Racing ratios for GeLo 2.5 SR

[matteo68]

The ‘74 type 915/06 5-speed gearbox from my T is soon to be rebuilt for GeLo with racing ratios, Quaife LSD and oil pump (but no cooler) to replicate the 915/00 ‘box the SRs were fitted with. However Angus and I are debating which ratios we should fit. The stock 915/06 ‘box was fitted with the following:

1st: 11:35
2nd: 18:33
3rd: 23:29
4th: 27:25
5th: 29:31
Final drive: 7:31

With Michelin TB15s (18/60’s (F) and 23/62’s (R)) on 8 & 9” Fuchs, we want to achieve strong acceleration (which it should have with 270 bhp @ 7,800 rpm & 960 kgs) and good top speed for mainly A and B road driving as well as the odd trackday.

I have looked at the ‘Airfield’, ‘High speed circuit’ and ‘Nurburgring’ ratios in ‘Information…for Sports Purposes’ and see that the respective ratios are:

Airfield circuit:
1st: 14:37
2nd: 18:32
3rd: 22:29
4th: 26:26
5th: 28:23
Final drive: 6:32

High speed circuit:
1st: 15:36
2nd: 20:32
3rd: 23:28
4th: 26:26
5th: 28:23
Final drive: 7:31

Nurburgring:
1st: 14:37
2nd: 20:32
3rd: 22:29
4th: 25:27
5th: 27:25
Final drive: 7:31

Has anyone had any experience of these ratios for a ‘fast road’ application and what would the advice be to achieve good driveability and shifting?

[hot66]

have you driven your stock S hard on B roads with the stock ratios ( and by that i mean most of the time full throttle , 5500 rpm to redline pulls out of each bend etc ? )

[matteo68]

Er, no - principally because I’m still waiting for it to be collected from Dr. K in Vienna and then have it UK registered!!

I’ve been extremely patient whilst his mechanic has replaced virtually everything you could call consumable, plus an incorrect ‘73-on fan strap and trombone oil cooler over the past 6 months, but now I’m really wanting it here to drive properly hard in this lovely weather we’ve got right now!

[jwhillracer]

Hi, I have quite a bit of info gathered over the years with our gearbox, the 915 equivalent of Trigger's broom, which traces back to an original Airfield ratio box that we originally used in 1983

If you can wait until this evening, I'll try to dig out some figures and information gathered since then ............

Cheers
JW

[matteo68]

Thanks, that would be great - no hurry, so this evening would be fine.

[hot66]

Er, no - principally because I’m still waiting for it to be collected from Dr. K in Vienna and then have it UK registered!!

I’ve been extremely patient whilst his mechanic has replaced virtually everything you could call consumable, plus an incorrect ‘73-on fan strap and trombone oil cooler over the past 6 months, but now I’m really wanting it here to drive properly hard in this lovely weather we’ve got right now!

reason I asked is that would have given you a good idea for road use what you might need. These cars are pretty short geared anyway compared to moderns .. 7200 red line in 4th for my car on stock gearing is about 100 mph. On b road blasts I never go into 5th as the 5th to 4th change is slow . Stock 5th is great for long runs and means my engine is just where it wants to be at 70-80 mph ( 3500 - 4k) . Without traffic I find the stock gearing to be great for fast road use. In traffic I always find I need to overtake in the 40-50 mph zone and here I'm kind of between gears .. nothing like starting an overtake off cam and then usually when side byside my power kicks in and away i go .. still frustraing though

[matteo68]

Thanks, really helpful!

I think my S will probably be here before we have to make a decision on ratios (it’s not happening imminently), so I’ll be able to get a good feel for the stock ratios beforehand but I do know that the 915/00 ‘box had ground gears and the stock ones weren’t, so this will be an influencing factor.

I also need to check whether the ratios in the ‘72 915/02 ‘box were the same as those in the 915/08.

[MBEngineering]

HI

the only diffrence is 4th gear between the 2 box's, the 915/ 02 is 1 ratio lower than the 915/ 08 box

with 270BHP that box will not last to long, even with the oil pump kit

you may need to check your gear ratios, for 4th and 5th that you have posted above

"The stock 915/06 ‘box was fitted with the following:"

1st: 11:35 mph 38 Max rpm 7,000
2nd: 18:33 mph 62
3rd: 23:29 mph 94
4th: 27:25 mph 128
5th: 29:31 mph 111 should be intresting when you change in to 5th from 7,000 RPM in 4th ?????????
Final drive: 7:31

regards mike

[johnM]

HI

the only diffrence is 4th gear between the 2 box's, the 915/ 02 is 1 ratio lower than the 915/ 08 box

you may need to check your gear ratios, for 4th and 5th and with 270BHP that box will not last to long, even with the oil pump kit

"The stock 915/06 ‘box was fitted with the following:"

1st: 11:35
2nd: 18:33
3rd: 23:29
4th: 27:25
5th: 29:31
Final drive: 7:31

regards mike

That’s 50 years of experience talking right there.

