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View Full Version : Effect of tire diameter on shift points, etc



jdando
04-25-2006, 05:47 PM
I got my track tires mounted today, Dunlop Super Sport (http://www.tirerack.com/tires/tires.jsp?tireMake=Dunlop&tireModel=SP+Super+Sport+Race) 245/35/18. This is a DOT approved track tire. The diameter is about 4" less than the stock 245/55/18. The revs/mile jump from 727 to 840. The speedometer reads about 9 miles fast at 55 (like I will drive 55 :lol:)

So my question is what is my "effective" rear end ratio? I have the stock 3.55 rearend.

Also are there any gotchas to watch for regarding the transmission shifting and lock-up points? What tells the tranny to shift, engine rpm or car/driveshaft speed? Also assuming I want to go really fast, is there an appropriate why to engage OD? Is it the upshift or downshifts that will kill it?

And yes these tires on all four wheels will light up the Traction Control.

Thanks in advance.
jeremy (just a brake job away from the track)

gdmjoe
04-25-2006, 08:50 PM
Given that the wheel diameter difference is 13.5%, the RAR would now be ~4.03.

juno
04-26-2006, 05:02 AM
Actually it is pretty close to a 410 using this.

http://www.svtoa-atlanta.org/cgi-bin/trans.cgi

You can plug in the 4r70w 3.55 and your tite size and see where your rpms/speed compares. Should wake the car up!!

Are you going circle track/ road course somewhere?

Zack
04-26-2006, 05:13 AM
http://www.mercurymarauder.net/showcase/files/4/0/4/TCoff.bmp
It is easy to change, you need the software.

shakes_26
04-26-2006, 05:18 AM
Also assuming I want to go really fast, is there an appropriate why to engage OD? Is it the upshift or downshifts that will kill it?


Jeremy,

Leave OD off, unless you run a big course like Sebring or Road America, where you carry higher speeds longer, I wouldn't mess with it. I played with it a bit this weekend, and really only used it on the straight and one high speed section. When I kicked OD off it gave a nice downshift, but it really made no difference except to drop my rpms. Generally you dont want to run it becasue you'll carry enough speed to keep it from downshifting, so your rpms are low and your car will lug, and you'll have to really nail it to get the downshift, which at that particular time might not be what you need.

One piece of advice, if you dont have a trans cooler.....get one. I saw trans temps around 210, and on Sunday when I really was beating on the car mercililessly (sp?) I saw 225, thats measured in the pan so AFTER the factory cooler. Needless to say, ATF replacement is in order this week.

Those tires should be real nice, let me know how you wind up liking them.

jdando
04-26-2006, 06:10 AM
Are you going circle track/ road course somewhere?
I will be going to Driving School at Brainerd International Raceway in Brainerd MN. This will be my second trip there:banana:

http://www.na-motorsports.com/Tracks/MN/images/bir/BIR.gif

The only part I did not like was the turn 9 exit has a wall on the outside. If you make the turn correctly it is a non-factor. For a spaz like me I was concerned, but it was never an issue.

jeremy

juno
04-26-2006, 06:17 AM
That looks like a great track! I did my first one last weekend and I think I am hooked!! It is hard on the ol' girl though!

Can you tell me why you picked that size and brand of tire?

Edit: And like Marc said, the tranny cooler is a must. For the price of one and the short time it takes to install it is one of the best preventive maintenance mods out there.

Keep the paint off the walls!!!!

shakes_26
04-26-2006, 06:27 AM
Awesome track!! My sister lives in Jacobson, near Grand Rapids. I was up there for the Loonacy races last year, but we had issues with her BF car and he wound up not going so we stayed by the lake. oh well.

Turn 1 and 2 have got to eb interesting, high speed sweepers...bet the car drifts out on those :) weeeeeeeeeeeeeeeeeeeeeeeeeeeee !

BruteForce
04-26-2006, 07:28 AM
(humongous file size BMP image removed)
It is easy to change, you need the software.

Here's a tip on posting images. First avoid BMP (bitmap). Open the BMP image in an image editor (Windows comes with MS Paint), do a File | Save As and choose GIF. It will take that 1.5 MB bitmap image and turn it into a svelte 50-100K web-ready image.

jdando
04-26-2006, 08:28 AM
Turn 1 and 2 have got to be interesting, high speed sweepers...bet the car drifts out on those :) weeeeeeeeeeeeeeeeeeeeeeeeeeeee !

For the "big boys" turn 1 can be taken flat out, turn 2 requires a little braking. The fast cars were taking turn 1 at 145mph and turn 2 at 120mph.

I was between 100 and 115 mph for turn one. Had to keep my eyes on the road for turn two so I do not know how fast I was going:cool:

jeremy