High low . Wanting to build a 600 - 650 whp 8.1 . I want to do this as cheap as possible , as if this motor doesn't hold up , I'll build a gen 6 that will . In thinking injectors , removing the baffle in the intake and only a cam if I must . This motor has piss poor heads , so basically need to force feed

I'm considering putting it up front passenger side . Will have a 04 duramax intercooler . 15 psi max , but prefer to be under 10 due to rings not being gapped and poorer factory pistons . Was thinking a single , but wouldn't rule out twins

This is for towing and a lil'bit of fun , but mainly towing , so high torque at lower rpms is best

 

The 8.1 was factory rated at 340hp @ 4200rpms, which works out to 81% ve. So that means 14psi required to make 650hp. Exhaust flow is 54lbs/min at 1.55 expansion ratio if your looking at maps.

From here you have a decision to make, twins that spool earlier, or a single that is simpler.

Twins a pair of knockoff g30-660 with 1.21 housings will do the job.

Single a standard s475/96/1.32.

I think the single would introduce more heat under the hood since there will be more piping, but its also probably the cheapest option.

 

There’s a world of difference between a quarter mile sprint and lugging 30k up a mountainside. You’re asking your motor to live under stress it was never designed for. I would advise a Duramax/Allison combo, even though initial purchase price might be a little painful. ‍♂️

 

I had a 16 duramax . i do not want that expenses of the repairs that comes with those motors . The 8.1 is a real work horse of a motor . Yes , the duramax will get better economy , but the difference in fuel and maintenance costs , the 8.1 is cheaper . I know of several people that have both and use them for daily hauling/work trucks

 

They may be work horses, but they need to be run like a gas engine... meaning they were never designed to be run at low rpm with the massive torque production forced induction adds, especially with the weak parts, and huge bore. You may get away a few psi at low rpm, but with 10-15 psi, it needs to come in after peak torque, and let it rev. Just my opinion.

 

the heads barely suck, so they suck ass . so you can't rev and run in the higher rpm ranges like most engines . all that power is below 3,600 rpms

 

i used to work on schwans trucks .maybe the hd version is configured differently but the lp injected engines advertised 404hp and did most of their work between 3000-5000 rpm.

 

I had an 02 1 ton with the 8100 with 30 some thousand miles and I wouldn't of traded that truck for any diesel, it pulled my 40 ft race trailer like it wasn't there. I've had all different brands of diesels and worked on them for over 20 yrs and with the expense of just a service I can't see the benefit.I'm no ford guy but I did have a 2000 1 ton 7.3, standard shift that really pulled good, plus it got 16 mpg with a trailer.These newer diesels suck to work on and cost you out the a$$ when you have a problem.

 

i was going to put a cummins in it , but i think now i'll put a turbo on it instead and you are very correct on that comment

 

BBI , if starting with some mods that made it 450 HP , how much boost for 650 HP ? Thanks

 

8psi in the manifold, 10ish at the turbo.

Assuming you can get to 100% ve at the same 4200rpms as the factory rating.

 

8.1 pistons like to chip, spend the gasket money and gap the rings for insurance. A 8.8 running core would be a better starting place, people have been cnc porting the 8.8 iron heads, perhaps its affordable now.

 

There's a guy on youtube Lawrence LT Tolman.

He put a single turbo in his bone stock 8.1L

Cranked it to 18 psi with no problems. Factory everything. 15psi is his normal boost as i recall. The only thing he modified was the Pcv breather port under the intake, as i recall he made the hole larger, and added a baffle. As it began to push oil amd blow smoke. Other than that. It made stupid reliable power

On 91 octane, and 13-14 psi he said it made 580hp and 905lbs of torque.

On e85 and 18 psi, it made 817hp and 1140 tq at 3500rpm

 

Yup , seen his vids . i'll be using this for towing , so not sure his luck will be mine . He runs it hard and then it has a chance to cool down . The heat and boost isn't like lugging up a mountain pass or a long pull out of a low valley . I know some guys that had RVs and they put a turbo on theirs . all ended up in extra parts . Lot's of them . I want to keep the psi as low as possible to build 700ish HP . I will tho gap the pistons . I think this was their main failure , but time will tell

 

gap the rings not the pistons lol. Exhaust manifold would probably stand up to the heat better than headers for the hotside

 

I'd get what ever size turbo that'll operate in its best efficiency range for your goal and for the rpm you're pulling. Then because you're towing and may be going slower up grades, an air to water intercooler may work better. And you have the bed to fit a huge reservoir. And or maybe some meth injection for long pulls. But I'm no expert.

700 hp seems like a lot for mostly stock, and reliably towing. His figure of 900ftlbs at 3500rpm on a 91 octane tune seemed ideal. Torque is what moves stuff after all.

My old man put a magnuson blower on his 2019 tundra and pulled a 24ft camper loaded from Virginia to Oregon. I think those are tuned at 7psi

A big oil cooler would probably be worthwhile for towing under boost

 

Torque is what makes things move... and breaks parts. Horsepower is how fast you get it done. @Mnlx nailed it, let it rev a little, make the power up top... let the gearing be responsible for getting it moving. You are pulling those hills at RPM without the turbo, it's how the engine was designed to work under load.

All the high towing capacity diesel trucks, and most semi trucks use air to air. You don't need a ton of speed to make them work, especially not with those giant mechanical fans pulling on the cooling stack.

 

Even stationary, or slow moving constant load heavy equipment run air to air for the most part these days. They have a large fan that's hydraulically, or engine driven.

 

As far as gapping: Towing gives a lot of time to build and not shed heat piston wise. I'm thinking towing you will be very happy with less than 10 psi. I'd stick it at 6 psi and see how it is. Get the tune right and conservative especially at peak torque. When the trailer isn't on the back, put 12 psi to it and enjoy the hp. JMO.
Now if the engine is getting pulled apart anyhow, well gap them while you are in there.