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3.8 vs 3.6

 
Old 11-21-2018, 06:51 PM
  #16  
Bill Verburg
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Originally Posted by nowata View Post


Steve can you provide a chart or graph that compares this? Like speed vs rpm for each gear? And is an LSD install the right time to do it?
What am I, chopped liver? if you guys don't care for my input I'd be happy to stop posting

This is an amended version of the one above, for years it's bugged me that there was an error in the above version, I finally found it.

A g50/30 is a factory racing transmission used in 993Cup & 993RSR, it has pretty much ideal splits for 3.8 running 7500 to 8000rpm. Used w/ a street motor limited to <7k it loses some of its edge
In 1 & 2 & 4 the stock /03 or /05 will give better acceleration, the beauty of the /30 is the closer spreads provided by the extra gear, to get the /03 or /05 to do that top end would be severely limited or a high step 5th used


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Old 11-21-2018, 08:11 PM
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Here's what a typical 993 3.8RS vram ina 964 w/ g50/03 looks like

Last edited by Bill Verburg; 11-21-2018 at 09:20 PM.
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Old 11-21-2018, 09:38 PM
  #18  
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Bill - interesting chart- seems like a wide difference though.
So, if I already have a close box - following is rough speed calculation
1 = 2.692 - 54 at 6500
2 = 1.991 - 75 at 6500
3 = 1.45. - 100 at 6500
4 = 1.13. - 128 at 6500
5 = 0.92. - .92. at 6500
Engine has 993 pistons, valves, Carroll rods, stronger valve springs, polished intake ports, 993 Varioram, custom exhaust, stock 964 cams and a Motec M84 with coil on plug. Dyno figures show 293-297 HP. Car has been lighten significantly as am at 2346lbs with 150 lbs in driver seat and half tank.
What I am reading here is the best I should expect is a 5% increase or around a 15 HP increase. So is there more to be had if compression is increased and I change cams?? Just trying to figure out where the best bang for the buck.
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Old 11-21-2018, 09:39 PM
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Ooops 5 is 158 at 6500
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Old 11-21-2018, 10:54 PM
  #20  
Steve Weiner-Rennsport Systems
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Originally Posted by nowata View Post


Steve can you provide a chart or graph that compares this? Like speed vs rpm for each gear? And is an LSD install the right time to do it?
Bill is nice enough to share a virtual boatload of data here on the effects of many different gear ratio options and I'd strongly recommend taking advantage of his generosity with that.

Indeed, this IS the right time to install a proper (Guard) LSD since that too, makes a very big improvement to the handling, not just wheelspin control.
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Old 11-22-2018, 12:54 AM
  #21  
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Bill knows his stuff. If I knew 5% of what he has forgotten, I’d be way ahead of the game. Thanks Bill!!!

mike
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Old 11-22-2018, 06:48 AM
  #22  
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Here's a question...given 993 6 speeds are impossible to obtain these days/ expensive even before a rebuild, how would you re-gear a standard 964 G50 5 spd to get the best out of a street/ race 3.8? Also can you/ would you change the final drive to 8.32?

Only essential parameter would be that 5th must be decent for motorway cruising - say 2500 rpm at 80 mph?

It would be great to see that on the same charts above/ against the same benchmarks.

Fyi Bob's knowledge is incredible...im sure there are many 'ghosts' who value every one of his posts - I know i do!
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Old 11-22-2018, 09:27 AM
  #23  
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Originally Posted by Gus View Post
Dyno figures show 293-297 HP. Car has been lighten significantly as am at 2346lbs
Awesome!
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Old 11-22-2018, 11:14 AM
  #24  
Bill Verburg
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Originally Posted by Gus View Post
Bill - interesting chart- seems like a wide difference though.
So, if I already have a close box - following is rough speed calculation
1 = 2.692 - 54 at 6500
2 = 1.991 - 75 at 6500
3 = 1.45. - 100 at 6500
4 = 1.13. - 128 at 6500
5 = 0.92. - .92. at 6500
Engine has 993 pistons, valves, Carroll rods, stronger valve springs, polished intake ports, 993 Varioram, custom exhaust, stock 964 cams and a Motec M84 with coil on plug. Dyno figures show 293-297 HP. Car has been lighten significantly as am at 2346lbs with 150 lbs in driver seat and half tank.
What I am reading here is the best I should expect is a 5% increase or around a 15 HP increase. So is there more to be had if compression is increased and I change cams?? Just trying to figure out where the best bang for the buck.
More important than the absolute value of the # is the shape and extent of the torque curve

Here are examples of 3 torque curves, the 3.8 is ~ 290hp at the rear wheels the, 3.6RS is ~270 and the stock 3.6 ~ 240
the mass that has to be motivated also enters in but for comparison purposes can be ignored
The 3.8RS w/ vram has a very wide flat torque curve, it benefits much less from close gearing because of this, it almost doesn't matter what gear is selected. The 3.6RS has much peakier torque curve(although it has the same cams as the 3.8RS the effect in the smaller motor is peakier) than the other 2 and so benefits most from gearing that can keep the revs in a narrower range. None of these engines have really extended rev ranges which tend to be peakier and which also benefit the most from close ratio gearing.


