(Yet another) 915 new gearing question

[NigelJones]

Dear all,

I would be grateful for some good advice for the 915 gearing on my 1973 RS replica.

There is a very good and very detailed post on a similar build here - Gearing 915 for Street Performance: I have read that post a good number of times and hope that I am close to the ideal solution below but would still welcome some feedback. In particular, I dont have the knowlede to know how to analyze the impact on the car's torque curve.

My criteria are very similar to White911SC's as set out in his post referred to above:
- 915/63 box, regeared on 2nd - 5th
- seeking tighter gear spacing without making it unpleasant on the highway

My situation is a little different though:
- the car is a 1973 RS replica with a 2.7l engine and 16" wheels
- the engine is being rebuilt close to the original spec (2.7l, RS cams, Weber carbs)
- I am using the car as a fast road car (around town, B roads and short sprints)
- I am not using the car for long distances (probably no more than 1 - 1.5 hours max) and will not be taking it to the track (I have other cars)

I followed a similar path to White911SC above - I used the Instant G gear model and started by looking at the original RS spec (seems to be about 3,600 rpm at 75mph). I then used a tighter gear set and had about 3,900 rpm at 75mph so did a second scenario with 75mph at 3,800 rpm.

In the pictures below, I show my understanding of the original RS spec, then scenario 1 and then scenario 2. I don't have the torque curve for the engine yet (still being built), but my mechanic assures me it will be close to spec. So, assuming a standard 2.7 RS spec engine, is scenario 2 about right for a fast road car or have I overdone it? Like White911SC, I wonder if 3,800 rpm is too much at 75mph. I have gone from 5th at 26:26 (scenario 1) to 26:25 (scenario 2) but I could go further to 27:25 etc if recommended.

Here are the numbers and pictures:

Input data:



Reference car - Original RS output:



Scenario 1 - output:



Scenario 2 - output:



I welcome your advice, many thanks.

Nigel Jones

[gliding_serpent]

Those look like good gears for rally, or a tight racetrack. If it were me i would look for gears, that when paired with the 8:31 r+p, mimic the original rs final ratios. Your options are short. I would go with a longer 2nd.. 2, or 1.93, and work up from there

[gliding_serpent]

I will do my best to add to this given my 1.5year old son is literally crawling over me.

First thing you do not need a torque curve. just google a dyno from a similar engine build.

Matt gave me a few great suggestions. I settled on 1.93, 1.38, 1.08, 0.889 for various reasons, but I have a 3.4 and track considerations were a factor (especially the length of third gear), as was day to day livability. In it's 3.2/chip/cat bypass days, my car had oem gears with a 7:31 R+P. Post 3.4 I went 8:31 with short gears. These gears were about 8% shorter than my cars stock gearing (but longer than with the 7:31 R+P), but with 30 or so extra hp, 250lbs less weight, and R compounds for our track, I did not want to run out of revs (or spend all my time near redline). My engine was also low revving with a very flat torque curve. I am not racing and you need to compromise sometimes. This sounds to be too long for your needs.

What I wanted to do, but chickened out of, was the following (again Matt advised)
2.0, 1.429 (on order last I checked, but he had 1.409 in stock), 1.125, 0.923
I think this gets you pretty close to the actual RS gearing if you run the numbers (I did not, just guesstimating). I think this would be good for you actually.

Another thought is a focus less on making the gear ratio short, and more on making it close. The idea here is a longer 2nd (your really short 2nd will not see much use at 2.059), and then short steps after that. Mind you, with a peaky engine, that 1st to second shift could be sluggish, unless you rev 1st high. Or go super short and close with your 1st 4 gears (Are you really planning on going over 120mph on the road? If you are, check your head), and then go with a longer cruising 5th.

Lots of options. Toughest decision for my build. Again, in the spirit of the rs replica, I would try to mimic the RS gearing (but with the 8:31, so shorter gears). Porsche knew what they were doing.

[NigelJones]

Many thanks for the quick thoughts, much appreciated.

The more I read and study the art of gear ratios the more I realise it is not easy ....

I think I started in the right place, by trying to see what Porsche had in the original RS. So I mapped the ratios in the first chart. I then just moved everything about 15 mph slower since, as you say, you can't go very fast on the road. There is a different CWP, wheels and tyres but I tried to make the gap in mph the same between gears and made sure the rpm drop gets smaller. There is another very good post explaining this point, so I followed that advice.

I understand your advice that the 2nd is too short, so you would recommend making the 2nd a little longer with 2nd, 3rd and 4th slightly closer together? Would you recommend 5th slightly longer (not so much to go faster but to reduce rpms at 75 mph on the highway)?

I can model another scenario at the end of today.

