MaxVelocity

I have read ZEX-Z's thread on his 400whp cammed LS1 with decked stock port 243 heads, and an LS3 intake manifold using a set of adapter plates to go from the cathedral port heads to a rectangle port intake manifold, and I'm wondering if anyone else has done this with better heads, more timing, and more rpms? I searched and only found others asking and no other real results.

Do the adapters hurt flow significantly on a ported head/big cam N/A combo with power levels like 540-550hp @ crank around 6600ish? ZEX-Z's pulls stopped at 5900, and that timing curve was tuned for 91 octane (I think) running stock port 243s.

Would the adapter plates allow that LS3 intake to keep flowing with 310cfm heads, 93 octane, and 6600rpms?

With supposedly 440whp, I figure my LS2 has gotta have some head work done and I KNOW it's got a healthy cam because it idles pretty rough at about 950rpm and doesn't like to drop below about 700rpm. I can pump the brake pedal a few times at idle and feel it start to lose vacuum assist. I'm guessing my stock LS2 intake is costing me 25-30hp, but I don't want to spend $1000 on a FAST when maybe the adapted LS3 that I already have will work, or a Dorman 615-901 with a bit of DIY porting and maybe radius rods will get there for sure.

Does anyone have any real evidence that the direction change and extra 1/2" of runner length induced by the cathedral head to rectangle intake adapters causes problems in the 550 CRANK hp/480WHP range?


*note: All of these numbers I'm throwing around come from either this forum, major auto media, or the totally honest guy who sold me the car(not sarcastic, he was great). I'm no dyno shop, and certainly not an expert on LS stuff....yet.

Spaceme1117

Instead of adapter plates, it would be better to get a set of used LS3 heads and LS3 intake manifold and do a top end swap.

This is basically what the L76 engine is in the Pontiac G8 GT. It is the same 6.0L LS block as the LS2. But it has the LS3 intake manifold. Has the LS3 heads but the valves are different (heavier I think). It has a smaller cam and AFM. I used to have G8 GT that I removed the AFM and installed an LS2 cam in. It ran great and was essentially and LS2 with LS3 top end.

MaxVelocity

The cost of used LS3 heads is about $800 a set here, which is about the same as the cost of a stage 1 CnC ported cathedral port set that have been rebuilt with the springs and retainers that I already have, but with the LS3 castings I'd gain about 10cfm at .600" lift, and the charge would be stalling after .500" lift. I'd probably lose a bit of low speed velocity and make my cam even more peaky with very little gain at the very top of the curve over a ported 243 casting. I need to go on and pull my intake to check for port work because with the car making 440whp, I'd guess that work has been done, and if it has, then there is even more reason to wonder about LS3 intake performance with the spacers. The other option is a ported Dorman LS2 with radius rods.

Not quite applicable here, but check this action out:
http://www.superchevy.com/how-to/ght...s-head-to-head

DMITTZ

How about a FAST 102, should work nice with ported 243 heads. If your on a tighter budget maybe try finding a used fast 90, I got a ported fast 90 with a LS2 fuel rail adapter kit off ebay for $500 to go with my ported 243 heads.

PCMusicGuy

I doubt you'll get much real world feedback here on the adapters. Most swaps are to the FAST or the LS3 intake and heads. The cathedral port heads aren't really the weak link from the stock LS2. It is the intake (air filter assembly and manifold) and the exhaust manifolds. If you already have the LS3 intake manifold, why not buy some adapters and find out?