915 gears ratios

Hi , Could you please let me know what are the gear ratios on a standard 915 gearbox ( 1972/73 2.4 S,E,T ).
I would like to get some advice regarding the best choice for high speed circuit ( Le Mans) .
Thank you
Cheers,
JDJ

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Originally Posted by jdj123

Hi , Could you please let me know what are the gear ratios on a standard 915 gearbox ( 1972/73 2.4 S,E,T )...

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The standard gears for a 72/73 5-speed 915 were 11:35, 18:33, 23:29, 26:25 and 29:22, with a 7:31 ring & pinion.
Some cars were equipped with 915 4-speeds.

For advice on the best choice for a high speed circuit, much more information is needed...
Vehicle information, engine information, tire information, type of differential, event rules and information, driver level and experience at the event, etc..
Is someone experienced with 70s Porsches and with this event assisting you?

Jon B.
Vista, CA

Hi Jon,
Thank you for your answer.
The engine is a twin plug 2.5 L with 906 cams , carbureted ( weber 46) dynoed at 269,8 hp @ 7250 rpm and a torque of 30,3 mgk/ma @6000rpm.
Cheers
JD

JD, your first question was easy, but your second question is very difficult without a lot of information.

The engine information helps, so now we know it's not your stock 73.5 T road car... :-)
Does the engine have a speed limiter, or were you told to limit the engine speed to a certain point?

Is the car a dedicated race car, lightened and stiffened with suspension improvements?
What type and size of tire, including the loaded radius to the ground?
Is the driver very fast and experienced?
A very fast driver or fast car will need taller gears than a slower driver or less improved vehicle.

Is the event being run on the full FIA 24 Heures du Mans circuit, or is it being shortened or slowed by comparison?
Anyone giving you meaningful advice will need to know all these things.

It's best to enlist the help or advice of someone experienced with this event, and even better with 70s Porsches as well.
I am only the latter.

If anyone else here has experience with this event, or can recommend someone who does, then please add your advice or suggestions.
Otherwise, if we are your last resort, we will try to help.

Jon B.
Vista, CA

The only thing I can add is that the fun factor goes way up when you change 4th something that is not an overdrive gear. ...26/26 maybe.

Jon and Jay ,

thank you for your comments, I appreciate;

Jon , find below answers to your questions.

The car is a dedicated race car ( 2.5 ST 1972 with FIA HTP, stiffened & lightened ( 1020 kg with 12liters of oïl and 100l of fuel) 19 & 23 torsion bars , 16 & 19 bars , bilstein dampers
MICHELIN TB 15 tyres ( 18/60 & 23/62 ) Fuchs 7" & 9" and/or AVON CR6ZZ ( 205/70 VR15 & 245/60 VR15 ) Minilite 7" & 9"
Good driving experience ( I drive classic 911 since the last 40 years as well as modern ones )
the event will be run at Le Mans Classic

currently the 915 gearbox is a 915 close ratios ( 915/35 ) for road racing with ZF LSD and Cooling sytem that I bought from Manfred Freisinger , the gears are:
14:31 , 20:32 , 23:29 , 25:26 , 27:24 .

I'm planning to buy a standard 915/02 ( 2.4 S 72/73) with the following gears : 11:35 , 18:33 , 23:29 , 26:25 , 29:22 .

Cheers

I cannot give you a whole lot of feedback but you should try to find someone that can build you a gear chart.

I have a twin plug 3.0 with mod S cams, mild port work that makes peak power around 6500. I shift at about 7k but it is rev limited to 7300. I'm running 205/55/16 and 245/45/16 tires. I have same 1,2,3 gears as you and then a 26/26 4th and 28/33 5th and also a 7:31 ring and pinion. It is a very fun box but set up for a short track or tight twisty roads. The car sprints to 120 and then it is a slow climb after that. In other words, I run out of gear up top. This is only a bench mark for you but I can tell you that your plan would be a big improvement over my set up for a high speed track.