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I saw in West Coast Corvettes latest that they carry 3.90 gears. Would a Z06 with 400+ rwhp and 400+ rwt be able to pull 6th past my current 177 mph top speed in 5th with the stock rearend? If so, what would you estimate it would pull before it became drag limited?
 

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Hmm, not shure. /phpBB/images/smiles/icon_confused.gif Ron, do you have a dyno sheet on your car?

Looks like when you shift from 5th to 6th at the rev limit, you drop to 4400 RPM (.56 / .84 * 6600).

With a 3.90, 5th gear will top out at 156.61 MPH instead of 178.59.

So I think the question is how much horsepower/tq do you make at 4400 and is it more than enough for 156.61 MPH.

FYI - Rev limit in 6th gear will be 234.91 instead of 267.88

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2002 Z06/Millennium Yellow/Black/All but PSMs
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Discussion Starter #3
Pat, thanks much! I am making about 320 rwhp and 380 rwt at 4400 rpms. Sounds like it would be enough to pull 6th at those speeds. Since you are good with ratios and rpms, what speeds would be pulled in 2nd, 3rd, and 4th at 6600 rpms with 3.90s? Thanks. /phpBB/images/smiles/icon_smile.gif
 

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With 3.90

MPH at rev limit (6600 RPM)
1st 44.29420739
2nd 63.5525584
3rd 91.99566
4th 131.5537959
5th 156.6117
6th 234.9175
Rev 40.10786

With stock 3.42
1st 50.51093825
2nd 72.47221575
3rd 104.9073333
4th 150.0174866
5th 178.592246
6th 267.8883689
Rev 45.7370386

RPM after upshift from lower gear at rev limit
1st-2nd 4600
2nd-3rd 4559.42029
3rd-4th 4615.384615
4th-5th 5544
5th-6th 4400

FYI - I've put together an Excel spreadsheet to compute these, let me know if you want it.

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On 2001-08-22 11:44, Rippied.Z06 spewed forth this drivel:
Pat, thanks much! I am making about 320 rwhp and 380 rwt at 4400 rpms. Sounds like it would be enough to pull 6th at those speeds. Since you are good with ratios and rpms, what speeds would be pulled in 2nd, 3rd, and 4th at 6600 rpms with 3.90s? Thanks. /phpBB/images/smiles/icon_smile.gif
If your car was geared to drag limit at the peak HP RPM of the engine, it could 'theoretically' go 177~178 MPH. This is based on a Cd = 0.31 and a frontal area of 21.3 ft^2 for the Z06.

At true top speed where you are drag limited, the available RWHP is equal to the sum of the aerodrag + rolling resistance to maintain that speed. In the model case of the Z06 with 320 RWHP, it will go 177~178 MPH in ~70 deg air at sea-level. Of the 320 RWHP to the ground, 240 RWHP is to overcome aerodrag, and 80 RWHP is to overcome the rolling resistance of the car on the pavement.

Stock Z06 gearing says 176 MPH in 5th gear at 6500 RPMs actual (not indicated, which could be off alittle bit). Peak RWHP is at ~6000 RPMs, so it seems that most stock Z06s will be able to pull near red-line in 5th gear which is what your seeing.

If you put 3.90 diff gears in it, the RPM at 178 MPH is ~5000 RPMs, which is below the peak RWHP hump. Therefore, I would say it will NOT pull any faster (and probably slower) than 5th currently does.

IMO, GM has already set the Z06 up to drag limit at near peak RWHP in 5th gear /phpBB/images/smiles/icon_eek.gif. Of course, if you had some mods and extra RWHP, then using 3.90 gears would get you more than 178 in 6th gear. My calculations say that even a 2002 Z with 350~355 RWHP would not benifit from 3.90 gears. Putting 4.11 gears in a Z with 355 RWHP would pull nearly 185 MPH in 6th gear.

pmontelo -- I think your numbers are a hair high. I get 264 MPH in 6th gear at 6500 RPMs with 3.42 gears, and 176 MPH in 5th.


On 2001-08-21 15:18, Rippied.Z06 spewed forth this drivel:
I saw in West Coast Corvettes latest that they carry 3.90 gears. Would a Z06 with 400+ rwhp and 400+ rwt be able to pull 6th past my current 177 mph top speed in 5th with the stock rearend? If so, what would you estimate it would pull before it became drag limited?

It would take a Z06 with 403 RWHP to go 194 MPH if geared perfectly, and you had a LONG road to get upto top speed. My calculations show that 3.90 gears may do it in 6th (194 MPH = 5450 RPMs), BUT 4.11 gears would probalby work better (194 MPH = 5750 RPM, closer to peak RWHP point).