Go back and look at the 964 torque curves, here the effect of aero is added to the raw thrust curves, drag has a significant effect more so in 5 than 1, the stock early 964 G50/03 is really pretty good for a stock motor, you want the tail of a given gears curve to just meet the next up's curve w/ no extra, 1 & 2 will almost never satisfy this condition look for it only in 3 up. When the motor is modified for more power the right end of the curve tends to be lifted, this may cause some separation which re gearing can address


Here is a comparison between a 993Cup w/ 7500rpm and a 964RSA, the extra rpm range really allows the gearing to work



Here a pretty healthy 964RSA w/ g50/30 note the gaps at the ends of the curves, either more revs or closer gearing are desired


I don't have a 964 cwp change handy but here's a 993, yes you get more thrust but the cost is less speed, there is no change what so ever in the gaps, using a shorter rear tire has a similar but less drastic effect, whether or not any of this is beneficial depends on the usage and specific track, My own 993(3.8RS w/ g50/30) is not ideal at a place like Lime Rock but is much better suited for Watkins Glen, In street use it is also great.
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Old 11-22-2018, 11:23 AM
  #25  
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Another comparison, My 3.8RS compared to a friend 3.9 w/ some really nice cams and ITBs topped by a GT3 intake manifold, this one really shines at high rpm and loves closer not necessarily lower gearing


Lastly I did this to compare the 3.6RS in my 911 to the 993, the 911 has a 915 trans and the biggest isse is that I am always tryig to grab a 6th than isn't there
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Old 11-22-2018, 01:16 PM
  #26  
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Great info - really like the Rsa 50/05 vs 993 cup chart. Here is that in real life in 2nd and 3rd gear at a race start a few years ago along w a 997 C2 and a 964 50/03 right next to me. I had a great race w the 964 C2! / g stock as I am in my RSA.


My biggest disappointment w my RSA since I bought it in 2003 is the gearing. The gap between 2nd and 3rd is terrible and 3rd and 4th are too long. The entire boot at the Glen is miserable esp the toe. Still love driving my car of course and gearing is about perfect for my home track, Summit Point. Couldn’t be much worse for the Glen though. The 03 is slightly better at the Glen. Wish I could run a 8:32/ 4.00 in stock class at that track. Tremper’s car zooms up the toe there in H prepared.
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Old 11-22-2018, 03:09 PM
  #27  
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For those that don't know it the "Toe" @WGI is a very special turn, 100' climb from 1460' to ~1560' in a U, In most cars in 3rd gear shifting to 4th at the top, it rewards light weight, gearing and torque

The other big climb is into the esses starting @2, this one allows you to carry a lot of momentum into and through the climb, very fast in 4 or 5 depending on how fast you enter #2, the fastest straight is the back because of the speed carried through the esses.
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Old 11-22-2018, 03:34 PM
  #28  
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Yes it is too fast for second and too slow for third- as are the next two corners - basically any 58-75 mph corner sucks w stock gearing. Iirc pre repave my min speed there was about 61-2 which puts the motor around 4K and is miserable going up that hill. 275/35/18– 25.5”.

sorry for the detour Gus!
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Old 11-22-2018, 09:14 PM
  #29  
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Gus- try out this app. I used it and while not full featured it was very useful for looking at power verses gearing. Input the dyno plot and gears etc and you can get a basic view of the wheel torque per gear and you could get an idea- pros and cons of adding power in a specific range.

Shift Rpm







Your gearbox seems really tight for the broad spread of power with the v-ram. I bet using this app you would see a lot of overlap between gears. Where the lines overlap shows the ideal shift point for power- the point at which the next gear is making more power.

It seems that if you’re looking for all out power you might benefit from boosting power in the higher revs and raising the redline. But who knows- looking at various graphs you may just as well find the money better spent on different gearing instead.

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Old 11-23-2018, 06:48 PM
  #30  
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Subscribed! Thanks for all this data guys. I sent Bill V. my gear ratios and dynapack data. I think it would be a nice addition to see how very short gears work with an internally stock varioram 3.6 with exh/tune.
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