Oil Pressure on 948/1098 vs 1275?

Any topics pertaining to the BMC A-Series engine.

Oil Pressure on 948/1098 vs 1275?

Postby Steve » Thu Jul 09, 2015 10:07 pm

According to the Factory Specifications the normal operating oil pressure for the 948 and 1098 engines is from 30 to 60 PSI. For the 1275 engine, the pressure is 40 to 70 PSI. The oil pressure relief spring, however, is the same part for both series of engines. Are the design clearances tighter for the 1275 engine? or is it because of a different oil pump? I have seen a number of comments on this forum about oil pressure for Sprite engines, but I don't think this has been explained. Thoughts?

Thanks, Steve
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Re: Oil Pressure on 948/1098 vs 1275?

Postby Sprite33 » Tue Jul 14, 2015 3:58 pm

The relief pressure can be varied by the depth of the valve seat in the block of the pressure relief system. If it is more shallow, it will have more pressure from the spring and provide high relief pressure. I don't know if this is the case between the 1275 and 948/1098 but it is a way to get different oil pressure with the same oil pressure spring part number. The spring holding threaded plug and/or the valve could also be different causing more tension in the spring.

I use a spacer in my race car to increase the tension in the spring to provide higher pressure relief. This work pretty well for about half a race and the oil gets hot enough that the main, cam and rod bearing clearances become the oil pressure relief system. I also run a higher capacity oil pump.
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