This winter I swapped from a T-5 to a 700R4. After the swap my trap speed decreased from 110mph to 105mph and I’m hitting the 6000rpm rev limiter with about 200 feet to go. All other factors are the same.

My questions are:

Is the gearing difference between 3rd in the 700R4 to 4th in the T-5 enough to cause a 5mph drop?
Is it a combination of gearing and power loss because of going through an automatic instead of a manual?
Is bouncing off the limiter causing the drop?
Would raising the limiter 500rpm and going from 26” to 28” tires be enough to cross the line without hitting the limiter?

Specs are in the sig but in case it doesn’t show: 426 flywheel hp, 4:11 gears, 700R4 trans, 26” street tires, 2.1 – 2.3 60’ times

 

The gearing is 1:1 in both transmissions when you're crossing the line, but you've got slippage in the torque converter with the automatic.

 

3rd gear in the 700R-4 and 4th in the T5 are the same. They are both a 1:1 gear. I'm sure bouncing off the limiter isnt helping anything. Did your ET change at all? BTW, you sure you have 11.9:1 CR? You running race gas with that?

 

If anything the slower speed is from the added drivetrain loss, mismatched converter, or bad shift points.

A 28" tall tire could solve your problem but if you were hitting 115 with the same tire WITHOUT hitting the limiter then you should still be able to. Something is amiss if the combo truely stayed the same.

 

What converter is in it?

Slippage will show higher trap rpm than what you'd see with stickshift.

I'd try raising shift point to see what happens. T5 and 110 mph with those gears/tires means you used to trap at 5845 rpms. Most converters will have 5% slip or so. Poor converters will have more than that. 5% slip puts you at 6137 rpm now for same MPH.

hitting limiter 200 ft out will really change the MPH

 

Very good point about the ratios. Had I been at home typing and not at work I probably would have realized that. With regard to the compression ratio, yes I'm sure it's 11.9:1 and due to conservative timing and lots of overlap I can run 93 without detonation but that's neither here nor there regarding this issue.

I had not accounted for converter slippage which would make sense and with the calculations posted by Orr I could see the 6k limiter being hit now and not before. The converter is a 10' Edge Racing piece with a 3000 stall.

My ET's were about the same at around 13.1 - 13.3. The ET's alone point to a lack of traction and loss of power at the long end but I'm thinking tires may fix both. Can we calculate the difference in rpm going from a 26' to a 28 tire? Thoughts?

 

Heres a calculator you can use. Go about mid way down and you can put in your tire size, gear ratio, speed, and trans gear ratio for whatever gear you want to calculate (doesnt have to be overdrive like it says).

http://www.fbody.com/calc.htm

 

in 3rd with the 700r4 and 0 slip and 28inch tall tires your top speed would be 120mph. so 118ish with a lil slip. Get tires. 2.1 is horrible

 

"Get tires. 2.1 is horrible" You're telling me! Last year I blamed it on the stick. I had drag radials at the time. This year I blame in on the tires.

According to the calculator built91Z28 gave I should be trapping at 5577rpm but I'm hitting the limiter meaning I'm over 6k so I assume this is due to converter/transmission slip?

But 500rpm of slip seems excessive to me. I'm not sure the math works this way but does 500rpm of slip mean ~9% at 5577rpm? That can't be it can it?

 

at 1:1 with 4.11 and a 26" tire I get you at 58xx rpm. How are you getting 5577?

 

I put the following into the calculator:

Gear Ratio: 4.11
Speed: 105mph - This is my new trap speed
OD: 1
Tire Diameter: 26

This represents my setup this year and says I should be trapping @ 5576rpm which is not correct as I am hitting my 6k limiter. Using last years speed of 110mph I get the 5842 you are getting which works. I believe if this calculator can be used as a tool it is telling me that I have some serious slippage of either the converter or the trans.

Seeing that the converter is brand new and from a reputable company I have to assume the transmission is slipping more than it should be. It feels great on the street and fine on the track but I'm no expert so I guess I could be mistaken.

Any other thoughts?

 

The transmission shouldn't be slipping at all. What brand and model is the converter?

 

Edge racing 10 inch, 3000 stall. Brand new.