46 replies to this topic

[minidave54]

From what i have been reading these AE 21251 pistons are low compression and would actually lower the compression of my engine which is already at 9.4:1. Maybe an upgrade to the 21253 +20 is what is needed.  

[Earwax]

Hi

Will the cylinders clean up at 1293 ( plus 20) ?  If you are now having to buy pistons anyway don't commit to plus 40 unless you have to.  It is a little bit chicken and the egg, but i always prefer to hand the machinist the actual piston going in for sizing, along with the specific instructions from the manufacturer regarding fit. ( as an example i think Omega forged pistons use the piston plus clearance value as their size --- so their piston is marginally undersize)  Others with more knowledge may comment on preferred measures.  

 

Sorry if the above comment seems to undersell your decision making ( that was not the intention) it is just sometimes when lots of different scenarios are being thought of,  the bundling of ideas can unintentionally mean some options are disregarded.

[ACDodd]

The compression will be too high with these as the compression height is much higher about 0.040". You will need these dishes out to keep the car at a sensible level.

Ac

[Cooperman]

The 21253 is indeed a much better piston.

Personally I would measure to find out what the volume above the 21253 would be relative to the existing deck level, then re-do the CR calcs to see what you end up with. It would be better to open up the combustion chambers in the head a bit to get the CR to no more than 10:1.

Most builders do prefer the 21253 piston and I would rather have an engine with just over 10:1 and with 21253's, than a lower CR and the 21251.

As before, if the CR is just over 10 you can always use 98 RON petrol and it will run fine so long as the dizzy advance is set correctly. Additionally, a bit of octane booster works well with higher compression engines.

I built an engine for an Innocenti 1300 Cooper and it ran with the 21253 pistons, a 10.3:1 CR and an Aldon 'Yellow' dizzy. With Shell 98 RON petrol it ran beautifully and with the standard BMC 510 cam it had simply lots of mid-range torque.

[minidave54]

I am new to engine building and the talk of CR calcs, deck levels and chamber volumes all a bit of a mystery. I was expecting a low compression piston or standard high compression piston to fit in a standard bore.

 

Sorry if I am missing something here, but if I have an SPi 1275 standard bore engine, that has never been skimmed and increase the bore by +20, can I not just fit a set of 21253-20 high compression pistons (I believe these are a modern version of the 21963 that were HC pistons and fitted to many minis) without any further work being needed.

The 12153's are quoted as having an 8.4cc dish and would be fitted with cylinder head with a 21.4cc combustion chamber.

[AndyMiniMad.]

I would go with whatever advice AC supplies. And indeed cooperman. They both have a massive knowledge on A series engines. For what its worth, I would fit standard comp plus 20 pistons and fit the other mods you have already listed. You will see a decent hike in both power and torque, the car will feel more lively for sure, The standard final drive on the spi is 3.1 and personally i would fit a 3.44. But long motoway trips will be more comfortable and indeed economical with the 3.1, you can always drop in a 3.44 once you return from IMM Germany if you want.

Edited by AndyMiniMad., 09 January 2020 - 06:45 PM.

[Cooperman]

I am new to engine building and the talk of CR calcs, deck levels and chamber volumes all a bit of a mystery. I was expecting a low compression piston or standard high compression piston to fit in a standard bore.

 

Sorry if I am missing something here, but if I have an SPi 1275 standard bore engine, that has never been skimmed and increase the bore by +20, can I not just fit a set of 21253-20 high compression pistons (I believe these are a modern version of the 21963 that were HC pistons and fitted to many minis) without any further work being needed.

The 12153's are quoted as having an 8.4cc dish and would be fitted with cylinder head with a 21.4cc combustion chamber.

 

It can all be a bit daunting at first, but measuring and calculations for CR's is not as difficult as it might sound.

You need a Dial Test Indicator (DTI) on a magnetic base, a set of feeler gauges, a Digital Vernier Calliper and a 10 cc syringe (from Boots-the-Chemist. A crankshaft protractor is necessary for checking & setting the cam timing using offset woodruff keys.

 

If you get a copy of David Vizards book 'Tuning the A-Series Engine' it is all in there, or we can help you on here.

 

Doing this measuring &  calculating is all a part of engine building and is a 'must' if a really good engine is to be built. For example, just considering the piston dish and the chamber volume does not give an answer as the distance from the top deck level of the block down to the top if the piston when the crankshaft is at top-dead-centre is a vital measurement in working out the CR. Actually, it is a very interesting part of engine building.

[minidave54]

Quick update. I have ordered a set of 21253-20 pistons from mini spares. When these arrive I will get them down to the engineering company. Then carry out the calculations to see what the CR is.

The camshaft timing will indeed be getting done. This is a job I have done before, though many years ago and on an old Ford escort. But the principle is just the same

[AndyMiniMad.]

I have all the equipment mentioned by cooperman for calculating your comp ratios. I will bring it this weekend for you when we are rebuilding the box and diff. I will bring the yellow bible,(David Vizards book),with me as well. Some bed time reading for you lol.

[minidave54]

The engine finally got built with a few changes to the components, such as the head and distributor. The final compression came in at about 10.2:1. I run the engine for the required 500 + miles and put it onto a rolling road for final tuning. The figures came in at a pleasing 78 Bhp at 5699RPM, max torque of 88.4 lbft at 3427 RPM

 

My worry is the ignition timing. At idle it's 12 deg, but at full advance it's up to 47 deg which i think is far too high. I would expect more in the range of 32 - 38 deg. Am i right in thinking this ?

When driving the engine does sound rough from 3000 RPM onwards, which i'm guessing might be caused by the high advance.

 

What are my options ? I have a standard 59D electronic distributor i could try. Adjust the idle timing down a few degrees ? Or am i looking at a new or re-mapped dizzy

 

cheers

Dave

 

Camshaft (Mini Spares)

Evolution 001

Valve clearances 0.014" on inlet and 0.016" exhaust. LCA 107 degrees.

DURATION INLET 264 EXHAUST 268, LIFT INLET 264 EXHAUST 289.

SEAT DURATION ON INLET 257 AND EX 252

CAM LIFT IN 264 EX 289

VALVE LIFT IN 327 EX 358 STD

 

Cylinder Head (Mini Spares)

C-AHT135

Valves 35.7mm inlet x 29.3mm exhaust

Combustion chamber 21.4cc

 

Distributor (Mini Spares)

Distributor Electric Evolution Performance to 276 cam profile

Ideal for our EVOLUTION001 camshaft

 

Carburettor

HIF44

Needle BDL

[sonscar]

If advance under load is indeed 47° I am surprised you have any pistons left.More experienced persons than me can advise but low 30s would be a ballpark figure.Is it not pinking?Be careful,Steve..

[Tones61]

Had dealings with a series race midget engines,max power was with 34/36 max advance,
always melted no.4 piston at 38

[minidave54]

Thanks for the replies. The engine has only done just over 600 miles since build, and most of that below 3000rpm, so hopefully i have no damage.

I'll get a look at the pistons through the spark plug holes. Next i will need to read up on setting the max advance around the 34 point.

[Turbo Phil]

Echoing the above comments, your never going to need more than 35deg.
Was it set at the RR to 47deg? If so I’d definitely be worried about their knowledge.

Phil.

[nicklouse]

Echoing the above comments, your never going to need more than 35deg.
Was it set at the RR to 47deg? If so I’d definitely be worried about their knowledge.

Phil.

exactly. most race engines want about 15 at 1500 and then progress to 35 ish.