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Now, I've tuned the PT7675 setup to run great. Had it running consistent 21psi at WOT, no knock retard and running 19 degrees of advance at WOT. A buddy of mine with a near new ON3 84mm took his kit off his 5th gen to swap to a supercharger. He gave me the 84mm to try it out. I put it on this past weekend with NO other changes. But when I took it out for some tuning sessions, it would only go to 16psi. I've turned the Turbosmart Boost Tee up a few clicks at a time to elevate the PSI and that's working. The car still feels as fast/quick as before and my scans show higher KPA in relation to RPM than I saw before. I'm just curious on why the 21psi setting for the PT7675 would show only 16 psi with the ON3. Higher back pressure? Have any ideas?
Doesnt make a ton of sense due to the same 75mm exhaust wheel in both. I mean I'd understand a couple psi, but 5psi variance is pretty significant. Are you planning a track outting any time soon to compare results?
How much larger physically is the 84mm jobber compared to your 7675? Curious if it will fit on a Huron V3 AC kit without modification for future upgrade potential.
Yes, I'm puzzled about the pressure differential.
Test n tune is tomorrow night at my nearby (7 miles?) Lebanon Valley Track but there's a small chance of Rain(20%). I suppose I should give it a try before switching back to the PT7675.
Both turbos have billet compressor wheels.
On the size, the 84mm doesn't readily seem to be larger. I haven't put the tape measure to it. The compressor outlet on the PT7675 is 3" but the 84mm is only 2.5". Both are T4 flanged.
I wouldn't be in a such a hurry to dismiss it as a less efficient turbo.
That being said, The billet 84mm matched with that small turbine is a really bad turbo combo. It would work well on a smaller engine, although a 5.3 with a triple 12 might be able to slide in there and make it work.
I'm just curious on why the 21psi setting for the PT7675 would show only 16 psi with the ON3. Higher back pressure? Have any ideas?
Less efficient exhaust side producing higher hot side back pressure, probably a less efficient compressor as well. Just because it says it has a 75mm exhaust side or that the compressor is bigger doesn't mean it's better. Remember the size of a compressor or turbine isn't the only factor.
If the 84mm is making less boost at the same settings as the original setup, shouldn't that mean there is less back pressure their for its not spinning as fast on the compressor side. Hence more exhaust pressure bleeding out the turbine. How many exhaust fins on the 84mm versus the pt7675?
If the 84mm is making less boost at the same settings as the original setup, shouldn't that mean there is less back pressure their for its not spinning as fast on the compressor side. Hence more exhaust pressure bleeding out the turbine.
Doesn't explain why the wastegate is opening sooner (have we actually verified that the gate is opening)? You can't have a gate open at a lower PSI with both less boost and less backpressure.
It is possible that the compressor is more efficient and is producing more power at a given PSI. The turbines on those 2 turbos are identical. More power would mean more exhaust, so more backpressure at a given intake pressure, even with the same turbine.
Yes, the gate is opening. It's vented to atmosphere and there's no mistaking that Jet whoosh like sound.
Originally Posted by JoeNova
Doesn't explain why the wastegate is opening sooner (have we actually verified that the gate is opening)? You can't have a gate open at a lower PSI with both less boost and less backpressure.
It is possible that the compressor is more efficient and is producing more power at a given PSI. The turbines on those 2 turbos are identical. More power would mean more exhaust, so more backpressure at a given intake pressure, even with the same turbine.
I switched from a On3 Cast 78mm to a On3 billet 84mm( I didn't get it for the size but for the 84mm wheel being lighter than the cast 78mm wheel witch would get me into boost faster) same 75mm exhaust and on gate 10ish# in hotter weather gained 2 mph over the 78mm in colder weather.
I switched from a On3 Cast 78mm to a On3 billet 84mm( I didn't get it for the size but for the 84mm wheel being lighter than the cast 78mm wheel witch would get me into boost faster) same 75mm exhaust and on gate 10ish# in hotter weather gained 2 mph over the 78mm in colder weather.
People Knock China made **** for one glaring reason. They don't know that almost everything is made there. The difference is that the stuff that gets filtered through more "reputable" brands has more levels of quality control, so the bad pieces don't make it to you. This particular 84mm turbo may just be one of the ones that would not have passed a good QC check.