[matteo68]

Yes indeed. And very much appreciated advice too.

[matteo68]

And thanks for the perfectly stamped paint tag as well John!

[jwhillracer]

Our gearbox started out as an Airfield ratio box in our road-going Targa for competition. The ratios were –

14:37 - 43 mph @7200rpm on 225/50 x 15 rears 49 mph with 8:31 Final Drive
19:32 - 57 “ 65 “
22:29 - 79 “ 90 “
25:26 - 100 “ 114 “
27:24 - 117 “ 134 “
7:31 Final Drive

I haven’t heard of a 6:32 final drive before, it would give you a lot of gearchanges for a maximum of about 100mph.
The above was a nicely spaced gearbox, we used it with a 2.7RS spec engine, and then a standard 2.0S engine for most of a season when a couple of rods made a bid for freedom on the 2.7……..
It was “interesting” to get off the line with the 2.0S, but we learned a lot about carrying as much speed as possible through corners, which put us in good stead the next year, when we had a 2.7 again. It was OK on the road for having a thrash, but quite wearing on trips up and down the M5 at 4300 on the legal limit.
I think the first thing that broke was the 19:32 2nd gear. It was quite weak, the smaller cog hadn’t much meat between the base of some teeth and the top of the internal spline. As we had nothing else, we replaced it with a standard 18:33. It wasn’t ideal in terms of equal spacing, but worked very well on hillclimbs, where the most important thing is to have a selection of gears to minimise changes, and get a good launch out of corners.
I broke the 7:31 CWP, probably inevitably given the grief that it was getting when we built the new (present) car, and swapped over to soft Hillclimb slicks. We replaced this with an 8:31, which is/was stronger, but more importantly, we had one! First gear was a bit high with this, so when it died, we replaced it with a 15:36, which returned the overall ratio closer to where we started. On the road, however, it was a bit of a pig in town to move off gently in traffic with the high first gear and sharp triple plate clutch.

I should explain that through this time we were competing in 20 – 25 hillclimbs and sprints a season, double driven with the same young lady, Laura Wardle, and with great support from Parks, the Porsche main dealer in Exeter at the time, and Crispin Manners from Oak Tree Garage, who would always get us going again – just in time! It sounds like a lot of breakages, but they are spread over nearly 40 years of competition with the various developments of that gearbox. I think the most embarrassing breakage was leaving a large pool of oil and various internal parts on the toes of our own 911Hillclimber on the start line at Loton Park, when the casing split in two top and bottom as the differential shot out through the side plate. The saving grace back then was that 915 gearboxes were a LOT cheaper and more plentiful then they are today!
The only change that we have mad recently to the ratios has been to replace the 22:29 3rd gear with a standard 23:29, not because anything had broken, but because we were hitting the rev limiter at Shelsley Walsh 30 or 40 metres before the finish line, at a fairly consistent 91/92mph. We lost time changing up to 4th, but the extra few mph from the higher gear has made all the difference – until we change something else.

Sorry, I’ve wandered off topic….
As James said, I would try your standard gearbox first. If you are going to do some track days, standard ratios will probably be perfectly OK with a revvy engine. Going to a higher 1st is better on the track, and not so good on the road in traffic. Closer gears in 2/3/4 will keep you on song on the track, without affecting road manners too much, and then you have to decide on the balance between the revs/noise/fuel cost of a low top gear on the road, versus fun on the track. As Mike has already said, and you can gather from the above, once you start putting serious power through it regularly, something will inevitably give way………
I hope that’s some sort of help. I’ve been lucky to have Crispin to help through the years to keep ours going, and he has gradually developed a stronger more reliable, now 3.2 based gearbox, which despite all the carnage detailed above, has actually done us very well.

If you want to see it in action, we hope to be at Prescott this weekend, 23/24th April, Harewood on 7/8th May, and Wiscombe Park on 14/15th May

Cheers
JW

[jwhillracer]

Sorry, my table didn't work out in the post.
The second figure for each gear in the inverted commas is the speed using the 8:31 final drive

JW

[matteo68]

Thanks very much JW!

I will positively digest all this and discuss with Angus - all comments received have been hugely helpful!

[BILLY BEAN]

Quaife ATB
Diff. 7/31
1. 11/35
2. 18/33
3. 23/29
4. 26/26
5. 28/23

TB 15s 23/62 on 9 x 15"
271 BHP at flywheel limited to 7200rpm
GE 40 cams. Car weighs around 900kg.
I can't claim to have selected the ratios as I inherited the gearbox which had seen track use in SA. Ratios appear properly spaced.
If anything 5th could be a slightly taller gear as the engine could take it. 70mph is around 3200 rpm in 5th.