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Old 03-16-2019, 06:47 AM   #1
v8_supercar
11.94 et @ 113.69 mph
 
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Unhappy 3.91 Anxiety !

Well after many days , nights , weeks , months of searching and reading everything ever printed about 3.91 gears in a GTO I'm officially a mental wreck. I've already have everything to do the install which is supposed to take place next week. My issue is what the engine RPM will be through the traps in the 1/4 mile. I have a bias ply and a drag radial tire I run both 26" tall. Mods are headers ,3600 stall , Svede and tune. Car is bracket raced, hardly on the street at all. Maybe 200 miles a year at best. Car has run a best of 11.94 at 113 mph. In the heat of summer it settles in the 12.20's. I run the car in 3rd so it stays out of overdrive . I wish I could remember exactly what it traps rpm wise now but I'm not sure. I've read so many different rpms through the traps with 3.91 in a auto with similar mods and 26" tall tires. Anybody out there able to tell me where I'll be rpm wise with 3.91 in my car going through the traps? As of right now what I have read has me cancelling the install for next week. Thanks
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Old 03-16-2019, 07:25 AM   #2
lejambcr
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I have a spreadsheet which does a pretty good job of capturing gear ratio and HP/torque impact on 1/4 mile performance. Assuming all things being equal in terms of 60 foot time etc.... I would expect your ET to drop .2 to .3 seconds, your trap speed to go up about 2MPH and your trap RPM to go up about 700 from what you have now. If the converter is tight at that RPM (about 4-5% slip) and your 3rd gear ratio is 1 to 1 which I believe is correct I calculate your trap RPM to be around 6200.
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Last edited by lejambcr; 03-16-2019 at 07:40 AM.
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Old 03-16-2019, 07:39 AM   #3
v8_supercar
11.94 et @ 113.69 mph
 
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Quote:
Originally Posted by lejambcr...View Post
I have a spreadsheet which does a pretty good job of capturing gear ratio and HP/torque impact on 1/4 mile performance. Assuming all things being equal in terms of 60 foot time etc.... I would expect your ET to drop .2 to .3 seconds, your trap speed to go up about 2MPH and your trap RPM to go up about 700 from what you have now. If the converter is tight at that RPM and your 3rd gear ratio is 1 to 1 which I believe is correct I calculate your trap RPM to be around 6200.

Thank you . Well I'm not wanting to drop 2-3 tenths in ET . So that's one for ditching the gear install.

Last edited by v8_supercar; 03-16-2019 at 07:41 AM. Reason: X
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Old 03-16-2019, 07:45 AM   #4
lejambcr
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Are you looking for more ET drop.....dumb question I guess? Then your going to need more HP or less weight.

Last edited by lejambcr; 03-16-2019 at 07:55 AM.
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Old 03-16-2019, 09:14 AM   #5
v8_supercar
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Quote:
Originally Posted by lejambcr...View Post
Are you looking for more ET drop.....dumb question I guess? Then your going to need more HP or less weight.

I'm sorry I totally took what you said the wrong way. Damn I have been thinking about this to hard I guess. My main reason for the gear change is to get this heavy car moving and out of the beams quicker . I figured it would probably be slightly quicker in ET which I'm fine with . I'm just not wanting to spin it to hard through the traps. Trans shifts at 6500 now so your trap rpm prediction of 6200 I think i can live with. Be nice if it was slightly lower . The car has a best 60' of 1.58 but averages in the 1.62 range . Thanks for the help. It's getting down to crunch time for racing season and I'm getting stressed .
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Old 03-17-2019, 09:50 AM   #6
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If your not planning on adding any more power without adding more rpm capability, you’ll be fine. Your cruise rpm at 70mph May not be too good though. You never want to use overdrive for full throttle anything with a 4l60. It’s not gonna hold it long. If you plan on going power adder latter, you’ll have to go back down in gear. I have stock 3.46’s right now and it won’t be long till I’m looking at going lower. Just keep all that in mind.


If your going to stay n/a then put the 3.91’s and if you want more power then put a cam that will allow you to pull it to 7000. You’ll probably end up in the low 11’s at that point.
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Old 03-17-2019, 02:06 PM   #7
v8_supercar
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Originally Posted by Kfxguy...View Post
If your not planning on adding any more power without adding more rpm capability, youíll be fine. Your cruise rpm at 70mph May not be too good though. You never want to use overdrive for full throttle anything with a 4l60. Itís not gonna hold it long. If you plan on going power adder latter, youíll have to go back down in gear. I have stock 3.46ís right now and it wonít be long till Iím looking at going lower. Just keep all that in mind.


If your going to stay n/a then put the 3.91ís and if you want more power then put a cam that will allow you to pull it to 7000. Youíll probably end up in the low 11ís at that point.

