I need taller tires !

[Hershal Byrd]

As most of you guys know I have the Audi 5N trans in my GT. The final drive is 4:11. That with a .73 fifth gear and 25.7 diameter tires ain't gettin' it. I'm thinking of putting on some taller tires to get a larger diameter so it will decrease the revs per mile on the tire thus giving me more top end.
I need to be around 760 to 790 rounds per mile. I have 16" rims and Yokahama advans. If I have to buy bigger rims I will but I would prefer to keep the 16's.
Is there anyone else out there who has bigger diameter tires on their GT? If so please let me know the brand and the diameter and if it helps your top end speed.
I'll run 20" rims if that will help.

Hersh

 

[Guest]

Hi Hersh

I think you may be better off trying to change your final drive ratio. 4.11 is high. Your tyre size diameter and 5th gear ratios are actually quite simialr to what is commonly used on standard GTDs with their Renault boxes. But our final drives are as low as 3.44. 3.88 is the other but less popular option. You must be whizzing through the gears! What are your speeds per gear?

The biggest tyre I can physically get in my wheel arches is the Dunlop Post Historic 1200 6 15 which has an overall diameter of 27.5 inches. Any larger and it would be fouling the fore and aft parts of the wheel arch. It is the largest diameter tyre I know of in use. However as my speedo was previously calibrated for the 265/50 15 Comp TA, when they were available, and which had a diamtere of 25.4 inches, ie pretty much the same as your tyre, this gave me a 10% increase in speed for the same rpm.

However if you want sensible cruising speeds/rpm you will need even a bigger tyre than the dunlop as 10% won't really gain you much, even if you had the 15 inch rims to put them on. Remember the final drive ratio differences is much more than 10%, at nearly 20%. Therefore I doubt you are going to get a tyre to solve your problem satisfactorily.

Can you see if Audi do the same box but with different final drive ratios? Maybe RF can advise better than me as you have one of their cars.

Your engine will drive a lower final drive ratio as my car with 345 bhp pulls the 3.44 fnal drive on 27.5 inch diamter tyres no problem.

Hope you get a solution

Malcolm

PS my theoretical top speed is about 190 mph but no chance ever of pulling that! Too much wind resistance to punch through. I would suggest I could get the car to about 165 mph if given a long enough legal bit of road to do it on. I have never driven beyond 155 as that is quite fast enough as it is for me and now there are speed cameras on that road! Once was enough. At Goodwood I pull about 130 on the fastest straight, proven by telemetry with a lap time of about 1 min 32 secs. Paul has reached 155 on the same straight with smaller diameter tyres but a much more powerful engine. He can reach that speed quicker than me so I need a longer straight. My lap times are acheived by not slowing so much for corners!

With my new sneaky Pete Quaife first and second gears being longer ratios I get 49 in first and 76 in second at 6250 rpm! Third remains at about 100 and 4th at about 130 plus (the straight ran out!). I haven't actually used my new longer fifth gear ratio yet except only to see that it worked!

[ October 17, 2002: Message edited by: Malcolm M ]

 

[Ron Earp]

The box is rotten for driving it appears. I ran these calculations:

1st: 3.60 2nd: 2.13 3rd: 1.36 4th: 0.97 5th: 0.73

Tire Diameter 1: 25.1 inches
Rear Gear Ratio 1: 4.11:1

MPH
RPM 1st 2nd 3rd 4th 5th 6th
1000 5 9 13 19 25 0
1500 8 13 20 28 37 0
2000 10 17 27 37 50 0
2500 13 21 33 47 62 0
3000 15 26 40 56 75 0
3500 18 30 47 66 87 0
4000 20 34 53 75 100 0
4500 23 38 60 84 112 0
5000 25 43 67 94 124 0
5500 28 47 73 103 137 0
6000 30 51 80 112 149 0
6500 33 55 87 122 162 0
7000 35 60 94 131 174 0
7500 38 64 100 140 187 0
8000 40 68 107 150 199 0


You can check for yourself here: http://www.smokemup.com/auto_math/mph_range.php

Just put in the ratios and it will produce the chart above in a more readable format.

