1. The Turbo Forums - The discussion board for both hard core and beginner turbocharged vehicle enthusiasts. Covering everything from stock turbocharger cars, seriously fast drag racers, boats, motorcycles, and daily driver modified turbo cars and trucks.
    To start posting in our forums, and comment on articles and blogs please

    IF YOU ARE AN EXISTING MEMBER: You can retrieve your a password for your account here: click here.

8.1 twin turbo build

Discussion in 'The Turbo "Builds" Board' started by sedgehammer, Jan 1, 2022.

  1. sedgehammer

    Joined:
    Apr 24, 2016
    Hello

    I want to do a twin after long tube header build. Yes, I know it won't spool up like if it was an engine bay mounted. Lots more work to to that. Looking for 600 hp range. Daily driver and will tow a 5th wheel. Will have 4, 2,450 cfm electric radiator fans. Can do 5 if needed

    I know sts makes these types of kits, but nothing for a 8.1, plus they're over 6k. I think that is way to high for what that build is

    Also, since the throttle body size is 4", shouldn't the intercooler be 4" ?

    Thanks

     
    Last edited: Jan 2, 2022
    CSteffen489 likes this.
  2. bbi_turbos

    Joined:
    Dec 8, 2021
    Looking at the factory dyno graph for the 8.1, it makes 340hp @ 4200, so I'll use that for our design point. To get to say 630hp (crank hp, unless you wanted 600 at the ground) you need to increase density 1.85x's, and will need around 65 lbs/ min of airflow.

    15 psi at 75% efficiency going through a 70% efficient intercooler comes out to exactly a 1.85 density ratio.
    Next we need to size the turbine side, 15psi is a 2.02 pressure ratio, assuming the egts have cooled off to ~1300°f at the end of the headers, that leaves us with 29lbs/min corrected exhaust airflow, with a pressure ratio drop of 1.6 needed.

    Looking at the garretts site, a pair of gtx3576s will move 32.5lbs/min at 75% efficiency, with either a 1.06 or 1.01a/r housing, would work. The 1.01 will give you earlier spool.
     
    Disney Lincoln likes this.
  3. B E N

    Joined:
    Nov 22, 2016
    No need for an intercooler that big, they are tough to find. When you are talking pressurized air your diameter requirements per HP go down. Get a core that's a little oversized for your power level since you are towing, heat soak will be a thing. Treadstone makes some nicer stuff that is still affordably priced. If I was doing what you are I would look for a used intercooler out of one of the Duramax trucks, you know it will drop in and they are huge.

    Since you are towing, and the vehicle is a truck you can go larger with the turbos than someone who is running a passenger car, you have better loading than someone with a stripped down drag car.

    In Borg Warner you could look at something like an s362, I don't know what your budget is, if you want on shore or off shore parts. I really think you want a bigger hotside on this, and oversized gates, towing long grades with too small of turbos is going to lead to trouble.
     
    Last edited: Jan 2, 2022
    Mnlx likes this.
  4. Mnlx

    Joined:
    Sep 20, 2009
    I personally would avoid the urge to try to make diesel like torque at low rpm. The stock lower end isn't built for that, and with the large bore, elevated cyl pressures are really going to push the limits of the rods. Choose turbos that make usable boost at 3k, and let it rev a little.
     
    38Pontiac likes this.
  5. sedgehammer

    Joined:
    Apr 24, 2016
    Thanks for the reply! That 15 psi would be if I did nothing else to the motor. I was hoping it wouldn't be that much boost. From my reading I'd get about 100HP from removing the restrictions from the intake, cam, long tube headers and a tune. Further reading shows that this motor stock can only handle 6 PSI or it'll crack pistons, so what size would I need if I wanted to stay around 6 PSI and what size if I still wanted that 600 HP ?
     
  6. sedgehammer

    Joined:
    Apr 24, 2016
    Good idea on the intercooler. I'll look

    I'm stuck at the 6 PSI if I don't do a rebuild and get some better pistons and rods. I will do that at some point, but for now I will try to do it within the 6PSI parameters

    What psi would that build ?

    Thanks
     
  7. sedgehammer

    Joined:
    Apr 24, 2016
    You know what sizes they would be ?

    That would take a cam grind to do that also. Ideas on that ?

    Thanks
     
  8. B E N

    Joined:
    Nov 22, 2016
    6psi is going up completely transform the truck. Start there, se how you feel about it.
     
  9. bbi_turbos

    Joined:
    Dec 8, 2021
    Extra 100hp means ve% bumps up to 94%, vs stock 78%.

    6psi is 1.4pr, @75% efficient temp is 131°f coming out of turbos. After intercooler your at a 1.22 density ratio assuming 2psi pressure drop in the intercooler.

    340hp + another 100hp from bolt ons is 440hp, 440hp x 1.22 density ratio= 537hp on 6psi of boost. Turbos need to move 28lbs/min @ 1.4 pressure ratio.

    When we move to the exhaust side we run into "trouble". Because the engine is now running a higher volume of air at a lower pressure, the exhaust corrected flow shoots way up to around 35-40lbs/min. A 1.4 pressure ratio on the compressor side will require a 1.25 drop on the turbine side, so the turbine needs to flow 35-40lbs/ min at 1.25 pressure ratio. This will take something like a gt45 turbine side with a 1.44 housing to properly handle.

