changed out the .81 with a .68.....i guess i need to change it back

[one eyed willy]

mpt70 on a 4.6 3v 2007 mustang built shortblock

so give me your thoughts....i changed out my .81 in hopes to get a little quicker spool, which is exactly what happened,but it would seem that now the top end is really choked up.i was fighting a back pressure issue the last time i had the .68 on the car, i switched out to 3" exhaust, remade some piping and added the .81 housing,all was good and i was happy....then i got the bright idea to throw the .68 back on becuase i figured my issue was probably all the piping and old exhaust.so now i have the .68 and did some datalogging,i wanted to know if this sounds pretty normal for too small of a exhaust housing.

when i un about 9psi,boost comes in,stays about 9 all the way thru until i shift,drops off maybe 1 psi,little more little less.

with the same settings on the boost controller that used to get me 13 psi with the .81,i now get a huge spike to 17 for a second then it drops off to around 9 psi

does this sound typical of too small exhaust housing?

[BottleFed70]

A .68 AR turbine housing would spool sooner than a .81AR.    Are you sure it spooled quicker/sooner with the .81? 

[jaredsamurai]

A .68 AR turbine housing would spool sooner than a .81AR.    Are you sure it spooled quicker/sooner with the .81? 

Thats not what he's saying. He's saying it does spool faster than the .81 ar.

[jaredsamurai]

Maybe your wastgate cant keep up because of all the extra BP from the smaller turbine housing? That free flowing exh you just added might have something to do with it. When I go open down pipe I get an increase in boost from full exaust being on. No other changes. Just going open DP.

How bad is the spool up on the .81 turbine? I would just go with that. Did you data log backpressure differences between the 2?

[one eyed willy]

Maybe your wastgate cant keep up because of all the extra BP from the smaller turbine housing? That free flowing exh you just added might have something to do with it. When I go open down pipe I get an increase in boost from full exaust being on. No other changes. Just going open DP.

How bad is the spool up on the .81 turbine? I would just go with that. Did you data log backpressure differences between the 2?

spool wasnt that bad with the .81, i didnt datalog with the .68 back pressure,i was at 2:1 ratio with the .81, 10pounds boost to 20pounds exhaust pressure.

[Marty Staggs]

If it was 2 - 1 with the 81, its REALLY bad now. 

[one eyed willy]

If it was 2 - 1 with the 81, its REALLY bad now. 

thats what i was thinking my self.....im gonna switch it back this weekend and thro a wotbox on it so it will spool at the line....

[one eyed willy]

i think i figured out why the boost was dropping off so bad.....ever heard of oberboost?lol, i just realized that the over boost light kept flashing after i was getting on it, im using a eboost street controller and the over boost psi is set to 15 psi, so when it sees this 15psi or abaove its cutting all the boost and dropping it back down, so all i really need to do is cut the amount of boost and try it out to see if it drops off again.ill try that out tonight....it would be nice if thats all it was and that easy to fix,lol.

[rdakota340]

 

[one eyed willy]

so i went and did some data logging today.....all is good with the .68 housing!! it was all in the boost controller, had the gate pressure too high causing a spike,then a drop off,the spike was right at the beginning of the boost,causing a "overboost",shutting down the boost controller sending me back to 9psi.

so i got that under control, which actually smoothed out the boost curve, and i could hold a steady 12psi to red line, then i just had to crank up the duty cycle till i got 15psi out of it....and it holds the 15 psi to red line now. so i have the first setting set to 10 psi and the second setting set to 15psi, both hold solid boost to red line!!

im happy now!!

[rdakota340]

Glad to hear it,it is a nice day to tune too I have playing around with my truck too 

[Orr89rocz]

Makes me wonder if .96's on twin 60's is gonnabe too much for my 401 sbc... got .68's now but i thought they were too small. But if a single .68 is good for 281 inches then i should be fine for a 200" motor

[one eyed willy]

i did some more datalogging today, the .68 seems to be doing just fine.15psi to red line no problems....

[rdakota340]

My .81  seems to pull hard on my 5.9 up to 6000rpm with no problem but i'm at 10#'s too.

[one eyed willy]

im wondering if by using the .68 at 15psi, if thats the same as 15psi with the .81?

[MONTEGOD7SS]

Probably not in the hotside it isn't the same.  Any way to measure backpressure with the new downpipe to see just how the .68 housing is working?

[rdakota340]

You will have less flow with the .68 than the .81 and the more boost the more back pressure on your hot side.

[one eyed willy]

yeah,i have the up pipe tapped with some copper tubing just so i can measure the back pressure.i need to go hook it up and take a look.

[TTZ06]

Can you describe how to measure backpressure. I would like to do that to mine

[Marty Staggs]

of the boost,causing a "overboost",shutting down the boost controller sending me back to 9psi.

OOOOPS!  
The first thing you do is adjust the OBS, then Gate Pressure.

Glad you got it sorted out.

[one eyed willy]

Can you describe how to measure backpressure. I would like to do that to mine

i used a 1/8th inch bung, welded in right before the turbo flange, then i used about 10ft of copper tubing coiled up and put a boost gauge on it. then you want to firgure out the ratio of backpressure to boost. 1.5 to 1 is good 2 to 1 is OK, that 2 pounds of back pressure to 1 pounds of boost...in my case, i run 10psi on the street, my .81 was gving me around 20 psi of back pressure.

[turbo6.0]

bigger housing =less back pressure smaller housing= more back pressure

[one eyed willy]

i under stand the back pressure deal,ive doen alot on getting rid of back pressure...

guess im just trying to figure out,if the car spool good,makes 15psi which is what i want then is there a reason to switch back to the .81?

i know i want my back pressure to be low, but if im making 15psi where i want then it cant be too high right?

[9t3svt]

bigger housing =less back pressure smaller housing= more back pressure

Glad you joined up so you could clear this big mystery up for us

[RyanMayo]

i under stand the back pressure deal,ive doen alot on getting rid of back pressure...

guess im just trying to figure out,if the car spool good,makes 15psi which is what i want then is there a reason to switch back to the .81?

i know i want my back pressure to be low, but if im making 15psi where i want then it cant be too high right?

Did you lose any power when you switched from the .81 to the .68?  How much faster does the .68 spool?

The thing I'm thinking about is this: at the end of the exhaust stroke there's always a little bit of exhaust left in the chamber.  Well, not always but lets figure on it being there.  With higher BP, you're going to have more exhaust left in the chamber, which when the intake valve opens will cause less air to be pumped in before you reach your pre-determined 15psi.  So by reducing BP, your mass flow of fresh intake air goes up.  So to me, as long as it spools well, I say do anything and everything to reduce BP.