Back Pressure....How Much Is To Much?

[weather man]

Set the stage. My setup at 9-10 pounds of boost produces 28-30 PSI of back pressure by 5,000 RPM and holding til shutdown.

There are cars making more boost that probably have as much back pressure.

How worried should I be about the exhaust valves/seats?

I am tempted to just drive it until I decide to replace with a better turbo, or go another direction. I would prefer not having to redo the heads.

Opinions?

 

[Kalatrax]

What does your exhaust consist of?

 

[weather man]

What does your exhaust consist of?

3" out to duals with Borla touring mufflers. You can drop a golf ball right through the borla's, they are not a chambered design.

 

[TurboX]

Subscribed

 

[rcm90]

I'm no expert.. But I don't think it's going to tear anything up if stays the same. Your giving up power and slowing down the spool with the post turbine bp that high (around 3:1). Have you tested the pre turbo pressure?

 

[weather man]

I'm no expert.. But I don't think it's going to tear anything up if stays the same. Your giving up power and slowing down the spool with the post turbine bp that high (around 3:1). Have you tested the pre turbo pressure?

The back pressure was measured just before the upppipe enters the hotside of the turbo. Post hotside was not measured. They have seen countless 3" setups like mine with no issue.

It seems to spool fine running on the 10# spring, which actually gives just shy of 9# of boost that tapers off to just over 7# by 6,500 RPM.

A salute to Lunati, I think my bottom end pickup is their turbo cam.

And compared to the N/A mod curve, the HP/TQ curve got picked up nicely. I just don't want to turn up the boost, since they did and the 3 to 1 ratio stayed unchanged.

 

[rcm90]

I see, I ASSumed it was measured after the turbo. What are the specs on your turbo's hotside?

 

[weather man]

I see, I ASSumed it was measured after the turbo. What are the specs on your turbo's hotside?

In post above and running dual TIAL wastegates. The turbo is a COMP T4 6765 .96 A/R

 

[JoshK]

The turbo piping on his kit is dual 2.5" To single 2.5" Up-pipe also. Just trying to get all the info in there to get more help for ya bud.

 

[weather man]

The turbo piping on his kit is dual 2.5" To single 2.5" Up-pipe also. Just trying to get all the info in there to get more help for ya bud.

Thanks FUEL. The concensus seems to be that at this boost level, that shouldn't be the issue, even though definately not the most optimal. I think if I was shooting for 700+, it would have to be addressed.

 

[rcm90]

2.5" is plenty enough pipe to make 700whp. My fox had a 331 with 2.25" primaries going into a 2.75" merge pipe feeding a 80mm t4 based turbo and it made just a tit over 920rw at 20lbs.

Hopefully CPRsm will chime in.

 

[weather man]

2.5" is plenty enough pipe to make 700whp. My fox had a 331 with 2.25" primaries going into a 2.75" merge pipe feeding a 80mm t4 based turbo and it made just a tit over 920rw at 20lbs.

Hopefully CPRsm will chime in.

We've been talking...lol.

 

[TurboX]

Brian, We exchanged PMs on the other forum. I share the same concerns and issues as you are facing. I just started reassembling my piping this evening(cracked pipe) and will have BP measurements very soon. From what Ive been told by precision and read on heard from many experts, anything over 2:1 is unacceptable.

I see you have 2 waste gates, if possible I might use that bandaid on my car. Do you happen to know what back pressure was before you added the 2nd waste gate ?

 

[weather man]

Brian, We exchanged PMs on the other forum. I share the same concerns and issues as you are facing. I just started reassembling my piping this evening(cracked pipe) and will have BP measurements very soon. From what Ive been told by precision and read on heard from many experts, anything over 2:1 is unacceptable.

I see you have 2 waste gates, if possible I might use that bandaid on my car. Do you happen to know what back pressure was before you added the 2nd waste gate ?

I don't, the 2 wastegates are because the original kit location sucked.

 

[one eyed willy]

3:1 is too high IMO......

What cams? What's your current turbo?

Wastegates have nothing to do with helping with back pressure....

 

[weather man]

3:1 is too high IMO......

What cams? What's your current turbo?

Wastegates have nothing to do with helping with back pressure....

Lunati custom turbo cams Part #21270724. This is 233/245 at .050, .500/.512 on the lift, and it is on a 114 LSA

COMP CT4 6765 ceramic ball bearing/oil-less


People say 3 to 1 is bad, but finding anyone with experience who can say: your exhaust valve seat/valves will be crispy critters in XYZ time/miles is hard to find.

 

[one eyed willy]

My first custom kit I built was 3:1.....it took about 2 weeks to bend a rod. Your motor is built so you won't have that issue. The higher BP will create more heat, I'm not sure how those seals will hold up.

That's why on my second kit I built I used 3" from the manifolds to the turbo, I had 1:1 at that point....now I'm switching to a THP hot side and not sure what I will have.

I don't understand why your BP is so high, mine was high because when I built my first kit I actually built it with a 2.5" down pipe, had the wastegate recycling to the down pipe and had 2.5" over axle pipes. I dropped that first kit to 2:1 with 3" down pipe, 3" magnaflow exhaust and vented the wastegate....your already there with all that.....

I'm no cam expert, but I believe the cams can have a lot to do with the back pressure.

What mufflers? Did they ever try dropping the down pipe and checking the BP?

IMO, drop theDP and check it....do some research on those cams. There is no flex pipe anywhere right?

 

[TurboX]

Brian, EGTs would better answer that question than a BP number. I know several guys on the diesel truck forums have burned up valves because when they turned up boost the exhaust was the restriction. Yes it was back pressure but the EGT is what you actually want to watch.

1250 is safe, 1300 is bad, 1350 is dangerous and 1400 for even a short time can kill inconal valve.

 

[one eyed willy]

That's a great idea in his situation, EGT info would be very helpful!

 

[weather man]

No flex pipe and Borla non-chambered mufflers. They did not drop the exhaust system. Shop did not think that was the issue.

I wonder if mounting a EGT in the same spot just before entry into the turbo would work?