View Full Version : Back pressure test results, when is it too much?


SPRAYED 01
04-16-2012, 05:26 PM
Ok so I made a real nice piece to install in an exhaust bung I welded in right before the flange to the turbo. I hooked it up to a boost gauge and did some street testing.

Comp 80/79 t4 on a 1.32ar
9.5 compression 370
Stock ported 241 heads
Stock ls6 intake and tb
232 236 115lsa 609ish lift cam
4 inch k&n right on turbo
Large ebay ic with 3 inch piping

9lbs intake, 20lbs exhaust

14lbs intake, 30lbs exhaust

Tried 18lbs intake but boost gauge only goes to 30. Id say it was at least 35 probably 40lbs back pressure.

Now I'm running a good size ar with a 3.5 dp. I think this is high but then again I bet everyone with a t4 has to be right around the same.

Is this something to worry about?

Alex

Johnv
04-16-2012, 05:34 PM
I would have thought 2:1 wasn't too bad ,
I'm sure mine would be way worse 383, t76 f168 exhaust wheel t4 1.32 ar

stevieturbo
04-16-2012, 05:43 PM
2:1 certainly isnt good. But on a low boost setup provided that pressure ratio offers fast spool, then wouldnt worry too much

However if it is laggy and with 2:1, something is wrong. It certainly isnt the d/p size.

I'd be much happier around 1.5:1 EGBP pre turbine vs Intake pressure

SPRAYED 01
04-16-2012, 06:12 PM
This is also running an open cut out about 3 feet after dp.

SPRAYED 01
04-16-2012, 07:27 PM
Stevie, what do you consider low boost?

INTMD8
04-16-2012, 08:55 PM
It's nothing to 'worry' about, and those numbers are pretty normal for a single T4 on a 370.

Also consider that for every PSI lost to the cold side is another pound of backpressure. (You can reduce exhaust backpressure by increasing cold side efficiency as the engine doesn't need to drive the turbine as hard).

It goes without saying that you could pick up a ton of power/rpm with twins or a good T6 based single.

mike13
04-16-2012, 09:25 PM
It's nothing to 'worry' about, and those numbers are pretty normal for a single T4 on a 370.

Also consider that for every PSI lost to the cold side is another pound of backpressure. (You can reduce exhaust backpressure by increasing cold side efficiency as the engine doesn't need to drive the turbine as hard).

It goes without saying that you could pick up a ton of power/rpm with twins or a good T6 based single.

Also wouldn't a T4 with a larger turbine wheel also help? I know guys make good power with the 75mm & 80mm borg warners because they use the 87mm turbine wheel.

INTMD8
04-16-2012, 10:24 PM
Also wouldn't a T4 with a larger turbine wheel also help? I know guys make good power with the 75mm & 80mm borg warners because they use the 87mm turbine wheel.

Absolutely a higher flowing turbine would help. :nod:

skinnies
04-17-2012, 12:45 AM
Any idea on actual power numbers? Awhile back I talked with comp on that turbo, they claimed it'd do over 1000rwhp on a 347 ls motor(thru an auto), but I just don't see it happening.

SPRAYED 01
04-17-2012, 05:52 AM
777 whp at 17psi with this setup.

SPRAYED 01
04-17-2012, 06:40 AM
Jim, I'm going to look into getting a better quality IC and correct the way the cold side is routed to it. I have a harsh 90 on both sides going to the IC.

stevieturbo
04-17-2012, 02:55 PM
Stevie, what do you consider low boost?

Anything below 15psi is pretty low really, and the 8psi he mentioned certainly is.

Ryan K
04-17-2012, 03:25 PM
In the reading that I have done:
(Intake:Back Pressure)

1:1 is the Ideal state

1:1.5-2.0 is what you would normally expect

1:>2.5 would be unacceptable

stevieturbo
04-17-2012, 04:50 PM
I would try and aim for 1.5 or less. But again it is very dependant on spool and power delivery.

I'd sacrifice some for a fast spooling setup at the expense of a little power. Or if you wanted ultimate power at higher rpm's you'd sacrifice some spool to achieve the lower pressure with a better flowing turbine.

icicleboy
04-17-2012, 06:49 PM
JohnV, what kind of power does your car make?

Johnv
04-17-2012, 10:17 PM
700 rwhp @ 16 psi, M6 & spools very quick....

bmech211
04-18-2012, 06:54 AM
In my opinion you should've measured back pressure downstream (at the downpipe), to get a better overall view of how effecient the turbo/exhaust system is as a whole. High pressures upstream is not as detrimental to performance as it is downstream. In fact higher pressures in the header helps turbo performance more than it hurts. Look at what a quick spool valve does and how much higher it raises perssure, yet improves spool-up.