Nigel Jones

[gliding_serpent]

It all depends on what you want really. There is no right or wrong, just what you want. I personally do not like the longer cruising 5th... simply because... because I like the idea of 5 gears designed to work together. Screw highway comfort, screw practicality, my car was becoming a hotrod and I was not going to "wimp out" of 5th. Like you, my car is for back road fun. That was not scientific however. With my stock gears and 7:31 R+P I ran redline in 5th once. It was fun. Did I say fifth? I meant 3rd. The fun of short gears is that you might actually get to experience all of them.

I designed my car's gearing around 4th and 3rd, key gears for our local track (you don't need 5th, and I wanted to avoid needing fifth, and 70-80% of our laps are spent in 3rd).

Really short gears is great in really tight spaces, like hairpins and such (rally and autocross). But think about where you cruise. Few public roads have 2nd gear corners, and if they do, it is a 90 degree turn with a stop sign. For me, the fun is around 60-90mph on the street. You can do that on good quiet twisty back roads easy. That is 4th gear, or 5th gear cruising if you are not interested in the car being quite so high strung. As such, your car does not see 2nd much at all, accept to get rolling, and maybe putt behind slow traffic. But then I enjoy carrying speed and being smooth more than anything when I drive. Heavy breaking and heavy accelerating is for the track.

Gear choice is not easy at all. And short ratio is not always better (it would be for your engine). Yes, you get more mechanical gearing advantage, but it is offset by less time spent in a gear (offset in your car with high redline and peaky powerband). Plus shifting slows you down.

also remember that if you go too short, you will just end up being one gear higher at any given speed, which kills the gearing advantage of short ratio a bit. If that makes sense. In other words, your 1:1 5th, is the same as the stock 4th. If you spend most time cruising there, you would do fine with a stock gear ratio. There is more to it than that, but I think you get my point.

If it were me I would "baby bear" this one. Not too short, not too long, just right. I think your options are pretty aggressive. What looks sexy on paper might not be as fun (relaxing) in real life. You also want more aggressive than the original RS. Split the difference and go with something between your, and the RS gearing. The good news is that any decision you make is correct, because this is not a track car.

My final say in this will be to say that I am not the one to advise gearing for you. Call Matt from Guard up. He will not lead you astray. Like you I am a learner. I am just hoping that I am helping you to think about your choice from different angles of pros/cons. That being said, your gear ratios look great, you just need to be sure you want to go that aggressive.

[NigelJones]

Thanks again for the input. I take onboard your advice to 'baby' the gears. You are right, this is a road car, I want to get a little more excitement but I don't need to go crazy.

Here is another scenario. This is what I have done:

- I have moved gears 2, 3 and 4 to the right (lower ratio) so that the gap between 1 and 2 is larger but 2, 3, 4 and 5 are closer together
- I left 5 as the previous scenario at 26:25 or 0.962 at this gives me 3,800 rpm at 75mph and I don't expect to drive so fast in this car (although I welcome advice if this is still too aggressive)
- I googled a torque curve for the 2.7 from analogueman. It looks like the best torque is 4 - 6,000
- I then looked at the speed in this band to see if the engine remains 'in the band' as you change gear. There is overlap, so it seems to work.

The input is:



The output is:



The torque that I found is (but not verified if correct for me yet):



And finally, here are the speed/ rpm in the 4 - 6,000 rpm band:



Am i getting this right?

My only question is whether the box is still too agressive and whether I should move all the gears to the right but the reality is in town and surburbia it is not possible to drive quickly these days.

Nigel Jones

[Bill Verburg]

Here is a stock '73 2.7Rs w/ 215/60x15 tires
7:31
11:35,18:33, 23:29, 26:25, 29:22


It's nice but if any change was made for mostly street use
3 could be a tad shorter and 5 a tad taller to eliminate the tails

for mostly track use
3 could be a tad shorter and leave 5 alone

shorter or taller tires won't change the gearing selection only the top speed in gear and acceleration curve slope in each gear

[Matt Monson]

I'm a big fan of the 1.93 2nd gear. It really wakes it up on a street rod. Has use around town and on back roads. Also helps to reduce synchro wear on 2nd gear because the 1-2 upshift is tighter. You will still want to rev 1st up a bit more than the 3000-3500rpm so many people shift at (mistakenly I might add), but if you do, it will snick into gear quite nicely.

I would also just leave 5th stock. Unless you are on the highway you don't use it or need it. Don't spend money on a gear that you don't use. Put top of 4th at 120-125mph and build it backwards from there. 1.93, 1.409 and 1.125 is a nice stack with stock 1st and 5th. You can go shorter if you like, especially with an RS spec engine.

[NigelJones]

Bill, Matt

Thank you both for your input.

