What Diff Ratio will get me 10.XX

[Brule]

Currently have a 650hp car doing 12 second passes.

The overall setup is very strong but I would like to what everyone thinks would be the best diff ratio to change to.

Setup is a 6266 turbo
Plazmaman intercooler and intake
3inch intercooler pipes
750cc injectors
Plenty of fuel pumps
Precision coils
2000hp powerglide transmission gear ratio 1.86 and 1:1
1000hp drivetrain parts
3.08 ford 8.8 diff

Best pass is 12.02 128mph 26psi boost (Hot weather)
60ft 2.15
330ft 5.53
1/8th 8.11@ 97mph


2nd best 12.31 121mph 17psi boost (cold weather)
60ft 1.97
330ft 5.07
1/8th 7.7 @84 (backed off super early)

What diff do you think will improve 1/4 et?

 

[Brule]

Photo of engine

 

[iminhell1]

60' looks more like a tire issue than a gear problem.
No trans brake I'm guessing? which is really hurting.

Looks like to get the speed you need 3.55 is the best bet. 4.10 you might not have the MPH on the big end you'll need, unless you really push RPM.

Is switching to a TH400 out of the question? that extra gear would really help here I'd say.

 

[NoGuru]

I have to agree, looks like a traction issue.
As a ref point, I have a DCT trans, around 600whp, and my 60' is 2.0 on PSS4S and 18" wheels. I cross at 11.6 doing 123 mph.

What is your wheel and tire setup?

 

[rac]

What rear tyre size, and what rpm are you happy to rev to?

I would have thought between 3.55 and 4.10. Assuming your running 265/35/18.

 

[Cruizinmax]

@rac is on the right track. Use this gear calculator and target your max speed in quarter. https://spicerparts.com/calculators/transmission-ratio-rpm-calculator

I'm guessing you're having a hard time getting out of the hole due to not making boost? How high are you able to stall with your current converter?

 

[gmagnus7]

With your currently setup the 128 mph run should have been hitting somewhere around 5300 rpm correct? If so that's way too low.

Your mph is okay but your 60 foot is really bad, overall traction is definitely your problem. Get a 26" slick and then a 4.10 would put you at 132 mph at 7000 rpm with 1:1. You should be able to hit 10's with that mph so you just need to fix getting out of the hole. You should be able to launch the shit out of it with those parts.

 

[Brule]

Yes it has a transbrake.
It has 10clutch packs rated for 2000hp.
Changing to a T400 is rediculous.
17 40 275 hoosiers
Crosses the line at 6200rpm

It is very hard getting quick 60ft
It can make 20+ psi of boost on launch pretty easy.
Any higher then 4500rpm will put on a smoke show.
I have been trying to launch at 4000rpm but as soon as I launch its a balancing act with the throttle for 50m or so sometimes 100m trying not to spin. If I launch on 4000rpm It will make target boost as soon as i put the throttle 100%.

For example I have done quicker 1/8th mile on 17psi then on 26psi.

 

[ShocknAwe]

What happens if you limit boost or torque in lower gears?

 

[Cruizinmax]

Yes it has a transbrake.
It has 10clutch packs rated for 2000hp.
Changing to a T400 is rediculous.
17 40 275 hoosiers
Crosses the line at 6200rpm

It is very hard getting quick 60ft
It can make 20+ psi of boost on launch pretty easy.
Any higher then 4500rpm will put on a smoke show.
I have been trying to launch at 4000rpm but as soon as I launch its a balancing act with the throttle for 50m or so sometimes 100m trying not to spin. If I launch on 4000rpm It will make target boost as soon as i put the throttle 100%.

For example I have done quicker 1/8th mile on 17psi then on 26psi.

Sounds like gearing isn't really the issue then if you can make boost off the line and your RPM's are high enough through the traps. Lower gears will just increase your torque at the tire making it more likely to spin (all else equal).

Which Hoosiers are you running? A drag tire or road racing tire? Really looks like fine tuning your boost curve and if you're willing to commit, a drag specific spring/shock setup that can transfer the weight is what you're needing.

 

[martymil]

Diff gears wont help you here, @Cruizinmax summed it up well.

 

[iminhell1]

Get a better converter.

And lets look at this. You're spending about 11+ seconds in high gear right now. Applying a 1:1 ratio to the rear (TQ to the ground).
If you add a gear you're now cutting that time down and adding a multiplier (1.48:1 from what I'm seeing).
TBH, you're not running near enough power to make a glide worthwhile.

And I just looked, you should be able to get a 3.73 out back too. If sticking glide, I'd go that route vs 3.55/4.10.

