73.5mm pistons question.
Turbo Mini Forums

I need about 18cc in the piston or at least 15 at a massive push which means I'll have to access a mill. I have a decent lathe and I'm willing to use it. I'm also willing to buy a set of cast new pistons (but not forged). Ideally I want to keep the squish, in fact won't lose it. So, what sort of bodging can be done to achieve this? Are there any alternative pistons available which could be bodged on. When I say bodging, I have access to the equipment and could modify virtually anything within sensible limits.

I could go 1293 but want to stick with the 73.5 bore I have. This project is using a very skimmed head which I HAVE to use. With a lot of buggering about I might get another couple of cc's out of it but doing this would leave a good chamber shape.

Anyway, all sensible input appreciated.

Stu

Bugger off, I'm getting there.

I did my last set of +60 1330 pistons on a lathe.

It takes some time to set up as the piston needs to be square in the chuck, you check it by setting up a vernier dial to see how out it is..

To keep the squish you need to keep the rim around the piston edge above the rings, take that maybe to 3mm wide.

The main thing is not to go too deep and to make the piston weak. It is an advantage to have an old piston of the same type to cut in half to see how much meat you to deal with.

Hpe that helps a little.

JF.

I know Ross Pistons in the US could maybe make some custom pistons ??

They arent as expensive as ya might think. They made my V8 pistons.
Obviously the exchange rate helps.

9.85 @ 145mph
202mph standing mile
speed didn't kill me, but taxation probably will

OMEGA do a 73.5mm 11cc dished piston

with a 28cc head and the rest .... this will work out around 9.5:1 C/R not ideal but there is some meat to come off these as john as stated above!

the easy option is ACRALITE 73.5mm forged 18cc dished pistons

with the bore you have and a 28cc head and all the rest you will end up with a CR ratio around 8.3:1

hope this helps but with the Acralite pistons it wont on the back pocket but its the w forward IMHO!

May have to put my hand in my pocket, but we shall see. What's a safe minimum thickness to go for?

Bugger off, I'm getting there.

i need to get a set of the short height 73.5mm pistons, do these have less meat in them to machine out? i need to find around 16cc in the piston to be safe with my 29cc head, hopefully though they'll sit down the bore a bit cos i dont think my block has been decked much at all.

anyone used the omega cast short height ones before?

Edited by Dangerous on 28th Sep, 2006.

Metro turbo weekend driver,Mini turbo in the making again!

The snag is squish. I don't want to burn the piston edges and shag my top ring lands. Having pistons which don't come to the top of the block isn't an option. I'm probably going to have to go for accralite ones but will look at what Ross have to offer. Ideally I was going to throw together a budget(ish) engine and wanted something I could poach or bodge to achieve the ends. I was thinking it would be nice to use short height pistons, get the block bored from the bottom and leave the top of the bores alone to leave the squish metal in place. As the gasket is quite critical, I can't really afford to use a decompression plate (with the squish left in that). I've got 101 other things to spend the money on and since it isn't a full race jobby, I don't really want to waste money on over speccing it. I'll see what I can get turned out of the pistons as they are. I suppose I can lose 1mm or so out of the squish band, maybe even turn a smidgen out of the valves. Got a decent lathe for the job which is one thing. Suck it and see I suppose. Stevie, I remember you saying you hacked off 6mm of your turbo pistons to bring them down. Am I right in thinking the wall thickness and crown thickness is the same and so effectively you ended up with a thick centre and perhaps a dangerously thin bit around the edge?

Bugger off, I'm getting there.

Somewhere - there is a picture on this site showing a section through a stock AE turbo piston if its of use. I would aim for 6mm crown thickness personally - and definately not anything less than 5mm...

A good place to start is the MiniSpares Mega range. These are super-tough pistons - even with a bit of machining :)

I'll have it to bits and take some measurements.

Bugger off, I'm getting there.

I think this is a link to thread pic Dave's on about, its also got a pic of Alex B's dished piston.

https://www.facebook.com/pages/Fusion-Fabri..._homepage_panel

www.fusionfabs.co.uk



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

From memory, the distance from top ring land, to top of piston was about 10mm.

Removing 6mm leaving an almost flat piston still leaves 4mm. It also does not touch the crown thickness at all.

obviously combustion chamber shape is badly compromised doing this though.

But its cheap. Aside from me melting a couple of pistons, I never had any issues with my bottom end, which was totally standard aside from the shopped pistons. Quite surprising considering the abuse I gave it, day in day out.

