Too much back pressure?
You should definitely look into a cutout of some kind as well.
Ideally one for each turbo but if space doesn't allow then perhaps close to where the two down pipes merge into one.
If you can do a 4" electric cutout right after the merge you'd see a big difference in spool and power.
I had a single 3" exhaust all the way out the back on my old 4th gen and adding an electric cutout right under the driver floor board made huge difference in spool and power, plus it sounds badass.
Ideally one for each turbo but if space doesn't allow then perhaps close to where the two down pipes merge into one.
If you can do a 4" electric cutout right after the merge you'd see a big difference in spool and power.
I had a single 3" exhaust all the way out the back on my old 4th gen and adding an electric cutout right under the driver floor board made huge difference in spool and power, plus it sounds badass.
Staging Lane
Joined: May 2021
Posts: 97
Likes: 11
Side note - Your back pressure could be the 4" exhaust. 4" pipe flow is 1.5sqin less than (2) 3" pipes feeding it. Just some thoughts.
A single pipe will also retain heat better (less surface area).
Speaking of heat, Fallguy, is your exhaust system wrapped after the turbine exit? It's probably not as big a deal as pre-turbine but it could help. Otherwise what was mentioned above is correct... Small changes in post-turbine back pressure can have big effects on spool time and power. I've seen it on other cars... All else being equal, go to a larger diameter pipe, or remove a restriction like a catalytic converter -> spool RPM drops and power goes up dramatically.
Last edited by Deude_Mann; May 11, 2021 at 04:37 PM.
You should definitely look into a cutout of some kind as well.
Ideally one for each turbo but if space doesn't allow then perhaps close to where the two down pipes merge into one.
If you can do a 4" electric cutout right after the merge you'd see a big difference in spool and power.
I had a single 3" exhaust all the way out the back on my old 4th gen and adding an electric cutout right under the driver floor board made huge difference in spool and power, plus it sounds badass.
Ideally one for each turbo but if space doesn't allow then perhaps close to where the two down pipes merge into one.
If you can do a 4" electric cutout right after the merge you'd see a big difference in spool and power.
I had a single 3" exhaust all the way out the back on my old 4th gen and adding an electric cutout right under the driver floor board made huge difference in spool and power, plus it sounds badass.
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Thread Starter
Staging Lane
Joined: Jan 2016
Posts: 56
Likes: 14
Thinking 3200ish
it’s not as light as he would think. I think factory it’s about 3100 pounds it’s an aluminum LS so that’s not adding too much. The car has a lot of sound deadening and I swear the electric leather seats must weigh 75 pounds each.
It’s also one of the few cars that started out as a convertible then they added a roof to it not removing any of the extra structure that was in place for the convertible. BMW at the time said it was the most rigid vehicle they had ever built
it’s not as light as he would think. I think factory it’s about 3100 pounds it’s an aluminum LS so that’s not adding too much. The car has a lot of sound deadening and I swear the electric leather seats must weigh 75 pounds each.
It’s also one of the few cars that started out as a convertible then they added a roof to it not removing any of the extra structure that was in place for the convertible. BMW at the time said it was the most rigid vehicle they had ever built
Staging Lane
Joined: May 2021
Posts: 97
Likes: 11
The straight 6 DOHC S52 (?) that is normally in that car is a pretty big engine. It weighs about 330 lbs.
It's kind of the same deal as a LS swap in a Miata (NA-NB). It really does not add very much weight, about 100 lbs or so depending on the transmission. The Miata BP engine has a hefty iron block (aluminum head), and a SUPER beefy forged crankshaft, so it is really heavy to begin with. With better rods and pistons (and FI, and a skilled tuner) that bottom end is capable of ridiculous power levels for a 1.8L, like 600-800 HP.
It's kind of the same deal as a LS swap in a Miata (NA-NB). It really does not add very much weight, about 100 lbs or so depending on the transmission. The Miata BP engine has a hefty iron block (aluminum head), and a SUPER beefy forged crankshaft, so it is really heavy to begin with. With better rods and pistons (and FI, and a skilled tuner) that bottom end is capable of ridiculous power levels for a 1.8L, like 600-800 HP.
