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Author Topic:   Racking brain on gear ratio
doubletrouble racing
Dirt Forum Racer

Total posts: 88
posted February 26, 2005 09:27 PM  
We run a Pontiac Grand Prix Metric car with 7.5 rear end.I am unsure of the stock gear ratio in it,all we did last year is install a mini spool.We run a TH 350 tranny with 2.52 first gear and 1.52 second gear (think thats correct ratios)We turn around 4700-5200 rpms in first gear.We seem to be out geared last year on the 1/4 mile track.Tire roll out is between 79" and 80".Was wondering if you guys are running 7.5 rearend and Th350 tranny combo,and what gear you are running,rpms turning in the 1/4.This might help me decide what I need to get.I am shooting for a final drive around 6.20 from what I heard around the pits but not sure if "stock" motor will take that kind of RPM's.Thanks for your help.

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The opinions,thoughts,and comments of double trouble are that of the car owner and are not affiliated with the driver "Dirt Track Girl" , the track Hill Country Raceway ,or any other outside person or institution

bubba0six
Dirt Maniac

Total posts: 146
posted February 26, 2005 11:03 PM  
you should prolly be looking for something more like 6.33 or 6.50

hoosier transplant
Dirt Maniac

Total posts: 104
posted February 27, 2005 10:47 AM  
You can check what gear you have now by marking the tire and rotate it thru a full circle and count how many times your drive shaft yoke turns. Say, you count 3 full turns and another 3/4 of a turn, you probably have a 3.73. My first gear is a 1.82 and I run a 3.50 rear gear which gives me a 636 final. But I run a 5/16 and a 3/8 track and also turn more rpm's. I would agree, you probably need a 650 final. Hopes this helps.

racer17j
Dirt Forum Champ
Total posts: 5390
posted February 27, 2005 01:58 PM  
Well if you are planning to run HCR... then 6.20 is not enough... I would look for at least at 6.88 for you... now if I ever run there again I would try a 7.22 or so...maybe more (probabably more)... so to answer your question you should be looking for a 2.73 rear end...to get a 6.86 a 2.56 will give you a 6.45 good luck...AJ

Pigpen
Dirt Maniac

Total posts: 141
posted February 28, 2005 07:37 AM  
Oh..by the way if you are talking about HCR then that track is not a 1/4 mile unless you are talking about with the pits included... but on a regular 1/4 mile track in my bomber I still run a 6.88 (only turned 5,200 with this at hcr) well good luck...

racer17j
Dirt Forum Champ
Total posts: 5390
posted February 28, 2005 04:11 PM  
HCR is a TINY track... I would bet 3/16 at best...and My tires were about 81" I tried to run the smallest tire I had... I am not kidding I would try to run as close to a 7.50 as I could find...but I want to get closer to 7,000 so I think a 6.80 is safe...thats what I run at I37...

doubletrouble racing
Dirt Forum Racer

Total posts: 88
posted February 28, 2005 09:37 PM  
Thanks Dancer for your replys will consider your ratio.Is your bomber have the same combo;th350 and the 7.5 rear?O and by the way my bad....my tire roll is 81" to 82" (dont know where I came up with those measurement)Dont want to get it to low this old worn out motor wont take to much.Will be hopefully landing a new "chevy 350" later but will have to get started with the OLDS until I can get extra chash togather.Taking all I have to aquire quick steer,gearing,new skins,& set up by chassis builder.Thanks to all for the help!!!

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The opinions,thoughts,and comments of double trouble are that of the car owner and are not affiliated with the driver "Dirt Track Girl" , the track Hill Country Raceway ,or any other outside person or institution

powerglides
Dirt Freak

Total posts: 374
posted March 04, 2005 07:08 AM  
If you turn the wheel 10 times and have someone count how many times the drivesaft turns you get a much more accurate ratio number. 10 turns wheel = 31 turns driveshaft is a 308 ratio etc.

racer17j
Dirt Forum Champ
Total posts: 5390
posted March 05, 2005 10:22 PM  
The motor I ran the last several years was with stock lower end which was balanced and put together with good fasteners. turn it 6500 every night with no problems at all on a 1/2 mile that carrys alot of momentum so rpms are pretty much sustained.