When I do strip mine, I'll be making enquiries with Ross or similar about getting a custom set made, along with rods.
Ive seen rods advertised for almost £1k from one of the vendors in the UK.

IMO thats a mental price. I'd like to think I could nearly get custom rods and pistons for that sort of money !!!

9.85 @ 145mph
202mph standing mile
speed didn't kill me, but taxation probably will

Be interesting what you can get out of Ross... As you may know JE make a big-dish forged piston for the a-series. Only problem is it sits something like 1/4" under the standard deck height...

I think this wasn't so much because someone screwed up - but because they don't / didn't have any dies for such a tall piston (the a-series piston is bloomin tall im comparison to diameter against every (?) modern piston...

End of the day though - someone decided it'd be allright chopping that much off the deck... Aside from the potential for weight saving - I'd prefer as thick a deck as possible...

So far, I calculate

Dish Size:CR
18-8.55:1
15-9:1
11-9.9
7-11:1

So, I'm not chopping the deck off, I'll have a play with some bowl models and then make a pattern up and then cut it out. I'll hope that my engine I'm using has 11cc pistons although I think they're 7's. (engine off a mate). Should be able to get 15 out of 11cc pistons and then lose about another 3 out of the head, reluctantly. I can't see the step being achievable to 18cc. Bearing in mind the stuff is 2.7gcm^-1 that's a fair load of swarf required to weigh that much. My reasoning for using a set of shitty old pistons was that I fancy lightening them by losing "a bit" of the skirt. I'm going to make a set of ultralight rods, have a spangly bottom end and then build a relative screamer. My mate who used to race minis in the 60's said this was a popular mod, they didn't have snags even though they weren't using undercrown cooling. I figure undercrown cooling with a thinner crown should improve heat transfer (but probably not operating temperature). Anyway, I'm just a cheap bastard. If I look at my set of pistons and they aren't do-able, it'll mean hands in pockets time. I'm going to get 9cc of blu-tack and see what it looks like. Probably fuckin huge! 4cc is probably do-able, especially with a piston that big. We shall see, there is no rush to finish the job as I'm using the car with the other engine and working full on at the moment. Can't remember when 1/2 term is. Well, with my track record, it'll probably be christmas.

Bugger off, I'm getting there.

Bloomin hell Stu. Try pressing the return key occasionally!



Here's a tip:
If you chop just one of the pistons up, you'll then know how to optimally machine the other three...

9.85 @ 145mph
202mph standing mile
speed didn't kill me, but taxation probably will

Long rods, long stroke, small (ish) bore. rods will foul bore at 90' BTDC or ATDC which ever way you like it.

Edited by stevieturbo on 29th Sep, 2006.

9.85 @ 145mph
202mph standing mile
speed didn't kill me, but taxation probably will

I think that the explanation why you can't buy an A series cam with a high LCA is because it would mean starting with a new billet rather than modding a standard cam.

You cant get the lift/lobe centres without going too small on the base circle.

Pure economics. No one would pay for it.

You sometimes find people offering billet cams but usually based on a design proved from a modded standard.

Pick up any book about modifying a small block Chevy from 20 years ago and they extol the virtues of high LCAs for street or Turbo use.

I used a 112 Degree LCA in my mild 350ci Chevy and it was great. You could drop the clutch in top gear from a standstill and go to over 135mph!

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."

9.85 @ 145mph
202mph standing mile
speed didn't kill me, but taxation probably will

Stevie;
Part of my swithching back to the MG metro grind for the injection was as much to make a 'baseline' as anything else. I reckon a mild stock cam is a 'safe place' to be when trying to get this injection event optimised.

Once the injection is sorted, then it'll arguably be easier to swing and adjust cams and see real changes after adjusting the air/fuel ratios on the laptop. I'm pretty sure no-one apart from Giallofly has actually tried swinging his timing about whilst on the dyno either (I don't know if he has - but have an inkling this is the case)

I agree that the ph2 seems to work - but was it optimised for a 1293 with a stg3 head running 14 psi through a standard spec T3 turbocharger - or something much more extreme?

One of our australian friends on here recently had a cam ground by Graham Russel over in Oz - and reported a MASSIVE difference - i think - over the phase2..

Have you got any 1st-hand experience of Turbine A/R vs overlap as regards HP / torque potential Steve?

Edited by turbodave16v on 29th Sep, 2006.

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."

9.85 @ 145mph
202mph standing mile
speed didn't kill me, but taxation probably will