What do you guys think about this twin turbo LT1?
08-18-2018, 03:40 PM
#1
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What do you guys think about this twin turbo LT1?
I hope to draw on the experience of members who have built or who have hired someone to build a twin turbo LT1 for their Z. I have an opportunity to purchase a recently built, twin turbo 383 LT1 engine backed by a PerfomaBuilt Level Three 4L60E transmission. The build includes a forged crank, as well as forged rods and pistons. The assembly comes topped off with Edelebrock ETech 170 heads, Holly stand alone EFI, and Garrett water cooled turbos. I was told that the setup put down 500 WHP on 12lbs boost before being put up for sale. Knowing what you know about the cost of these components but also considering that the build was done by a local shop with decent yelp reviews but no real reputation for performance builds. What would you be willing to offer the owner for the setup if you were in my shoes? Thank you for all for your advice.
08-19-2018, 04:31 PM
#2
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Re: What do you guys think about this twin turbo LT1?
In my opinion (I don’t have any experience with LT1s or turbo LT1 builds, but I do have a built turbo car with a standalone EFI system and a lot of generally applicable knowledge):
500rwhp from a forged 383 on 12psi? I smell a significant flow restriction somewhere between the turbos and the cylinders. That 383 should be 350-400rwhp by itself (assuming low compression for boost and an appropriate, mild-to-medium cam choice), and an honest 12psi in the cylinder should almost double that — I’d be looking for at least 600 and preferably close to 700rwhp on a build like that, assuming you can still build 12psi once it can flow. The turbos might be too small for that though. Do you know what size the turbos are?
The fact that it’s getting only 500 down tells me that the cylinders are only seeing 4-6psi. Something between the turbos and the cylinders is cutting the boost down by 50-70%.
Resolving the flow restriction could involve intake plumbing, throttle body, intake manifold, heads, cam, valvetrain, or any combination of those. That’s a lot of places to investigate!
Also, it’s possible that it has a REALLY conservative tune and no flow restrictions. That’d be a little weird, though — who leaves 100+ horsepower on the table? I’d rather have fuel and timing set right and use a different spring in the wastegate to keep boost down.
Also, that 4L60E isn’t going to last, especially if the flow restriction gets resolved. Almost nobody can build a robust ‘60E.
As such, I don’t think I’d buy that particular combo at any price.
500rwhp from a forged 383 on 12psi? I smell a significant flow restriction somewhere between the turbos and the cylinders. That 383 should be 350-400rwhp by itself (assuming low compression for boost and an appropriate, mild-to-medium cam choice), and an honest 12psi in the cylinder should almost double that — I’d be looking for at least 600 and preferably close to 700rwhp on a build like that, assuming you can still build 12psi once it can flow. The turbos might be too small for that though. Do you know what size the turbos are?
The fact that it’s getting only 500 down tells me that the cylinders are only seeing 4-6psi. Something between the turbos and the cylinders is cutting the boost down by 50-70%.
Resolving the flow restriction could involve intake plumbing, throttle body, intake manifold, heads, cam, valvetrain, or any combination of those. That’s a lot of places to investigate!
Also, it’s possible that it has a REALLY conservative tune and no flow restrictions. That’d be a little weird, though — who leaves 100+ horsepower on the table? I’d rather have fuel and timing set right and use a different spring in the wastegate to keep boost down.
Also, that 4L60E isn’t going to last, especially if the flow restriction gets resolved. Almost nobody can build a robust ‘60E.
As such, I don’t think I’d buy that particular combo at any price.
Last edited by JakeRobb; 08-19-2018 at 04:35 PM.
08-19-2018, 08:15 PM
#3
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Re: What do you guys think about this twin turbo LT1?
I think the out-of-the-box Edelbrock E-trek 170cc heads only flow a bit more than a stock LT1 head. I've seen them referred to as an upgraded, raised runner, aluminum version of the Gen 1 SBC Vortec heads, and flowing 238 CFM @ 0.600”. Could be the flow restriction. High boost in the intake, but it's eaten up as pressure drop through the heads. At $875 each, $1,750 for the pair, they could have had a stock pair of LT1 heads with a LE3 280 CFM port job, good for ~420 rwHP without forced induction.
I make the above comment based on limited hands-on with boost..... I'm a nitrous guy. Maybe some of the forced induction guys, if they ever come here any more, can clear this up.
Other thing seems odd.... I'm not even sure there's an LT1 (Gen 2 SBC) version of the E-tek head, which means someone would have to modify the coolant ports in the heads to match the ports in the LT1 block. That would allow them to use a Vortec carb manifold if it’s running a conventional distributor and the Holley Sniper throttle body EFI. But if it’s running a distributor and Gen 1 heads, might have been more practical to use a Gen 1 block.
I make the above comment based on limited hands-on with boost..... I'm a nitrous guy. Maybe some of the forced induction guys, if they ever come here any more, can clear this up.
Other thing seems odd.... I'm not even sure there's an LT1 (Gen 2 SBC) version of the E-tek head, which means someone would have to modify the coolant ports in the heads to match the ports in the LT1 block. That would allow them to use a Vortec carb manifold if it’s running a conventional distributor and the Holley Sniper throttle body EFI. But if it’s running a distributor and Gen 1 heads, might have been more practical to use a Gen 1 block.
08-20-2018, 06:30 PM
#4
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Location: San Diego, Ca, USA
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Re: What do you guys think about this twin turbo LT1?
I agree that the dyno numbers are quite low. I have a 350 with Advance Induction 190 LT1 heads and cam and a Vortech supercharger making 8-9 lbs boost and it made 470 rwhp. The engine was also somewhat restricted with 1 5/8" shorty headers. A 383 should make close to 520 rwhp based on displacement alone and substantially more with the additional boost. I agree that there is a restriction in the heads causing the horsepower to be capped and the boost to increase over what it would be with a better flowing head. I'm curious at what rpm the power peaked? I originally had a Flowmaster 50 muffler on my car and the power peaked 25 hp lower and at only 4800 rpm. After switching to a lower restriction Dynomax muffler the peak was 1000 rpm higher at 5800 rpm and power was up 25 hp. I'm sure an intake restriction would act the same way.
08-21-2018, 07:44 PM
#5
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Re: What do you guys think about this twin turbo LT1?
Thank you all for your posts. This is exactly the insight that I am looking for. I thought that 500 WHP was low. The owner stated that torque converter problems (before the PerformaBuilt tranny) prevented any clean dyno runs. I am honestly more concerned about the custom twin turbo system. While I confirmed that top tier components were used, plumbing may be the actual culprit with the turbos mounted low enough behind the motor that a return oil pump is required. Still, while considering the possible challenges to extract the system’s full potential. I could still use some help coming up with a fair offer for a fellow enthusiast who invested a lot of time and money in this setup. Thank you.
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