1st is good for all of 30mph and could be used to pull stumps from the ground. 5th around 135-150. Not good.

 

Since your first report on the box i have been looking into other options as well. The best i could come up with was getting a diesel tranny (AAZ) but the diff is still only 3.88 (i think) or try and swap the guts of the porsche version into yours, again i think the diff is a 3.88. Don't know if that will make enough difference but sooner or later someone will be the ginea pig. Let me know how it goes.

 

[Hershal Byrd]

Malcom,
As always you're a wealth of helpful info. I think you are correct about the 10%. From your expierence and with what I have learned at top speed this tranny comes up short. Also I'm around 3800 RPM at 80MPH in 5th. That's not good. It's not even close to what the chart says.
Here in Arizona we have great open stretches of interstate highways. I was on t one of these highways cruising along at 70MPH. on my left a new red ZO6 vette pulled along side. Being that we were the only two on that section of road we started playing . He and I accelerated from 70MPH and I stayed in 5th gear. He was obviously in 4th gear. I
stayed even with him up to about 115MPH. then he started pulling away from me. We shut down at 125MPH and he was about ten car lenghs ahead of me. I have been over 130MPH in this car once and it felt like there was more left but the engine was at 6000rpm's not at 5500rpm's like the chart shows. I
don't think the vette ever shifted to 6th gear. I pulled along side the vette up the road and gave hime the well deserved thumbs up and he did the same, it was good fun.
For a short track or a tight autocross this trans would work just fine. But I want to drive it a lot and our interstate speed limit is 75MPH and most do 80 to 85. Out here you can stay at that speed for hours. I feel that 3800 RPM's is way to high for cruising at long periods.
I know exactly what I'm gonna do but I must wait awhile. I will purchase a Quaif or an RBT. I hate to pay that kind of money for a trans but unfortunetly it's the only show in town.

Hersh

 

[Mark Worthington]

I believe the innards of the AUdi/Getrag gearbox are identical to those used in Porsche 944s. Has anyone looked into the availability of aftermarket (or Porsche OEM, for that matter) ring and pinion sets? Has anyone inquired of the shops that specialize in Porsche transmissions? There must be some 944s that run in events like the Silver State Classic...what do they use?

 

[Jim Glickenhaus]

This the way to go. The T44 has a set of quick change gears in the back that can change the "rear end" from 4.56 to 2.99. That way it's your choice. Rubber in all four gears or 223 on the Mulsenne... On the street I find 4.56 much more fun. 60 to 100 is very fast and thats what you need to pass especally with right hand drive.

 

[Guest]

Thank you, Hersh for your kind comments, it is nice to know that people read my numerous (!) postings. Perhaps you could consider the Renault box. It comes from England the same as the Quaife and is much cheaper. I know nothing of the other box you refer to. The Renualt UN1 is also tried and tested. Adaptor plates exist as well as drive shafts etc etc. LSD too! Costs could be say 700 for a recon box from say Chris Cole, 50 for an adapter plate second hand from club mag, 120 per drive shaft, 40 per CV joint, 650 for LSD, 120 for taller fifth gear. The flywheel could be a ford item if you used the Tornado bell housing else a Ford V6 transit flywheel redrilled etc will work. Custom made steel flywheels are about 400 although Roy got fed up waiting for one so made his own! Uprated clutch parts are AP items and cost about 150 for plate, 50 for bearing and 150 for cover. The rubber mounts for the box are tricky to find but lots of alternatives must exist out there. RF may use something else than what we have on GTDs. I would strongly suspect you would have change having bought all the above out of what it will cost to buy just the box from Quaife. Oh and to make a solid link gear change is reasonably straight forward too for the UN1, it can be left or right hand depending on your choice.

Malcolm

PS the costs mentioned above are UK £ and are approximate as I have not checked suppliers for specific item costs. Maybe the costs of the above would be less than a new set of good rims to change your tyre diameters?

 

[Neal]

The lowest ratio I could find was a 3.89. This is available from the dealer. Custom r/p are $$$.

