Help, only dynoing 300RWHP

[EvanZR1]

Ok, as I posted in NachoZR1's post over on CF, we were at the dyno last night, and my car only did 300RWHP. Car has (supposedly, need to find receipts from previous owner) a Corey Henderson chip, and top end porting from him. Also has 4.10 gears, but that shouldn't affect dyno readings. I was going to check the secondaries by turning the power key to normal, but apparently with Corey's chip it is full power all the time. The car runs really well, and pulls strong all the way to the top. If I hadn't seen the dyno on it, I'd say it was putting down a little more than my LT4 Firehawk (330RWHP).

So my thoughts for what might be wrong are:
collapsing intake duct
failing/clogged cats
secondaries not opening

Any other ideas?

Here's the dyno:

[Pete]

I had gotten 270 rwhp with secondaries off.

I would first fix your intake duct.

I would take secondaries completely out and be done with them.

Pete

[EvanZR1]

Yeah, I'm thinking about just removing the secondaries completely. Corey has already offered to update my top end porting for free, so I think I'm just going to have him pull the secondary system completely and burn a new chip while he's doing that. He thinks I'm probably down about 80HP from where it should be, which would be great. Now I just have to figure out when I can get the car down to him for a couple of weeks.

[Pete]

Originally Posted by EvanZR1

Yeah, I'm thinking about just removing the secondaries completely. Corey has already offered to update my top end porting for free, so I think I'm just going to have him pull the secondary system completely and burn a new chip while he's doing that. He thinks I'm probably down about 80HP from where it should be, which would be great. Now I just have to figure out when I can get the car down to him for a couple of weeks.

"Do It" "Do It"

Better find the time and way to get it to Coreys as fast as you can,heck walk back home if you have to

Corey is probably spot on about the HP loss.
If you think the car feels fast now wait till after he's done with it.

Pete

[Locobob]

Here is a dyno of a 91 before and after porting, no headers. If you compare this to your dyno you will note that yours is down on power over the whole rpm range - not just at the top. So whatever issue you are having is effecting the whole curve, probably not a collapsing air duct as that usually just shows up at the higher rpms.

[Pete]

Originally Posted by Locobob

Here is a dyno of a 91 before and after porting, no headers. If you compare this to your dyno you will note that yours is down on power over the whole rpm range - not just at the top. So whatever issue you are having is effecting the whole curve, probably not a collapsing air duct as that usually just shows up at the higher rpms.

Peaks at 5600
Some details please.

Pete

[Locobob]

Originally Posted by Pete

Peaks at 5600
Some details please.

Pete

My first porting job so the runners were a bit more conservative than what I do now but it's mainly the lack of headers causing the early hp peak. Might also be breaking up a bit at the top cause of lack of tuning - you can see some "seismic" activity starting at around 5600rpm. Car was a pretty stock 350cid with Power Effects exhaust.

Back to the comparison: You can see on my graph the car is making 200hp around 3300rpm vs your 165ish - that says a lot about the nature of your issue - look at Nacho's dyno and you'll see the same trend = Your problem is across the board. Secondary issue.... good possibility.

[EvanZR1]

Yeah, I was comparing Carlos' (NachoZR1) graph to mine last night, and noticed I was down over the entire rev range.

[Kevin]

is the duct collapsing?

[EvanZR1]

Not sure, the dyno was an above ground unit, so I wasn't able to get up next to the car while they were doing the run.