If you plot out engine RPMs vs. Speed in Excel, you can see that there is a HUGE gap in the gear ratios when going into 6th gear. I don't think GM ever intended 6th gear to be a top speed gear. If you where actually only interested in top speed runs, it would actually be better to gear the diff UP and stay in 5th gear. As you can tell, gearing is all dependant on how much RWHP you have -- every time the RWHP goes up, typically so can the gearing to take advantage of the extra HP. Unfortunately, for a stock Z06, 5th gear is already at or close to it's rev-limit at drag limit, and 6th gear is a far cry too tall compared to 5th. The best solution might be to change 6th gear ratio in the tranny if possible.

BTW - I like crunching numbers if you couldn't tell /phpBB/images/smiles/icon_biggrin.gif
 

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On 2001-08-22 20:30, ZeeOhSix spewed forth this drivel:
pmontelo -- I think your numbers are a hair high. I get 264 MPH in 6th gear at 6500 RPMs with 3.42 gears, and 176 MPH in 5th.
ZeeOhSix, you are correct, I was using 6600 RPM. I should have said @ rev limit instead of @ red line. /phpBB/images/smiles/icon_wink.gif I don't think Ron is afraid of bumping the rev limiter occasionaly. /phpBB/images/smiles/icon_biggrin.gif

ZeeOhSix - I think you missed it, but Ron has 421.7 peak rwhp, 410.6 peak rwt.

I think Ron would probably be able to see a MPH increase with the swap. Not shure how much.

ZeeOhSix can you pull that calculator back out of your pocket protector and tell us? /phpBB/images/smiles/icon_lol.gif As you said, the acceleration in 6th may be painfully slow.

Ron, what RPM are your peaks at? (No, this is not a personal question /phpBB/images/smiles/icon_lol.gif )

ZeeOhSix, we need to exchange spreadsheets!

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1985 Coupe/Red
2002 Z06 Order# DKFFMZ - TPW 8/6 - DOA /phpBB/images/smiles/icon_frown.gif
2002 Z06/Millennium Yellow/Black/All but PSMs
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Discussion Starter #7
Pat and Zee, good info. Pat, I have somewhere between 410 and 421 rwhp at 5800 rpms depending on the dyno. Hp stays over 400 rw to the rev limiter. Rwt peaks at 5050 rpms at around 400 to 410.

Zee, as far as taking a long time to get to the rev limiter in 5th, it came up so fast my wife did not have time to yell! /phpBB/images/smiles/icon_cool.gif
 

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On 2001-08-22 21:37, Rippied.Z06 spewed forth this drivel:
Pat and Zee, good info. Pat, I have somewhere between 410 and 421 rwhp at 5800 rpms depending on the dyno. Hp stays over 400 rw to the rev limiter. Rwt peaks at 5050 rpms at around 400 to 410.

Zee, as far as taking a long time to get to the rev limiter in 5th, it came up so fast my wife did not have time to yell! /phpBB/images/smiles/icon_cool.gif

pmontelo -- ya, I did miss on how much HP he had ... see below /phpBB/images/smiles/icon_biggrin.gif

Ripp -- yes, I could see it going to redline quickly in 5th gear, mostly with RWHP like yours /phpBB/images/smiles/icon_eek.gif. 6th gear is a whole 'nuther animal -- its ALOT higher spaced away from 5th than any of the other gear spacing.

Here's my slide rule results:

Using avg of 415 RWHP at 5800 RPMs, the Z could 'theoretically' go 196~197 MPH if geared perfectly to take advantage of peak RWHP at drag/roll limit. This is when 415 RWHP 'balances out' with the aerodrag and rolling resistance of the Z06.

Now gearing.
With 3.90 diff gears, at 5500 RPMs in 6th gear the speed would be 196 MPH. This is just a hair below your peak HP RPM, so I'd say she would pull ~195 MPH /phpBB/images/smiles/icon_eek.gif /phpBB/images/smiles/icon_eek.gif. 5800 RPMs with this gearing would be 205 MPH, but 415 RWHP is not enough power to overcome aerodrag and rolling resistance at that speed.

Most people don't believe me when I tell them this, but if you did the high speed run on a hot day (like 90 deg F) at near sea-level, you would probably hit your 196 MPH or even a few MPH more. The reason for that is because the air density reduction due to the hot air significantly reduces the required HP to go top end. You ask 'what about the reduction in motor HP due to hot air'. Well, I've factored that in, and the numbers say (using the SAE air correction formula) that the reduction in engine HP is less than the reduction in HP to go this fast in hot air. The result is that you will go alittle faster on a hot day. Everyone thinks that cool days are the best for top speed runs, but they are not. Cool days are great for 1/4 mile races, but not for true top speed runs.
 

