What's my boost limit with this combo?
Thread Starter
Joined: Apr 2006
Posts: 3,890
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From: Grand Rapids, Michigan
What's my boost limit with this combo? So, now that I'm getting a little better hang of tuning my Terminator X system (and being really conservative about it), I'm eventually going to want to turn the boost up. Question is, I am not sure what a reasonable boost limit would be. This is my first turbo build, and I'd like to not murder my engine pushing the envelope.
So, those of you with more experience than me, chime in on where you'd put a hard limit with this combo: aluminum LH6 block, Summit ProLS pistons and rods, stock crank, AFR enforcer heads, BTR small bore LS9 style gaskets and ARP head studs. Turbo is a VSRacing Gen 2.5 78/75 with a billet wheel 1.25 A/R, 4" dump pipe.
I myself was thinking 22PSI, but maybe it could hit 25? I think the turbo will be out of steam at 25 lbs. Either way, can the short block handle it?
So, those of you with more experience than me, chime in on where you'd put a hard limit with this combo: aluminum LH6 block, Summit ProLS pistons and rods, stock crank, AFR enforcer heads, BTR small bore LS9 style gaskets and ARP head studs. Turbo is a VSRacing Gen 2.5 78/75 with a billet wheel 1.25 A/R, 4" dump pipe.
I myself was thinking 22PSI, but maybe it could hit 25? I think the turbo will be out of steam at 25 lbs. Either way, can the short block handle it?
Thread Starter
Joined: Apr 2006
Posts: 3,890
Likes: 1,137
From: Grand Rapids, Michigan
Well, I can do both, and I plan to sneak up on it a little at a time. But this engine has a maximum amount of boost it can mechanically handle and stay together, and that's what I was curious about.
Personally, mechanically I think you could be fine at well over 30+ psi.
Fuel will be your limiting factor.
There're several guys on YB I follow running forged 5.3's on methanol running in the 30-40 lb. range and more.
Another gent routinely runs 30ish lbs. on E85 out of the barrel on a Gen IV SBE deal.
I intend on running around 22 psi on pump and meth with a rod/piston LS1 so with your fully forged deal, you'll be solid.
Fuel will be your limiting factor.
There're several guys on YB I follow running forged 5.3's on methanol running in the 30-40 lb. range and more.
Another gent routinely runs 30ish lbs. on E85 out of the barrel on a Gen IV SBE deal.
I intend on running around 22 psi on pump and meth with a rod/piston LS1 so with your fully forged deal, you'll be solid.
Thread Starter
Joined: Apr 2006
Posts: 3,890
Likes: 1,137
From: Grand Rapids, Michigan
Yeah, I'm going to have to upgrade my fuel system and turbo to really push it. Most people are saying 25-30+, so I guess I shouldn't worry too much. I do wonder where this FAST intake's limit is, though. It's one of the original ones from like 20+ years ago. I may have to upgrade to a BTR before I really pour the coals to it.
A bone stock gen4 short block would handle it. If yours was put together well, it will handle it easily.
With decent flowing heads that turbo will be the limit. I doubt it will push 25lbs efficiently. Just depends how well the top end flows. Like you say, I bet that turbo is out of steam around 20-22psi. It will likely still pick up a little after that, but It won’t make 25-35+hp per pound it should.
Many times this is where people hurt things. As soon as the turbo stops making its usual gains per pound, its done. If you aren’t class limited, chasing more power after that isn’t wise. If you want more, get a more capable turbo. Only so much you can do with an under driven 75mm turbine wheel and a T4. I’d suggest T6 S400 line or similar. A legit T6 Borg S480 (even a cast unit) will make 1300 crank all day. Still the best all around bang for your buck 1000whp turbo out there IMO. It will also make more power per pound across the board. Often spooling better than the poorly designed 78/75 units. (depending on the 78/75 model). Happel picked up 100whp at like boost going from a 78/75 to a T6 S480. (believe it was a gen1 78/75)
With decent flowing heads that turbo will be the limit. I doubt it will push 25lbs efficiently. Just depends how well the top end flows. Like you say, I bet that turbo is out of steam around 20-22psi. It will likely still pick up a little after that, but It won’t make 25-35+hp per pound it should.
Many times this is where people hurt things. As soon as the turbo stops making its usual gains per pound, its done. If you aren’t class limited, chasing more power after that isn’t wise. If you want more, get a more capable turbo. Only so much you can do with an under driven 75mm turbine wheel and a T4. I’d suggest T6 S400 line or similar. A legit T6 Borg S480 (even a cast unit) will make 1300 crank all day. Still the best all around bang for your buck 1000whp turbo out there IMO. It will also make more power per pound across the board. Often spooling better than the poorly designed 78/75 units. (depending on the 78/75 model). Happel picked up 100whp at like boost going from a 78/75 to a T6 S480. (believe it was a gen1 78/75)
Last edited by Forcefed86; Apr 15, 2025 at 08:59 AM.
