Compression question
Turbo Mini Forums

Hi, in finally starting my turbo build now, and just getting everything ready to send to the engine shop for machining.

I used to have a spreadsheet but can't find it for working out compression ratios.

Would someone be able to tell me what size head chambers I would need for 9:1?

Bore - 1275 +60
Pistons - 21253 +60
Deck height - block will be decked so pistons sit flush
Headgasket - BK450

Head is a 12G940 which came off by old 1.3 Maestro - has the 35.7mm inlets for some reason! Will covert to unleaded first.

It will be a fairly low boost build (7/8psi intercooled GT1752).

1994 Mini Mayfair, 1330, Mspd Ph2 cam, Benross turbo head, 9:1, GT1752, TT Intercooler, MegaJolt, BMW M3 leather seats.

Discarding ring land volume, with, 1330cc, standard stroke, 8.4cc pistons, 3.8cc in head gasket, pistons flush will require 28.8cc in the head for bang on 9.1.

Phil.

WWW.TURBO-MINI.COM

I've cut some 21253 from 8.4cc dish up to 15cc. I haven't run them yet, but know that other people on here done... Maybe another option instead of taking the head to 29cc.

Factory turbo'd a-series ran 7-8 psi with 9.4:1, low octane fuel, mechanical ignition. As a factory build, this was probably conservative too. You may not need to go down to 9:1.

Decking the block about 0.5mm will loose about 2cc. Pistons will still be the 1mm below the head due to the gasket. 29CC head will require opening the chamers up A LOT so decking the block may just make your CR harder to reach, with little gain.

Which engine shop you using?

OK didn't realise I would need that much out of the chambers! Was planning 9.5:1 but was told that was too high for 8 psi. AC Dodd suggested 8.75.

At the moment my engine (with 21251+60 pistons currently) has the pistons sat 60 thou down the bores. Have been told that the higher you can get them towards the top of the block, the better the burn, and less likely to experience pinking (assuming similar CRs).

What would the compression be if the 21253s sit 60 down? Is there much benefit to getting the pistons as high as possible on an A series. The advice I was using was based on an N/A Triumph straight 6 engine.

1994 Mini Mayfair, 1330, Mspd Ph2 cam, Benross turbo head, 9:1, GT1752, TT Intercooler, MegaJolt, BMW M3 leather seats.

OK that's a good point. Will have to do a mock build and see how high the 21253 sit. I thought the 21251s were just a generally weaker piston.

Was using a local guy called Roy that CRM use for machining, not sure his full name, think he does freelance. Is there someone local you would recommend?

1994 Mini Mayfair, 1330, Mspd Ph2 cam, Benross turbo head, 9:1, GT1752, TT Intercooler, MegaJolt, BMW M3 leather seats.

I had a good look at both pistons in Minispares. Visually, the castings are almost identical. The 2 differences are the wrist-pin height (giving different CRs) and the 21251 has much longer slots behind the oil control ring. It's suggested that the slot make the skirt weaker and more likely to fall apart under high RPM. If you're building a performance NA engine that needs to rev higher, I can see this being an issue. It's hard to shake common beliefs, but IMO it's probably less of a concern for most turbo builds.

I also use a freelance guy called Roy for bits and pieces like head skims, centre-cap machining, or anything slightly out of the ordinary that a normal engine builder doesn't want to take on. He's resident at a business (can't remember the name) on Sapcote Business Park, Small Heath Highway. Is that the same Roy?

I was actually after suggestions myself - Willenhall Engine Centre is not too far from you - please avoid them !!; They made a mess of one of my blocks.

Edited by PhilR on 19th Apr, 2015.

I used Roy for Shauna's engine, his place is in Dudley opposite Cals old unit. Never had a problem with any of his work, the crank regrind he did was spot on. He will eventually be doing my machine work, there's nothing he can't do, he has a full machine shop and recently acquired a hot dip tank for chemically cleaning blocks and heads etc.

I have two sets of 21251s removed from engines that are trashed.
One set broke a ring land, the other had the ring lands open up.

One of the more eminent engine builders fitted a set to a performance engine and decked the blosk to suit instead of using proper pistons.

Metric is for people who can't do fractions.

There are trashed examples of every piston available.

In fact, considering 21251s is one of the most common pistons out there, there are plenty of examples of them working perfectly.

Personally I wouldn't use those pistons. I know that 25 quid sounds like a bargain, (and it is) but I just feel like they are not suitable for anything other than a super mild engine, and the problem with that is boost is addictive... Are the metro turbo Pistons still available in +040?
They are a good "economy" choice that have demonstrated capable of a lot of abuse over many many years...

Think I would rather stick with the 21253's, as at least they have been proven in Turbo engines. I think Bud666 was running mad boost with his?

Question:

If my 21253's sit 10 thou down the bores, and I use a standard head (23.5cc?), what size would the piston dish need to be for 9:1?

1994 Mini Mayfair, 1330, Mspd Ph2 cam, Benross turbo head, 9:1, GT1752, TT Intercooler, MegaJolt, BMW M3 leather seats.

Dish would need to increase from 8.4cc to around 13.25cc.

If you cut the dish out flat and no deeper than it already is at the centre , you can get to 15cc. This will reduce the crown thickness from 6.35mm to a touch over 4mm at it's thinnest point.

Of course there's no reason you can't take cc out of both the head and pistons. This is about 11.5cc dish :

Turbo + 40thou pistons should be still available. Bought a set few months ago. They are 12cc dish if I remember correctly. Problem is Ian is already running + 60thou pistons.

Made in Poland build thread:
http://www.turbominis.co.uk/forums/index.p...tid=570363&fr=0