Hay guys. Ok here it go's' I took my car to get it dynoed and tuned. it did real good they got the carb all tuned and the timming diald in.But the power stoped going up at 4500 rpm and stayed there un till 5600 rpm. I have a 350 vortec heads, comp cam xe268h (witch is good to about 5900 rpm) rpm airgap,msd digital 6 with a pro billit , holly 750 vac sec. and a stock bottom end. what would cause the power to level out . it didn't change but 2 or 3 hp through 4500-5600.any replys will help thank.

 

Auto tranny with loose converter?

Rich

 

Your carb is on the larger size for your combo, but it's OK because it's a vacuum unit.
The 350 Vortech heads are nice as is the RPM Airgap.

But your cam is WAY too small for your combo.

OK, maybe "way" is too harsh, but honestly...
You should have a bigger cam. That one will quit too early to take advantage of your other parts, especially the RPM Airgap.

 

Sounds like a VE problem...what exhaust system do you have?

We see this a lot with older vett's with side pipes, the motor will peak at 4500 and make the same power till 5500 or so, we have gained as much as 110rwhp by dumping a customers too small exhaust.

I'll print out one of those dyno sheets to give an example...

 

Well the converter is a b&m holeshot 3000, the exahust is dual 2 1/4" flowmaster mufflers and 1 5/8" headman headers.as far as the cam would changing the cam timming help? And if not them what cam do you guys think I should get. One guy told me I didn't have enough compression for the cam. that's why the power was stoping. I have about 9.2 to 1 I thought that was enough for the cam.

 

Well there's the problem. . . I, personally, have gained 40 rwhp by getting rid of a FlowSmasher exhaust. Like MachinistOne, I've seen a 100 rwhp gain by dumping an exhaust system. Try that first.

Next, make sure the secondaries are opening in the carb. I've seen vacuum secondaries stick closed on several occasions.

I'd leave the cam alone. We've had good luck with that cam, especially with low-ish compression. DCR is very high with it.

How much power did it make, anyway?

 

well It mad 221.4 hp at 5250rpm and 247.3 tq at 4500. But it's a 3600 lbs car and it ran 13.2 sec with a 1.7 60' I don't know how 221 hp can run like that. And with it not making power at the rpm it's should it stops pulling at about the 1000 foot. I don't understand how the mufflers can do that to the power? if you could explain that would help. just so I can learn somthing.

 

oh ya I also spun the tires alot on that run.

 

That power is low. My friend had a very similar set up a while back:

8.5/1 350
'70's smog heads (882?), non ported
XE268 cam
Holley 750 vacuum secondary carb
1 5/8" headers
TH350 and 2500 converter

He made 300 rwhp.

If you think yours is laying down at 1000 ft, then check fuel pressure also.

So now you have 3 things to check:
- Drop the exhaust.
- Make sure secondaries are opening.
- Check fuel pressure.

 

I thought the same thing about the fuel pressure but somone said that it would lean out if there was no fuel. is that true? I expected 300-315 hp I was really shocked when those numbers came up on the screen.

 

The definition of "lean" is less fuel than you would like to have. So, yes, low fuel pressure causes it to lean out and lose power, typically at the other end of the track.

 

but would that show up with the a/f meter?

 

Obviously.

 

the reason i ask is we had one hooked up on the dyno and it the a/f ratio was fine. and I checked the secondaries are opening so the leads me to think it's the exhaust. thanks alot for your help.

 

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