IMHO, boost doesn’t hurt engines, timing does….
So, keep your timing in check and add boost until it seems to run out as mentioned above.
Then, slowly add in timing to get the power you’re looking for while reading the plugs after every change.
But, I’m no expert.
Best to you!
So, keep your timing in check and add boost until it seems to run out as mentioned above.
Then, slowly add in timing to get the power you’re looking for while reading the plugs after every change.
But, I’m no expert.
Best to you!
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That is a pretty small turbo and your engine should easily handle 1000rwhp which that turbo won't make. Get used to that turbo and then upgrade to something bigger when you are ready.
We are always conservative with timing on turbo LS setups and run only pump e85. Don't get greedy with timing and always run as much octane as possible if pushing it. The turbo inlet should be getting the coldest air available as the intercooler can only do as much work as it can.... so don't grab hot underhood heated air as your source.
We are always conservative with timing on turbo LS setups and run only pump e85. Don't get greedy with timing and always run as much octane as possible if pushing it. The turbo inlet should be getting the coldest air available as the intercooler can only do as much work as it can.... so don't grab hot underhood heated air as your source.
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So, now that I'm getting a little better hang of tuning my Terminator X system (and being really conservative about it), I'm eventually going to want to turn the boost up. Question is, I am not sure what a reasonable boost limit would be. This is my first turbo build, and I'd like to not murder my engine pushing the envelope.
So, those of you with more experience than me, chime in on where you'd put a hard limit with this combo: aluminum LH6 block, Summit ProLS pistons and rods, stock crank, AFR enforcer heads, BTR small bore LS9 style gaskets and ARP head studs. Turbo is a VSRacing Gen 2.5 78/75 with a billet wheel 1.25 A/R, 4" dump pipe.
I myself was thinking 22PSI, but maybe it could hit 25? I think the turbo will be out of steam at 25 lbs. Either way, can the short block handle it?
So, those of you with more experience than me, chime in on where you'd put a hard limit with this combo: aluminum LH6 block, Summit ProLS pistons and rods, stock crank, AFR enforcer heads, BTR small bore LS9 style gaskets and ARP head studs. Turbo is a VSRacing Gen 2.5 78/75 with a billet wheel 1.25 A/R, 4" dump pipe.
I myself was thinking 22PSI, but maybe it could hit 25? I think the turbo will be out of steam at 25 lbs. Either way, can the short block handle it?
Thread Starter
Joined: Apr 2006
Posts: 3,890
Likes: 1,137
From: Grand Rapids, Michigan
I've been running 16-18 lbs. of boost with no issues, but my timing has been very soft, I believe around 14.7 degrees. Which is fine with me, I'm still sorting this car out and don't need to put all the power down at once. I figured a bigger turbo was in the cards eventually, not sure if I want to go up to a T-6, though. This is and will always be a 99% street car and it's already got more power than it can put to the ground, but I eventually would like to see 1000+ to the wheels just because. I know there are a few T4 frame turbos that will get me there.
I've been running 16-18 lbs. of boost with no issues, but my timing has been very soft, I believe around 14.7 degrees. Which is fine with me, I'm still sorting this car out and don't need to put all the power down at once. I figured a bigger turbo was in the cards eventually, not sure if I want to go up to a T-6, though. This is and will always be a 99% street car and it's already got more power than it can put to the ground, but I eventually would like to see 1000+ to the wheels just because. I know there are a few T4 frame turbos that will get me there.
. I figured a bigger turbo was in the cards eventually, not sure if I want to go up to a T-6, though. This is and will always be a 99% street car and it's already got more power than it can put to the ground, but I eventually would like to see 1000+ to the wheels just because. I know there are a few T4 frame turbos that will get me there.
A properly sized T6 will not only make that 1200+ easily but it'll have room to grow. That T4 would be damn near at the limits and back pressure would way up in the clouds
Well I will say that I just upgraded my T6 turbo a few days ago I went from a VS Racing 80mm to a True North Turbo 88mm with a 88x96 9 blade turbine wheel in a 1.15 AR housing and that thing lights off faster than my 80mm ever could dream of. Timing I'm about 14* @ 17 psi on 91 pump with meth. You could definitely bump it up a bit
I've been running 16-18 lbs. of boost with no issues, but my timing has been very soft, I believe around 14.7 degrees. Which is fine with me, I'm still sorting this car out and don't need to put all the power down at once. I figured a bigger turbo was in the cards eventually, not sure if I want to go up to a T-6, though. This is and will always be a 99% street car and it's already got more power than it can put to the ground, but I eventually would like to see 1000+ to the wheels just because. I know there are a few T4 frame turbos that will get me there.
