Piston slap

[ian.stewart]

I have just had my mate round asking Rover questions, He has a Twin Turbo 5ltr RV8, the question was how much does a bore expand at normal running temps, the bore is 20+, he has bought some expensive forged turbo pistons from a west london specialist and had the block bored by them to match the pistons, the skirt clearance is between 4 & 5thou,the engine is so slappy he has stripped down the engine to check for damage after less than 1000miles, he described it as sounding like he had left a rod cap loose!!!, Personally I think there is too much clearance and have told him so, He has another 4.6 block with a standard bore which has been pressure tested at 50psi for 3 days without loosing any pressure, this wes done cold, and he is hoping that its ok. what I have suggested to him is get the new block bored and piston matched to 2thou clearance, the company who made the pistons recomend between .0015 and .004 clearance for skirts depending on application, and have said the larger for a turbo race engine, which this engine is not, its a daily driver,
I realise the bigger stroke engines will be prone to slap due to the shorter rods giving greater rod angularity and shorter skirts. any suggestions???
next question??? are RV8 injectors high ot low impedence ?

Ian

[softdash3.9]

Hi Ian,
Regarding pressure testing of the block.

My porous block was pressure tested internally to 120psi, the pressure subsided down to 80psi, but would hold this pressure when cold and not fall any further:!:. This was repeated several times to identify the source of the leak, it was even submerged in a tank of water but the leak couldn't be found, it was eventually seen when the block was pressurised and put in the engine hot wash(microscopic bubbles at the top of No3 liner ).

I suggest therefore that 50psi internal pressure is not great enough to test a block for leakage, obviously combustion pressure is far in excess of 50 psi.

Not saying that the replacement block isn't OK but 50psi doesn't tell you anything

It is very painful stipping your engine for the second time, talking from personal experience, especially if you have high performance expectations, but hope the engine block of you friends is OK

[kev_the_mole]

Have you thought of PTFE piston buttons to control the slap? Although a bit 70's/80's the VW boys still use this trick on big bore conversions where the cylinder diameter changes greatly due to the greater temperature range of air-cooled motors. This would allow him to use his proven original block too!

If you're interested I can let you have a bit of PTFE bar for experimentation.

Cheers,

Ian

[Richard]

Are RV8 injectors high ot low impedence ?

High impedence.

[jwriyadh]

Ian, be wary of the injectors.

Hotwire injectors are "high impedance", measure about 14-16 ohms.

Flapper injectors are "low impedance", measure about 2-4 ohms.

jw

[Boosted LS1]

Ian, I've built a few engine's with custom forged turbo pistons and clearences of .006-7". Surprisingly they don't slap whether hot or cold so I think he's got something wrong. The bore will expand but by less then the pistons. Also, it's hard to lower c/r for a blown turbo 5.0 so I'd guess he's running pistons down the hole? Thats not a good way to do this, they'll probably be very short in overall height which makes them less stable. If the pistons are above deck height they could be hitting the heads once past a certain rpm, just a thought.

Oh, that variation in clearence is to great. The piston will reach maximum temperature/expansion sooner or later whether racing or on the street so it could seize at the lower clearence.

Boosted.

[stevieturbo]

Do whatever the piston manufacturer reccomends.

Though a little loose, will do no harm.

A little tight....will.