Possible causes of power deficiency above ~5000rpm - Page 2 - GM Forum - Buick, Cadillac, Chev, Olds, GMC & Pontiac chat


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Old 04-05-2007, 07:09 PM   #11
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6 degrees is a lot, most cams do not like to be very far from 0. Retarding the cam should shift the peak higher in the revband but the peak may not be as big.

I agree valve float could be an issue and tends to feel like you hit a wall.
KR and injector duty cycle are other things I would look at.

Reality is that most non-sports car engines are not designed to breathe well above 5000rpm.
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Old 04-05-2007, 11:24 PM   #12
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Ok, so I searched and Ben does have LS6 springs.
I remember the cam specs, and there is no way he has valve float.
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Old 04-06-2007, 01:38 AM   #13
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Quote:
Originally Posted by 95naSTA
Ok, so I searched and Ben does have LS6 springs.
I remember the cam specs, and there is no way he has valve float.
Were they new or used?

What'* the fuel pressure like? Can you scan your injector readings for us?
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Old 04-06-2007, 01:55 AM   #14
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Quote:
Originally Posted by 95naSTA
Ok, so I searched and Ben does have LS6 springs.
I remember the cam specs, and there is no way he has valve float.
What are the cam specs?
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Old 04-06-2007, 03:51 AM   #15
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Let me answer some of these questions....

1. I installed used LS6 valve springs (90# seat pressure w/valve closed) when I did the cam. They were supposedly low mileage (under 40k IIRC), so they would still only have 60k-70k miles on them.

2. Cam specs are as follows:
--------------------------------------
Lobe Center Sep 110.8*
Valve Overlap -11.9*

Intake
Valve Open -5.9*
Lobe Center 110.1*
[email protected]" 210.2
Max Cam Lift .30622"

Exhaust
Valve Open 36*
Lobe Center 111.5*
Valve Close -5.9*
[email protected]" 210.1*
Max Cam Lift .30642"
----------------------------------------

These specs are very similar to a ZZP GT1, and with my LS6 springs there should be no float. Also, when I said we retarded the cam 6 degrees, I meant we changed it 6 degrees from the previous timing. It had been 6 degrees advanced, so all we did was return it to stock timing.

Dyno chart for the previous cam timing (6 degrees of advance):
Name:  DynoResults.jpg
Views: 0
Size:  48.1 KB

Valve float was never a problem when doing this dyno run, so I don't think I would be experiencing float now.


3. As far as fuel is concerned, I haven't checked fuel pressure in about a year. I was getting a little KR on the top end after returning the cam timing to stock, but Thomas added a little fuel with HP Tuners and the KR went away. As of our last tuning session I have no KR. My fuel sender is a little wonky (fuel gauge is completely inaccurate), so maybe the pump is having problems too? I also have 25# injectors, which are a few pounds higher flow than stock L36.

4. I am intrigued by the theory that my tranny pump might not be keeping up. Does a gear ratio change affect pump RPM, or is the gearing independent?

5. The possibility of not enough air flow will soon be addressed by the installation of a stock L67 throttle body that has been modified by Doug to adapt to the L36 UIM. I will also be doing some wideband O2 tuning with Thomas (PDXGTP) to make extra sure the A/F ratio is right.


Sorry for the long post. If there is anything else I can clear up, please ask. I'm willing to explore many different possibilities at this point. The car doesn't necessarily feel slow... I think "unimpressive" might be a better term to describe the upper RPM range. This thing should be set up to accentuate the high RPMs, but instead it seems fairly lackluster. I might be running the 1/4 mile in current form, so I may have some numbers to relay to you all on the 13th. I just want to get this thing figured out and running in top form.
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Old 04-06-2007, 08:01 AM   #16
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Ben..I'd never say it if not pushed... Bob thinks it'* the lack of supercharger under the hood.

I think you are on the right track. Check the fuel pressure...but check it driving the conditions where you see the drop off. I extended my FP gauge and was able to see fuel dropping off in higher RPM at a point which led me to a high flow pump. (I can get the info if you'd like)
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Old 04-06-2007, 10:58 AM   #17
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Quote:
Originally Posted by BillBoost37
Ben..I'd never say it if not pushed... Bob thinks it'* the lack of supercharger under the hood.
I vaguely recall making this statement before in a similar topic. So I would like to back up really quick to what Firebuick was saying....

What exactly do you think the point of failure would be? I can't see much difference other than the TC softening the blow, and if it locks up after it shifts, then the shift shock is taken out.
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Old 04-06-2007, 11:05 AM   #18
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Just a guess
Quote:
Originally Posted by MyLittleBlackBird
Size 10.5 Riddell. Them puppies cost $4.00 and don't owe me a thing at this point. They are still quite comfy.
(I love that one)

When you say failure, meaning a top swap or the current issue. IIRC no one in AZ has tested his fuel pressure lately and it'* possible. We know KR is under control... so that probably rules out pressure..worth checking though.

What about the timing change. It'* not easy to do..but.. if the car was seriiously strong before the timing change at that rpm..Hmmm Only other thing I think of is the gear change..
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Old 04-06-2007, 11:16 AM   #19
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Trans pump efficiency drops off at higher engine rpm'*, and is unaffected by gear ratio of the trans.
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Old 04-06-2007, 11:20 AM   #20
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now that you say you are just unimpressed with the high rpms i say, probably a restriction. I would say HV3. I am going to say if it isnt a fuel pressure issue then maybe your butt dyno isnt registering right, with the gears and tq convertor you are coming into power quicker and maybe your low end goes by quicker making you just "feel" your top end is lacking. That is just total guessing but the track will tell. Also will those gears dont you loose a little top end speed anyways. I know a few guys that went 3.69'* in gtps and lost like 2-3 mph in the 1/4. So that may be an issue. OR it is just an act of god buying me time to get my tranny before you set down some nice numbers at the track that way you dont pull too far away from me .
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