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Best 4 speed gearing with 500+HP

What would people in the Know suggest for gear stack for a four speed. The motor should have around 500 or better HP.
Old 05-14-2010, 07:13 PM
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Stock.
You'll have plenty of power for those long gears.
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'78 911SC Widebody, 930 engine, 915 Tranny, K27, SC Cams, RL8 Headers & GT3 Muffler. 350whp @ 0.75bar
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Old 05-14-2010, 08:26 PM
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My two cents:

I do know that they can make a car much faster by keeping one at a higher HP level more of the time. However, it is best if they fit your build and intended goal.

Might want to build your motor first and get the dyno HP curve. With this, your final rear tire size and a clear goal on what it is going to be used for and you can then work up gears that will maximize your performance.

Think: street, street/track, track/street, of track gears.

I agree with Brian that with that much power at some point if it is a good mid range HP build that many seem to grow into the stock gears to a degree.
Old 05-15-2010, 06:37 AM
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Rule of thumb;
The "peakier" (is that even a word? ) the torque curve, the more the platform requires close-ratio gearing.

The flatter the torque curve the less important gear spacing, or the number of gears for that matter, becomes.
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Old 05-15-2010, 06:52 AM
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Even with high hp I find the gear spacing to be too large. First gear is fine but the jump to 2nd, 3rd, and 4th could be shorter. Unfortunately I haven't had the budget to change gears so I don't have any real world experience.
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Last edited by David; 05-15-2010 at 02:57 PM..
Old 05-15-2010, 08:00 AM
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I made a few assumptions with my short answer and should state them.
> 500WHP means you will use EFI and headers and should have a very broad torque range.
> This is a mostly street car as track cars use completely different gearing that is not fun on the street (tall first and short 2,3,4). I just went through this with a fellow very dissatified with his 500HP street car with track gears.
If these assumptions are not correct we need more info.
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'78 911SC Widebody, 930 engine, 915 Tranny, K27, SC Cams, RL8 Headers & GT3 Muffler. 350whp @ 0.75bar
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Old 05-15-2010, 08:04 AM
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For referance, assunming about a 6500rpm shift point a stock 4 speed's shift points are about as follows:

1st to 2nd is a shift at about 53mph with about a 2700rpm rpm drop.

2nd to 3rd is a shift at about 94mph with about a 1900 rpm drop.

3rd to 4th is a shift at about 137 mph and about a 1800 rpm drop.

Red line at 197 mph.

It is fun to look at your dyno graph and plug in your HP numbers at the begining and end of each gear.

If one built a 930 motor that has a shift point of say 7200rpm this exadurates the rpm drops and the first to second shift is more like 3000rpm.

Not many builds including turbos have that wide of a power band. Most build make great power over about a 1500rpm to 2000rpm range.

----

IMO the large 1st to second rpm drop is the biggest problem with a 930. That drops the motor down to about 3800rpm at just under 60mph which is right in the middle of where most street wars are going to take place. On the track, this is probably going to kill ones speed right as they enter they leave the slowest corner that is right at the begining of the most important long straights on a track.

Even with a monster build that is going to see a greater needed drop in performance with the shift into second on a stock 930 box.


When doing a trans build I would first ask myself what I want first gear to look like. Stock, higher, lower.

Second question is, what do I want top speed to be.

Third, question if it is a street car is what rpm at cruse speed can I live with.

From there, 2nd and 3rd are best fit to keep the motor in its best power band depending on use. It could be near equal rpm drop spacings or lean toward better acceleration early or better acceleration down the longest straight.

Some street builds might end up with closer 1st, 2nd, & 3rd for max street racer acceleration and then a tallish 4th for cruse.
Old 05-15-2010, 02:37 PM
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I never thought about the bigger drop in rpm due to shift point. I shift at 7000 or so which may be why the 2nd gear shift feels like so big a drop.
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Old 05-15-2010, 02:48 PM
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RPM in any gear directly correlates to speed. While a drop in RPM between gears is static the feel is dependant on where you fall in the torque curve after the shift.
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Old 05-15-2010, 03:15 PM
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David,

What is your Peak HP, HP at 7000rpm and HP at 4000rpm which is about where you come in when you shift to 2nd?

Also, how about your TQ at the same points for those that like to think in TQ.

It would be an interesting reference.


Brian et al,

Please, please lets try to not get into another TQ v HP discussion. It will just ruin yet another potentially good thread.

Maybe we can just agree to disagree on that point or refer back to another thread where we beat the vocabulary and math of it to death and almost come to the understanding that we are kind of talking about the same thing.

Respectfully, Thx.
Old 05-15-2010, 03:29 PM
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I don't have my dyno chart handy but my peak hp is about 6500. It still pulls strong to 7000. I try to shift before 7000, but that's a little difficult in 1st.
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Old 05-15-2010, 04:57 PM
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Well the original poster hasn't come back so no pertinent information to plug in anyway.
Re-gearing a 930 tranny is an expensive endeavor. I wouldn't do it myself until I was certain that the engine was never going to be fiddled with again. Every time you change the level of power or the RPM range where power is made you affect how the gearing works for you. Think diesel truck vs motorcycle.