SPRAYED 01
04-18-2012, 10:46 AM
Hmm, not sure if that's right. I always thought you measure before turbine because it would show a restriction to the turbo.

stevieturbo
04-18-2012, 05:52 PM
Hmm, not sure if that's right. I always thought you measure before turbine because it would show a restriction to the turbo.

You are quite correct, you want to determine how efficient the turbine is.

bad6as
04-18-2012, 06:34 PM
i know a guy in va with a 365ci that has a 80mm compressor and a 87mm turbine t4 1.25 ar and at 11 lbs boost he had 14psi bp iirc

SPRAYED 01
04-18-2012, 07:17 PM
That's really good, what turbo?

bad6as
04-18-2012, 07:22 PM
borg.

SPRAYED 01
04-18-2012, 08:33 PM
Im so limited with space, that thing is probably alot bigger than my compact comp turbo. Comp said they have a 1.52 ar housing I could try. I wish they could fit a larger turbine wheel I there.

bad6as
04-18-2012, 08:41 PM
You need a bigger turbine. Going to a 1.52 ar isn't worth it and you'll see small small gains in hp and a small drop in bp.

stevieturbo
04-19-2012, 02:09 AM
Precision have a range of cea turbines that Flow better for any given size, or you could maybe have the turbine wheel clipped. Larger housings aren't the only option.
I think the turbine wheels used in Garretts billets also flow better than old wheels.

SPRAYED 01
04-21-2012, 07:41 PM
Just got my 60 psi boost gauge so ill have some more accurate results tomorrow

TurboS10
04-22-2012, 05:46 AM
Any idea on actual power numbers? Awhile back I talked with comp on that turbo, they claimed it'd do over 1000rwhp on a 347 ls motor(thru an auto), but I just don't see it happening.

The billet version maybe. With a different turbine setup on it.
The OP already proved the 79mm turbine billet compressor combo won't work on a 370. Comp exchanged it for this one.

Comp spec'd a 1.15 on an 8079 would work for me on a 347 for 1khp.
I ended up finding a 6.0 30 miles from me so I couldn't pass it up.
Based on the OP's numbers, I'm going to sell my 8079 due to BPR.

Based on the BP number the OP has seen, it looks like about how a 76HPSH would run on a 355. Just a little more power.

98Z28CobraKiller
04-22-2012, 10:29 AM
In my opinion you should've measured back pressure downstream (at the downpipe), to get a better overall view of how effecient the turbo/exhaust system is as a whole. High pressures upstream is not as detrimental to performance as it is downstream. In fact higher pressures in the header helps turbo performance more than it hurts. Look at what a quick spool valve does and how much higher it raises perssure, yet improves spool-up.

Ideally, you want to see 0 backpressure in the DP. Backpressure in the DP would slow spool, limit max boost pressure and create additional pressure ratio in the hot pipe. But that said, backpressure ratio is measured as close to the exh housing as possible (the way the OP did it).

SPRAYED 01
04-22-2012, 11:45 AM
Turbo s10, so what turbo are you going to with? Also keep in mind before you sell it. Not many test bpr and post results. If your in need of a small frame turbo due to space limits you don't have many options that are going to have less bpr.

TurboS10
04-22-2012, 07:15 PM
I bought a GT4788r -
71 Nova- not worried about cutting anything, I'll make it fit.

I could have went and got a 5.3 and made it a 5.7. Probably should have....
But money wise it will be a wash since the GT after selling the 8079 will end up costing me about what a 5.3 would.

I think the 8079 is perfect for a 5.3 w/ported heads. If my overtime hadn't dried up I would have tested it just to see....I need to sell the 8079 before curiosity gets the better of me and I stick it underneath the jimmy! I already need to cut the exhaust a part and want a different muffler...

jmop- but I like your cam as a 236/232. It's what I plan on running.
LS1Curt has one in his S88 370 build. I'm basically copying 98Z28Cobrakillers' build.

SPRAYED 01
04-23-2012, 08:01 PM
Ok got my brand new boost gauge in that goes to 60psi and did some high way testing. Looks like it 2:1 at every boost level. Intake pressure: exhaust

9.5:20
14.6:30
17.6 35
19.3:38

Id say thats pretty normal for a t4 turbo on a 370 with ported heads and a cam.

Any one else have results?

bad6as
04-23-2012, 08:31 PM
Is your bp sensor before or after the wastegate?

bad6as
04-23-2012, 08:32 PM
Also what's the lowest boost you can run?

SPRAYED 01
04-23-2012, 08:39 PM
Its before the waste gate, I can run 8psi on a 10.5lbs spring. Haven't tried a lower spring

bad6as
04-23-2012, 09:10 PM
What's the bp at 8 lbs boost?
What the most boost you plan on running?

