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1200 Vintage Road Racer / differential question
Not too shy to talk
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Hope I am posting this in correct spot.. I read a lot, but seldom post as I don't have much useful to ever add to the conversation.

question for those of you who know more than me. I have a 1200 road race car I use for vintage racing in California, USA (i.e. C Sedan). Its used on Thunderhill, Laguna Seca, Sonoma, Buttonwillow, etc. Not used on Willow springs or large / long tracks like that.

as it sits today: Full race A12 engine. close ratio comp 5 speed with 1:1 5th gear. H190 welded Diff with 4.38 gears

I have a buddy with a couple diffs I can have for "buddy pricing"
1. H165 LSD with 4.88 gearing
2. H190 LSD with 4.62 gearing

The H165 sounds nice as its a bit smaller / lighter.. but would require quite a bit of work to swap in.
The H190 would just be an easy direct swap since I already run an H190

Question is on the gearing... want thoughts from those of you road racing... would the 4.62 gearing work well for the tracks I am running on with my setup ?

Posted on: 6/6 21:26

Edited by DirtyEvo on 2018/6/6 21:58:32
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Re: 1200 Vintage Road Racer / differential question
No life (a.k.a. DattoMaster)
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From Las Vegas USA
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Steve here are my gearing charts with the Hoosier TD 20.6 diameter tire. As your race A12 is making in around what my hotted up A15 is I think it would be close. the A12 is likely revving higher so the top speed is likely good. Note that I'm using overdrive transmissions so you need to look at the 4th gear column.

4.62 R&P 60 series 5 speed
20.6 diameter tire


RPM 1 2 3 4 5
4000 15 24 38 53 65
4500 17 27 43 60 73
5000 19 31 48 66 81
5500 21 34 53 73 89
6000 23 37 58 80 97
6500 25 40 63 86 105
7000 26 43 67 93 113
7100 27 43 68 94 115
7200 27 44 69 95 116
7300 28 45 70 97 118
7400 28 45 71 98 119
7500 28 46 72 99 121
7600 29 46 73 101 123
7700 29 47 74 102 124
7800 29 48 75 103 126
7900 30 48 76 105 128
8000 30 49 77 106 129
8100 31 49 78 107 131
8200 31 50 79 109 132

60 series 5 speed 185/60D13 4.38 R&P
20.6 Diameter tire
RPM 1 2 3 4 5
3500 14 23 36 49 60
3600 14 23 37 50 61
3700 15 24 38 52 63
3800 15 25 39 53 65
3900 16 25 40 55 66
4000 16 26 41 56 68
4500 18 29 46 63 77
5000 20 32 51 70 85
5500 22 35 56 77 94
6000 24 39 61 84 102
6500 26 42 66 91 111
7000 28 45 71 98 119
7100 28 46 72 99 121
7200 29 46 73 101 123
7300 29 47 74 102 124
7400 29 48 75 104 126
7500 30 48 76 105 128
7600 30 49 77 106 130
7700 31 50 78 108 131
7800 31 50 79 109 133
7900 31 51 80 111 135
8000 32 52 81 112 136
8100 32 52 82 113 138
8200 33 53 83 115 140

Posted on: 6/7 0:02

Edited by Rallytwit on 2018/6/7 1:09:30
Edited by Rallytwit on 2018/6/7 1:10:12
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Re: 1200 Vintage Road Racer / differential question
No life (a.k.a. DattoMaster)
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I have an H190 with a 4.38 LSD now that id consider swapping for a 4.62 if that helps you any. I'd have to run the numbers on a 4.88 gear to see its to low for what you are doing.

EDIT: I ran the numbers for a 4.88 final drive with a 1-1 5th gear your top speed at 8000 rooms with a 185/60-13 radial would be 106 and with 205/60-13 would be 111.

111 is what I'm doing at the top of the hill at Spring Mtn. I sometimes run the motor to 8200 which your A12 would easily do so you could probably get as much as 115mph out of it.

Posted on: 6/7 0:04

Edited by Rallytwit on 2018/6/7 1:15:07
Edited by Rallytwit on 2018/6/7 1:28:36
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Re: 1200 Vintage Road Racer / differential question
Not too shy to talk
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Thanks so much Tom... so... seems this H 190 LSD with 4.62 gears would be a good swap for me... easy swap, going to give me the type of performance I like better and not hurt me on top speed in any significant way that would matter to me. perfect info.. thanks much!

Posted on: 6/8 0:19
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Re: 1200 Vintage Road Racer / differential question
No life (a.k.a. DattoMaster)
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The 4.62 would be my pick for sure. Always glad to help out.

I'm working on my A15 replacement, I ran the trusty mildly tuned A12 at a PCA track day and the 20hp drop was painful. I know won't make Buttonwillow but I may try Willow Springs in November.

Posted on: 6/8 5:34
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Re: 1200 Vintage Road Racer / differential question
Not too shy to talk
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Cool.. hope to see you at Willow Springs.. I will be there with the Wagon.. hopefully driving well. I was a mess at Pahrump.. combination of the car being a suspension mess and horrible driving.. but we corrected all the suspension problems and I corrected some of my driving problems and last time out at Buttonwillow the Wagon and I bonded a bit... so I hopefully can keep it going the rest of the season. hope to see you there. And I will get the 1200 out to Spring Mountain in 2019! So you can teach me how to drive it!

Posted on: 6/11 18:59
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Re: 1200 Vintage Road Racer / Differential Question
Home away from home
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So, Tom...will that replacement A15 for your trusty steed be the dry sump version, lowered 3" and pushed back in the chassis as far as it will go?

Posted on: 6/15 16:18
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'73 1200 coupe vintage racecar
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Re: 1200 Vintage Road Racer / differential question
No life (a.k.a. DattoMaster)
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From Las Vegas USA
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Stan as my car would take some serious correcting to get it back to true B-sedan, the mid term plan is to stick with GTL . So yes I plan to lower the motor and move it back as far as it will go.

I've also been out in the garage notching valve reliefs in the pistons so that I can run a slightly more aggressive camshaft.

If I can work out the pieces for the crank fire ignition I may do that as well

I'm going to install the larger wheel cylinders in order to get slight more rear brake bias. This is going to sound shocking but I'm not braking as deep as I'd like, it tends to lock the inside front when I transition from threshold braking to trial braking.

The 63 series box will go in as well.

Posted on: 6/18 1:27
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