 

[Coupes66]

Hi Brule. You are running the 275/40R17 Hoosiers which are 25.7" diameter. By my calculations, with the 3.08 ratio diff and top gear being 1.00 ratio, at 6200 rpm and no slippage, you should be doing 155mph. At 128mph, you should be at 5,120rpm. Has the TC got too much stall and slipping throughout the run. What pressure are you running in the Hoosiers? To get the ideal traction with these tires on a drag strip, you should be running 16-17psi. If the pressures are much higher than this and the road surface is a rough aggregate finish, you are not going to get your car to hook up like it would on a prepped drag strip where the surface is smooth and sprayed with traction compound.

 

[Brule]

So looking at a few calculators.
This is results from the tci calculator for best diff for power/mph

for 128mph 3.8 diff
130mph 3.78 (6266 might get here)
135mph 3.64 (7675 will get here easy but pistons may not)
140mph 3.51 (duck got here 3.46 gears)
145mph 3.39 (goal is here)
160mph 3.076 diff (never getting here on n54 so 3.08 gotta go)



What I dont think alot understand about getting traction with a 3.08 diff with 1.86 gear, it is very difficult to launch. Imagine loading your car up with 15psi -20psi of boost in 3rd gear and when you put throttle past a certain point the tyres just get blown off not just slip or bog if to low. This is where a higher diff will help.

If it had a higher ratio there would be less wheel speed per rpm. It has enough power but it bogs or smokes the tyres not much grip and go area.

Yes I need more power to pump 10s easy.
No problem 7675 is going back on. This 6266 should still do 10s easy with higher diff ratio and good tyres might need 28-30psi to do it.


Yes the converter is slipping alot with a heavy car and 3.08 diff gears. It doesnt ever get to rev up in 2nd it just stays pretty flat rpm. In 2nd I just checked logs 5900-6600rpm.

The PB 60ft on 17psi boost was on 20psi tyre pressure.
The latest runs were on 26psi boost (not that i used that boost in 1st gear) 25psi tyre pressure.

So yes lower pressure tyres at 18psi will be next. Yea bad roads dont help but its good to get improvements so I can go faster.

Pretty much drop tyres down to 18 again and atleast get in the 11s with thos diff.
See how the converter goes with 7675 and it might have to go back to loosen it up more.

Converter is from well a know converter workshop. Pretty popular in the Australian drag racing scene. So it can be remade to suit getting the 7675 turbo and new diff whatever its going to be.

I always knew 3.08 was crazy high but thought it might pull really strong at the big end and still be fun but reality is you really need the power to get it moving.

 

[rac]

If 3.78 s an option id start there or in that ball park. Your going to spend a lot of time working out traction issues so don't waste that effort on gearing that is not going to be close to your final destination and then have to do it all again on another ratio. If your definitely going bigger power then maybe drop to the 3.55 so you don't do that twice. The problem with the stock ecu is you have fk all traction control options available to you except boost curve. At least that was the case last time I checked. Drop boost for 1st gear and build it up as traction allows through the rpm trial and error.

 

[rac]

After reading your last post sounds like 3.55 is sound starting place for where you want to head....

 

[iminhell1]

Ya if RPM is flatlining and you know the converter is slipping, that is the #1 place you should start. You're just pissing in the wind on anything else.
Gotta remember that what other drag racers are doing isn't what you're doing. The same parts they use aren't going to work the same for you. Converters are TQ and RPM and pump pressure based. They probably aren't making as flat of a TQ curve as you or at the RPM. You're really going to need a custom spec one to get the best results. And trust me once you find that perfect converter the car will really come alive.

 

[Cal1415]

I recently changed from Hoosier DR2 275/40/17 to Hoosier 28x10x17 bias ply and the difference in grip was incredible. Crossing the line my rpm is now 600 lower but trap speeds are the same. My 0-60mph went down by 0.4 seconds.

 

[gmagnus7]

Your mph doesn't make sense for your gearing and rpm. Something's not right, probably your torque converter like Couples66 said. In my opinion, you're not fully utilizing your setup if you're not finishing in the upper RPMs. A 3.9/4.10 would be ideal for a 26" slick and 4.10/4.3 for a 28" slick, depending on how fast you're expecting/wanting to go. For higher speeds (145-155 mph) you could go a bit taller (3.73 or 3.55) but you need the power to push it that fast. I also agree that bias plys will help your setup for sure, stick to 18-20 psi because 25 is too high.

 

[Cruizinmax]

You can certainly spend money if that's your prerogative. A lower gear in the back and probably a tighter converter would help. Based on the information you provided you are seeing about a 20% slip in the converter at the top end. Ideally you want less than 10%.

Honestly there's quite a bit left in your current setup without spending money on parts as well. You need to dial in a boost curve that will allow you to go full throttle out of the hole. You're not doing yourself any favors by attempting to peddle the car especially with how dampened the factory throttle pedal is.

Just as a reference point my last car was a 350whp FWD (manual trans) on all season street tires and ran mid 12's with a 2.0 60". You should be cutting 1.7-1.8's in a RWD vehicle on slicks. Dropping tire pressure and getting the front to lift in order to transfer weight to the rear is where I would focus my efforts if I were you.