Thanks for the time to respond. I've always raced it in 3rd since my first pass down the track 8 years ago. No plans for a power adder at this time but if I do it will be small nitrous shot. It's usually trailered to the track and if I do drive it to the track I'm only on the freeway for about 20 miles. No plans on a roll bar so 11.50 is my et limit. Cam maybe in the future. I had one picked for the mods and weight of the car. Cam Motion Titian 4. Thanks again for the response I appreciate it.
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Old 03-17-2019, 03:46 PM   #8
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Quote:
Originally Posted by v8_supercar...View Post
I'm sorry I totally took what you said the wrong way. Damn I have been thinking about this to hard I guess. My main reason for the gear change is to get this heavy car moving and out of the beams quicker . I figured it would probably be slightly quicker in ET which I'm fine with . I'm just not wanting to spin it to hard through the traps. Trans shifts at 6500 now so your trap rpm prediction of 6200 I think i can live with. Be nice if it was slightly lower . The car has a best 60' of 1.58 but averages in the 1.62 range . Thanks for the help. It's getting down to crunch time for racing season and I'm getting stressed .

If you don't feel comfortable with your trap RPM you can always drop it 250-300 RPM by adding another inch of tire diameter, although that will also make you a little slower out of the beams.
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Old 03-18-2019, 02:19 AM   #9
v8_supercar
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Quote:
Originally Posted by lejambcr...View Post
If you don't feel comfortable with your trap RPM you can always drop it 250-300 RPM by adding another inch of tire diameter, although that will also make you a little slower out of the beams.

Yea, this may be a option down the road. If the car didn't already have a brand new set of 26" Mickey's Street R bias tires with the stickers still on them I'd look into that now.

At least these tires are 26.3 tall as my Street R drag radials on my 17" stockers are only 25.7 tall.
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Old 03-22-2019, 08:49 PM   #10
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Or go in between with some 3:73s instead. I know several GTO owners with automatics who went that way.

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Old 03-23-2019, 07:24 AM   #11
v8_supercar
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Quote:
Originally Posted by yowzerman...View Post
Or go in between with some 3:73s instead. I know several GTO owners with automatics who went that way.

Tom.

Thanks Tom , ya I may have to drop to a 3.73 . The 3.91's are in now. Car is still at my friend's shop getting some other mods finished up. I hope to have it home Monday and then off to my tuner.
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Old 04-14-2019, 05:28 AM   #12
v8_supercar
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Well I'm finally able to update this post. Got the car tuned yesterday at Proxes Tuning. As luck would have it Proxes Tuning is 20 minutes from Kilkare Dragway where I occasionally race at. After the tuning we hauled it to Kilkare and entered into the bracket race as there are no test and tune on race day.
After the long day was all said and done the car traps through the finish line at 6100-6200 rpm. Only got one time run but the car was so good we made it down to 3 cars (semi finals) but red lighted away my chance for the final by .005 .
Anyway on the Dyno it made 390/402. Previous #'s we're 371/366. The only engine mod that was changed was a Fast 92 intake.
Et and mph wise it went a best of 11.85 @ 114 mph with a 1500' DA. Haven't had the car on the freeway yet to see rpm at 70 mph. The gears are motive and I was less than thrilled at the possibility of having the horrible sounds I have read that motive gears are known for. But I'm hear to tell you they are dead silent. I was shocked to say the least . My buddy that has been building racing drive trains for 35 years is the best! Anyway that's the update , thanks .
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Old 04-14-2019, 06:14 AM   #13
lejambcr
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Well done! Looks like my spreadsheet got pretty close. How did your 60' times react to the gear change?
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Old 04-14-2019, 06:37 AM   #14
v8_supercar
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Originally Posted by lejambcr...View Post
Well done! Looks like my spreadsheet got pretty close. How did your 60' times react to the gear change?

Yea thanks. Your spreadsheet works well!. Had my best sixty foot 1.56! There is more in the car . I'm sure it will hit the 11.70's sometime this season.
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Old 04-14-2019, 06:44 AM   #15
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Cool, how much ET room do you have before they make you put in a rollbar?
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Old 04-14-2019, 07:29 AM   #16
v8_supercar
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Cool, how much ET room do you have before they make you put in a rollbar?

I'm good to 11.50 so right now 3 tenths.
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Old 04-15-2019, 06:36 AM   #17
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Is any weight taken out the car? lightweight drag wheels?
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Old 04-15-2019, 07:25 AM   #18
v8_supercar
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Originally Posted by Kfxguy...View Post
Is any weight taken out the car? lightweight drag wheels?

The only weight reduction really is muffler delete, holeshot 15x4" wheels on the front so I can run a shorter front tire for reaction time. I have a whole new light weight racing wheel set up waiting to go on the car. The 11.85 pass was with the stock 17" with 245/45/17 drag radials. Run was also with full tank of 93 and everything the car came factory with in the trunk.
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Old 04-15-2019, 01:07 PM   #19
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If you're willing to share your buddies shop info, pm me please.
I've been looking for an installer.
Got my tune at Proxes, also. 25k miles and 4 or 5 years ago.
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