Looking to get more out of my setup and looking for advice. Staying NA. With a tune I was only at 406 rwhp which seems low.

383 all forged internals
11.1 compression
PRC as cast 225. 62cc
EPS 236/242 .604/.604. 113 +3
1.75 headers to Y-pipe to 3.5 catback, no cats.
ls6 intake and TB
6 speed
8.8 rear 3.55

 

ls6 intake and TB your intake and tb is not helping. You should be making a lot more hp than 406. See my sig.

 

I would think you would be around 440ish the way it sits but like stated get rid of that Tb and intake. Fast 102/102. Which headers and y pipe do you have? Maybe a flowmaster merge at the end of you y pipe would help.

 

I have a swap, headers are JTR basically long tubes. 1.75 into 3 inch collectors, custom y pipe, into a 3 inch catback. My exhaust is too low in one spot so I plan to get that fixed, would going to 3.5 inch single over the 3 inch improve things much with my setup?

As for the intake, best I can do is the Fast 92, the 102 requires too much cutting of the hood.

I’ve read people with 383s closer to 500 rwhp, is my cam/heads too small to get closer to that? I know it needs to breathe better, guessing I’ll need to go with a ported fast 92. I am also SD tuned but not sure that matters much.

 

Dynojet or mustang dyno?
are you in the mountains?

 

Dynojet and only around 1500 ft elevation

 

The Fast 102 is just a little bit taller than the LS6. It will fit under a 5 gen f body cowl and hood. I thought the Fast 92 and 102 were the same height.

As far as "cam/heads too small" you have bigger cam, head and displacement than I do, so I don't think that is your problem.

 

What’s the build bore/ stroke and deck height?
gasket thickness?
what lifters and did you measure the pushrods what’s the pre load? What injectors?
all cylinders firing?

not sure what the ls6 78mm throttle body flows and how much it’ll support. I’m sure someone does on here though.

 

Can you post a graph?

 

Elevation isn’t great but that’s not what’s killing the power that much compared to what he should be. Like Kcs said post a graph. Maybe it’s falling off early for some reason too. Also what car is this engine in?

 

Depending on the car, that little bit may cause issues. The FAST in my Miata **barely** fits under the hood. It actually rubs a teeny bit in places. Some of us don’t want to cut our hoods or run non-stock appearing hoods.

 

3.905 bore X 4.00 stroke
.045 cometic, 3.910 bore
3cc domed pistons with relief cuts
Fast 36lb injectors
not sure on deck height, I didn’t build the long block, just know what’s in it.

it’s in an RX7, the fast 92 will require some firewall manipulation, will probably go with a Motion race TB since they are shorter and will allow more space between it and the hood although up for recommendations there too.

I had a print out of the dyno results but can’t find it, I remember it being decent across the board though.

 

Have you pulled the plugs. I’m wondering if you have a cylinder not firing.

 

I haven’t pulled them recently but it was firing on all cylinders last time I checked. Definitely when it was dyno’d. Guess I am looking for what mods will help the most, seems intake makes the most sense.

 

I still think you have something else going on. Every setup is different but still a 383 with a nice set of heads and cam like that should still be pumping out more then 400 hp. The dyno graphs would help still if you can find it. There’s so many different factors and things it could be like valve float, exhaust, intake setup up, tune related things, converter locked and unlock for autos and weird things that might pop up on a dyno graph.

 

Did you happen to check to see if the valves are sealing as the engine is rotated over. Long pushrods will hold the valves off the seat. Putting a engine together without checking everything usually ends bad.

 

Check your throttle body cable to make sure it's opening all the way. Also dual exhaust would help.

 

As BBond stated, the Ls6 Intake and TB. You got 383 cubes, good flowing heads, and only 78mm for incoming air. A choker.

The other things by others are also worthy of consideration.

Never heard of JRT Headers and since a Swap, the design may or may not be as free flowing and they're 1 3/4s. 1 7/8s would be better, but probably not made, unless custom.

Catback exhaust if not mandrel bends could also affect power. You mention some clearance issue. Spintech makes some pipes that are oval and could help with any clearance issues, but a little pricey.

A Fast 92 would be best ported if 102 doesn't fit.

 

Take it to the track and run it. i only made 389 on dyno and been 10.58 in a 4th gen.