Are the manifolds holding me back?

[Josh_luther]

I know the answer is yes and I already ordered the headers but is it all the manifolds?

I recently dynoed my 408 with l92,l76 combo 232/236 cam and c6 manifolds.

The torque numbers were nice 450 ftlbs at 3500rpm but HP peaked early and really low only 378@4700. I know it is partially do to being a little rich, and the manifolds just don't flow but is there more?

What can I log to see how bad the exhaust is? Dynamic cylinder air? What should it look like mine starts at .91@3000 peaks at 1.02@5000 and drops from there.

[69lt1bird]

Yes, the lack of headers is hurting you but there must be something else too.

Post a good log (WOT and normal driving) and your tune.

[WS6FirebirdTA00]

What headers did you get? I gained 8 hp peak with QTP headers over Pacestter headers and 30 ft lbs at 3200 RPM. The right header will make a huge difference.

[Josh_luther]

There are really no choices for headers for my car they are Schoenfeld 13/4 primaries with a 3.5 collector.

Here is a log from the dyno it has three pulls on it.
And the SD tune I am currently running.

There is a little knock up top and the A/F is rich but, I think I am down at least 50 hp and I am short on my power band 1200 rpm.

[WS6FirebirdTA00]

Yeah, didn't look at the sig when I asked that ? lol

[69lt1bird]

I don't see anything glaring. Who installed the cam? Was it degreed? Just something to think about.

[Josh_luther]

I built the motor myself.
The knock was never there when the motor was in my truck and I ran more timing. I am hoping a good flowing exhaust side will really wake it up.

[midgetracr]

You said max torque occured at 3500, max HP at 4700 and max cylinder air at 5000.

It seems very strange to me that max cylinder air occurs at a higher RPM than Max HP. Normally torque continues to rise until VE falls off, then HP continues up as torque falls off slowly. Then HP falls rapidly when torque dives down because the engine is running out of air.

Are you sure that your cam is timed correctly? Is the dyno OK?

Just my thoughts.

[Josh_luther]

The dyno sheet is hard to read but it looks like around 4800 it peaks.

Here are some pix.

Dyno sheet



Dyno sheet with log

[WS6FirebirdTA00]

Yeah, it is possible the cam is incorrectly degreed, or those manifolds are chocking it down horribly.

[69lt1bird]

After seeing the dyno graph and your HP tuners scan, I don't get it. Your scan showed you ran it up to 6200 rpms and 137 mph, the dyno stops at 6000 and is in the toilet by then. The numbers in your scan look good MAP hits 99 kPa, fueling looks good, the knock you get is nothing. How does it feel, SOTP? Is there another dyno in the area, I think the dyno has the issues, not your car.

[WS6FirebirdTA00]

Could be that type of dyno. What do other cars make on it?