What Diff Ratio will get me 10.XX

[Brule]

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

Yea if I was planning on keeping a small turbo I would get a diff around 3.78.
That would be alot better for 650hp or whatever is is.

 

[Brule]

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.

The diff ratio is for good for 160mph or 1200hp-1400hp
Any converter would be struggling.

Once I have a diff suited to my mph I might get the converter changed if it still slips heaps. Its not off the shelf part so it can be altered refreshed ect.
Changing the converter for a ratio suited for 160mph and 1200-1400hp wont work.

 

[Brule]

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.

Theres no doubt bias ply will give you more grip. It would definitely give you a better 0-60 all things being equal for both radisl and bias ply tyres.

For my case I had 25psi in the tyres which is obvously to much and now i know.
My pb 0-60mph was on 17psi boost and 20psi tyre pressure at 3.75 seconds.

I think I will try lower tyre pressure and stick with 17 inch radials. If im still struglling to get 1.6 69fts Ill go to 15 60 235, I wont even go 275 after seeing low 9s and 8s donr on 235 radials.

 

[Brule]

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.

Yes tyres will be 18psi next test hit.
Ill keep away from bias ply. Radials will be enough. Ill try lower psi on 17 inch then 15 inch next.

 

[Brule]

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.

Theres no point pushing with an incorrect diff ratio and blaming a converter.

Diffs gears are only $4-500 delivered.

I will be racing at roll racing next Friday and maybe try another 1/4 with 17psi tyres but then the diff and turbo will be changed for next setup.
Roll racing is ok as 1st gear absolutely killed every car even a 700kw t400 ute and 850kw manual supra but then they chopped me half track.

 

[Brule]

Overall....
Drop tyre pressure to get 11.xx easy but not 10s.

A diff around 3.8 would be best suited for 650hp.

17 inch radials will be used on 18psi tyre pressure.

 

[derekgates]

Wow, wild thread.

I wish you luck!

I went from a 2.56 to 3.15 with M3 rear swap on my DCT. Haven't taken it to the drag strip but top speed is certainly reduced.

 

[rac]

Overall....
Drop tyre pressure to get 11.xx easy but not 10s.

A diff around 3.8 would be best suited for 650hp.

I have ordered 3.55 and 3.31 diff gears.
17 inch radials will be used on 18psi tyre pressure.

Dont forget the n52 exhaust cam. There's 2 seconds in that mod alone.

 

[typedRew]

stop over complicating things and just get a decent converter and a proper tire

 

[Brule]

It has a dominator converter (well known in Australia drag racing and recommended by transmission builders around the country) and Hoosier drag radials.

Diff gearing is to low for this power and cars weight.

Tyres hooked at 20psi tyre pressure on 17psi of boost and did a 330ft time of 5.07 on the street. Thats less then .2 slower then some guys in the 10s. The car just doesnt have the power to keep pushing such a low gear.

 

[gmagnus7]

5.07 330ft is not terrible at all (especially for the street) but I'd expect slightly better with your setup/mod list if at the track. Sounds like you know where to start though. If you plan on keeping your current tire setup (and not getting bias plys) I would personally go with a 3.9 ratio. Crunching the numbers it should put you around 6700-6900 rpm & 131-135mph at the finish which is pretty good and definitely do-able for 10's if that's your main goal.

 

[Coupes66]

Running 93 PULP (98 RON here in Australia), I was getting 4.8-4.9s for 330ft and 1/4 times of around the 11.5s. As your time is only a tenth away from mine and with your set up, it surprises me you wouldn't pick up that time plus more for the remainder of the run. It indicates that you are not just losing time at launch but also at the top end. To me that indicates too much slippage in the TC which isn't allowing the power to get to the wheels.

 

[Brule]

Converter shop said it will be totally different with a 3.55 gear.
Theres no point using a 3.08 unless your cruising it in a hotrod with a 454 big block.
If its slipping to much with 3.55 we will tighten it up.

Diff gears first.
Maybe converter next.
Rods might come out before a new converter.

Map 7 28psi at roll racing it is.

 

[Brule]

This did 11.2 8xmph backing off at the 1/8th and 10.7 122mph backing off 300m mark.
4.10 gears
26psi

Needs 3.73 gears as 4.10 ran out of rpm.

This was done last year running the same power doing 12s at 128mph.
So if you have a similar setup try 3.73 or 3.55 gears.