as per the race cars) ( We have seen these systems glow red hot from turbo face to exhaust tip for 12 hours (what a sight to see). Very few, if any other, exhaust systems could withstand that).

SMB Full Exhaust Systems-Mazda RX-7

BLUE TII

iTrader: (9)

From my personal experience I have found the same.

I had full 3.5" turbo back exhaust on my TII with a S5 based stock hybrid turbo (60-1 compressor wheel, T04B HiFi housing and P trim exhaust wheel).

I put the 3" Racing Beat Rev TII system on because I was going to move to the SF bay area.

Totally killed turbo response. Full boost was at 4,000rpm now instead of 3,400rpm and much less boost/torque from idle to 2,000rpm.

So, I adapted my 3.5" downpipe/midpipe to the RB 3" Rev TII "cat back" section.

Full boost back to 3,400rpm and the boost/torque back from idle to 2,000rpm.

Dynoed same power as full 3.5" as with 3.5" DP/MP to 3" RB exhaustl, though RB did weigh 30lbs more.

Problem I had to work out running the full 3.5" exhaust was boost creep.
But I could have run the 3.5" DP/MP to 3" RB "cat back" section and avoided that whole "fix the boost creep" adventure as well...

Eventually, I went back to the full 3.5" exhaust again when I got another daily driver and the TII became the race car (because single 3.5" was 30lbs lighter).

peejay

Hugh MacInnes detailed the turbulent-outflow problem in his book, as well. He recommended 18" or so of large diameter tubing before necking down.

IMO, if you're finding that 3" isn't enough at the end of the exhaust, 3.5" isn't a big enough downpipe either at that point.

WANKfactor

iTrader: (1)

Where did you find those figures? People have been making 400 rotary rwhp on 3" exhausts since forever, myself included. Thats a LOT more than 339 at the crank.

BLUE TII

iTrader: (9)

Diameter > Pipe Area (in^2) > Total CFM > Max Hp (Crank) Per Pipe
3" 6.49 747 339
3 1/4" 7.67 882 401
3 1/2" 8.95 1029 468


I think those figures are for cross plane NA V8s.

They have a problem of **** firing order/collection which means the pulsating flow (two cylinders firing at once and a gap) need bigger pipe diameter to make the same power.

So, you get sizing charts like the one on this page that recommends single 3.5" to make just 350hp

What Size Exhaust Pipes Work Best? - Hot Rod Network

On a turbo car with constant exhaust velocity after the turbine you can make much more peak power on a smaller exhaust.

----------
On a turbo piston engine car with a big turbo like a 500-700hp 2.0L Evo for instance, you can go too big on exhaust diameter after the turbo because the car spends a lot of its rpm range as an NA engine.

If you go too big on exhaust in this case it affects the spool up since the engine makes less power in NA mode, so less exhaust is available to spool the turbo.

We don't have this problem on the rotary. Plenty of exhaust energy and rpms. Bigger exhaust will help the low-end and mid-range power.

RGHTBrainDesign

You are both right, I'm an idiot. Either way, I'm still going to say the EFR9174 needs a 4" for my specific application. I'm really trying to chase that sought after 1:1 EMAP:MAP ratio since this motor will get a LOT of hard miles. :P

Shainiac

iTrader: (7)

With my EFR 9180 and street port 13BT, I had a 4" downpipe, 3" Vibrant resonator, and oldschool 3" Racing Beat catback (circa 1987!). I welded a 60mm wastegate into the downpipe before the resonator and plumbed it on a solenoid to have a boost-activated exhaust cutout. With the cutout open, it picked up 60whp! I would say that there is definitely something to be gained from going above 3" out the back. Granted, this was at over 500whp, I think on a ~350whp car it would be pretty pointless.

Also, SirLaughsALot, If you're looking for a 1:1 exhaust ratio, Why not get the 9180? You'll gain nearly 25% more flow on the exhaust side by going to the larger turbine.

peejay

Which is funny because, exactly because of the uneven exhaust pulses, V8s need larger dual pipes than they would otherwise need if they were a single, so you CAN'T just say 'it needs Xin^2 for each dual pipe, so you need 2Xin^2 for a single".

I've personally made 750hp at the crank on a single 3" system with V8s. Twice. One with a cat. I only instrumented the first one's exhaust but it had only 1psi backpresssure at max power.

A lot of the "you need X pipe to make Y power" charts are theoretical, and crap. I made something like 200hp N/A with a stock 12A midpipe, which is 1 7/8". Changing to a 2.5" system didn't add any power to a meaningful degree. Of course, some people on the Internet would say you need a 3" front-back system to make that power.

RGHTBrainDesign

It's not about what power you made, its about what you're leaving on the table for a boosted application. Naturally aspirated experiences have no point in a Single Turbo discussion. I want empirical data (EMAP vs. MAP ratio is a critical piece) as to why.

I still think the bottom line for any boosted system is run as big as you can fit. Budget shouldn't be a factor because you're leaving SO much on the table (as per Alex's 60whp). If I could, I'd run a slow merging 3" to 5" downpipe to full 5" exhaust...

peejay

The V8 applications were boosted

Actually, come to think of it, I also did a V6 that out diown 565whp on a Dynojet, but to be honest I don't remember what exhaust it had, I'd like to say single 3.5" to 3" cat to dual 2.25".

