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Spi Compression Ratio

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#1 clem_c


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Posted 01 July 2019 - 01:04 PM


I am currently rebuilding the engine of my SPI mini. (Initially an economical low compression 1275cc SPI sealine).

After a few modifications, the engine has got the following specs :

  • +0.040” piston overbore, so the swept volume is 327,4 cc/piston.
  • Pistons are minispares ref P21253-40, giving a measured 9cc in the block with a piston at TDC.
  • The head gasket (AF470) has got a 4cc compressed volume.
  • The head cylinder is a Minisport stage 3 with a chamber volume given for 21cc, however I have measured it to exactly 20.53cc.

The bore increase, and the combustion chamber volume being on the low side of 21cc, have bring the static CR of my engine beyond the value that I was targeting (10.5:1) :

This set up gives me a static Compression ratio of 10.76:1.


I am afraid it is a little too high to run  95octane fuel.

I would like to know if someone has got a similar set up, and if they have had any problem running on 95 octane fuel?


I am using a Kent MD274 camshaft : intake timing is 12/56, to compare with the standard SPI camshaft intake timing 9/41**.

With the MD274, the intake valve closes 56-41=15deg later that with the std cam, this should lower a bit the dynamic CR. (next thing that I will calculate)


Has anyone been in the same situation?






**according to : http://www.theminifo...identification/

Edited by clem_c, 01 July 2019 - 02:42 PM.

#2 clem_c


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Posted 01 July 2019 - 02:35 PM

So i have now calculated the dynamic CR of my engine.

It is 8.6:1 with my Kent MD274 camshaft. Good news!

I am surprised to see that it is lower than a standard 1275cc engine equiped with standard cam and a static CR of 10,5:1. Such an engine has got a dynamic CR of 9,3:1, according to my calculations.


I have just found this interesting article :



I am still interested to hear about people having fitted a Kent MD274 camshaft to their engine.

What compression ratio do you run, and what fuel do you use?

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