I currently have a 2004 GTO with a forged LQ4 408.

Wiseco -8cc pistons
Eagle Rods
K1 crank
Trickflow As-cast 220 heads (62cc)
.051 gasket
etc...

Compression is ~11.6:1

I would like to run somewhere around 10 PSI of boost without meth injection and I'm trying to decide how best to make that happen. - Haven't decided on whether to turbo or supercharge yet, but leaning towards mid-size twin turbo.

If I were to keep my current heads, I feel like I would need to replace the pistons with something more boost friendly for a lower compression, but obviously that is a pretty involved process considering I just rebuilt it about a year ago and the build only has about 1500 miles on it.

If instead, I replaced the heads with something like 317, or aftermarket 72cc chamber heads I could get the compression down to ~10.4:1, but I'm not sure if that is low enough to safely run 10 psi on 93 octane fuel.

I didn't mention current cam specs because I would be buying a new cam to compliment whichever route I take.

Personally, I'm leaning toward the head swap, but I'm open to suggestions.

 

Why no meth? Kits are cheap, easy to hookup, and you can refill for cheap from any parts store and most gas stations, not to mention the huge huge safety margin.

 

Added complexity, system failure could potentially lead to engine failure, don't want to have a system I need to refill frequently (gas is bad enough...LOL) and that's also why I want to go FI instead of just putting a big shot of nitrous on it.

 

Swap on LS3 heads so you don't notice the change flow from the 220s. They have the same size chambers as the 317s.

 

Putting myself in your shoes I would experiment first, but that's me, in your shoes. & I would also like to know the low-down on the cam specs. Also, how much timing you can get away with now is an important factor for later on.

I would build the system to run efficiently at close to your target max manifold pressure & the engine as is as a start. Ofcourse get your fuel system up to snuff so that isn't an issue. Then start off at 4psi with very low timing & work your way up in pressure as you learn the safe timing limits. If you get to say 8psi & no matter how little timing you run it still get knock then you know something else needs to be addressed. Whether it's static/dynamic compression, chamber shape, fuel delivery (meaning how good the injector is atomizing the fuel) etc. since you are contemplating doing the head swap, doing it later, if absolutely necessary, isn't that much of a big deal.

You may find ~8psi to be a limit, it may not. If it is & it makes your target power goal, are you going to cry if your gauge doesn't read 10psi?

Either way you go it will be a riot.

 

4032 pistons hate detonation, I'd either drop the CR or just add a meth kit and get the best of both worlds

 

I like the way you think gtfoxy. I hadn't really considered it that way.

Truly, I'm not shooting for an astronomical HP number. I dont care what the boost gauge says, I just want 650-700 HP and a consistent mid-low 10's car. It makes 475 N/A as is. Current cam is a 239/247 112+0. It has a lot of overlap for boost, but.....you never know.

I run about 24* timing to be safe year round on 93 octane. I can run as much as 28* if I have decent weather, but I'm in Florida, so its a no-go in summer.

I could certainly leave the compression alone, though it is a lot high for boost in most people's opinion and maybe just go with a cam that will reduce cylinder pressure and run low boost. Hmmm.

 

They are 2618. Wiseco K394X3 is the current part number.

 

If you like my train of thought allow me to keep it on movin on the rails...

To me finding an ideal combo isn't always an absolute. But for sure the only way to get close is having a baseline. You have that. Once you start changing stuff that baseline is gone. Poof. Where'd it go? I don't know, but it's gone. (Old farmer voice from Barnyard is how that sounds in my head, Ha)

You know you make 475 at the wheels now. Are you looking for 650-700at the wheels or total? It's also a torque monster at almost 500lbft. Given you are running an auto let's start there. If your drivetrain efficiency is between 80-85% that means your motor is making between 560-590HP & 575-610lbft.

General rule for me, with your altitude, is add 8psi & you should "theoretically" gain 50% in power. That would get you close to 840-885HP & 860-915lbft. Which works out to, given the same drivetrain loss, 700-715WHP & 750-775Wlbft.

Timing loss may be a factor but if the timing can be lower & make the power that is not a bad thing. It just means you have less pumping loss increasing the net gain.

If you go & change a bunch of stuff right away your baseline will change. You will never know if later you go to the "turbo" spec'd cam, or even heads, if it really gained you anything. Something is only good or bad when compared to something else. So get your baselines & work smart to know the impacts changes make.

Just to throw it out there, not knowing your current A/F or the total timing curve you are running, you should be able to have it start with an 11.5-11.75:1 running about 12-15* total timing on 4psi. Size your injectors based off a .6BSFC, based off the higher numbers, & see where the duty cycles land once you get it up & running.

You will need to have your converter tightened a bit I would guess. Only way to know is, again, baseline it. If you get locked in second & your blowing the converter away, have it ratcheted up & go from there.

 

That all makes sense to me.

Just for additional info, I typically run a safe 12.7:1 AFR at WOT and around 24* timing in all those cells. I can ramp it up after peak torque to 27* safely, but just leave it alone to make sure there are no issues if I get crap gas. Cruise is more around 35-38* and idle in the mid-high 20's. I run OLMAF with no narrowband O2's, just a wideband. I'm within +/- 3% of 14.7: outside of WOT at any given time and I scan frequently. I can run OLSD also, its fully tuned for it, but like the smoother transitions with the MAF.

Looking to get 650-700 at the wheels...yes and maybe more later if the budget allows for any necessary adjustments or I throw some race fuel in and turn up the wick a bit. I'm certainly not an HP junky so I wont be running ragged edge on anything...getting too old for all that, but I do like to play a bit

This converter is the tightest 3600 I've ever run. It really acts more like a 3000 IMO. Its the third one I've had on the car and this one is tighter than the 3K I had before. It's a billet performabuilt unit.

But totally get your point. You have to start somewhere right? Why spend a bunch of cash getting to a new starting point.

 

Exactly. Put the effort into the system & get it up & running.

One last thing to consider is dyno numbers. It's not a bad idea to factor in a few percent on the base when dealing with extrapolated data. It won't hurt you other than jumping a hair in injector.

You'll be limiting changes to your combo with only two things: The fueling of the injector in conjunction with the turbo system.

Both of those are going to have big impacts on the system but maybe not necessarily bad. How the added chamber pressure & increased back pressure plays nice with your squish is more than likely going to limit your timing. Again, less isn't necessarily a bad thing if you can burn what you get in there & transfer the power. Running less timing allows you to lean it out a bit. When being able to make good power on 11.5:1 means a small jump in A/F ratio equates to some nice gains. Find the happy medium & work with it.

Keep in mind too rich can get ugly but that's why you start off with real low boost with what you're doing. You can run it, get some logs & read the plugs. Then go from there.

If you can find a heat range colder it wouldn't be a bad idea. Haven't pushed the compression limits on an LS that high to need them, but it would be something to look in to.

This is where good turbine wheels come in. Limiting how much heat stays in the cylinder makes for a happy motor when pushing the compression limits.

Your motor would love it some E85, BTW.