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gears???.

Old 12-16-15, 09:55 AM
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gears???.

ive just put a 12a bridgeport in my 70 mg midget that i run s.c.c.a reginals in Arkansas.....still finishing up,remoded my intake an run a eld 500cfm 4 barrel....works great, runing a ford irs 7.5 diff,posi,with 327 gears, no power below 3,000 rpm so a gear change is nedded. thinking of 410 gears.....but not sure if i should go lower,running 275/35/15 hoosers an if i leave at 4,000 she will smoke em good but then boggs when it catches up till im again above 3,500 so i know i need lower gears,just not sure how low.
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Old 12-17-15, 06:08 AM
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over 200 views an no commets??? i thought surly someone would at least give some advice on what gears to run from there experance........still thinking i need 410 or 456 gears to get above my 3,500 rpm bogg without having to blow the tires off everytime i leave the line...just dont want to go to low an have to constently shift between 2nd an 3rd.
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Old 12-17-15, 10:52 AM
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Is the car strictly for scca? At this point I think anything is better then what you have in there right now. I believe you mention you have a 3.27 rear ??

Last edited by heynoman; 12-17-15 at 10:58 AM.
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Old 12-17-15, 11:45 AM
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yes,327 was factory gear for irs rear,think ive desided on 410
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Old 12-17-15, 02:56 PM
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You could just about work it out mathematically, based on what RPM you are seeing at the end of the main straight in top gear with your 3.27's and what RPM you want to be seeing at the end of the main straight, but then taking into account you will be getting there quicker too.
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Old 12-17-15, 03:25 PM
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What top RPM...gear could be anything from a 4.56 ot a 5.12.
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Old 12-17-15, 03:53 PM
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Its a s.c.c.a. track car......so not really any straights,all corners an an sweepers an such,motor will turn around 9 to 10,000 I think....but for longgiveaty I'll keep her below 8,000. So I'm thinking 410 will work,hopefully that will let me run the full range (3,500 to 8,000) rpms on track.
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Old 12-17-15, 07:59 PM
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When gearing for autox you need to set a car up to run out of RPM at the maximum estimated speed for the car your are running on course. This can vary by region and lot size. Here is KC top speeds for fendered cars is around 65MPH. A Solo Nationals course will probably top out in the low 70s.


Back when I ran a C Prepared Mustang at Solo Nationals I set the car up to run the entire course in 2nd gear after launching the car in 1st.. The car would do 72MPH at 7000 and could be stretched to 7600 without going to 3rd. During the 13 years I ran a solo nationals I ran either a T5 manual or C4 automatic and I just changed the rear end gear as needed to keep the car in 2nd. Shifting gears during a run wastes time. The advantage of a V8.


This strategy doesn't work in an RX7 because you will have to shift gears. However, you can still help the situation by using two gears that are close together. Assuming you are using a stock gear box, setting the car up to launch in 1st and then use 2nd and 3rd on course. This would be better than just 1st and 2nd because of the wide gap between those gears. Below is a gear calculator that will help you predict speeds and shift points. BTW my RX7 has a 4.88 and I have ran a few autox's with it using 2nd and 3rd most of the time.