I took your advice and did the following:
- I kept the stock 1st (3.182)
- I took Matt's advice on 2nd (1.933)
- I struggled on whether to use the recommended 1.409 or my prior choice of 1.429, so I modelled both and then mapped out the power band of 4 - 6,000 rpm and felt that the overlap of the 1.429 is slightly better (I show the table below)
- I took the advice for 4th (1.125) which is what I aready had in the model
- for 5th, I looked at the standard ratio in the 915/63 box (0.821 23:28) but felt that it was just way too fast with the 8.31 CWP, so I amended my prior choice of 0.962 to 0.926 (25:27)
- this leaves me with: 3.182 1.933 1.429 1.125 0.926

The ratios and rpm drops are:



which brings the rpms at 55mph and 75mph close to the original 1973 configuration (so I know that my ears will be fine on the highway)

The rpm data used to choose 3rd is:



And the output graph is:



Hopefully this is close to the best solution for a fast back road car with some use on the highway. Any more feedback would be most welcomed.

Nigel Jones

[gliding_serpent]

Sounds great to me!!!

[Bill Verburg]

Originally Posted by NigelJones

Bill, Matt

Thank you both for your input.

I took your advice and did the following:
- I kept the stock 1st (3.182)
- I took Matt's advice on 2nd (1.933)
- I struggled on whether to use the recommended 1.409 or my prior choice of 1.429, so I modelled both and then mapped out the power band of 4 - 6,000 rpm and felt that the overlap of the 1.429 is slightly better (I show the table below)
- I took the advice for 4th (1.125) which is what I aready had in the model
- for 5th, I looked at the standard ratio in the 915/63 box (0.821 23:28) but felt that it was just way too fast with the 8.31 CWP, so I amended my prior choice of 0.962 to 0.926 (25:27)
- this leaves me with: 3.182 1.933 1.429 1.125 0.926

The ratios and rpm drops are:



which brings the rpms at 55mph and 75mph close to the original 1973 configuration (so I know that my ears will be fine on the highway)

The rpm data used to choose 3rd is:



And the output graph is:



Hopefully this is close to the best solution for a fast back road car with some use on the highway. Any more feedback would be most welcomed.

Nigel Jones

That proposed gearing w/ the original 2.7RS engine looks like this

The issue I see is the tails where 3 crosses 4 and 4 crosses 5, you have wasted rpm when you see those



here's one that is better w/ less wasteage, it is based on MM's 1.93 2nd suggestion, I'd have to go dig out the 3-5 ratios.

You engine may or may not have a similar torque curve so yours may want a different setup.

[gliding_serpent]

Also remember crossing tails is not the end of the world. On the flip side, "wastage" can be seen as giving you flexability, or options. Running to redline will be just as good as short shifting. Choose what works best based on the corner you are approaching (you may not want that extra shift up). I also see crossed tails as being more realistic for real world driving.

In a perfect world the tails never cross, just touch, and you shift a 7200rpm (or whatever your refline may be) everytime. Great for racing. No gearing wastage.

Now, back to the street world, the huge majority of shifting is at ~4 k rpm. You might go up to 5, even 6 now and then. For your car you might go to 7, but i sm sure not on a regular basis, maybe a few times a drive.

I studied other local driver videos at my track to guide my gearing choices. One very experienced driver in a very fast 3.6 sc hotrod short shifted every time by 500-800rpm. His redline was 6800rpm and he would shift at 6, maybe a touch higher. He did not lack speed. If anything, his car was grip, not power limited, so the high rpm stress on the engine was not worth it. Mind you... The 3.6 has a fat torque curve so he can get away with it.

What i am saying is, adjust the redline to where you will shift in the real world (or evrn a more realistic 200-500 below redline), and those tails above in Bills charts will no longer be crossing, and may in fact be perfect for your needs. Now, if you do plan on redlining at 7200rpm for every shift on the street... You have adjustments to make (gearing, and police scanner). Plus, it is hard to shift the split second before the rev limiter kicks in and really kills your power.

Or another way to look at it, if you pick a gearset with no tails crossing, and dont shift at redline everytime, you are leaving performance on the table, where as a gearset with crossed tails gives you options to short shift with less of a performance hit, and keeps your rpm's higher post shift. You are still leaving performance on the table, but you are being more realistic about your driving habits.

Everything is a compramise. No answer is right or wrong. Just build things based on how you will drive it.

Food for thought

[NigelJones]

Bill

Thank you for the advice.

Just to make sure that I am understanding your graph correctly, could I confirm that your graph is showing torque against speed? And, that when you aim for no crossed tails you are aiming for a 'box that allows you to get to [redline?] or the end of the 'meat' of the torque curve and that when you change up a gear you are then right at the bottom of the 'meat' of the torque curve? Did I get that right?

