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Discussion Starter #1
I had my truck out at test and tune today. New combo.

383 SBC
Turbo 400
9" rear 4.30 gears
New PTC 8" converter - stalls at 5300 rpm on the t-brake
28x10.50 MT slicks

Going thru the traps (1/4 mile) at 7500 rpm. Does it sound like my converter is slipping too much? I thought I'd be going thru at about 7100-7200 rpm.

I had three runs at a 10.580, 10.562, 10.563 at just over 125 mph. 60 foot times were 1.451,1.488,1.455. I'm currently running 5+ year old slicks. I want to buy a new set, but think I'm going to have to go to a larger diameter slick to keep the rpm down. Peak HP is at 6600 rpm. I was shifting at 6800 rpm. There is only a 4 HP spread from 6400 - 7000 rpm, so I thought setting the shift light at 6800 would actually be shifting at about 7000-7100.

Any help would be appreciated.


Thanks,
Jeff
 

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My car has 4.10 gear and goes through at 125-126 mph at 6900.I have calcluated it out to 8-10% slip whitch is not bad for a Nat Aspirated combo. Sounds like your around 11-12 % with the 4.30 gears.What is your shift extension,or drop in rpm between shifts.Im at 900-1000 rpm.Ideally you want to go through the trap at 500 above peak HP.
 

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Discussion Starter #3
My car has 4.10 gear and goes through at 125-126 mph at 6900.I have calcluated it out to 8-10% slip whitch is not bad for a Nat Aspirated combo. Sounds like your around 11-12 % with the 4.30 gears.What is your shift extension,or drop in rpm between shifts.Im at 900-1000 rpm.Ideally you want to go through the trap at 500 above peak HP.
I'm not sure what my rpm drop is. I'm math challanged.

I too have heard that you should go over peak through the traps. That's why I was shooting for 7100-7200 rpm. With my old setup I was trapping 115-116 mph at 7200 rpm.

Looks like I'm going to have to step up to some 29.5's.


Thanks,
Jeff
 

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Discussion Starter #5
i calculate you are getting 14% slip.
That seems like quite a bit. This is my first professionally built converter. I don't know if that is common or not. I'll have to give PTC a call about it on Monday.

Thanks,
Jeff
 

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Discussion Starter #6
After talking to PTC today they recommended that I verify the rear gear ratio seeing as I didn't install the gears. They came with the truck, and the owner told me the gearing.

Today I went down to the shop and jacked up the rear and had one of the guys slowly turn the rear tire as I counted the drive shaft turns. Looks like it has steeper gears than I was led to believe. The drive shaft spun almost exactly 4.75 times for one revolution of the tires. Might explain why it was spinning up to 7500 rpm with the 28" tall slicks.

Looking through the Summit site it looks like it has 4.71 or 4.86 gears. Time for some 29.5" tall slicks...

I placed an order with Summit's Race Shop today for a fresh set of MT slicks, a window net, and a new seat belt harness setup. My old ones expired 4/10. They tend to look at you a little closer when you get into the 10's.

My friend that was running the video camera said that during my 3 trial passes to the 60' marker I was pulling the front tires up out of the lights about 10" on the last two. He said the front tires didn't turn until it was a ways out of the box. I had the 2 step only set at 4000 rpm on the first pass, then went to 4300 rpm on the next two passes. When I raised it to 4500 rpm and higher is was spinning the slicks too much to hook.

It should be interesting to see how she leaves with some new slicks on it. Not to mention it should pull all the way down the track in the power band. I can't wait.

I just wanted to update so everyone would know that it wasn't that the converter had too much slip. It was my fault for not verifying the gear ratio.


Jeff
 

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when you are making the mph/rpm/convertor slip calculations, are you accounting for probable tire growth? I'd suspect that the tire would gain atleast 2 or 3 inches of rollout at that speed. just a thought.
 

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Discussion Starter #8
when you are making the mph/rpm/convertor slip calculations, are you accounting for probable tire growth? I'd suspect that the tire would gain atleast 2 or 3 inches of rollout at that speed. just a thought.
When I talked with MT, they said to figure approximately 1.5" of tire growth. The main problem was that I have steeper gears than I was told I had. It threw all my numbers off.


Thanks,
Jeff
 
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