Houseman Autosport: Testimonials


Regarding max RPMs....my Stock port 12a was always shifted over 8000 during club races and my street port 13b is around 8500 also on a race track. Autox is not as taxing on an engine as road racing - so rev it up.
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Old 12-18-15, 04:47 AM
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cool.thanks for the info mustanghammer, this is the first roatry ive ever ran and am amazed at the power these things make in such a lil package! i dont drive the car much off track but testing....but my job is a mile down the road an i live in town,so id like to keep it somewhat streeable.that being said my main gole is to blast around on track an in running a 4 speed with a 6 puck clutch. when i buoght the 12a (for $175) it came with 2 4 speed trans an mony went to rebuilding an bridgeporting ($1,100) so 4 speed it is. hopfully in a couple years i can upgrade to a 13b bridgeport big turbo motor.KEVIN BALTZ built it for me is why it was so cheep,around here he is a ledgend with roatrys,hie dayly driver of 3 years is a 74 mazda pu with a 13b bridgeport turbo that makes a lil over 500hp to the rear wheels on the dyno!!! i think he has a lil over 70,000 miles on it right now.....any way he said mine would last a lot longer if i kept it around 7,500 rpm,i have turned it as high as 8,200 an was still pulling hard...but even with good seals i had a few small flaking spots he cleened up the worried me about prolond rpms above 8,000....money is tight !but im hoping to grab a 5 speed soon,so ill probly go with the 456 gears seeing as how there all the same $$ anyway. one of the reasons i picked the 90 ford thunderbird irs rearend was gear changes are less than $200 for ring an pinion and it was cheep to buy an $100 and is a factory posi.i was a master mechanic for 17 years before fibromailga made me stop,and for past 12 years have been a tool an die mechinest with my shop 1.2 miles down the road....so fabbing parts is no big deal.
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Old 12-18-15, 10:21 AM
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Remember...horsepower does not move things, torque does. With that in mind, I would look at a 5.12 since you are turning the motor 8k. On a FC 5 speed, speeds are 31, 1st, 53, 2nd, and 78, 3rd. Looks good for most autocross courses. That assumes a 225-45-15 tire, 3.45 first, 2.002 second, and 1.366 third.
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Old 12-18-15, 10:36 AM
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My mg only weighs 1350lbs,I'm assuming a lot lighter than a rx7,Ive had 488 In it with the 56hp motor that came with it...so I don't think I need quite that low with the HP I have now.my power to weight is greater than this 12a in a rx7
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Old 12-18-15, 01:30 PM
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I have twice the weight but three times the horse power. With the lower gear, you put more torque on the ground, which is what moves the car.
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Old 12-18-15, 02:08 PM
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Yes....but I still think a 1350 lb car with 175 ish HP(its what the engine builder says) doesn't need lower than 456 gears to work well on our short road course( most times limited to really big parkinglot) especially with just a 4 speed,as I'm trying to use 2nd gear mostly like the miatas an Porsche's do.
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Old 12-19-15, 10:14 PM
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Do you know what the gear ratios are for the 4spd that you have now? This information would also assist you with determining a final drive ratio.

My RX7 has a 4.88 in it because with a stock port 12A it allowed me to red line the car in 4th at the end of the longest straight on all of the tracks I raced on in Midiv. It also made 3rd work better in the slower corners on the same tracks.

Now my car is running in a different class with more top end potential so I will use the 4.88 on some tracks (Hallet, Hastings) and a 4.44, 4.30 or 4.10 at other tracks (KS Speedway, Heartland Park, RPM, Gateway). The goal is the same, run out of RPMs at the end of the fastest straight. So not a weight issue - a speed/RPM issue. Note that on slower tracks I am using MORE gear, not less.

Back to my old Solo Mustang. In that car, with a 25" tall tire, the magic ratio was 7.2 to 1 because if allowed the car to use all of the motor on a Nationals course. This ratio is calculated by multiplying the 2nd gear ratio by the final drive ratio. To illustrate the effect the transmission has, notice the rear end gears I had use with each transmission. With a T5 - 1.93 2nd X 3.73 rear end gear = 7.19. With a C4 automatic - 1.48 2nd x 4.88 rear end gear = 7.2
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Old 12-20-15, 06:09 AM
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i can see where it would benifit you,but our reginal is a much slower an smaller setup.even our fastest cars(one is a boxer with a camin s motor) dont run aything lower than 456 gears.most set up there cars to run the course in 2nd gear only so as not to have to shift but 1 time.all the guys that have ran an won at a pro solo event and race the courses that were limited to (including my roatry builder) tel me 410 should be good,but im going to use 456 just to play it safe,i cant aford to multi gears to change,this is just a hobbi that i can barley afford.i was just gettin all advice i could before i desided on what gear best sutes what i want to do from race car to some driveabilty. like the fact that the motor will turn 9,500 rpm but would require a rebuild moore ofton...so im not turning it higher than 7,500 so hopfully ill get at least a couple years out of it before i need to tear it down. its far from a perfect race car...but it does work good an is a blast to drive! and my gole is to have as much fun as posible with the least amount of money spent.its the most fun an cheepest way i can continue my speed/fab passion.idealy it would have a store bought susp insted of my own custom stuff an a 13b bridge big turbo.....but im making due with what i can and having fun...isnt that what its all about???????
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Old 12-20-15, 06:13 AM
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oh yea...forgot to mention on our corses our longest streight is maby 2 car lenghts...not much of a chance for long wide open throttle.
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Old 12-21-15, 12:03 AM
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Kind of apples to oranges. But when I was running Solo2 in my 1790lb 510 with 198hp I was running 4:88 gears. It put me between 2nd and 3rd gear. Depending on the track.
I also know of 2 others cars (510s) that were running 5:90s and doing very well.