Perhaps put another way, with my simple thinking in the table above I have highlighted the speed through each gear where the rpms are in the range 4 - 6,000 rpms (highlighted in yellow) as I understand that this is the 'meat' of the torque curve. I am an enthusiastic amateur driver with limited skill so I thought that it is best if I drive the car up to maybe 5,500 rpm and then when I change up a gear the engine will be at about 4,500 rpm so I remain in the 'meat' of the curve. So, by my logic I was thinking that overlap is better (or at least easier to drive). I can see that for a fast (high speed) car this approach does not work as you run out of speed ... but for a more moderate speed aims, it should be easier to drive?

Please do correct and guide me on this if I am back to front.

gliding_serpent ... have I just said in a long winded way the same advice that you gave to me?

Nigel Jones

[Matt Monson]

On an RS engine peak hp is 6300 and peak torque is 5100. No point in building an RS Spec engine that revs to 7200 rpm if you are going to shift so low. It won't be cammed right and you will be losing a huge amount of acceleration. If you really are going to use the shift point you mention,I would reconsider the engine spec. Put in a torquer less pesky cam than an RS spec one.

Couple other comments. Aftermarket 5th gears will make noise on the highway. I tell you that. My competition kind of glosses over it and pretends it isn't the case. Don't get suckered by a salesman.

Also, when the speed limit is 75, guess how fast I cruise in my Carerra? 80-85. I think this is common. Where I live that is just the interstate speed unless you camp in the slow lane getting your windshield pelted by rocks from the tractor trailers.

[gliding_serpent]

Nigel, bill can answer better, but he post acceleration/thrust curves vs speed, and it takes in account things like drag. More valuable than torque alone. I am not sure what redline he is using (the right sided tail of each curve) but he often runs 6800rpm in his generic calculations.

The redline used in his curves is key to know.

Also remember we are starting to really split hairs here.

Your logic above is on the right track. However as bills accelleration graphs above show, for max acceleration you often stay in gear beyond the rpm giving peak power/acceleration, because even as acceleration/power falls off as rpm's go up, the lower gear mechanical advantage for acceleration is still better than shifting up, even if it brings you back into the meat of the power curve by shifting. The cost is engine stress.

[Matt Monson]

Bill runs thrust curves. That is the power to the ground at speed for each gear. When they overlap you are not using maximum efficiency of the gearing, which as gliding said, isn't a always a bad thing. But it is not the way to quickest straight line acceleration.

[Bill Verburg]

Originally Posted by NigelJones

Bill

Thank you for the advice.

Just to make sure that I am understanding your graph correctly, could I confirm that your graph is showing torque against speed?

No, the ones that I posted in this thread are acceleration vs speed

net thrust after drag is added will have the same shape, ie maxima, inflection and end points just a different magnitude at any given point. I find these to be the most helpful for choosing gearing when you know what your engines torque and drag curves look like.

Originally Posted by NigelJones

And, that when you aim for no crossed tails you are aiming for a 'box that allows you to get to [redline?] or the end of the 'meat' of the torque curve and that when you change up a gear you are then right at the bottom of the 'meat' of the torque curve? Did I get that right?

when the tails cross it is a sign that you should shift up, you have less acceleration by staying in the lower gear, the net thrust curves are all limited by the 7200rpm red line except for 5 which is aero limited to 152mph, w/o aero the theoretical top speed is 157mph(215/60x15 tires & 7200rpm, shorter tire will have a shorter theoretical top speed)

any torque curve looks like the top of a mountain, some are narrow and peaked some are broad and flat, the engines torque curve combined w/ rpm is what generates hp, the hp peak is always to the right of the torque peak for this reason,hp is the rate which torque is applied, acceleration comes from the rate that torque can be applied ie hp. The goal is it maximize the area under the hp curve which means keeping the rpm in the range bounded by the red line on the right and the torque peak on the left

for a 2.7RS 5100 to 7200 is the 'meat' of the rev range which you want to stay in
Now we look at this view of the transmission


here we can see that the max drop is 3051 going from 1 to 2 this keeps the revs is the desired range
all the succeeding drops are smaller keeping the rpm in the range which maximizes the area under the hp curve except for 4 to 5, 5 was chosen as an Autobahn gear where the car will be aero limited, if you are ok w/ a higher cruise rpm, then a slightly shorter 5 would be chosen which would eliminate the aero restriction and reintroduce the rpm limit as in all the other gears

[NigelJones]

Matt, Bill, gliding,

Got it! Many thanks, this is a very helpful walk through the technical analysis and is a useful explanation for me. Hopefully this is of value to those that follow.

Matt - I will PM you on prices and availability etc.

Thanks,

[Bill Verburg]

Just for your information, here's the factory hp and torque curves for a 2.7RS


The part of the hp curve you want to stay in for max accel is ~5200 to ~7200, friends w/
these engines claim to be able to rev to 7800 but the potential for damage is high, this engine actually ran a few laps w/ the snapped crank



This one didn't run far at all