any gearing gurus here?
 

My '74 gearbox is in the shop in need of repair. I'm considering changing to a lower 4th and 5th.

The 3.0 w/ Mod S cams and carbs makes between 210 and 215 hp at around 6300 peak.

The current gears are
1st 35/11
2nd 33/18
3rd 29/23
4th 25/27
5th 21/29

7:31 r/p

I think these are the same gears as the 73 RS.

The shop that has the box has a 26/25 4th and 24/27 5th that he recommended.

How does this sound?

You have to factor your rear tire diameters to make any decisions about gears,...:)

I was hoping you'd chime in Steve.

The car wears 16x9s with 245 45 16 Toyo RA-1s. I'm not certain of the diameter at this point.

Empirically I thought previously that 4th was a tad tall and 5th offered nothing other that a way to drop the RPMs and cruise on the freeway.

Thoughts? and Thanks in advance.

here's your stock setup

http://forums.pelicanparts.com/uploa...1202398287.gif

here's w/ the 2 alternate gears, pretty ugly
http://forums.pelicanparts.com/uploa...1202398389.gif

If you just use the 26:25 4 then you shorten 4 to the 120 seen in the second set, leave 5 alone for cruise. The only downside is the bigger 4-5 gap but if you aren't using 5 in anger, so what?

Hello,
Based on what I see in the initial post, I have a sneaking suspicion that confusion around how 915 gears are traditionally listed versus how G50 gears get listed have led to a misunderstanding with respect to exactly what gears the shop is offering.

When I chart it, there just doesn't seem much point in installing the 4th gear they seem to be suggesting ( QP 26:25 0.96). This is the gear that Bill has charted.

A far more likely option is that they have a PQ 25:26 1.04 4th that they are trying to plug in there. That's a bit tighter on the 3-4 shift as well as offering a reasonable RPM drop on 5th that makes their suggested 5th quite useful in pulling hard up from roughly 110-130mph.

I suggest you go back to your builder and ask him specifically the letter designation of the gear to clarify what you are getting. If it's a QP, I wouldn't bother. If it's a PQ, they are steering you in the right direction and it's a good option for 4th regardless of whether you go with their suggested 5th or something a bit taller to preserve some highway cruising ability.

---Understood more-or-less. This is all new to me as is the nomenclature and/or numbering.

I'm not sure which number should be the numerator and which the denominator.

That said, Assume I have an 11/35 first then the shop is recommending the 25/26 4th not the other way around.


Bill, Does this look "less ugly"?

Thanks again

JMZ,
Looks like you have a nice sporty 3.0 in your car. Changing your 4th gear to a "QQ" 1.00 will make 116mph @ 7000rpm and changing your 5th to a "SN" 0.82 will make 141mph @ 7000rpm. This also reduces your RPM drop from 3rd to 4th, will now be 1,448rpm, and a drop of only 1,249 from 4th to 5th. Way more sporty in the high gears now. Just my suggestion.

it is a pretty fun little engine ...the hp figures above are at the wheels...
so what can i expect if I go with the PQ that the shop recommended in 4th? How about the 5th?

Bill, I'd love to see one of those cool charts.

Would the PQ or 25/26 be too low?

gears can be speced either way, but based on the above the first # is the numerator the second is the denominator. 1-3 are underdriven 4 & 5 are overdriven.

following the format previously used this is an underdriven 4 and an over driven 5

changing that to 25/26 overdriven 4 and retaining the overdriven 24/27 for 5 gives this


http://forums.pelicanparts.com/uploa...1202421330.gif

I like the short drops in 4 & 5 but the speed in gears seems way low

I like that idea a lot!


http://forums.pelicanparts.com/uploa...1202421732.gif

Keep in mind evenly divisible gears are to be avoided when possible. With a 1.000 ratio, the same gear teeth are meshing every single time.

http://forums.pelicanparts.com/porsche-911-technical-forum/241180-theorys-deciding-short-gear-ratios.html

And you shouldn't create unintended ratios by exchanging gears. For instance, a overdriven QP 25/26 should not be reversed to "create" an underdriven PQ 26/25. Only a PQ with the gears arranged/cut as intended should be used. I wasn't implying the guys in this thread are suggesting that. Just mentioning it for someone else who may consider that idea.

Agreed the last setup gives nice spacing and the car will be in the powerband at redline shifts from 3-5. Just don't forget about 5th cruising speed if you drive your car to the track. The SN gives you almost 5000 rpms at 80 mph. That could get tiresome. If you don't drive your car on the highway, then who cares about cruising rpm?

I get 3800 in 5th not to be a pita here. Am I missing something?
Also, Kevin can you explain more about evenly divisible gears? I am slow on the uptake today.
Thank you! Great thread
Bob

Thanks guys I appreciate your shared knowledge.

I need to learn how to do this stuff.

I wonder how the car would be tranformed if I put a 8:31 r/p in it instead...

I assume with the shops 4th 5th it would work pretty good but I don't know if I want to sacrifice that 2-3 excitement the car has now.

[QUOTE=KTL;3754934]Keep in mind evenly divisible gears are to be avoided when possible. With a 1.000 ratio, the same gear teeth are meshing every single time.

good information but a question. Don't all the SC's have a 26/26 4th? I don't recall hearing about an abundance of 4th gear problems with those cars:confused:

Kevin makes a good point about the 1.00 ratio, evenly divisible, each of the same teeth see contact with each other every revolution. This might cause a fast wear pattern on the teeth. But in 4th gear seems OK compared to having a 1.00 in 5th gear position. You stay in 5th under load way longer that all the other gears. And yes, QQ 4th (1.00) was a stock gear in some of the 915's.

Well, with the 901 box guys flipped gears all the time. However, for a 915 performance box it is not desirable to have an overdrive gear short of 5th, so it isn't an issue.

SCs from '75 through 86 all had a 1:1 4th.

Walt

[QUOTE=jmz;3755014] You don't seem to hear about it all that much but it's definitely an issue. In fact it impacts all 3 of the 4th gears we've been discussing here. I've got stacks of 1.04's, 1.00's and 0.96's that are worn beyond use from this problem...

As others have mentioned, the QQ 26:26 is probably a better option if the car is going to continue to run a slightly taller 5th to allow for highway cruising. It's always a balancing act with these things. There's no really such a thing as a single best set up as much as a best compromise for the conditions.

Higher average highway speeds and more power have really changed things in recently years. It wasn't that long ago that a stock 901 "X" 0.88 5th gear was considered an acceptable highway gear. That's essentially the same ratio jmz's shop has suggested with the RO 5th and we can all agree that it's too short for the highway today. But it's still a great top gear for a short track or novice racer who isn't driving the car to the event.

OK, I'm liking this idea and think it is the best approach.

Now Does anyone have or have a source for a QQ (25/25) or a SN. I think this would be a (28/23) ...same as a (39/32) ...right?

For that matter would a (26/26) as from an SC Gearbox be the same as a (25/25)?

Thanks all.
I think this made a good thread on the subject.

Cheers,
Jay

looking at an old post of Bills and one of his charts I have to ask.

Is the gearbox that 356RS recommends just the same as a stock 915/61 63 just with my 7:31 Rp in in it? I assume that a 25/25 is the same as a 26/26 correct or not? the s/n is a 28/23 right?

Getting more confused by the second!!!