Head and Compresion Ratio Questions
Turbo Mini Forums

Hi

I'm trying to work out the compresion ratio for my engine and have a few questions. I want to get it down to around 8.5:1 for super charging with around 12PSi. I've been searching, but can't find any real specifcs for my pistons.

The engine in question is a 1275 +0.20" over bore with AE 21253 pistons sat flush to the top of the block at TDC. Aparently they have a 6.5cc dish.

The head I'm hoping to use is an MED Stage 3 with 35mm + 30mm valves. It looks to have had about 1mm taken of the top and bottom as the thickness is now 68mm.

I've been using AF Racings compresion Ratio spreed sheet and the following measuremnts:

Bore - 71.12mm
Stroke - 81.23mm
Head chamber volume - 20.5cc
Head gasket - 3.8cc
Ring land 0.5cc
Deck volume - 0cc

Giving 11.02:1 compresion.

Firstly, have I got my figures right? I'm not sure about the Ring Land volume for these pistons and where that fit's into their spread sheet.

According to the spreadsheet, to get 8.5:1 I need to loose 10.82cc. Which is a lot. I guess I can loose 5 - 6cc from my head if I'm lucky, but where do I find the other 4?

Is the head suitable with those size valves? Or would it just crack with them being fairly close together?

If you can help I'd appreciate it, but if you want to call me names cos I'm thick then that's fine too

Thanks

Andy

It seems like your basic problem is you are starting with a specification for a high performance Normally Aspirated engine.

ie, low piston dish volumes, a skimmed head and zero deck height.

AF Racings calculator is the best I've seen and, although your outcome is probably right, I would actually measure the piston dish and head chambers, and not just assume values from what you've heard about the pistons and what you think has been skimmed off the head.

Actual physical measurements are always the best starting point IMHO.

Schrödinger's cat - so which one am I ???

You're right about the engine. Came out of an ebay scrapper, so I'm working with what I've got.

The head is marked as 20.5cc, but I did re-measure it. Bloody NHS siringes! Oil wipes the 'kin markings of! It looked like 20.5 was still accurate.

I'll have another go at measuring the block tomorrow. Hopefully our lass is borrowing me another siringe from work...

Would it be easier to use the CD & siringe method to measure everything in the block in one go, then add the head and gasket to that?

I found a lot of the maths to work out piston dish etc when I searched earlier, so I'll have a go at that tomorrow.

Schrödinger's cat - so which one am I ???

So... the pre-A plus 1380 that's sat in my shed might be a better option then??

From memory, it has a fair deck height, a 73.5mm bore, pistons with a 6cc dish. Some quick maths with measurements from memory on AR's spread sheet gave it a CR in the mid 9s with my stage 3 head as it is.

I would have to loose the Kent 286 cam, but that's not really a big deal. And it would mean I would get the light weight pre-verto crutch too.

wont the pistons be larger that 6cc dish if they are standard? my 998s were 6cc

I run a supercharger and I don't care the TB is on the wrong side.
VEMS + 12 PSI + Liquid Intercooler = Small Bore FUN!

Schrödinger's cat - so which one am I ???

I need to dig it out tomorrow and do some proper measurements, as the ones I've based my calcs on are from memory and I haven't really touched it for a year.

I think Graham is probably right about the piston dishes.

Like I said before, ar's spread sheet had the CR in the mid 9s, so that must be better than 11.03:1 that the other engine was giving??

i calc it up to 10.85:1 way too high,
i could get you another 8 cc in the head chambers but this would give you a cr of 9:1 still a tad high!

you would be better off to go for OMEGA cast pistons +20s with 10cc in this togeather with a 28cc head will give you 8.3:1
just right

the pistons are avalable from MED at a cost of £280 inc delivery
ii can provide the head service for you if you want please pm me ... your not far from me in yorkshire

the 1380 route its getting the pistons for it there are noo accralite 18cc dish pistons in stock unless someone commissions 10 sets and a 10 week wait but ........ MED do a froged 18cc 73.5 dish Omega around £400
the choice is yours ?

Edited by BENROSS on 9th Dec, 2008.

I'll get the engine out tonight and do some measuring. From memory the head that I took of it had had a lot of meat taken of the face, which (in my mind) implies that the engine had quite low comp to start as it came out of a n/a grass tracking car.

Edited by apbellamy on 10th Dec, 2008.

It was a hill climb car that it came out of Andy.

If something is worth doing, it's worth doing half of.

Same difference.

I've dragged the 1380 out of the shed and done some rough measuring. Again, CR's are worked out using AF's spread sheet.

The pistons are marked 1D12 W08 STD, so are probably standard pistons out of something else similar.

Bore - 73.5mm
Deck Height - 1mm giving 4.2cc ((pie * 7.35^2) /4 )* 0.1
Piston Dish - 61mm x 7mm - TurboDave's post gives 60mm x 7mm as 10.1cc, so I'll use that for ease.

Which makes for 9.81:1 and a short fall of 6.85cc in the chambers. :)

So 8.5:1 is acheivable with that block, but I'm not sure that's the best route. From what I know, the pre A+ blocks are not that strong, it's not been of set bored, two of the pots have some slack in the rings and the cam would need changing.

I'm not really wanting to spend too much on this engine as it will just be a put me by until I can build something proper, so I need to look at another engine.

Fortunatly, John (minimadmotorman) has offered to swap me the 1293 for his bog standard metro 1275 and a standard MG Metro cam to go in it. I need to measure his block, but it should have either 9.4:1 or 9.75:1 as standard, so if I took Benross' kind offer to fettle my head I should be able to get down to the low 8.xx:1s.

From the little I know, the MG cam is a good one for forced induction.

Can somebody check my maths and let me know if what I'm guessing about the Cam and the third engine is correct?

Thanks

I've been over to John's house and measured his engine. It's a standard bore with the pistons flush to the top of the block. The pistons have a 13.1cc which makes a compresion ratio of 9.4:1.

So I've nicked the MG cam and will throw that motor in the back of the Metrovan the first chance I get

Thanks for all you're help.