I always planned ahead for the goofy huge turbo. Going straight T6 is just more economical/practical IMO. Vband housings save a lot of room/weight as well and are pretty common these days.
I've been running 16-18 lbs. of boost with no issues, but my timing has been very soft, I believe around 14.7 degrees. Which is fine with me, I'm still sorting this car out and don't need to put all the power down at once. I figured a bigger turbo was in the cards eventually, not sure if I want to go up to a T-6, though. This is and will always be a 99% street car and it's already got more power than it can put to the ground, but I eventually would like to see 1000+ to the wheels just because. I know there are a few T4 frame turbos that will get me there.
I want to say it was a PTE turbo with an 80mm compressor and 96mm turbine in a T4 housing.
It was definitely smaller than a typical T6 turbo, Turbo Tech used them on their kits and the turbo was placed down low next to the engine on the passenger side so no way it was a T6.
Honestly, I'd leave it alone or maybe pony up for a PT 7675, those little bastards make a lot of power for what they are.
PT claims 1,450hp on their website so even if it only got you to 1,200hp you'd be able to hit your 1,000whp goal.
Gotta remember they toss those wheel specs around a lot and they mean different things form different manufacturers. A “96” wheel is an 87/96. The exducer is an 87 which is how most (aside form borg) rate the wheels. Soo the first unit mentioned would be an 80/87 if you are comparing like exh wheel diameters. (really closer to an 86mm when I measured them)
As for PT stating the 76/75 is a 1450hp turbo… That’s also massaged information. It won’t make 1450 on an LS. Though it might on a 2 liter engine. Precision and the like are some of the best units out there no question. But they are 2x the price and half as durable in my experience. Super sensitive to material being passed though the bearings etc. I’d run a pre-turbo filter if I invested in a precision. Personally, I think a quality S400 frame setup with the 87/96 exh wheel is a much better bang for your buck for a “1000whp” turbo. With a Borg (or s400 clone) I wouldn’t run smaller than the 87/96mm exh wheel with that goal on a v8.
The PT 8284 promod turbo is a bad little T4 if budget isn’t an issue. It will do an honest 1350 crank hp on a 5.3.
As for PT stating the 76/75 is a 1450hp turbo… That’s also massaged information. It won’t make 1450 on an LS. Though it might on a 2 liter engine. Precision and the like are some of the best units out there no question. But they are 2x the price and half as durable in my experience. Super sensitive to material being passed though the bearings etc. I’d run a pre-turbo filter if I invested in a precision. Personally, I think a quality S400 frame setup with the 87/96 exh wheel is a much better bang for your buck for a “1000whp” turbo. With a Borg (or s400 clone) I wouldn’t run smaller than the 87/96mm exh wheel with that goal on a v8.
The PT 8284 promod turbo is a bad little T4 if budget isn’t an issue. It will do an honest 1350 crank hp on a 5.3.
Gotta remember they toss those wheel specs around a lot and they mean different things form different manufacturers. A “96” wheel is an 87/96. The exducer is an 87 which is how most (aside form borg) rate the wheels. Soo the first unit mentioned would be an 80/87 if you are comparing like exh wheel diameters. (really closer to an 86mm when I measured them)
As for PT stating the 76/75 is a 1450hp turbo… That’s also massaged information. It won’t make 1450 on an LS. Though it might on a 2 liter engine. Precision and the like are some of the best units out there no question. But they are 2x the price and half as durable in my experience. Super sensitive to material being passed though the bearings etc. I’d run a pre-turbo filter if I invested in a precision. Personally, I think a quality S400 frame setup with the 87/96 exh wheel is a much better bang for your buck for a “1000whp” turbo. With a Borg (or s400 clone) I wouldn’t run smaller than the 87/96mm exh wheel with that goal on a v8.
The PT 8284 promod turbo is a bad little T4 if budget isn’t an issue. It will do an honest 1350 crank hp on a 5.3.
As for PT stating the 76/75 is a 1450hp turbo… That’s also massaged information. It won’t make 1450 on an LS. Though it might on a 2 liter engine. Precision and the like are some of the best units out there no question. But they are 2x the price and half as durable in my experience. Super sensitive to material being passed though the bearings etc. I’d run a pre-turbo filter if I invested in a precision. Personally, I think a quality S400 frame setup with the 87/96 exh wheel is a much better bang for your buck for a “1000whp” turbo. With a Borg (or s400 clone) I wouldn’t run smaller than the 87/96mm exh wheel with that goal on a v8.
The PT 8284 promod turbo is a bad little T4 if budget isn’t an issue. It will do an honest 1350 crank hp on a 5.3.
