13 replies to this topic

[pdnz]

Hi all,

I am new to this forum and have been watching it for a while. I have found some very great advice so far and for the first time the search function has drawn a blank for me.

I am currently building an A+ 1275 engine and have it completely stripped ready to be machined. My questions are:

1. What size over bore? Do i go to 1340 or 1330 and how many thou is that over?
2. Any recomendations on pistons to use / places to look for them?
3. How do i determine a good compression ration? Fuel here is 98 octane (best) and in the long run i am planning to supercharge it.

Any help would be truly appreciated.

Thanks again!!
terry!

[TimmyG]

Hi there,
Being as your looking to supercharge in the future, I would say you would be best off going with the smallest re-bore possible that will clean your bores up, so +20thou-1293cc if possible, then +40 thou etc. 1330cc is +60 thou re-bore, and i think 1340 is a rather special one in that it is a de-stroked 1380?! not sure on that one except that it isn't acheivable on a standard re-bore. The extra cc gained from going straight up to the bigger re-bores though will not gain any noticable extra power and just results in less potential future rebuilds. Plus it will be easier to keep the compression at the lower levels required when supercharging, i.e. less cylinder head work. I'm pretty sure minispares sell suitable pistons in all oversizes and in different dish volumes to achieve the compression desired. The compression ratio needed would depend on the boost pressure your going to use, with bigger boost pressures you maybe looking as low as about 7.5:1. It would be wise to use a programmable ignition conroller such as the megajolt so that you have tight control over ignition timing and reduce the likelyhood of detonation. This may also allow you to use more compression without causing problems especially if your going to use 98 fuel..
Sure someone will be along with more detailed info and experience soon but hope this helps a bit.

[pdnz]

Thanks TimmyG! Very helpful indeed! Gives a good idea as to where to start! Any one else got any ideas?

[Guess-Works.com]

+20 - 1293
+30 - 1301
+40 - 1310
+60 - 1330 ( or as some call it 1340 but it's not )
73mm - 1360
73.5mm - 1380
74mm - 1400

Depending on your budget, go for 21251 (LC) or 21253 (HC) pistons on the over sizes... however how confident are you about the supercharging actually happening... basically as you have a choice later to lower the base compression of the engine by fitting either or all of, new pistons, modified head and possibly a decompression plate...

So my suggestion is build the engine for what you want to do with it today, and not what you may do with it in the future.

[pdnz]

Thanks heaps mate! Thats really helpful and I think I will do that. I was planning on gettign the engine running before supercharging anyway. Without the super charger what compression ratio would you say is good to go for (98 fuel)? Do i have to worry about compression ratio when buying pistons or do i get any pistons and then modify the head or dish the pistons etc? Also, what brand are the pistons above? Sorry, this is my first project!

Thanks!

[Guess-Works.com]

Aim for about 10:1...

The Pistons are Hepolite AE ( std replacement pistons )

21251's are low compression variants eg http://www.minispare...ty=pb&pid=37695
21253's are High compression variants. eg http://www.minispare...ty=pb&pid=37864

21253's are a better piston and run as standard @ 9.75:1 which is close enough, but a skim off the top of the block with quickly raise the CR

If it's your first time build then I would suggest taking the short engine to a respected mini engine builder who should know all the necessary machining dimensions to achieve a 10:1 CR with either set of pistons, however these may be a little sparse in NZ.

[minidaves]

if your gonna run 10.1 compression then remember that the 21251's have a limited life span but of course they are cheaper

dave

[pdnz]

Thanks guys! I will have a look at those pistons and see what my budget allows. I am hoping to get the better options available so might require savign a bit more.

Dave, you said the cheaper pistons have a shorter lifespan. how short are we talking? Does that imply that the expensive ones will last the entire life of the engine and the cheaper ones wont?

Thanks again guys for all your help! I truly appreciate it and any more to come!

[Cooperman]

The 21251 is not a bad piston, it's just not as good as the 21253. I've run the 21251 at compression ratios of up to about 10.5:1 with no problems. It will last well so long as the revs are kept below about 6000 and the ignition timing is set accurately and the distributor has an optimum advance curve.
However, I would never use a 21251 on a forced induction engine. For forced induction Omega do an excellent piston with a 10 cc dish. I've used them on a Turbo car running 7.5 psi boost and it's good with about 115 bhp at 5600 rpm.
To go from standard induction to forced induction requires a significant dropping of the comp. ratio. To achieve this you can open out the chambers in the head by the required cc's, change the pistons for ones with a lower gudgeon pin height or a bigger dish, or a combination of all these things.
Once your engine is built without forced induction, it's easier to improve the power by 'normal' tuning methods, although that won't give the better flexibility and mid-range torque that, say, a good supercharger will.
It's a personal thing really and you just make your choice. Some feel that a supercharger is better on a Mini than a turbo-charger, just because the supercharger runs cool, whilst the turbo has lots of heat involved with it and it can be difficult to get that heat away from under a MKini bonnet. Both can be made to work well though, so again, it's just personal choice.

[minidaves]

well after all the stick i got on this forum and on the internet and from an indpendant inspection of an engine i fitted the 21251 pistons too (because of budget) , I would not recomend any more than 9:1 compression with them, they are a cheaper piston, they are not terrible, but they are not a performance item and I would only ever use them in a standard road type engine and nothing with any major bhp, evan though in the past I had never had a problem. Thats my experiance, and as always you can have two of the following cheap powerful reliable.

[GreaseMonkey]

Go with the 21253s, they are quality pistons. I've just bought a set.

See here for more Click me

[pdnz]

Thanks guys!

Seems like I will go with the 21253 - 60's (Going for a 1330cc). No other brands I should consider or is it just not worth it for a daily road vehicle?

Im looking for something that will have a kick but im not planning to race it. Is MiniSpares the place to buy from or are there any other options?

So far ive found - minispares, minisport and minimania. I have to ship them to NZ so more than anything im looking for best shipping prices

[pdnz]

I have just been suggested to by a mini shop here in NZ that Hypatec pistins are also a very good consideration. Does anyone have feedback on these? How do they compare to the 21253's?

[drmini in aust]

I have just been suggested to by a mini shop here in NZ that Hypatec pistins are also a very good consideration. Does anyone have feedback on these? How do they compare to the 21253's?

 

I have used Hypatecs in high performance engine builds for many years, they are a good piston. They come with a Hastings ring pack, these are imperial width unlike the AE ones which are metric. Their oil rings work much better than the AE ones. Available from www.minisport.com.au or www.minis.com.au in +.020 +.040 +.060 73mm and 73.5mm sizes, [however right now they are both waiting on stock to come in for the 20 40 & 60 ones.] EDIT they are now back in stock.

 

Another good piston is the Russell Engineering ones, which come in 72.5, 73.0, 73.5 and 74.0mm sizes. These come with a Nippon ring set (same width as the Hastings) and have the rings a little closer to the crown than the Hypatec.

Hypatec is better if building a stroker. Both pistons have a thick crown so can be decked lots if needed.

Contact russellengineering [at] bigpond.com for info on his pistons.

 

Both the Hypatec and Russell Engineering pistons are a 12% silicon alloy.

Edited by drmini in aust, 26 June 2013 - 12:31 PM.