SPRAYED 01
04-23-2012, 09:20 PM
Not sure on 8 but id imagine about 16. I plan on running 19psi max,so 38lbs bp

got-a-ls1
04-23-2012, 11:23 PM
i have bungs in my headers but not at the turbo merge. if i measure backpressure at the collector at the header will the results be any good? id like to measure, but dont wanna hack into my cross over just to test as it runs good, im just curious. so basically anyone measure at the collector area and compare to right before the turbo?

turbociv910
04-23-2012, 11:47 PM
Not sure on 8 but id imagine about 16. I plan on running 19psi max,so 38lbs bp


It doesn't work like that. Turbine can only flow so much before it breaks 1:1 barrier, after that, the turbine inlet pressure rises faster than your boost level. And why plan for a 1:2 ratio, get the correct turbo.

But Got-A-LS1 it doesn't matter if its before or after waste gate, Exhaust Manifold pressure is measured at the turbine inlet.

This is where they should be ideally. Only using 2 due to divided housing.
http://i116.photobucket.com/albums/o12/ZedxElite/100_3959.jpg

stevieturbo
04-24-2012, 03:45 AM
Pressure can be monitored anywhere between the turbine and the exhaust valve. Pressure throughout the actual tubes wont vary that much.

2:1 certainly isnt great, but again if it's a fast spooling setup I wouldnt be too concerned. If however it is still quite slow to spool then it's just a bad setup.

SPRAYED 01
04-24-2012, 06:38 AM
Turbociv, what is your setup and back pressure results. If I could fit a larger turbo I would,i have no space at all.

SNOOP
04-24-2012, 08:22 AM
You guys are scaring me.... How about this 427 LSX with twin CEA 6766 (T3) V-Band inlet .82 A/R with 3in V-Band discharge???? Will I be ok?

SPRAYED 01
04-24-2012, 09:14 AM
No idea,is your setup up and running yet? If so its easy to check

SNOOP
04-24-2012, 04:26 PM
No idea,is your setup up and running yet? If so its easy to check

Not yet, about 2 weeks out...

turbociv910
04-24-2012, 08:24 PM
Pressure can be monitored anywhere between the turbine and the exhaust valve. Pressure throughout the actual tubes wont vary that much.

You theoretically could monitor pressure anywhere, but would not be a correct reading if you have an external gate, especially twin gates.

spoolin turbo
04-24-2012, 09:40 PM
Turbociv, what is your setup and back pressure results. If I could fit a larger turbo I would,i have no space at all.

What do you mean you have no space at all. You already have the CT5 cover which is the big cover. If you went full CT5 only the exhaust housing would change slightly and the chra gets a tad longer maybe 1/2" longer. Not much difference.

How does the car spool? fast/ok/slow?
Anyone else running a similar setup with another brand turbo? What kind of hp they making?
Crank the boost and see what she does and check the bp.

Im curious to know if Unleashed measured their bp. Maybe they can lend some insight.

SPRAYED 01
04-24-2012, 10:32 PM
I'm not running the ct5,i tried and it wouldn't fit. Im running their ct43 80/79. If I ditched the ac condensor I could fit the 5.

My car spools very fast, faster if not the same as my old setup,347 and tc78.

TurboS10
04-25-2012, 12:38 AM
I'm very happy you posted your results. It saved me some work down the road doing an upgrade to a T6. I thought the x trim might be just enough for what I want.

Once you swap to the CT5 w/bb the price goes way up...that's why I went to a fresh used GT47r vs trading/upgrading my 8079.

I'm not running the ct5,i tried and it wouldn't fit. Im running their ct43 80/79. If I ditched the ac condensor I could fit the 5.

My car spools very fast, faster if not the same as my old setup,347 and tc78.

Sounds like a bad ass street occasional strip setup due to the small turbine and t4. It looks like it will work better than a T76 s trim.
You should be able to get it into the mid 9's if desired an it's not too heavy.

What's the vehicle and what more do you want?

Twin 6262's or similar are about all that fits in similar space w/more power.

Nitroused383
04-25-2012, 01:49 AM
When is back pressure too much? My answer to that is when you turn up the boost but the Hp does not increase. The boost pressure may increase but it's not a true boost increase as the excessive back pressure is pushing back into the intake side causing a false boost reading.

SPRAYED 01
04-25-2012, 05:54 AM
Its an 01 ws6 that weighs 3850. Full suspension, t brake, and a bunch of other stuff. My goal is to just run 9s.

skinnies
04-25-2012, 06:16 AM
Its an 01 ws6 that weighs 3850. Full suspension, t brake, and a bunch of other stuff. My goal is to just run 9s.With that power it should go 9's without an issue!

