New pistons advice
Turbo Mini Forums

Hi!

Ive been running my turbo engine for 10 years (allthough only about 10000miles) when my diff gave up in the middle of Lithuania after IMM 2015.
Since I needed to sort the diff I decided to rebulid the engine. When removing the piston I found cracks in the ring land on 2 of the pistons!!
Pistons looks mint except for the cracks....

The pistons are +40 Mini Spares Mega machined to 11,3cc.
CR 8.5 and max 13psi pressure. No detonation/knock

Seems like Im not the only one with ring land problems on Mega pistons... so, looking for something better to replace them with.

+40 12cc pistons would be ideal and would keep my CR, but I cant find any.

Omega cast pistons seems popular and available in +40 10cc.
This would give me a CR of 8.8
Would it be safe to run 15psi on 8.8 in CR?
If so, how high would be safe with 8.8?
(Im using std Metro turbo Dizzy)


Both MED and Avonbar sell +40 10cc Omega cast pistons, MED@ £330 and Avonbar @ £270, large price difference!!
Are these the same pistons, or am I missing something?
http://www.med-engineering.co.uk/products/...ast-pistons-040
http://www.avonbar-racing.co.uk/1575/Omega...0-10cc-Dish-set


Have a nice day!
/Jonas

My best guess is we have VAT at 20% (value added tax) would take it up to a similar price
Also avonbar aren't a massive firm, just one guy if I remember correctly who can be very hard to get hold of so message or call them before buying!

In fact if the pistons where cracked are you sure the bores are ok?
Turbo motors run pretty hot and that would cause expansion
I'd be tempted to double check that and if it was me I'd bore to the next size up for piece of mind

I think the bores are fine, the crosshatch honing marks are still visible over the entire piston stroke area.

15 psi on 8.8 is possibly pushing it a bit, i would look to get it down a bit to about 8.5, then you have room for turning up the wick in the future,

you can get a few cc taken out of the pistons cheaply, that should get you where you need to be.

You'll probably find on an unskimmed block that the cast Omegas usually sit around 0.040" down the bore. So though they only have a 10cc dish, there's extra volume in the deck height.

Phil.

WWW.TURBO-MINI.COM

Thanks for the answers.

Ive allready have the 0.040 down the bore in the calculation.

I have 24cc in the head, Ill might be able to take some more of.

(I would like to avoid machining the pistons this time.)

8.5 compression? is it running a locked dissy or something why so low? make it so it will run on crap fuel or something?

There is alot of opinions on what CR to have....
8.5 is what I felt was right 10-15 years ago when I built the engine, and I havent had any problems with it until now...

What compression do you suggest?

there sure is

i really don't know what is right or wrong but it seems that in general not just minis but most engines i think people tend to use more compression than back in the day with the aid of modern ignition maps being able to add and reduce timing based on manifold pressure throttle position rpm

lower compression throws quite a bit more wasted pressure and heat energy out the exhaust when the valve opens

tuning has come a long way in 5 10 15 years

i only know enough to get my self into trouble so its just my opinion that it seems low

its also dependent on the cam a late closing exhaust valve will reduce the cylinder pressure quite a lot

ive had bad luck for Manny reasons but i don't think its my 10.5-1 "high" compression sure it makes the piston/cylinder run hotter but thats because the energy has been used to drive the piston down not to heat the exhaust pipes

the trick is fuel and timing management around peak torque to avoid det and excessive heat i did not do this and the result was a meted piston.. but to trade a little performance around peak torque to gain/flatten the torque curve is in my opinion better

not saying its the only way or the best way its just my thoughts

there are plenty of engines that run really low compression with a bunch of boost to make up for it and there are guys doing the reverse high compression and low boost

I'd rather run a bit more cr and better fuel & a big intercooler .The CR is a function of cam duration, intake temps,fuel octane levels,boost levels & propensity of the chamber to make trouble. People run Honda's with 11.5 CR and oodles of boost without problems, so it's not always clear cut.

That sir, is not rust, it is the progressive mass reduction system

http://aseriesmodifications.wordpress.com/

Thanks for you opinions!

Since I want a reliable engine (that will make it to/from IMMs etc around Europe) I rather have a low CR than being on the edge of detonation.

I dont think a engine with dizzy and carb, developed in the 50s is comparable to a modern engine with injection, electronic ignition, cam sensor, TPS etc...

good call,

if you ditvhed the dizzy and went progamable ingition, i would say you would get away with the 8.8, providing the fuel in your area is good, cooling good, intercooler good etc.

also if I was pushing the CR up i would be looking at fitting a knock light.