40 replies to this topic

[Minigirl]

When buying the mini the previous owner said it was a rebuilt high compression engine but after all the problems with it I was starting to thing the rebuild was a lie (eg engine only done 20 miles or so yet water all rusty colour and oil black...)

Anyway the head is off now and I can see inside. The pistons have AE 20773 on them with +1.016 BVF. Im guessing that means its a +40 piston (or +0.040)

Are these pistons for a high compression engine? They are dished. Thanks

Edited by Minigirl, 01 July 2015 - 09:32 PM.

[mini13]

nope, they are 8.3:2 pistons as std, and not a grat piston eaither, 4 rings, and not advised for use over 6k...

[Cooperman]

My data sheets show that to be a piston for a 998 with 6 cc dish. It is not thought to be suitable for sustained revs over about 6000 rpm.

The compression ratio will depend on 3 things, not just the piston, although a flat top piston is normally used in a high-comp 998 unit:-

 

The distance from the block deck to the piston top at TDC,

The dish of the piston

The volume of the combustion chambers.

 

With the head off you can measure all these and do the calculation. Allow 2.8 cc for a 998 head gasket.

[Minigirl]

What can a 998 rev safely to anyway? Would I see any benefit from installing a 266 cam still with these pistons?

[Minigirl]

I think the blocks deck has been machined down as the engine number has been removed

[Minigirl]

I think the engine has just been thrown together... it came with a HIF44 Fitted, Aldon yellow dizzy without vacuum advance, non ballast coil running on a ballast feed, 2 grades colder spark plugs, facet fuel pump fitted in the wrong place etc...

[Cooperman]

A 998 should rev safely to around 6500 rpm.

You really should do the CR measurements & calcs whilst it is in pieces. If you just do the measurements and send them to me I'll do the calculations if that helps.

You will only get an improvement if the head is effectively gas-flowed and probably has larger inlet valves fitted. The gas-flowing of the head is the key to better performance with the A-series engine.

A 266 cam will work well giving maximum power at about 6000 rpm, so you'll need to use these revs often to get the full performance, assuming a better head and a C.R. of  around 9.75:1 to 10:1 is done.

A 998 Cooper 3-into-1 ex. manifold will be needed plus a better inlet manifold and an HS4 carb (or HIF38).

[Cooperman]

I think the engine has just been thrown together... it came with a HIF44 Fitted, Aldon yellow dizzy without vacuum advance, non ballast coil running on a ballast feed, 2 grades colder spark plugs, facet fuel pump fitted in the wrong place etc...

My last post missed this. The carb is ideal, but needs a good manifold. Dizzy is good. Non-ballast coil is good, but the white/pink wire should be removed. The FACET pump needs to be at the back under the rear seat base.

[Minigirl]

Yea ive redone all that, I moved pump from engine bay where it was high up on bulk head online with the carb to the boot lower than the fuel tank.

I Removed the ballast feed and replaced with a 12v ignition live feed.

Im planning on adding a vac advance to the aldon but it also says the aldon is for engines with differnet cams fitted to standard

The prev owner did say there was work done to the head, I could measure the valves with my caliper gauge

[Minigirl]

The chambers on the head seem to be around 25cc - 26cc each 

 

what other measurements do I need to take? thanks

[gazza82]

Have a look at this ..

 

 

http://www.csgnetwor...m/compcalc.html

 

 

it shows you all the measurements you need ...

[Minigirl]

Thanks. My digital caliper gauge has decided to go loopy before taking the last measurements. What would you put down for hg thickness and hg bore width?

Only thing I can think is the strong magnet from the dti gauge has messed something up in it when I was measuring tdc and the piston deck clearance!

[Cooperman]

_{Just allow 2.7 cc for gasket volume on a 998 and you won't be far out..}

_{Also measure how far down the bores the piston tops are at TDC.}

[Minigirl]

gauge working again now... after entering it all I was given something like 8.3:1 (Thats if I entered the correct stuff..) ... Is that figure for a low compression engine? Is this why The car felt underpowered untill about 3500 - 4000 rpm??

[Minigirl]

Posted at the same time lol....

 

Do these measurements seem right that I got?

 

Bore Size - 65.3mm

Piston Stroke - 76.2mm

Head Gasket Bore diameter - 68mm (I went with that as a guess due to the egg shape of the hg (70mm one way, 66mm other so 68mm was in the middle :) )

Compressed headgasket - 1.1mm

Combustion chamber - 26cc

Dome volume (negative for dished) - -6cc

Piston deck clearance - 0.13mm

Edited by Minigirl, 03 July 2015 - 02:04 PM.