Short stroke engine
Turbo Mini Forums

Ive been reading and reading and reading, and there seem to be loads of options available but im unsure what my best option would be.

So far I believe a standard SA crank can be ground to a stroke of 78.3mm (from 81.3), using 970 rods (which are rare/expensive), would I be able to use AEG625 ('S' rods) and keep that length stroke? The 625 rods seem easier to get hold of than the 970 rods?

Im looking to run increase the deck height (lower piston down bore) and at the same time, shorten the stroke so this seems like it MAY be right?

Is there much advantage of building 1293 short stroke or would I be best starting with a 998 block, crank etc as the their won't be much difference in CC's after stroking etc?

If the CR is too low after the fettling i was thinking of using flatops pistons to increase the CC's, I believe TurboTim used flat tops on his engine last year!

The end result im looking for is a low compression (around 8.3:1 ish) short stroke, wihout having to sell my organs to pay for it. Lol

Appologies for the amount of questions, just trying to get my head around things!!

ANY help, advice and corrections what be mucho appreciated.

Matt

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1/4mile in 13.2sec @ 111 terminal on 15psi

the SA cranks have 70.2 mm stroke as standard and let you use std A /A+ rods. if you destroke a std crank you can get down to 78.3 or thereabouts. Then you need S rods. If you dont want to skim loads of metal of the top of the block to get the pistons up you need longer rods (custom rods or 970 rods). I am using a SA crank and have skimmed 7 mm of the top of the deck

you will be better to go with the 1293 option. large bore and short stroke getting nearer over square

you cant seriously call a 1293 over square? :) I have 73,5 mm bore and 70,2 mm stroke. That is oversquare, the 1293 is not. or did you not want oversquare?

Well I want to get it as sqaure as possible with utilising the parts I already have e.g. using 1293 pistons. etc..Lol

Using a 78.3mm stroke it should give me a 3mm deck height at the top so I should have plenty to play with to get my CR to what I want.

Any ideas what effect the additional deck volume would have? Im guessing its the same principle as having an increased head chamber volume, or are there other problems caused?

Im using a turbo block so it should have a decent amount of material to skim off without making the surface too thin!

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1/4mile in 13.2sec @ 111 terminal on 15psi

you loose squish, get the pistons all up and use dished pistons. Its no good to run the pistons down the bore...

I'd have to use 18cc dishes to get the CR any where what I want and they would need valve reliefs cutting out, not sure whether it could be done?

Im currently using 12cc pistons,standard stroke and 28cc chambers with a CR of 8.3.

new spec is 21 cc chambers possible 78.3 stroke and aiming for similar CR?

Any suggestions?

Edited by matty on 14th May, 2007.

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1/4mile in 13.2sec @ 111 terminal on 15psi

Saul Bellow - "A great deal of intelligence can be invested in ignorance when the need for illusion is deep."
Stephen Hawking - "The greatest enemy of knowledge is not ignorance, it is the illusion of knowledge."

everything is possible with boost

The valve cut outs may be very small when using 18cc pistons?

Edited by miniminor63 on 14th May, 2007.

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1/4mile in 13.2sec @ 111 terminal on 15psi

miniminor63 i said getting NEARER! over square

okay Mr Benross, point taken :)

i should have said ....
miniminor63 i said getting NEARER! over square My friend

that sound better

matty, a mate of mine is running flat tops (same ones tim had i believe) down the bore, with a std size head and the block skimmed a bit he's running 8:1cr and 160ish banana's @ 15psi. thats on a std stroke and +20 bore though.

Yea just got a pm bud

Can you still get hold of them "s" rods, out of the sprites for a bargain price?

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1/4mile in 13.2sec @ 111 terminal on 15psi

Any preferences which is the better rod AEG521 or AEG625 or if there the exactly same?

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1/4mile in 13.2sec @ 111 terminal on 15psi

I've been thinking about this since our conversation at the Riv and have decided that I am not happy with the idea of leaving the pistons down the bore.

Tonight I had a conversation with John K about the SC head gasket and I think there are other things to consider.

The low CRs everybody on here is used to is down to the combustion chamber shape, carbs and ignition controlled.

I was considering something in the region of 8.8:1 to 9:1 and was a little unsure if it would hold up but after talking to John K I now understand why the thickest gasket is 1.6mm and more importantly am happy that I can make it work with that.

If your going injection and 3D ignition then you should be able to get away with safer running at higher static CRs. Must be careful with the cams though to get the dynamic CR where you want it.

I'm going to do a few more sums and will post once I have something that I am happy to share but I'm sure we need not worry about getting into the low 8s even if 18-20 psi (may well be more) is your aim.

Every day is a school day ...........

How fast and how expensive ...... the same question...

i'm running 8.7:1 on my lump at 12psi for the time being but hopefully i'll have wound it up to around 16/17 before avon park. my advance is quite conservative at the minute though, think my max is 24 degrees on boost.

Yes i do run my pistons down the bore, and yes they are flat too.
Seem to give me great results, but one persons idea of a good engine set up is different to somebody elses.
Best thing to do is just try and see.

16V project underway

if you do go the stroked route, you probably already know this but when using the Con Rods and the stroke you mention, pending on the gearbox casting you will need to die grind a bit out of the gearbox or the endcaps on the rods will catch. i used a GT crank and had it stroked. another lil 2peneth would be to get everything balanced, pistons, rods, crank and flywheel............but that is just me :)

Edited by Richie on 16th May, 2007.

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he is talking about de stroking, do not need to grind the gearbox casing then...:)

Yeah destroking.

From rough calcs this morning, the cc reduction would be 11.4cc (ish) per cylinder making it about a 1246, thats with the pistons sitting 3mm down the bore and a 12cc dish. CR would be 8.0:1 to 8.3:1 depending on how much needs taking of the block to clean it up.

Am I correct in thinking that the flat tops are generally shorter in height than "normal" pistons. If this is the case then there would be further clearance but the same combustion chamber volume as there would be no dish?

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1/4mile in 13.2sec @ 111 terminal on 15psi

the flat tops my mates using are the short height ones for long stroke motors, rather than std height flat tops.

Matty, you won't need it down in the low 8s, not with full management.

As I said before I'm aiming at 8.8:1 (18psi boost target) which I feel is safe, John K tells me I'm being a big girls blouse .... I think he means a little conservative so I can only guess at what he'd run but I think it is safe to say it's 9:1 or greater.

Every day is a school day ...........

How fast and how expensive ...... the same question...