 

[andys]

Hershal,

It appears to me that your speedo and "the numbers" don't seem to quite work out. Have you calibrated your speedo? AAA offers a speedo check as a free service to members.

At the salt flats, we would often gear with the tires for optimizing top speed, though we used roll-out as a means for comparison. The 10% could be a lot, depending on your focus. In your case, @2500 RPM with your gearing and Malcolm's 27.75" tires, you would realize about a 5 MPH increase.

Andy

 

Back on the idea of pillaging a porsche tranny for parts, I found someones website with a list of porsche tranny gear ratios. It isn't %100 specific on application but it shows a 944turbo with an 016r or s as having a diff ratio of 3.375, 1st 3.5 2nd 2.059 3rd 1.4 4th 1.034 5th .829, now i havent pulled mine apart yet but looking at pics in a manual i think the diff could swap.
I do remember reading somewhere way back about someone attempting to mate the 2 on this forum.

 

[Robert Logan]

Hershal,

The 944 boxes have a worse 5th. gear than the Audi. They have torque tubes from the engine up front to the gearbox (positioned where the diff usually is) and with the first gear part of the input shaft this makes building 1 from 2 very difficult.

There are many different gearboxes used in the Audi's available and the diesel turbo boxes are the best as the diesel engines turn slower than their petrol equilivents. These boxes are very popular in Europe and this is where they are best sourced.

I spoke with BRT yesterday and they were realy nice and very helpful. They will sell you a 5 speed for $ 8800 or a 6 speed for $ 16,000 or a recon ZF for almost $ 7000 and yes all the prices are in US dollars.

I am working on an Audi six speed and because of the problems with the first gear ratio we will be replacing it with a new first. The complete ratios are not yet set as we are still working on it but if evertone is patient I am sure this will be the BEST option for ratio's and bucks. (I do wish I had more time to do all the R&D I want to do)

Hersh, turn that engine to 7000 revs which Gordon assures me it will do and get one of your old mates from the police to clock you on radar and let me know what she is realy pulling.

Best wishes,

Robert

 

[Dan W]

Robert , what source is this 6spd from Audi out of? Can It be aquired used at this time ? What is a good guess at costs to get one and after modification of the ratios. Just wondering from a potential RF customer.
thanks ,, Dan

 

[Jim Glickenhaus]

Dear Hershal
Drove the MK-IV and Lola today. FYI MK-IV 70MPH at 3000rpm with 4.56 rear and 27 DIA. rears. Lola 85 MPH at 3000 RPM with 3.22 rear and 26.5 rear DIA. Mk-IV 4 speed T-44.
Lola 5 speed ZF.
Best
Jim

What a day for driving!!!!

 

[Bill Musarra]

Have any of you guys talked to Renegade Hybrids. They have probably done alot of the ground work with the porsche units. I haven't talked to them yet as my car hasn't arrrived yet and am not sure what mods or changes I'll need. Seems like it would be worth a contact.
Bill

 

[Mike Dozier]

Hersh

Sounds like you've identified a real "issue"
with the Audi gearbox. I wonder what a specialty manufacturer like Strange would charge to make a small batch (30?) of
custom ring sets to a more user friendly
ratio? I'm sure between RF and CAV and others, we could easily get commitments
if the price was "reasonable".

Neal/Robert/Anyone...

Have you actually priced out custom R&P sets
for the Audi 016 trans?

MikeDD

 

[Hershal Byrd]

Guys,
Thanks for the input. It is most appreciated. All I really need is a .60 to a .67 fifth gear and I will be happy. I'm really not bothered that much about first gear being so short. My main concern is the cruise factor with the RPM's well over 3000.
I drive the car on long trips and I think this is way too excessive RPM's for 75MPH. Extra wear on the engine and the fuel burned at that speed is a little too much. I know that there are some audi gears that I can buy and swap it out .
In a week or so I plan on setting up the car for a test run and get good accurate figures so I really know where I stand. This trans is clearly a good one and so far with 1300 miles it has performed well with the exception of the overdrive.
MK-IV J6, Thanks for the figures they will help when I double check all my figures with gears and tire diameter.

Hersh