BMW MINI supercharger on a real mini DIY
Discussion
I think there are two 'worst cases' for piston loads. The highest compression load occurs just below peak torque. The highest tension load occurs at peak RPM. These tend to be a similar order of magnitude. I guess it will depend on the spec of the engine which of these is worse, but remember that forced induction will increase the compression loads. (On mine, boost is constrained by the compression loads on the pistons.)
>> Edited by GreenV8S on Saturday 15th January 22:30
>> Edited by GreenV8S on Saturday 15th January 22:30
I'm sure that compression loads aren't really an issue unless you are pushing a silly amount of boost. Compression would burst the crown of the piston if it was excessive (after detonation). Anyway, we should have a lot of stats in due course as FI is fashionable at the moment.
I would expect worn ring lands on forged pistons before the cast items have given up....
I would expect worn ring lands on forged pistons before the cast items have given up....
love machine said:
I'm sure that compression loads aren't really an issue unless you are pushing a silly amount of boost.
On mine the standard pistons are good for about 7 psi (I'm running less than that at the moment). Yours may well be different, but it's an issue that is worth checking.
I would have thought that as long as you don't have any detonation, they would be able to cope with the pressure loads. I thought piston failure was either detonation induced crown holes and subsequent disintegration, or the skirts dropped off/cracked due to inertial loads. God knows. I would be interested in knowing more though. I would only used forged pistons if I built a screaming race engine running over 12:1 but I woudn't do that as they eat bearings for a pastime.
My suggestion would be to blow an ordinary engine and see what happens. Look in the Haynes Metro manual find which engine code is for the low compression engine and go and pick one up from the scrapper for about £20. Should be 8:1 out of the box. Gear it to 10psi at 6000 which should see 7 at 4000 and it should all hold together fine.
My suggestion would be to blow an ordinary engine and see what happens. Look in the Haynes Metro manual find which engine code is for the low compression engine and go and pick one up from the scrapper for about £20. Should be 8:1 out of the box. Gear it to 10psi at 6000 which should see 7 at 4000 and it should all hold together fine.
The standard cast RV8 pistons are apparently vulnerable to gudgeon pin failure, and collapse above the top piston ring. Both of these are compression failures. Clearly this may not apply to other engines, but it would be worth finding out what the vulnerable areas are on yours. Would be a shame to find out the hard way and do unneccessary damage.
thansk for the info guys, especially the MR2 blower ill keep my eyes peeled! luckily i have an un modified 12G295 head in my garage which has got the most stupid sized chambers so im probably going to use that, i think ill look around for a second 998 engine with low compression as at the moment i have a CR of 10.3:1 which is lovely for a zingy engine but not for forced induction. im running a stupidly long 2.9 FD at the moment which is brilliant for top speed etc so if i get a more powerful motor im probs going to keep that running in it
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