Sorry Keith, you simply cannot have a discussion about gearing without bringing up the torque curve. That would be like trying to calculate your speed with no regard for tire height. There is a direct relationship.

I would suggest that the original poster get ahold of Bill Verburg. He has a ton of great info on the subject.
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Old 05-15-2010, 06:00 PM
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Quote:
Originally Posted by RarlyL8 View Post
Well the original poster hasn't come back so no pertinent information to plug in anyway.
Re-gearing a 930 tranny is an expensive endeavor. I wouldn't do it myself until I was certain that the engine was never going to be fiddled with again. Every time you change the level of power or the RPM range where power is made you affect how the gearing works for you. Think diesel truck vs motorcycle.

Sorry Keith, you simply cannot have a discussion about gearing without bringing up the torque curve. That would be like trying to calculate your speed with no regard for tire height. There is a direct relationship.

I would suggest that the original poster get ahold of Bill Verburg. He has a ton of great info on the subject.
+1

Go to this link and start at post #34. Torque Effect

Bill Verburg has done a ton of research on the subject and clearly knows the difference between Torque and Horsepower. And there is a difference, race fans.
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Old 05-15-2010, 06:20 PM
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I have not finished the build yet getting close so the hp figure is just a dream. It's a 3.4 with carrera intake. The heads flow a little better the big port SC heads. Twin t3 t4 turbonetics turbos, 993 heat exchangers, twinplugged, GT2 evo cams and knock sensor system.

Last edited by turbo nut; 05-16-2010 at 06:26 AM..
Old 05-16-2010, 01:34 AM
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I am just saying, lets not debait the my TQ can beat up your HP.

They are good frends joind by RPM and work togeather.

Maybe we do not need to go through all the math again.

Last edited by 911st; 05-16-2010 at 11:18 AM..
Old 05-16-2010, 07:07 AM
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Quote:
Originally Posted by turbo nut View Post
I have not finished the build yet getting close so the hp figure is just a dream. It's a 3.4 with carrera intake. The heads flow a little better the big port SC heads. Twin t3 t4 turbonetics turbos, 993 heat exchangers, twinplugged, GT2 evo cams and knock sensor system.
That should easily be over 500 rwhp. If you've got that much into it, you may just want to wait to see how the 4 speed feels. With twin turbos your torque may come on soon enough that it's not a problem. Or you may just want to go with a G50.
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Old 05-16-2010, 08:01 AM
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+1 David.

For a street car it would be fun to have such a build where 1st could be long, power/boost come in early enough for good starts off idle, and the power curve could naturally match traction or if needed be shaped by tapering boost in first such that at WOT traction can just barely be maintained (traction control?).

Then have the rpm drop in to second where we are not droping down to about 3800 rpm unless we are making so much power there that in will overpower the tires.

It dose not make any sense to reduce a gear split if a build is already overpowering traction.

Maybe for a monster Twin Turbo the big first to second split can work to one's advantage.

Thus, please see post #3 above.
Old 05-16-2010, 11:49 AM
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Quote:
Originally Posted by turbo nut View Post
What would people in the Know suggest for gear stack for a four speed. The motor should have around 500 or better HP.
What are going to do with the car - street or track?

I have 525hp with a 4sp. I increased the R&P to an 9.39:1 (don't quote me on the exact size) so my top speed is 155mph. Around town, it'll jerk you out of your seat and light 'em up. At the track, it runs out of legs on straights (not power though and you'll get there in a hurry) so stock R&P is best. If you like technical tracks, the G50 is the only way to go.
Old 05-16-2010, 04:43 PM
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It's a street car. Might go to the track once in a while. Tire size is 275-40-17
Old 05-16-2010, 06:38 PM
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It seems none if us here have really done gears except Don though we know it can help and have thoughts on how to approach it.

My two cents is I would get the motor/car built first. Dive it some to see where you want to see improvement like off the line and/or see if first is to short. Make note of what RPM you can live with on the freeway as this is often what keeps one from going with a lower final gear. Also, your Dyno plots will come in handy. Might note if you have enough power at the beginning of 2nd to spin the tires at will. If you do there is not as much reason to narrow the rpm drop into 2nd.

Cheaper improvements could be a lower R&P but from what I am hearing from Don and others that have gone there with big HP motors, this can make first to short and you end up with a weaker rear end on top of that.

Another could be to just move first up to reduce the 1-2 split but that would make your off the line pre-boost potentially slow.

The 2-3 and 3-4 gear splits are about 1800 - 1900rpm which not bad for a turbo. For a race car they could be shortened some to make an incremental improvement.

The best.
Old 05-17-2010, 07:00 AM
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