39 replies to this topic

[Jimmyarm]

Good thread, like your writing style Maga7ine

One thing i dont understand is, when shortening the stroke, do you have to skim the block ? or is it done with longer con rods and a shorter rotation radius (if that makes sense !) on the crank ?

Jimmy

[The Matt]

Good thread, like your writing style Maga7ine

One thing i dont understand is, when shortening the stroke, do you have to skim the block ? or is it done with longer con rods and a shorter rotation radius (if that makes sense !) on the crank ?

Jimmy

I am not 100% certain about decking the block, but you basically offset grind the crankshaft, so effectively the big end of the rods are going through a shorter radii stroke. The position of the top of the piston in relation to the top of the block will depend on the pison I guess. But is is one thing I always wondered about, where does the piston sit, what centre-centre length is the rod etc. etc. The same applies to longer stroke engines, do you have to skim the top of the pistons to get them to sit at the right level when at TDC??
Or is it a case of getting pistons with the hole for the gudgeon pin in a different position, to suit the stroke????????

[Jimmyarm]

Offset grind ? I dont understand how that would work (or what it means ! lol), surely you have to physically reduce the size of the crankshaft to achieve a shorter stroke, like in my pretty picture;

Circles are the crank radius (where the big end would attach) strange lines eminating from the circle are the con rods and the lines to the right of each picture are the distance of travel. See my skill with paint !

 untitled.bmp   576.05K   21 downloads

[Bungle]

i cant draw pics on computers

but imagine your pic with a smaller circle inside the big circle and off set so 1 side of it touches the outer circle

[The Matt]

as I understand it:

the red big end journal in the second picture is going to be destroked:

the grey area in this first picture is where the new offset ground journal ends up, thus de-stroking the crank. Or decreasing the diameter of the big ends rotation. The problem is that you have to remove metal to offset grind, therefore you must use rods with smaller big end journals (cooper or MG Midget, or expensive aftermarket ones).

EDIT: sorry about the non-scaled models, didn't have a proper one to hand to measure!

[Oldskoolbaby]

Smart ass!!

[Bungle]

thats what i said

[The Matt]

Smart ass!!

thats what i said

I still don't know if it's right though!

[Oldskoolbaby]

Looks good to me mate but with a stroke as long as your pic shows, itll displace about 2000cc lol............

[Dan]

As you said MMM you have to use alternative rods to destroke. The different rods are sometimes longer. It depends on how far you want to de-stroke the crank really as there are two different standard lengths available at Cooper S big end size. The rods for a 1071/1275 are 5.75" centre to centre which is also the standard A+ 1275 length and the rods for a 970 are 5.875" centre to centre giving an extra 1/8" length and so lifting the piston up the bore. Also if you use the very expensive custom steel rods that are available (such as Carrillo's) you can pretty much get a custom length made up if you're willing to pay.

Incidentally even at maximum re-bore and maximum de-stroke the A series/ A+ engine will still be long stroke. The only way to get a true short stroke motor is by using a 970 or 1071 crank which are NLA over here as forgings but billet ones are available.

Without meaning to sound like I'm having a go at anyone, it's so nice to read from an engineer who can actually speak English and isn't completely closed minded, thanks for the info Maga7ine.

As I understand it the difference you are describing in the charatceristics of the different strokes ("What you talkin 'bout Willis" ) is being caused by what the Americans refer to as the rod ratio? Is that it or have I missed the point again?

As for measuring the amount of shock experienced by the piston as it rounds BDC have you considered fitting a doppler radar emmiter into the bottom of an in-line A series sump? It should be possible to orient it so that it is aimed at the bottom of the crown and missing the crank and gudgeon, and time it so that it only reads at BDC for that piston. Obviously that is getting quite complex now and needs some exotic gear.