Speed Racer X
Dirt Maniac

Total posts: 115
posted March 07, 2005 06:03 PM  
We ran a Turbo 350 in 2nd (1.52) with a 4.10 rear gear for a 6.23 final ratio. On a fast 3/8 track with a roll out of 83"-84" on the rear tires, we turned in the 6500 RPM range.


doubletrouble racing
Dirt Forum Racer

Total posts: 88
posted March 07, 2005 09:38 PM  
Guess Im looking for a 273 rear end gear for the 7.5 rear-end does anybody have one for sell.or do I need to go on a journey through the junk yards.

Butch Stage
Dirt Roller

Total posts: 11
posted March 07, 2005 10:55 PM  
i'm running 4:10 rear gear with a th350 running in 2nd turning 5500 rpm to 6000rpm
on a 1/4 mile does ok

baddracing42
Dirt Roller

Total posts: 20
posted March 09, 2005 11:35 PM  
we run 1/4 here in mississippi we have the same set ups we run around a 700 final i run a 650 final little brother runs a 719 i run a 283 sbc he runs a 350

pmiracing
Dirt Full Roller

Total posts: 44
posted March 10, 2005 05:19 PM  
I've got a similar question I ran 273 rears last year at my track and turned about 5000 rpms. is there a way to tell the aprox. rpm I would run with 308's?

racer17j
Dirt Forum Champ
Total posts: 5390
posted March 10, 2005 08:14 PM  
Ok. The tire thing has me all screwed up. I turned my tire once and my driveshaft turned a little more than 1.5 another guy did one and it turned 1.75 how is this possible there isnt any way that this thing could be right. The car came out with around a 3.56 what are we doing wrong. Its on a 4 cyl rwd i dont know the name of the rearend but its just like a nine inch. Turned the right side wheel limited slip rearend.

51racing
Dirt Maniac

Total posts: 138
posted April 04, 2005 09:59 AM  

purestockgearing1.zip

 
Here's something I've found useful when figuring gear ratio's out. It's an excel spreadsheet that I made just for use for us pure stock/bomber drivers. It's set up for a th350 in first or second, but you could plug any ratios you want into it, and the formula's are there. I had to zip it, so you'll need winzip to open it, then MS excel to read the file. It beats sitting around with a pencil, paper and calculator though.


19J
Dirt Freak

Total posts: 370
posted April 04, 2005 02:37 PM  
Thanks but my comp isnt smart enough to read that. It could be me who knows lol. Thanks anyway.

51racing
Dirt Maniac

Total posts: 138
posted April 05, 2005 07:15 AM  
quote:
Originally posted by 19J:
Thanks but my comp isnt smart enough to read that. It could be me who knows lol. Thanks anyway.


I'll try to just paste the info here then, you can copy and paste it into any text document, you just won't have the formulas available so you can't just modify the ratios for other gears, hope this works.

Rear Gear Tranny Gear Overall Gear
2.29 2.52 5.77
2.29 1.52 3.48
2.41 2.52 6.07
2.41 1.52 3.66
2.56 2.52 6.45
2.56 1.52 3.89
2.73 2.52 6.88
2.73 1.52 4.15
2.93 2.52 7.38
2.93 1.52 4.45
3.08 2.52 7.76
3.08 1.52 4.68
3.23 2.52 8.14
3.23 1.52 4.91
3.42 2.52 8.62
3.42 1.52 5.20
3.73 2.52 9.40
3.73 1.52 5.67
3.90 2.52 9.83
3.90 1.52 5.93
4.10 2.52 10.33
4.10 1.52 6.23
4.30 2.52 10.84
4.30 1.52 6.54
4.56 2.52 11.49
4.56 1.52 6.93

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