    You will be better off keeping the engine as is and running the same gt35 turbos at 6psi. This will keep the exhaust side in check better, keeping drive pressure low as well.

    Then when you do the rebuild, you can do the bolt ons and bump the turbos up at the same time to keep things in check.

    Low psi and high volume are not what turbos are good at, that's supercharger territory. So your better off to keep volume low and psi up to keep it turbo friendly.
    Hope this helps, I try to help explain the results the math gives so you can make the best decision. The math isn't hard, but it really helps on the hot side stuff, which is really the critical part of the turbo system.
     
  10. sedgehammer

    Joined:
    Apr 24, 2016
    Only way I'll do the turbo mounting is behind the headers, so that's one bolt on unit I'll do. Will also do the intercooler, as I don't want to do any piping twice. Most likely will do the intake, but the cam can wait. That way maybe I'll have enough coins to do a set of none ferrous heads or at least new pistons. Then I can run one of the higher lift cams

    If I was going to run no more than 10 psi even after the build, would it be the same turbos ?

    What waste gates would you run ?

    How about a separate sump/pump/oil cooler for the turbos ?

    Really appreciate your help !
     
  11. C-lab

    Joined:
    Aug 18, 2021
    Good to see another 8.1 guy around here. I’ve got one swapped in a ‘76 K5. I’d like to throw some boost at it in the future. But my plan is to do a cam first. I like the grunt but need more RPM to work with. I’d like to have closer to the top end of the Gen II LT1 I took out.

    Looking forward to seeing your build if it comes together!
     
    sedgehammer likes this.
  12. Weedy

    Joined:
    Jan 31, 2019
    Me too, following now.

    Had an 8.1 but sold it and got a 8.8, going to start with small twins that I own, Dart heads, Dart intake with elbow, forged bottom etc. will upsize turbos if necessary.
     
  13. sedgehammer

    Joined:
    Apr 24, 2016
    What head from dart ?

    Dart intake w/ elbow ?

    Crank or just rods ?

    Thanks for your post !
     
  14. Weedy

    Joined:
    Jan 31, 2019
    Dart 15400170 8.1/8.8 cast iron head, 2.25 x 1.88 valves 320cc
    Dart 41616010 - Dart Dual Plane Intake, haven't picked an elbow yet
    4.5" Eagle forged crankshaft and Manley 6.7 H-beam rods
     
  15. 20112011Cummins

    Joined:
    Jul 4, 2014


    Might be worth skimming through LT's build. He has some dyno runs both NA and at low-ish boost for reference on a stock 8.1

    Does a Duramax intercooler requires a core support swap and an HD hood? That's a good looking truck, I'm in for updates.
     
  16. sedgehammer

    Joined:
    Apr 24, 2016
    Thanks!

    You have part numbers for the crank, rods & pistons ?

    Thanks again
     
  17. Weedy

    Joined:
    Jan 31, 2019
    Dart crank 945645006700
    I chose Manley H beam 6.7" rods with ARP2000 bolts (good for 1200hp) other BBC rods would work too.

    Pistons: my block is a PSI tall deck oem 4.350" bore, I am going with a conservative 4.440 overbore (lots of ring packs avail) I am getting custom ceramic coated 2618 pistons with -30cc dish for a 8, 8.5 to 1 CR I don't have a part number, the machine shop is dealing with procuring them. Diamond or JE.

    The block, crank, heads, studs, valvetrain all came via the usual online suspects over the last couple years. I am building this to power a boat so I have full water intercooling available at all times.

    Intake choices seem to be limited or expensive so Im gonna try the dual plane 4 barrel manifold with an elbow first.

    I have Holley EFI ECU with EGT probes to help with tuning.
     
    sedgehammer likes this.
  18. sedgehammer

    Joined:
    Apr 24, 2016
    With those dart heads, do you need special push rods or do you know if using them on a stock 8.1, would one need push rods ?

    Thanks for all that info !
     
  19. Weedy

    Joined:
    Jan 31, 2019
    If you use stock length valves, I believe so. PDF for heads at dartheads.com I think.

    I made/modified custom valves, 160-480# springs beehives etc. And recently Santa found me some NOS Jesel shaft rockers so for me that bus has left the station, I will measure and order correct length p-rods when that day comes.

    Cheers
     
    Disney Lincoln and sedgehammer like this.
  20. 91turboterror

    Joined:
    Mar 17, 2013
    Santa has been nice to you lol. Jesel is a great upgrade for the piss poor valvetrain on those 8.1 engines. One of the GM engineers that designed the valvetrain on 8.1s was all proud of his work until a friend of mine a GM technician burst his bubble when told that they fail on the regular lol
     
    sedgehammer likes this.
Loading...
Similar Topics - twin turbo build Forum Date
1968 mustang twin turbo coyote build The Turbo "Builds" Board Aug 25, 2021
Build: Ford 521 Twin Turbo The Turbo "Builds" Board Jun 19, 2021
2011 GMC Sierra Denali Twin Turbo Build The Turbo "Builds" Board Apr 6, 2021
Loading...
bridal-shoal