# MGB Roadster conversion



## 67BGTEV (Nov 1, 2013)

All,
I'm now planning for my next Conversion, which is going to be a 1979 MGB Roadster. This is a rear wheel drive. 

I wanted this to be a fast and high performance drive. I just bought a Remy HVH250. I'm close to getting a rinehart pm100dz. I'm planning to run it close to 500V.

I need your guidance on the reduction ratio for this setup. MG diff is 3.9:1. I read up few threads about 2 speed Powerglide trannies and I don't think it is a good idea to go for Powerglide tranny, as its final ratio is 1:1. I don't want to spend too much money on tranny/reduction 

Miz, RobA, Duncan, and all other experts, please share your thoughts


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## Duncan (Dec 8, 2008)

Hi
I don't think I'm an expert but here is my tuppence worth

Top speed - what do you want?
What is the motors max rpm?

Divide A by B - and you get your maximum reduction ratio

any greater reduction than that will reduce your top speed

Sit that to one side

Weight of vehicle (MGB) - weight on rear axle 
Size of tires

My tires are almost exactly 2Ft in diameter - so each Lbft of axle torque = 1Lb of force at the road surface (and at the axle)

General rule of thumb road tires have a max coefficient of friction of 80%

So if you have 1000lbs on the back wheels the most force you can apply is 800lbs
So the most torque is 800ftlbs (2ft tires)
If your diff is 4:1 
You need 200ftlbs at the motor - to MAX out less is OK 

Just plug your numbers in - see what you want and then have a look at what you can get


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## jwiger (Oct 18, 2014)

After you figure the motor speed vs vehicle speed limits, you could look into a Gear Vendors under/overdrive. I'm confident you could set up a good final drive ratio for cruising with one. The Lone Star EV guys are using gear vendor units stacked in their Assult & Battery dragster.


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## Moltenmetal (Mar 20, 2014)

Wow, this will be fascinating! Good luck!

I know nothing other than what I've read here, but I have heard lots of rattling here about serious trouble with permanent magnet motors, heat being the big issue (for reasons that seem obvious to me). How is the Remy motor cooled?


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## Moltenmetal (Mar 20, 2014)

Fascinating read about this on the motor forum- never seen these Remy motors before now. Appears that the highest output ones use active (forced) oil cooling, including flow between the rotor and stator if my brief scan of the thing was correct. Apparently they sell motors that are designed for 50% water/ethylene glycol (WEG) cooling which have sealed bearings, and oil lubricated + oil cooled units which give 40% higher heat dissipation performance because of the vastly greater surface area of cooling surfaces available when nonconductive oil is used as the coolant.

Which cooling system is your motor designed for? If it's one of the cartridge units and it's oil cooled, there's a whole lot of work required to integrate this into a housing, as the oil drains out of holes in the end of the motor cartridge...


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## 67BGTEV (Nov 1, 2013)

Thanks guys. I'll do some Calc and decide on the gear ratios. I'm leaning towards using a powerglide tranny without torque converter. Anyone thinks I shouldn't use tranny? 
MM - Thank you and I hope mine is WEG cooled, I don't have it with me yet, but will let you know once I have it.


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## 67BGTEV (Nov 1, 2013)

Moltenmetal said:


> Fascinating read about this on the motor forum- never seen these Remy motors before now. Appears that the highest output ones use active (forced) oil cooling, including flow between the rotor and stator if my brief scan of the thing was correct. Apparently they sell motors that are designed for 50% water/ethylene glycol (WEG) cooling which have sealed bearings, and oil lubricated + oil cooled units which give 40% higher heat dissipation performance because of the vastly greater surface area of cooling surfaces available when nonconductive oil is used as the coolant.
> 
> Which cooling system is your motor designed for? If it's one of the cartridge units and it's oil cooled, there's a whole lot of work required to integrate this into a housing, as the oil drains out of holes in the end of the motor cartridge...


I did some more research on the motor that I'm getting - Its HVH250 90SOM which is *S*eries wound, *O*il Cooled. I need to figureout how to take care of Oil drains..


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## 67BGTEV (Nov 1, 2013)

Duncan said:


> Hi
> I don't think I'm an expert but here is my tuppence worth
> 
> Top speed - what do you want?
> ...


Thanks Duncan.
Remy can go upwards of 10000 rpm. So I'm not worried about Max Speed. My interest is to keep a good torque in freeway speed (~60mph) as well 
Here are few numbers 
Constant Torque up to 4000RPM is 184 ftlb
Diff Ratio	3.9
Rear end Weight 1100 lbs
Tire height is 2 ft


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## 67BGTEV (Nov 1, 2013)

I wanted to see if I can do direct drive, today I saw the Latvian RX8 thread, which assures me of going Direct Drive. With rear diff ratio of 3.9:1, with 2ft tall tires, Rear weight of 1100 lbs, Max Torque of 880 ftlb here is what I got:


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## Duncan (Dec 8, 2008)

Hi
Can you find a higher ratio diff?
3.9 would work 
But 7.8:1 would be better!
Up to 36mph - 1400ftlbs - tire spin city
By 55 mph torque has dropped to 862ftlbs - still spinning
By 73 mph torque has dropped to 574ftlbs - now the same as the 3.9:1

Anything between those would be good - higher motor revs = better cooling

What is available for your rear axle?


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## 67BGTEV (Nov 1, 2013)

Duncan, I don't think they have much choice in mg, I've to go custom for that kinda ratio.


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## Duncan (Dec 8, 2008)

Hi
The MGB used a lot of Austin/Morris parts as MG was part of the Austin/Morris/Leyland.... conglomerate

I think you will be able to go to at least 4.4:1 with standard bits
And with the long history in motorsport there may be higher ratios available

It may be worth "Going Custom" as you will end up with impressive performance


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## 67BGTEV (Nov 1, 2013)

Thanks Duncan. Will see some local shops this weekend


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## Moltenmetal (Mar 20, 2014)

Beware- the motor may be happy to go 10,000 rpm long term, but your diff might not like that for long...bearings are usually designed for a range of surface speeds, and 10k RPM probably is well outside that range- ok for short bursts but not for steady highway trips. A custom gearbox might be ok with that.


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## 67BGTEV (Nov 1, 2013)

Yes, I'll limit the motor rpm under 7K through controller


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## Ai! (May 9, 2014)

Isn't this motor and controller combo from ebay auction that was ended half a month or so by accepting an offer?


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## 67BGTEV (Nov 1, 2013)

No, that's the VW Doka project. There is at thread in the forum


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## 67BGTEV (Nov 1, 2013)

Duncan said:


> Hi
> The MGB used a lot of Austin/Morris parts as MG was part of the Austin/Morris/Leyland.... conglomerate
> 
> I think you will be able to go to at least 4.4:1 with standard bits
> ...


I found out that I can swp the diff with a Ford 8.8 Diff. I can get the Gear Ratio of 5.71:1
Here is what I can get and I think this is gonna work better fine for me..


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## Rf 4.3 (10 mo ago)

Duncan said:


> Hola
> No creo que sea un experto, pero aquí está mi valor de dos centavos.
> 
> Velocidad máxima: ¿qué quieres?
> ...


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