Bottom line is to try and be in your RPM range and not to have to shift.
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Old 12-21-15, 05:25 AM
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yea....thats my ultamate goal. but just looked it up an 456 is lowest gear i can find for my rear end...so 456 it is,may have to find me a 5 speed if i ever deside to drive it very far though.as i said,only gave $175 for complet motor an 2 4 speed trans when i got it,and spent $1,100 to have it bridgeported an built ( i did all clean up an helpd to put together). i didnt want to build it myself the first tim as ive never built a roatry before.still should preform good as my car weighs 1350lbs an is 4.5 feet wide an 11.6 feet long...quit a bit smaller than a rx7 !really shows in the corners as i dont have to turn as much.
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Old 12-21-15, 04:32 PM
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I have a ford 8.8 in my turbo FC with 315/30/18s and downforce and decided on 5.14s to start with.

They work really well, but on longer straights I run out of steam in 2nd (the redline is set to 9000 rpm). but I have a lot more power than you do, and probably more cornering ability.

I'm going to be swapping in 4.56s this winter to get a bit more mph in 2nd.

I would put the lowest gear you can in it (highest numerically). 4.56s look good.

but without knowing what gear ratios your transmission has it's all a guess.
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Old 12-21-15, 07:05 PM
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Originally Posted by eage8 View Post
I have a ford 8.8 in my turbo FC with 315/30/18s and downforce and decided on 5.14s to start with.

They work really well, but on longer straights I run out of steam in 2nd (the redline is set to 9000 rpm). but I have a lot more power than you do, and probably more cornering ability.

I'm going to be swapping in 4.56s this winter to get a bit more mph in 2nd.

I would put the lowest gear you can in it (highest numerically). 4.56s look good.

but without knowing what gear ratios your transmission has it's all a guess.
I bet that will run really well with your new turbo setup.


FWIW my V8 car revving to 7000 will hit 78 MPH in 2nd with a 3.73 rear gear. With the stock 4.10 ratio the car would run 71 MPH in 2nd. My rev limiter is at 7150. I generally shift at 6600-6700, but will hold to 7000 for a corner if needed.

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Old 12-22-15, 05:56 AM
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my 4 speed trans gears...1st-3.674,2nd-2.217,3rd-1.432,4th-1.0,and gonna run 456 gears,currently running 327 gears.
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Old 12-22-15, 09:53 AM
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Probably will work well for you. Max speed in 2nd is 54, and in 3rd is 83 @ 8000rpm.
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Old 12-22-15, 05:07 PM
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Nothing to add to the conversation. Like they say on Shark Tank............I'm out.

Last edited by jr2dad6888; 12-22-15 at 07:03 PM.
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Old 12-22-15, 06:13 PM
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sorry...you must have not read my 1st post .....as ive stated i have a 7.5 irs ford rear end,custom upper an lower control arms,orginaly had mg rear with 390 an i chaged to 488....snaped that 1st time i got on it with the roatry.
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Old 12-22-15, 07:02 PM
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Well there you go then. Makes sense. Sorry I posted. Back to lurking and minding my own business. Good luck.
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