Hey guys, I know this thread is kinda old but I wanted to get some opinions. I have a L98 that is stroked to a 383 with AFR 195 HEADS, ZZ9 cam, Mini-Ram intake, and Doug's shorty headers. I'm thinking of adding a turbo but forced induction is new territory to me, this is my first engine build so I'm learning as much as I can. Going into a 1991 Z28, automatic.

 

I moved your post to a new thread. I'm sure lots of folks will chime in and give you some advice.

One thing missing is what is your compression ratio currently?

-- Joe

 

The engine is currently at the shop being machined but I believe it will be 10.5:1

 

That's a bit much for a turbo build, especially with a zz9 cam. I'd be shooting for closer to 9.25:1 or even lower, depending on the boost.

-- Joe

 

Okay, I don't know a lot about turbos so I'm just kind of testing the waters to see what I'd be able to do if I decide to go down that road. How would I change the compression ratio to 9.25:1? My goal was to build a bada** L98 that was NA but now I'm just kind of wandering into Forced Induction.

 

It really depends on your goal. Set the goal, then pick your poison. A bad @ss L98 means so many different things to people nowadays. NA four banger Honda's are running in the nines these days, so does bad @ss L98 mean beating one of them, or does it mean just an L98 built bad @ss to your specs, meaning what you would consider to be bad @ss...

- Rob

 

Do you have E85 around?

What crank rods and pistons are in it?

if you dont want to change pistons then you could change heads to 75cc chambers. Just buy another afr head.
thicker head gasket can help some.

 

For decent power on turbocharging I would be aiming for 8.5 - 9:0 compression. Most of the stock turbocharged "tuner" vehicles out there are in the neighborhood of 8.2 to 8.9. If you want success and ability to run on pump gas without fighting detonation you want to keep it low.

You can run higher compression with E85 but you will be largely stuck with E85 if you do.

GD

 

Can try it at 10.5 to 1, just be careful with the tune and keep it richer. I would strongly recommend air water intercooler . You can get away with alot more on cold air charge. Air water is most effective

make sure your rad and fans are capable of keeping it cool! Try keeping water temps below 180 if possible. That can help keep detonation at bay

ive seen 10.5:1 late models take a fair amount of boost. My bbc at 10:1 is fine at 12 psi and would take more.

i believe the afr heads are very sensitive to timing. I ran the 195’s on my 401” first generation turbo build and i pulled alot of timing compared to my other set of heads. Now i never dynod that car to see what timing was best but i did try the factory knock sensor system which isnt ideal since my bore was larger than factory. Maybe i was conservative but better safe than sorry. I think on like 14 psi i was down 16-17 deg timing. 34 made best power na on my 383.

 

When you guys run way low compression like that do you undersize the turbos so they spool quicker? I'd imagine anything 8.2-8.9:1 would be awfully laggy when not in boost.

-- Joe

 

Don't really have to with modern turbos. They spool pretty quick generally. We do a ton of Subaru's and all of the 2.5 engines are around 8.2 to 8.4 stock - their stock turbo will push 300 HP at all four wheels. Some forged pistons bump that to 8.9 but we generally use the stock compression forged replacements. We have done builds that produce 500 AWHP on E85 with the turbo spooling around 3600. These engines pull to 7500 before they start to drop off and rev to 8200.

And don't forget - the exhaust volume is relatively unchanged - the swept volume of the engine is still the same, and a loss of compression from 9.5 to 8.5 is only about a 5-6% N/A HP loss overall. The additional boost you can push on the top end though is SIGNIFICANTLY higher. On a 9.5 engine we would be unlikely to go more than 10-12 psi tops. On 8.5 we could easily push 30 psi.

Of course the DCR plays a significant role here also - but generally you don't use HUGE cams with a turbo setup because....... why? You can simply push more boost with lower compression and a larger turbo and get all that top end NA power the big cam would make and a WHOLE LOT MORE, without the low end driveability and poor idle.....

GD