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Rotary Sprite

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Don that's a lot of quality build you have done there. You tore that Bugeye right down to bare bones. Coming along really nice. Good luck. Dave
 
It's not perhaps the most cost efficient route, but I'd look for a housing from an S10 which might have the gear ratio closer to what you want. Most of the Monza and Vega type GM compacts had ratios up around 2.78 or so for higher mileage with automatic transmissions, I swapped the gears in mine with some from a Camaro. If you could find a Monza/Vega rear for the axles, backing plates and drums then add those to the narrowed S10 housing you'd have one of the lighest 10 bolt housings with axles having the correct 4 on 4" lug pattern and drum brakes that have rear cylinders that are compatable with the Spridget master cylinder. The Monza/Vega housing is pretty heavy as these cars used a torque arm mounted to the rear axle for location and that added a lot of mass to the third member casting. Here's a photo of my narrowed Monza unit showing the added mass in the pinion area.
 

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I think stock RX7 is somewhere around 3.54. I think he might be o.k at 3.7, especially w/ an 8K redline and a 5 speed. (I think the RX used a taller tire though)

It might not be the best for milage, but then again, a rotary isn't a gas sipper. It might be alittle buzzy, but it'll sure be fun !!!

I don't think I'd want a spridget with the power and gear to possibly pull 140 MPH.

(I didn't do any math, and it might be an exaggeration, but I wouldn't want to go over 85 in one of these things !!!)

I'm cheap and lazy. I'd run a 3.7 easy at first to see if I could live with it and then decide before I spent any $$$ on tougher spridget stuff.

If you decide later you don't want to go 3.7, you could easily sell it here and go to whatever axle you choose.
 
Regarding ratios; More powerful motor implies more displacement and therefore more torque. More low-end torque means a lower ratio can be used. This does not mean that a more powerful, original sized motor will have more low-end torque. Usually making the original engine more powerful means producing more torque at high speed. There are a lot of factors that go into the optimum ratio, and for which purpose.
 
WOW that looks sweet Bill. I'm looking around to see if I can get similar set-up and what it might cost me in the end. Thanks for the pic it gives me a great perspective of what I'm up against. Looks tight. I removed my gas tank and install a Jaz Cell in the rear cargo hold as I need more gas for the thirsty rotary so this help. Thanks again for all the ideas and help, I need it. Dave
 
kellysguy said:
I think stock RX7 is somewhere around 3.54. I think he might be o.k at 3.7, especially w/ an 8K redline and a 5 speed. (I think the RX used a taller tire though)
I don't think I'd want a spridget with the power and gear to possibly pull 140 MPH. (I didn't do any math, and it might be an exaggeration, but I wouldn't want to go over 85 in one of these things !!!)
I'm cheap and lazy. I'd run a 3.7 easy at first to see if I could live with it and then decide before I spent any $$$ on tougher spridget stuff.
If you decide later you don't want to go 3.7, you could easily sell it here and go to whatever axle you choose.
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There are some good point there. And I agree, I thinks I'm going to leave it a as is until I get it on the road. In the mean time I will start looking for a replacement so once it pops I have something in standbye. I don't really want to go over 85 but I would like to do a little light to light spanking, and a few donuts here and there. Thanks for the input. Dave.
 
Yeah there's my problem, I'm not sure what would be best for this motor and car set-up. I'm just looking to play around on the streets and show the car at local cruises.
 
DWeeB said:
Yeah there's my problem, I'm not sure what would be best for this motor and car set-up. .
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If you don't know what's been done to the motor, then just put it toghether with what you have and go from there.

Yeah, you might have to spend a little more time to get it exactly how you like it, but it sure beats spending more money.

I hate unknown variables.

Just don't hammer it till you know what you're going to do.


Ask the RX guys, they should have a pretty good idea of where you might need to be, even if you don't know what's been done to the engine. Don't forget you'll probably be running a shorter tire.
 
DWeeB said:
kellysguy said:
This may be helpful:

https://www.rx7club.com/showthread.php?t=779485

Modded 13B w/ 210 H.P and 150#'s

I don't know what mods he or you did, so this may not apply to your engine.
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Jeez, 210 at the wheels! Must be turbo'd. Cool thanks that was good stuff.
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Read the thread, it'n not turbo as it states it's N/A when talking about the dizzy weights. Sounds like just maybe some port work as they say he still has more potential. Contact the guy and ask him what mods he has.

I'm SURE he won't mind talking about his toy, would you?

Also note where he started tearing drivetrain mounts and pieces off of the car while running it hard.
 
kellysguy said:
DWeeB said:
kellysguy said:
Read the thread, it'n not turbo as it states it's N/A when talking about the dizzy weights. Sounds like just maybe some port work as they say he still has more potential. Contact the guy and ask him what mods he has.

I'm SURE he won't mind talking about his toy, would you?

Also note where he started tearing drivetrain mounts and pieces off of the car while running it hard.
Click to expand...
I bookmarked that thread and will look thru it. I'm hope'n for maybe 140 bhp and will run an aluminum flywheel to easy on wheel spin. I'm figuring the rear-end will suffer before mounts since I'm using poly and there pretty much solid. I have a lot to learn. Thanks Dave.
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the 13b in the rx-7 thread is a peripheral port....lots of porting work there! In talking with the guy building the 12a for my sprite...we were looking at doing a bridgeport and that was going to net us about 200-225hp at the flywheel.

most of your stock rotaries and mild port jobs arent going to produce enough torque to be detrimental to the drive train and motor mounts. I talked to a guy recently that had a bugeye with a stock 13b and he was running the original rear axle with no problems.

I guess if its a mild rotary and if you can temper your right foot, then the factory rear end may hold up just fine. emphasis on "may".

https://www.mazdarotary.net/porting.htm <-- lists many of the different types of porting jobs that can be done to the rotaries

we have decided to scale things back and will be likely going to with an extended port. that way it still has a bit of overlap between the intake and exhaust creating a bit of that cam loping type of sound.
 
Hey Branson my 13B is a six port 1987 motor with EFI converted to side draft carb. The 5th and 6th port valves have been removed and will be run wide open. I'm hoping the your right and the rear end will be fine, but I'll let you know for sure very soon. Thanks for the advice. Dave
 
LOL thats a cool comparison. Like a dumb @$$ I didn't take pic's of the original motor, so thanks. I'm going to copy that pic of yours if it's ok and put in my build portfolio.
 
the 1275 that came in my '65 was that bright blue color.

Im thinking of painting the 12a the appropriate bmc green....just to see if anyone notices anything strange.
 
LOL that's green is hideous. I'm sure you get a few WTF looks for sure!
 
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