I switched from a 78/75 to a pt8847 and my issue was now my same 45mm wastegate with a 10psi spring will not hold less than 18psi. My thought was the turbo flowing much more air and the gate cannot keep up. The opposite is happening with you, maybe although it's the same size turbine could the new turbo not be as efficient (higher back pressure?)
It's hard to say .So many dynamics are in play, even with all other attributes being the same. As JoeNova offered, it *might* be that the additional air provided by the larger compressor, is pumping more air than before and that is loading more air into the exhaust as well.
The local tracks test n tune got rained out tonight so I didn't get to try it out. Hopefully I can get a test soon to see what difference the 84mm makes.
I was sent to this thread by a friend and we are also thinking of doing back to back dyno testing in a similar manner.Currently with my on3 billet 84mm from 5500 to 6700 redline,I lose boost pressure by nearly 2.5psi.My previous on3 78/75 was the same. I attributed this mainly to backpressure since I run full 3 inch exhaust and a cutout just before the overaxle pipe. Ive tried running a combinations of wastegate springs to see if that would be part of the issue going from a 7lbs spring to12lbs and adjusting the boost controller which netted very small improvements.
The thought is that the On3 being a "less efficient" turbine for flow is holding me back from making more power throughout the curve and that just doing a turbo swap on the same day with a newer design PT should improve flow,decrease back pressure and maintain boost would result in more peak power aswell as increase my average power throughout the curve.
I will say that the On3 so far reliability wise has been great.No issues or shaft play after alot of abuse.But its also very quiet of a turbo.Im interested in seeing results of this and may still do it myself in the near future.
I was sent to this thread by a friend and we are also thinking of doing back to back dyno testing in a similar manner.Currently with my on3 billet 84mm from 5500 to 6700 redline,I lose boost pressure by nearly 2.5psi.My previous on3 78/75 was the same. I attributed this mainly to backpressure since I run full 3 inch exhaust and a cutout just before the overaxle pipe. Ive tried running a combinations of wastegate springs to see if that would be part of the issue going from a 7lbs spring to12lbs and adjusting the boost controller which netted very small improvements.
The thought is that the On3 being a "less efficient" turbine for flow is holding me back from making more power throughout the curve and that just doing a turbo swap on the same day with a newer design PT should improve flow,decrease back pressure and maintain boost would result in more peak power aswell as increase my average power throughout the curve.
I will say that the On3 so far reliability wise has been great.No issues or shaft play after alot of abuse.But its also very quiet of a turbo.Im interested in seeing results of this and may still do it myself in the near future.
Your exhaust might be why you are losing boost up top what cam do you have?. 2.5# is a huge drop IMO. I do not lose any boost up top through a single 3in DP on 14/15# boost on the on3 84mm it stays flat through out the hole run. The picture of my log is one of my 9.x passes
Like I said, it still feels strong but I can't tell if its just what I want to feel or if it's actually performing better. If the weather gets better, I'll have a chance to see. They'll only let me run the 1/8th w/o a rollcage, etc at the local track. Best last year with the PT7675 was a 6.4 @ 112mph. So at least I'll have something to compare to when I get a chance. Not that the 6.4 was all the PT7675 setup could do. I'm still on street tires all the way around and heavy ZR1 rims. Had about 50lbs of tools in the back too. Just drove there, let some air out of the MT Drag Radials to 17psi, then made the 1/8th mile passes. Leaving on the brake with about 5 PSI, 2 step set to 3,000 rpm, if memory serves. I set the 2 step to 3,500 for one pass last year but it just sent the tires up in smoke on the leave.
Originally Posted by 1CAMWNDR
Same Turbine inlet (Outer) Diameter on both? Same Turbine housing A/R on both?
Last edited by mightyquickz28; 06-28-2018 at 07:49 PM.
Your exhaust might be why you are losing boost up top what cam do you have?. 2.5# is a huge drop IMO. I do not lose any boost up top through a single 3in DP on 14/15# boost on the on3 84mm it stays flat through out the hole run. The picture of my log is one of my 9.x passes
My exhaust is pretty tight.3" dp to a punched out 3" cat,to my e-cutout just before the over axle to my gmmg muffler.My cold side is all 2.5 piping.On high boost my cutout is always open,generally I run 7lbs on the street.
06-26-2018, 07:29 AM
Took off my PT7675 to try a friends ON3 84MM. 
