LSJ / L61 hybrid - Boost Forum

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LSJ / L61 hybrid
Monday, February 12, 2018 12:48 PM
Alright so I'm just wrapping up with some major winter rejigs on the car.
Next in sight is finally getting a built engine to swap into the cav so I can crank up the boost.
My original idea was to find a donor LSJ and swap the internals into my L61 with a 2.2 eagle crank and external trigger wheel. But now I'm considering swapping either the LSJ head, or the whole damn thing into the car.

I guess my question is, is it worth the extra work to swap the LSJ into the cav, or is my original route the one of least resistance? I would rather tear down and rebuild the engine on a stand instead of a whole whack of in-car wiring. And since I'm only shooting for the 4-500hp range, I'm sure the L61 could handle that no problem with some beefed up guts.

Turbocharged '04 Cavalier

Re: LSJ / L61 hybrid
Tuesday, February 13, 2018 9:38 AM
You can't swap cranks like that. You need to use either complete rotating assembly in either block. The lsj crank has the wrong trigger wheel so you'd have to address that either way. Easiest way is forged rods and pistons in the l61 and run it. A little harder but doable would be running the LSJ head with that forged l61 bottom end.

The rest is my personal opinion:

Running the complete LSJ is possible but just more issues. The up-side to an LSJ/LSJ crank is that you get much better clutch options. Ie, the GMPP upgraded clutch that is cheap and reliable at those numbers (assuming torque is in the same ballpark). I'll get flamed for this, but I don't suggest an external trigger wheel for the crank sensor (assuming that you plan to use the stock p11 pcm). If you search you'll find that someone makes a bolt-on trigger that goes on the crank internally in place of the original 58x. That's the way to go.

I know guys that built l61s and rev them to 8400rpm and keep making power. If you look at the rod stroke ratios, the LSJ should rev better and be happier up there. It's really up to you. Stock for stock, I miss my LSJ bottom end at 7500rpm. With an LSJ head and cams on both bottom ends, the LSJ bottom end was definitely peppier and more fun. That said, L61 compression is higher and stock sized forged pistons are a lot cheaper than customs for the LSJ to make up the difference. You may say that with boost you should lower compression but at 17psi the LSJ could have used that extra bump that the L61 has. I suggest aiming for at least 10:1 if this is going to be a street/strip motor.

Keep in mind that the LSJ has bigger wrist pins, piston squirters, 8 bolt flywheel (GMPP clutch compatibility), better rod/stroke ratio (if revving is what you're planning to do), and an oil cooler that you could run an adapter on for an aftermarket front mount cooler (the stock cooler doesn't fit with the f23 trans linkage, assuming this is a 5 speed car).

L61 bolts right in, has the right trigger wheel, is easy and cheap to replace.

LSJ head is nice but doesn't really get you anything for the trouble until you port it. It has more meat so that you can port larger than an L61. It also requires you to figure out a cam sensor cover plate (cam sensor is unused by the p11 and doesn't fit over the master cylinder), and you either have to heavily modify your l61 coil pack or convert to LSJ individual coils (which will kill your cruising mileage because you'll always be running in batch fire mode without the fake cam signal from the jbody ICM).

Think that covers most of it.

"In Oldskool we trust"
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