SPRAYED 01
04-25-2012, 06:56 AM
I hope so, I've been stuck in the 10s for sooo long. Always some issue at the track. It went 10.4 at 134 with the tc78 and no suspension or t brake. I have about 150whp more now and qa1s with the brake. Hopfully I can get some clean passes.

rotary1307cc
04-25-2012, 10:59 AM
id like to see you bolt a 76gts on there

turbociv910
04-25-2012, 03:05 PM
When is back pressure too much? My answer to that is when you turn up the boost but the Hp does not increase. The boost pressure may increase but it's not a true boost increase as the excessive back pressure is pushing back into the intake side causing a false boost reading.

:confused:

This is how myths are created. That is an incredible theory though. LOL

SPRAYED 01
04-25-2012, 03:36 PM
I think it would be right around the same with the gts76.

spoolin turbo
04-25-2012, 04:14 PM
Someone lend him a billet 7675 and lets see the rwhp and the bp figures.

Nitroused383
04-25-2012, 04:20 PM
:confused:

This is how myths are created. That is an incredible theory though. LOL

No myth at all, do some research. Happens quite often when maxing the turbine side out. How do you think guys end up with carbon build up in the intake manifold?

Here straight from DrTurbo http://www.yellowbullet.com/forum/showpost.php?p=8188688&postcount=3

SPRAYED 01
04-25-2012, 04:21 PM
Id be down for that. Some one has to have tested bp on a 347 or 370 running a gts 76. I think the power is right on with the 76gts, right. 17 psi 777 whp through auto and 9 inch. Would a gts really do more?

Nitroused383
04-25-2012, 04:33 PM
I made 720 rwhp through my 76GTS with 11.5lbs of boosst on a 346. Through a 9" and on slicks but mine is a 6 speed. I will try to log back pressure this year. NicD made 810 rwhp on 15.5lbs of boost on a low compression 347 with the 76gts.

SPRAYED 01
04-25-2012, 07:54 PM
I don't really think there is going to be much of a difference in power from a 370 to 346 so 346 results count to.

Thanks for posting those results. I'm still on stock,241 heads that are mildly ported. The gts is a bad mofo for sure. Id love to,see bp results

rotary1307cc
04-26-2012, 10:00 AM
yeah i would just like to see the pressure differences. i just dont think the gts would be at those levels down by 10psi, perhaps it is, i dont know

i will get mine tapped and tested. dont know how relevant it will be though since my heads are worked over alot and i think mh cam is a bit bigger

SPRAYED 01
04-26-2012, 11:20 AM
I think it would be relevant. Not every setup is going to be exact

Not sure if you posted but what are your numbers rotary?
.

NicD
04-26-2012, 11:57 AM
I have what is essentially a 76GTS .96 ar on my 346 and it's donzo at around 18 psi and while I haven't measured the backpressure I'm sure it's gotta be up there. I am trying to figure out what I can do to help get that exhaust out whether it's clipping the exhaust wheel or trying another turbo but since I've got a TTi race kit my space is VERY limited and kind of excludes anything much larger than a 76GTS.

Heck I don't even know why I'm trying to make it faster or whatever, it's not like it techs for what it runs now LOL! I guess I just want to optimize what I have further.

rotary1307cc
04-26-2012, 12:11 PM
never been on the dyno, just the track twice

at just over 3000lbs it was 149mph on the limiter and 12psi, so whatever that is

i have regeared now and will add boost to see what the turbo does

it would be interesting to get yours to the track and see what the real world power is from the trap/weight

it may be making a good bit more than you think



.

SPRAYED 01
04-26-2012, 12:54 PM
I trapped 134 with the tc78. I'm thinking this will do closer to 140mph. And hopfully a 9.80 or so.

bmech211
04-26-2012, 01:36 PM
Ideally, you want to see 0 backpressure in the DP. Backpressure in the DP would slow spool, limit max boost pressure and create additional pressure ratio in the hot pipe. But that said, backpressure ratio is measured as close to the exh housing as possible (the way the OP did it).

I can definitely agree to that. However, including pressure measurements from downstream @ the DP will give you the actual PR across the turbine itself. Knowing just how high of a pressure drop you've got between turbine inlet and turbine outlet let's you know how effecient your trim/housing selection is at converting that high pressure to kinetic energy.

Sorry for going "Steven Hawkins" on ya..

rotary1307cc
10-05-2012, 09:31 AM
Here is another 80/79 with issues on a small mod motor even

http://www.yellowbullet.com/forum/showthread.php?p=9788923&posted=1#post9788923