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MG TD TF 1500 - Reduction gear for SC12

Have front mounted a SC12 to an XPAG, with I think 1141 cm3 per rev. However, this gave a pressure of 7.25 psi. As I find this too high and have done it mainly as a technical challenge and not for power, I am in the process of installing a V-belt reduction gear. Dia. of pulleys are 95, 110, 95 and 95, that ought to give about 13.4 % reduction. The belts are of Z type i think.

My question is, will it work, or will the SC draw more power than the belts can take.

How much Power will a SC require?

There are speedlimits of 70 kph almost evrywhere I drive.

Anyone who knows?
YS Strom

According to calculation by a belt supplier the gear will be good for 6 kW at 5 000 RPM, which will be more than adequate.

YS Strom

With a reduction of 13.4 % I now get a pressure of 6 psi. I would have liked it a bit less, but it may do. There are speed limits of 70 kph almost where ever I go, so the risk of blowing the gasket is small.
YS Strom

Another way to cut your boost is do like NASCAR and throw a "restrictor plate" (just a gasket with smaller hole) in between the carb to manifold (or manifold to blower). Much easier to tweak.

I had to machine a custom pulley for a S.Co.T blower. Not sure how it would pump on a 1466CC engine, I machined the blower pulley deeper to accept thinner belts for a smaller effective diameter and made the engine pulley as large as physically possible to maximize boost for what I had to start with. If it has to be reduced, I can recut the engine pulley much deeper and can go with a wider belt so they ride out farther on the blower pulley.

...or I'll just add a restrictor plate.


By the way, if you have a choice, let me recommend a single belt vs dual, as the groove dimensions are really critical with multisheaves. Even if the machining is perfect, identical belts can vary in dimension significantly. Mismatched grooves and belts will make the belts fight each other increasing friction and heat, the same way mounting different sized tires on dual rims can increase wear drastically and lead to very early tread failure.




JIM N

Strom,

More pictures, please!
JIM N

Using only single belt drive. That will be enough.

Spot welded a pulley to the main pulley for the generator. Then used a triple pulley and reduced the diameter of one of the groves and installed two ball bearings in it. The original pulley on the SC had splines. Turned that into a bush shaped piece and fitted it in a universal joint. Only one joint is required with this set up. Hope the pictures will be of some help.


YS Strom

One more Picture of the set up without gear. I have published it here beore.

YS Strom

Hope this will be enough.

A friend of mine said 6 psi was for beginners and cowards, so it might have been a wasted effort?? But as I am not in a hurry, I Think I am happy with it.
To give the SC 12 a reasonable shape, it takes a bit of sculpturing and new oil pipes.


YS Strom

JIM

In another home made installation I also used a single belt with a splitted pulley that could be shimsed to give desired dia. and speed. It worked fine for two seasons, but now it is fitted to my completely restored spare engine.

YS Strom

Hello again JIM

Using an orifice restrictor I guess will mainly reduce compression at high RPM, but will reduce the pressure only marginally at low RPM. Thus max. pressure in the cylinder will be almost the same I am afraid.
YS Strom

I understand your concern and it is a valid point. We can debate consequences of boost @ 3000rpm/35hp vs 6000rpm/70hp. Your boost gage likely isn't going to show high boost at low rpm unless you are lugging it in too high a gear.

There's a factory dyno plot that goes as high as 6psi boost and that was on low octane fuel. We cruise freuently at 4psi boost and once in a great while, get up to 5psi, on regular gas with no knocking.

I wonder about your friend. Maybe he likes turbos. Excessively high boost can be hard on the blower, generating temps that can expand the lobes from a microscopic clearances to an interference fit!!! OUCH!

I think you hit the bullseye right at 6psi!
JIM N

Interesting JIM

Relieved to learn you think 6 is not excessively high .

If I press the accelerator at idling, it takes just a second to get 6 psi. Haven't tried it on the road yet. It is far below zero here now. But with direct drive it also gave 7.2 almost immediately and not much more at higher revs. Admit I rarely exceeded 4 000 RPM. Will test it more thoroughly in may I hope.
YS Strom

This thread was discussed between 05/03/2019 and 13/03/2019

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