I also worked with an 810whp V6, but to be really honest I have no recollection at all what the exhaust was on that car. Tuning it on the street rebooted my brain enough times to give me temporary retograde amnesia Boost response was like the gauge was tied to the throttle pedal. Same 3.5" downpipe that everything else in that application uses, too.

Stuff like that is why I laugh at the OMG RACECAR guys who insist they *need* a 18" pipe off the turbo through the fender. No you freakin' don't, I've done cars that went faster, with less boost, through a full exhaust with mufflers that actually quieted the exhaust. And had functional air conditioning, too.

j9fd3s

iTrader: (3)

this is kind of what got me thinking to ask. if FD stock turbos want a 3.5" DP than, how about my 60-1?

j9fd3s

iTrader: (3)

+1 the average person would be surprised at how stock our race car is, and the only thing fabricated is the wall around the air intake. its wrapped in the bag blue apron meals come in, it works too

WANKfactor

iTrader: (1)

While were all here I might get you guys' opinion on something.
Im currently running a 3" exhaust, 3" SMB resonator runs pretty much alongside the g'box, up and over the diff and into a big 3" in , dual 2.5" out muffler i had SMB make me (plug, they really do a beautiful muffler). Ive got about 600mm of 3.5" coming off the turbo (efr8374)
Would it be worthwhile getting a 3.5" resonator and extending the 3.5" all the way to the diff? Will it change the tone, or the note or whatever? It kinda sounds good the way it is.

The way it is, since the turbine outlet is 3", the down pipe is basically a reducer cone, some fat pipe, then reduces down to 3"again - itkinda looks like a 2 stroke expansion chamber - it doesnt look like it would flow more than straight 3" in such a short length

RGHTBrainDesign

I was talking to Abel Ibarra about this exact type of downpipe design and through his experiences he states it's nothing but packaging (but mainly marketing).

Through my fluid mechanics and heat/mass transfer classes, I took the time to calculate what actually should be happening and it agrees with the theory of piping down a turbo like that is detrimental to power potential (increases exhaust pressure which works against the turbine wheel -> which works against the compressor wheel since it's connected).

Could you post a picture of how you set it up? The SE (Sudden Expansion) of the 3" turbine wheel (74mm is like 2.92" or something like that) to a 3.5" is the first issue we should try to combat. We want something like a 10 degree linear taper, +/- 4 degrees (depending on smoothness -> check into cerakoting the downpipe) or of course a gradual bell-shape, so our boundary layer at the edge of the pipe stays in better contact and doesn't result in reversion.

Written in simplified Bernoulli's terms: With Sudden Expansion, the velocity must reduce and so the pressure increases. Turbulent eddies occur which give rise to the local head loss. ---> Reversion leads to pressure increase which leads to power potential loss. Some of this is chicken before the egg/egg before the chicken though.

I think of it this way: You lose power potential both when you suddenly expand AND when you suddenly contract (3.5" to 3" for our example). So overall, you're better off slowly piping up in size and staying there.


From my experience using almost identical mufflers on various cars, here's what I see commonly:
As far as sound characteristics, that's going to be preference, but a dual muffler system is likely going to be a beefier midrange note and likely louder than a single system. The single will also likely be bassier (lowrange note). Are you losing or gaining power in either? Well, it depends on your Y-Pipe design. Hard to even throw an estimate out there. Power difference that far down the line is likely negligible vs. the earlier transition.

WANKfactor

iTrader: (1)

Thanks for the reply, I just worked out the reducer angle from the off-cuts - its about 7 degrees, pretty shallow.
The muffler and resonator are straight through, was thinking about using the same type of resonator again except bigger.
Havnt got a pic of the down pipe in its current guise, but it pretty much looks exactly like this, except with a cone reducer to 3.5" directly after the top bend, and reducing back down to 3" somewhere just aft of the bottom v-clamp, if that gives you a visual.

Would it be a worthwhile difference if it was 3.5" all the way to the diff?

BLUE TII

iTrader: (9)

It is a 7-12 degree expansion rate not a 7-12 degree angle from a fixed point.

If you look at the IndyCar motorsports versions of the EFR turbos you will see it is incorporated into the exhaust housings.

early 6758



later 7163





Throwing an internal wastegate into the mix will affect everything.
You need to incorporate the IWG flow into the turbine flow and aid the IWG instead of letting the turbine flow impede it.

Or, you can get fancy and divide the two flows.

Or fancier and go external waste gate.

But if you are talking about ideal turbo exit flows there will be no internal wastegate in the equation.

RGHTBrainDesign

The IWG section isn't always open though. I think what you did with dividing the charge and re-entering it MUCH later down into the downpipe is the way to go. That's what I calculated at least.

Here are some CFDs we did, interpret them as you wish:
















We modeled these on an EJ255 w/ 15psig MAP using various downpipe designs on the market.

WANKfactor

iTrader: (1)

So guys, 3.5" mid pipe. Should I?

lastphaseofthis

iTrader: (8)

yes i went 3.5 on my 7670.. in and out.

Sleeper 6

Have a V6 here. 3" downpipe to a dual 3" exhaust. Keeping things simple works great.

RGHTBrainDesign

Open dump breh. POST-Turbine Exhaust Pressure is very critical. Make it as low as possible and the Pre-Turbine Pressure (engine tuning) as high was possible. :P Port timing.