Savage Sierra

What would be the best shifts points for max power on a 87-95 C1500 w/5.7, TBI, 4L60E, 3.72 final ratio, stock swirl port heads, stock (crap) cam?

The truck has some bolt on mods ie: 670 cfm TBI, Headers, Cold Air Intake, shift improver and Corvette servo.

I've tried to get it right via custom tune but still its not right!

Originaly I had 1-2 & 2-3 WOT shift points set @ 5000 rpm
Now its set @ 5000 1-2 & 4600 2-3.

I'm still getting flat spots (speed/acceleration) from 1-2 and not as much from 2-3. Someone told me that WOT shift points should be around 3700-3800 with stock cam and heads.

Any tuners in here with an opinion or any body who has a 87-95 with a custom tune that has good luck with thier shift points?

YenkoST

iTrader: (11)

I always took the 1-2 shift to 5k. It stops pulling about 4500k but b/c the 1-2 shift is so tall compared to a 2-3 shift, I always let the engine go a little further in the rpms on the 1-2 than 2-3. So, 5000k 1-2 and a 4800 2-3 is what I did with the stock 350 TBI engine.

Savage Sierra

Another tuner tells me he sets his shift points between 3700 & 3800 rpm on the 87-95~5.7 TBI's. He said that's where the powerband comes in and while its still pulling.

I had mine set at 5000 and 5000 and it would flaten out around 4500. I reset it to 5000 from 1st-2nd because its shifts hard enough to chirp the tire and I thought that was a good thing. 2nd-3rd I had it lowered to 4600 rpm and still it hits a flat spot.

I went out yesderday and ran a few passes with the TTS DataMaster plugged in. I took the shifting into my own hands and shifted at 4000 and 4200 rpm and as far as the TTS data I was more than a second guicker than with the higher rpm shifts.

I'm going to try again today and shift at diferent points. I know according to the data my peak hp is at 4100 rpm and peak trq is at 3600 rpm.

Do you want to shift at the peak/top of the power curve, in the middle or at the tail end as it starts to fall off?

zippy

I would say around:

4700-5000 for 1-2
4500-4800 for 2-3
4500 for 3-4 (if you can pull it off).

Savage Sierra

Thanks, I'm already through the traps before 4th gear so 1-2-3 is all I'm worried about. I still think 4700 is out of my powerband. I'm thinking 4000 for both.

Your truck probably revs past 6000 rpm. Mine red lines at 4000 and blows up at 5000.

zippy

LOL, I know what you're dealing with. I've work on TBI's. 4700 is out of where it's making much power, but it gets you past the nose dive that happens on the shift.

Wide Open

I normally set the shift point 500 rpm past peak power. That drops the rpm's into the meat of the torque curve. It sounds like you have found the sweet spot to be a bit lower than that though. Let us know where you finalize your shift points and the performance comparisons.

Savage Sierra

Ok, this is gonna be a tough pill to swallow and I can already hear some people saying NFW but this is the real deal. I did some experimenting with manual shifting and a data logger then I confirmed it at the dragstrip.

I lowered my shift points to around 4000 rpm in 1st-2nd and 2nd-3rd. When the rpms were shifting at 5000 I was way out of my powerband, peak hp and trq were dropping like Britney's panties. The data logger showed my peak hp at 4150 rpm and peak trq at 3700 rpm. To keep the upshift drop within the powerband I settled at 4000.

I made several data logger 1/4 mile runs and three dragstrip runs all were within a few .100ths of each other. Note also I've had some tip in voltage problems which causes a bad hesitation at initial wot. Also my ABS system has been sick, so I did not powerbrake to pre-load the chassis. Right foot from brake to pedal best RT .197 and a bad stumble at the launch.

1/4 mile times before shift adjustments:

60'..................> 2.39 @ 28 mph
1/8 ET.............> 10.74 @ 63 mph
1/4 ET.............> 16.98 @ 78 mph
0-60 mph.........> 8.87 seconds

1/4 mile times after shift adjustments:

60'.................> 2.21 @ 30 mph
1/8 ET............> 9.84 @ 67 mph
1/4 ET............> 15.78 @ 83 mph
0-60 mph........> 7.53 seconds

For those of you that say NFW remember these trucks redline at 4000 rpm and max at 5000 rpm. After peak hp it drops of the chart and does not pull at all in fact it was hiting a wall at 4600 rpm and just going flat. Also shifting at 5000 rpm was causing sever trans speed slipage.

In drag racing one second is an eternity but in the real world half a second delay on both shifts is all it took to slow me down and put me in the 16's. So it goes to show sometimes the most simple and overlooked problems can make a huge difference at the track. {Stock Swirl Port Heads, Stock Cam, Stock Intake Manifold, Turbo Muffler}

Wide Open

Sounds completely reasonable to me. The old TPI cars were exactly the same. They used brute torque to accelerate not high rpm hp.

zippy

I can believe it. What I have found normally with the TBI trucks though is that the one's with headers on them will pull a bit higher. Either way, it's good to see you're getting somewhere with it.