TECH Resident
Joined: Sep 2013
Posts: 994
Likes: 119
From: Mechanicsburg, PA
I never figured them to be featherweight cars. German cars are built like tanks. That's not a bad weight at all. Love that car! Had a chance to drive a whacked out Z3 M coupe back in the day, but I was wearing my filthy work uniform and couldn't bring myself to hog it up. Totally regret that, lol
Thread Starter
Staging Lane
Joined: Jan 2016
Posts: 56
Likes: 14
I never figured them to be featherweight cars. German cars are built like tanks. That's not a bad weight at all. Love that car! Had a chance to drive a whacked out Z3 M coupe back in the day, but I was wearing my filthy work uniform and couldn't bring myself to hog it up. Totally regret that, lol
Defiantly NOT the turbine housing. I run the .63 T3 China GT35s (billet 64mm comp versions) No intercooler on an sub 9:1 5.3 and baby cam. Making 560/600 on 11-12lbs. Haven't been back to teh dyno yet, but I run 19 now and its a whiole new animal. Far from optimal setup but making substantially more power.
I'd go back to basics. Not that the new housings are bad... But look at your cranking compression, plug gaps, etc... Also fuel consumption is a biggy.
Also for what its worth the cast $110 Gt35's with .63 T3's have made 854/989 @ 29.5 lbs on a mild SBE 5.3.
I'd go back to basics. Not that the new housings are bad... But look at your cranking compression, plug gaps, etc... Also fuel consumption is a biggy.
Also for what its worth the cast $110 Gt35's with .63 T3's have made 854/989 @ 29.5 lbs on a mild SBE 5.3.
TECH Fanatic
Joined: Dec 2019
Posts: 1,129
Likes: 478
Another strong vote that it isn't the turbine housing here, from multiple instances of personal experience.
The ONLY possibility that it is backpressure making you lose that much power is from a camshaft that handles it poorly.
The ONLY possibility that it is backpressure making you lose that much power is from a camshaft that handles it poorly.
Thread Starter
Staging Lane
Joined: Jan 2016
Posts: 56
Likes: 14
Defiantly NOT the turbine housing. I run the .63 T3 China GT35s (billet 64mm comp versions) No intercooler on an sub 9:1 5.3 and baby cam. Making 560/600 on 11-12lbs. Haven't been back to teh dyno yet, but I run 19 now and its a whiole new animal. Far from optimal setup but making substantially more power.
I'd go back to basics. Not that the new housings are bad... But look at your cranking compression, plug gaps, etc... Also fuel consumption is a biggy.
Also for what its worth the cast $110 Gt35's with .63 T3's have made 854/989 @ 29.5 lbs on a mild SBE 5.3.
I'd go back to basics. Not that the new housings are bad... But look at your cranking compression, plug gaps, etc... Also fuel consumption is a biggy.
Also for what its worth the cast $110 Gt35's with .63 T3's have made 854/989 @ 29.5 lbs on a mild SBE 5.3.
i think I was using 60% if the fic 1000cc injectors. But at 11.0 afr.
i wonder if your housings are as restrictive as these. And if my 10:1 compression just pushes that much more air.
also at 13psi I’m sure I could if made 500+ but we didn’t add much timing. The last degree we added picked up 20whp. Then we added more boost because we thought it would of made more at that boost level.
Or maybe an improperly labeled hotside? I had the standard flange t3, not the Vband. Those do look pretty small, but mine weren't exactly huge. China is pretty bad with that. Ive had some china .8x T3 hotsides that were MASSIVE. Likely 1 1.25 or larger T4 equivalent.
This is my .63AR GT35 T3 next the ".83AR" 72/68"
Attachment 712962
I was running a 220/226 @ 112 .525 lift cam. Believe the malibu had a stage 2 btr but I'd need to verify that.
This is my .63AR GT35 T3 next the ".83AR" 72/68"
Attachment 712962
I was running a 220/226 @ 112 .525 lift cam. Believe the malibu had a stage 2 btr but I'd need to verify that.
Thread Starter
Staging Lane
Joined: Jan 2016
Posts: 56
Likes: 14
well it’s a BTR stage 1 tt cam. Did I install it a tooth off?
If you did, it would be really obvious when you check cranking compression. That's not a large cam by any means. Id expect 180-190 cranking depending on altitude and engine health. My 10:1 5.3 with the 212/212 @112 .560 trip/12 cam spit out 190-200 on all 8. I did advance it 3* or so. It was a mean raspy ****.I've had motors spit out low 140's... clear indication of a cam timing problem. (if you don't have a massive cam)