Is my car making peak HP at to low an RPM?

[evil666]

Would like to know if you guys think that my car is making peak HP at to low an RPM? When I put my car on the wheel dyno it peaked HP at 4677 rpm (607 hp), which I thought was low. I was sure that my car was going to peak HP at over 5000 rpm, considering that it is not stock and that it has an after market cam. Would like to know at what RPM most of you guys peak at, just to compare. I uploaded a picture of my dyno sheet, so you guys could give a look at it to see what I mean.
Thanks,
Marco

 

[evil666]

Forgot to upload the picture, here it is:

 

[delcowizzid]

we used to make max power at 5200 rpm on 14psi cranked boost to 21psi made max power at 4000 think its turbine backpressure issue or head flow of the exhaust port issue.about to fit an oil pressure gauge to the header to measure backpressure

 

[bison]

Maybe the rpm indicated is incorrect? Usually you drive through the converter enough to not show peak power below 5000rpm if you are throwing some power into it.


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[hensleyt]

If the tach was set for 8 cyl on a 6 that would be about 6200 @ the 4700 rpm

 

[evil666]

Maybe the rpm indicated is incorrect? Usually you drive through the converter enough to not show peak power below 5000rpm if you are throwing some power into it.


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Hi Bison
The rpm that it is indicating is correct. I don't really understand the part where you say that you drive through the converter enough to not show peak power below 5000rpm if you are throwing some power into it.
I spoke to Dusty and he thinks that maybe my valve springs are week a little and not holding enough pressure and that is why I am peaking HP that low. He says that on a flat tappet cam I should have 110 psi and that if I have 90 psi I am loosing some HP cause he says that with the cam that I am running, It should peak at over 5000 RPM.

 

[evil666]

If the tach was set for 8 cyl on a 6 that would be about 6200 @ the 4700 rpm

It was set for a 6 cylinder.
Marco

 

[SCOOBY DOO]

Marco...ask Joe the valve spring pressures we run on my flat tappet engine. Time and build is in my sig.

 

[Dusty Bradford]

Looking at the graph, it's possible you have weak springs. It's worth looking into. That's why I always prefer roller cams. When you start turning the boost up you have more spring pressure to work with. When all we had was flat tappet cams I would replace springs every year, sometimes twice a year. Once they dropped 20-25psi of seat pressure, the car would slow down.


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[SCOOBY DOO]

Dusty is correct. I got tired of swapping springs out twice a year. The spring pressure I run now would scare most.

 

[bison]

Hi Bison
The rpm that it is indicating is correct. I don't really understand the part where you say that you drive through the converter enough to not show peak power below 5000rpm if you are throwing some power into it.
I spoke to Dusty and he thinks that maybe my valve springs are week a little and not holding enough pressure and that is why I am peaking HP that low. He says that on a flat tappet cam I should have 110 psi and that if I have 90 psi I am loosing some HP cause he says that with the cam that I am running, It should peak at over 5000 RPM.

By driving through the converter I mean that at the torque/rpm indicated the converter would stall to a higher rpm and therefore not show peak power at the lower rpm. If the converter was locked then this would not apply. Id be running more than 90lbs on the seat for sure.


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[20psiofevil]

What did the boost curve look like? I'd suspect back pressure. Peak should be around 5200rpm with a .63 precision housing.

 

[evil666]

Ok guys here is the story. The heads that I have on my car belonged to Eric Shirtz from Dyno Tech Performance, not sure I spelled his name right. He sold those heads to my friend Claude (Too Fast For You on turbobuick) which he then sold to me six years ago. The springs on the heads were for a roller cam set up. I gave the heads to Joe (JB racing on turbobuick) to do a valve job and he removed the inner spring to lower the psi so I could use them with my flat tappet cam. 6 years ago he had told me that by removing the inner spring the pressure was fine for my cam, but to be honest I don't remember what the spring pressure was. So I guess its safe to say that the springs probably need to be changed and that is why I am making my peak HP that low an RPM. But I think what I am going to do instead of changing my springs, is take Dusty's advice and upgrade to a roller cam set up. I just have to start reading up on roller cams and find out which one will be the best one for my combination. When I started this tread, I also wanted to see where other guys peak in HP for comparison.
Thanks everyone for your help and advice.
Marco

 

[evil666]

What did the boost curve look like? I'd suspect back pressure. Peak should be around 5200rpm with a .63 precision housing.

I would have to post the file from my Fast XFI when I dynode the car to see the boost curve. What I do know is that my boost always starts low and works its way up. When I use to have my stock long block, it would peak in first gear and then the boost would drop down.
Marco