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On 2001-08-22 21:37, Rippied.Z06 spewed forth this drivel:
Zee, as far as taking a long time to get to the rev limiter in 5th, it came up so fast my wife did not have time to yell! /phpBB/images/smiles/icon_cool.gif
Yeah, that 4th-5th RPM is the key. After the shift to 5th, your just 250 RPM below your peak HP.

A couple things I'm thinking:
1. With the swap, you will have a higher top MPH, but getting to 178 will probably take quite a bit longer than it did with the 3.42. At 4400, your 1400 RPM away from your peak. So your wife will be screaming! /phpBB/images/smiles/icon_biggrin.gif You may get very few chances to actually see that top MPH.

2. At the track, you will need to reprogram your brain to get used to the new shift points and increased torque multiplication to avoid breaking the tires loose. Based on what I've seen you say about your top speeds at the track (150?), the new top in 5th sounds pretty well matched. You'll get there quicker than before. 6th will probably be worthless on the track.

3. If you do the swap, please don't get your self killed trying to do that top speed run! /phpBB/images/smiles/icon_frown.gif 178.592246 MPH is still very fast and your car gets there quick. /phpBB/images/smiles/icon_wink.gif
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1985 Coupe/Red
2002 Z06 Order# DKFFMZ - TPW 8/6 - DOA /phpBB/images/smiles/icon_frown.gif
2002 Z06/Millennium Yellow/Black/All but PSMs
Try #2 - Order# DNFM0T - TPW 8/20!!!
 

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Zee, a top speed of 195! /phpBB/images/smiles/icon_eek.gif /phpBB/images/smiles/icon_eek.gif /phpBB/images/smiles/icon_eek.gif

Pat, I agree that it may accelerate slower once I shift to 6th, but boy would she scream up to 158 or so mph. /phpBB/images/smiles/icon_cool.gif Now as far as killing myself going for the top, well like they say, if you're not living on the edge you are taking up too much space. /phpBB/images/smiles/icon_wink.gif
 

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I agree, the 3.90 diff would also be better for track use. It will give you more torque when coming out of corners, so you should be able to accelerate better between corners and have higher peak strait away speeds /phpBB/images/smiles/icon_biggrin.gif.
 
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Rippied.Z06 said:
I saw in West Coast Corvettes latest that they carry 3.90 gears.
I just looked online and I didn't see a part number. Is this the whole Getrag Rear? How much was it?

Thanks!
 

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milleniumZ said:
How about 3.73's? :p
In what respect?

With 3.73 diff, speed at 6500 RPMs is as follows:

4th = 135 MPH
5th = 161
6th = 242

You would need 721 RWHP at 6500 RPMs to pull red-line in 6th gear!! :eek: -- which means you'd probably have ~750 RWHP at peak 5500~6000 RPMs.

3.73 diff gears compared to 3.90 would allow alittle faster in 5th, but it wouldn't pull as good in 6th. Like I said above, 4.11s would actually be better for a 6th gear pull -- 6th is just too damn tall :roll:

:cheers:
 

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speedy-gonzalez said:


I just looked online and I didn't see a part number. Is this the whole Getrag Rear? How much was it?

Thanks!

Speedy, part # is WCC877 (West Coast Corvettes). Price is 1695 with exchange and includes reprogrammed pcm to calibrate the speedo. Says it is a pro rebuilt unit using Timken bearings, GM seals, and "our exclusive 3.90 ring and pinion." Comes with a 1 year warranty. Core charge of $500 and stock unit must be returned within 2 weeks.
 

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ZeeOhSix said:


In what respect?

With 3.73 diff, speed at 6500 RPMs is as follows:

4th = 135 MPH
5th = 161
6th = 242

You would need 721 RWHP at 6500 RPMs to pull red-line in 6th gear!! :eek: -- which means you'd probably have ~750 RWHP at peak 5500~6000 RPMs.

3.73 diff gears compared to 3.90 would allow alittle faster in 5th, but it wouldn't pull as good in 6th. Like I said above, 4.11s would actually be better for a 6th gear pull -- 6th is just too damn tall :roll:

:cheers:
Thanks for the info ZeeOhSix! Although I already purchased the 3.73's I am not looking into going a million miles an hour, just looking to get there faster ;) I just didn't think that the 3.90's were necessary for what I am looking for, but you guys that want top end speed, I guess this is the way to go :eek:
 
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Thanks for the Part number and price info!

My main question is, which is best for 1/4 mile times? I'm assuming 3.90 over the 3.73.

Aren't the stock gears 3.45? (at the top of this thread it compares to 3.42 ratio).

Thanks!
 

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Rippied, Just remember that with the 3.90 gears, that a down shift will really kick up the "R"s. A LOT more than it does now- and while racing that will get more than your attention:eek: :eek: Good luck with the gear change. Let us know how it works out and how it runs. I wish that they would have had a 3.90 rear end complete as with the 3.73s???:)
 
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