Head swap with no dyno gain...what's wrong?
 

I recently swapped my stock 241 heads for a set of 799s on my '04 GTO. Given they would at minimum raise the compression ratio, I assumed I'd see at least a little gain.

The dyno spit out exactly the same numbers as it did a year ago when I had it tuned for my cam - 347hp/357tq. Same tuner, same dyno, similar weather. I was thinking I might see something around 370 or so.

My setup is fairly simple. TR224-114, PAC 1218s, Summit underdrive pulley, JBA shorties, JBA catted mids, JBA cat back, K&N CAI, ported throttle body, Vigilante 3000 stall. I used stock GM MLS head gaskets with the head swap.

The 799s are unported with stock valves, but I had a Serdi valve job done by a very reputable local machine shop so I trust it was done well and certainly not any more worse than the stock valve job. If 799s are the same casting as 243s, I essentially have an LS6 with a bigger cam. Shouldn't I be seeing more than 350 to the rear wheels?

My initial thought was the tranny might be slipping, but the fluid looks and smells good and it hasn't been giving me any trouble (ran a new PB a couple months ago). I also assume that a tranny problem would have shown up on the dyno.

Thanks for any suggestions.

It's difficult to say without seeing the dyno or the tune. Can you post up the before and after graph?

Get some long tube headers, if you let the air in, it has to get out. You won't make more power without them.

Eh, I put 799 heads on my GTO as well but I have a tr230 cam. I made a whopping 389whp. LOL

Here is a good read for the short tube header crowd.

short tube GTO with cam, poor gains

There's also a guy with shorties with a GTO that made like 460whp with a vrx5. :engarde:

I know I've got some power left on the table with the shorties, but I'm not convinced that's the whole problem since the Z06 had stock manifolds (if that's even an apt comparison). I should have seen something from these heads with the compression bump right?

I'm still waiting on a good copy of the dyno graph. All I have is a crummy phone pic from the other day so I'm not sure it will help much but you can see the old graph and new graph are nearly identical.

http://www.ls1gto.com/forums/attachm...1&d=1266444053

Here's the one from last year:
http://www.dragtimes.com/images_dyno...c-GTO-Dyno.jpg

A4 or M6? What kind of exhaust?

I dont think to expect a 20+ Hp gain from a set of unported heads is reasonable, with a slight bump in compression you are not gonna gain much of anything. The power comes from more air and flow going thru the engine and doing it more efficiently. Now if you have the heads opened up and optimize the setup then you will get the increase. Just my 2cnts

One easy check is ask the tuner what the MAP reading was with key one, engine off, then find out what it is at WOT, 2-3 kPa drop means you have a good setup. If you know the intake tract is flowing good and the MAP drops at WOT you are probably being restricted.

Depending on the cam specs, you may not be scavenging enough with the short tube headers.
You can't compare two completely different cams and say well he got 500whp with shorties, the OP's cam is not the same or may not be the same.

It doesn't matter to me if people use shorties or long tubes, long tubes will always provide better gains.

Vigilantes can eatup some dyno #s. I expect it munched down on that little HP gain you pickedup doing the head swap. I would be more concerned about track #s.

Good info, thank you. I'll check that out.

I have a pretty standard fare cam - Thunder Racing 224/224 .563/.563 114.

I have an 04 m6. I have a tr230 cam, unported 799s, kooks 1 3/4 w/o cats, magnaflow catback, forged bottom end with diamond pistons, eagle rods, etc etc. Compression should be ~11:1. I still could only manage 389whp. Car feels like shit too.

Did you have it retuned after the head swap?

Yep. That's what it was back on the dyno for. He did 4 or 5 pulls with tweaks between before he decided he wasn't going to get anything else out of it.

Good to know that I'm not the only GTO owner with a car that runs like shit. LOL

:lol: I wouldn't say mine runs like shit, I'm just not getting what I thought I would out of it. I'm try to see if I can borrow an HP Tuners setup this weekend and log it.

I'm hearing two different things at this point. One camp says you should see 15-20 hp from a 243/799 head swap even in their unported state and the other says you won't see jack. I guess both are possible depending on the setup.

Then again, maybe posting your mods on a forum automatically adds 15-20hp because no one wants to admit they didn't gain anything ;)

Click my signature for my setup. Unported 243 heads milled .020" for 11:1 compression with a 224 cam. LS7 clutch and no underdrive pulley, 80 hp gain over boltons.