Possible causes of power deficiency above ~5000rpm - Page 3 - GM Forum - Buick, Cadillac, Chev, Olds, GMC & Pontiac chat


Performance, Brainstorming & Tuning Talk about modifications, or anything else associated with performance enhancements. Have a new idea for performance/reliability? Post it here. No idea is stupid! (please use Detailing and Appearance for cosmetic ideas)

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Old 04-06-2007, 12:12 PM   #21
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That'* not a super aggressive cam. My bet is if you dyno it now, with the cam at 0*, the low end will soften up a little but above 4500rpm will be near identical.

The intake and exhaust are tuned for daily drivability, low to mid range. I think you may just be trying to pull past the "tune" of the system. An intake insert may be all you need.
I am by no means a 3800 expert but the principles and design trends are fairly consistent across the automotive market.
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Old 04-06-2007, 12:41 PM   #22
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By calculations, but no physical evidence, the peak should have risen past 5600 to about 5900 with about 10 more hp. The gears and higher stall TC were with this in mind to basically skip the bottom of the band.
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Old 04-06-2007, 02:36 PM   #23
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One of the things that may be contributing is the higher stall. I wouldn't say that is the problem by any means, but, higher stalls do give up a little HP up top.

Neglecting fueling, the cam timing and stock intake runners are starting to become more believable as the culprit. Think about it. Before, your powerband should have peaked lower, and started sooner. The cam and intake runners were limiting it up top but the powerband was more broad. Also, the gears were longer which means you were in your powerband for longer.
Now that you have retarded the cam, the powerband should be shifted to the right. The power will come on at a later rpm, and depending on flow, should peak at a higher and later rpm. But, what if your intake runners can't flow much more past where you were at?
The power is now coming on later and right as its beginning to make it'* peak, it'* being choked up top. So, you may be feeling the power starting to ramp up and then get choked.
This is all happening quicker now too due to the gearing.
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Old 04-06-2007, 03:04 PM   #24
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The TC stalling in the top of the gear is why it needs to be locked. It'* wasting power by not locking up in the top end.
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Old 04-06-2007, 03:50 PM   #25
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So as we sit right now it seems we've mostly ruled out valve float and fueling issues. I can believe that maybe it'* running out of breath at the top end because of the intake setup, but I can't be sure until I get a new LIM/UIM design on there. I've also been wondering if the stock L36 TB is too small for this engine, so we'll see if anything changes when I swap on the L67 TB and tune with the wideband. If you watch my burnout video from just after the tranny swap, you can hear a slight hesitation and a "huffing" sound as I go WOT. I think it allmost sounds like it'* not getting enough air when I stomp on it.




Still, if there are any theories about tuning the transmission to put the power down, let me hear 'em.
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Old 04-06-2007, 03:57 PM   #26
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Have you changed the tune since the dyno run you posted?
Look at the A:F on that run; big lean spike at the start. That is going to be a big factor in the jumping around low on the torque curve and why you have a bit of a flat spot/bog when you go WOT.
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Old 04-06-2007, 05:58 PM   #27
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I'm not expert in tuning but I agree with Greyhare that big lean spike is gonna hurt you down low. Also maybe your trans pump isn't faring so well up top like was already mentioned. And that huff you mention; I think thats normal mine does that when I go WT and others I've heard too.


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Old 04-07-2007, 01:48 AM   #28
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I'd have to agree now, intake runners aren't tuned for the cam you're using, and I'm guessing your car hasn't been dyno tuned yet (you mentioned waiting for the Wideband to tune) to perfection.


Perhaps try the DIY UIM Mods that KDG was doing in P&B when his car blew up?
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Old 04-07-2007, 12:25 PM   #29
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I wouldnt do adiy uim mod. It is much more efficient to buy the hv3. For the price you might as well just spend the 200 on it. Thats just my opinion
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Old 04-07-2007, 12:36 PM   #30
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I wouldn't change ANYthing on the car until you get it tuned the best you can, or find out why you can't.

You don't want to throw another variable into the mix.
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