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partymn 06-28-2013 07:09 PM

1 PSI boost to 3.5 PSI back pressure???
So my car is having some issues with post turbo back pressure as it climbs higher in the boost. My tuner is saying that my down pipe and possibly the turbo are to small for my LS3, 67/67 turbo and 3 inch down pipe with 2 1/2 inch pipes all the way back with no cats. I really wish that I would have realized this when I first installed the turbo but I was so excited to finally get it that I over looked this detail. On the bright side its nice to have my car in the hands of a shop that knows what they are doing. The initial shop that tuned the car missed this.

So maybe its time for us all to get better educated on this topic. Any of our experts want to throw out some information on this?
Am I heading down the same road as GbrilliantQ with bent rods and blown out engine blocks?

GbrilliantQ 06-28-2013 07:31 PM

Hmmmm.... I've been without my car for almost 8 months now. I better not have this problem.

From my Note

partymn 06-28-2013 07:43 PM

Its hard to say if you will or not, I sure hope not.

killmode_on 06-28-2013 07:47 PM

Holy crap, that's really not good.
You have a single 67/67 with a single 2.5" down pipe? That is totally unacceptable. A single 2.5" pipe is good for less than stock HP.

partymn 06-28-2013 07:53 PM

Sorry for the mistake but the down pipe is 3 inches and the merge pipe is 2 1/2 inches, I will change the original post. However, its still not good.

JLE58 06-28-2013 08:17 PM

The turbo would be fine for 7-8 psi. Anything more than that is to much. I've said it more times than I can count a 67mm turbine wheel is to small for a ls3. That kit was never supposed to have a 67mm turbo of course it also wasn't supposed to be crammed between the motor and radiator either. Either way I wouldnt think backpressure would be a big problem at your boost level.

partymn 06-28-2013 08:30 PM

Its 29 PSI at 8.13 PSI of boost. Everything that I have read says this is not a good thing.

JLE58 06-28-2013 09:10 PM

I wouldn't think it would be that high. All of the turbonetics kits have a 67mm turbine turbo I believe. I'd see what some of those guys are doing to keep backpressure down. I'm sure most of them have never tested it. A larger down pipe would be the first step. That turbo does have a t4 .96 ar exhaust housing correct? You don't have much room for a larger turbo....

partymn 06-28-2013 09:29 PM

Yes it does and no I don't have much room left. I think SSDIAZ is running a 72mm but I am not sure. All I know is that the back pressure is high as hell and it is limiting me to 546 HP and 600 on torque at 8.13 PSI.

partymn 06-28-2013 09:35 PM

Are there other ways to help this problem that don't require me to have a down pipe made and a new tune after that.

killmode_on 06-28-2013 11:57 PM

You need a bigger turbine wheel #1.
And a bigger down pipe or at least expansion chamber right out of the turbo #2.

Single on a 6.2L motor should be a minimum of a 75mm turbine wheel if you want everything to work HAPPY HAPPY HAPPY.

dekan513 06-29-2013 07:59 AM

killmode , hit it on the head. im not a turbo man but . he thinking just as i am.

JLE58 06-29-2013 08:57 AM


Originally Posted by partymn (Post 6730027)
Yes it does and no I don't have much room left. I think SSDIAZ is running a 72mm but I am not sure. All I know is that the back pressure is high as hell and it is limiting me to 546 HP and 600 on torque at 8.13 PSI.

Your not going to make much more than that even with a bigger down pipe on that turbo. If it was mine I would move the turbo over to the driver side bring it up and point it towards the head light. Build a bigger cross over and down pipe and put a bigger turbo on it... That would be the way to make it right would also help with the drain issues. What turbo is on it Comp? I'm not sure how Diaz got the turbonetics turbo but my guess would be Chris pulled it off another car and charged Diaz full price for it. Either way I would bet money he never bought that turbo new from turbonetics.

Unreal 06-29-2013 09:57 AM

Even with a bigger downpipe the turbine is too small. Sell it to a honda/evo/subaru guy and get a proper setup.

GbrilliantQ 06-29-2013 11:14 AM

You know its funny how no one mentioned or thought this turbo was to small over a year ago when Chris introduced this kit. Now everyone is saying it....

From my Note

JLE58 06-29-2013 11:28 AM

It has the same turbine wheel size as the turbonetics kit... I think even Chris said the kit was for 550 Rwhp. The kit was supposed to have 76/75 turbos originally. I'm not sure when the 67mm stuff started I still wouldn't have thought back pressure would have been that high.

partymn 06-29-2013 12:49 PM

You know I really didn't want this to turn into another HPS bash session. I think that we all agree on what type of shop it was and have had enough fun blaming them for this kit.

I would like options to correct the problem with the kit set up the way it is. I cant afford to redesign the kit right now nor can I afford to buy a larger turbo. There has got to be a cheaper way to lower the pressure that doesn't require me to change the whole design of the kit. I need to get the backpressure down to a reasonable level to keep my engine alive.

I have a set of exhaust cutouts that will be going on when I get the car back, will that help while they are open? Will it need retuned afterword? I know that it normally wouldn't but I am not sure if it will in this situation.

Could the stock cans that I still have on the car exaggerate this issue? Could they be damaged internally and cause this high pressure? If so those can go away for some flowmasters.

If I get the backpressure down to, lets say, a 1:2:1 ratio will the car need to be retuned? I probably already know the answer to that but you never know.

Could my lack of a catch can also exaggerate this issue?

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