14 replies to this topic

[minipower101]

Can anyone help me decide which type of Omega cast pistons I need for my engine. What are the differences between these:
• 73.5 x flat top Std compression height
• 73.5 x 7cc offset dish Std compression height
• 73.5 x 11cc central dish Std compression height

My engine is 1380, stage 3 head, 296 kent cam (I think), 1.5 roller rockers.

Thanks

[samsfern]

id go for the middle ones, with the 7cc offset dish.

[Cooperman]

You need to measure the combustion chamber volume and do compression ratio calculations in order to decide which best suits your application.
You also need the 'gudgeon pin' height from the piston top to calculate where it will sit relative to the block deck when assembled.
Then decide what your compression ratio is to be and go from there.

[minipower101]

You need to measure the combustion chamber volume and do compression ratio calculations in order to decide which best suits your application.
You also need the 'gudgeon pin' height from the piston top to calculate where it will sit relative to the block deck when assembled.
Then decide what your compression ratio is to be and go from there.

Im sorry, I dont have a clue how to do that?? Thats getting a bit technical for me Any advice would be much appreciated.

[Cooperman]

Are you building the engine yourself or paying someone to do it? A professional engine builder should know how to do all this, but if you are doing it yourself then you need some assistance plus some measuring gear and certain special tools.

[minipower101]

A bit of both really. Have got the help of a mechanic and had the block professionally honed and crank re-ground etc. Will see if they can help with that. Do you know what difference the dish makes to the compression ratio. Is it the bigger the dish the higher or lower the compression? Thanks for replies.

[Liam Mini 35]

A bit of both really. Have got the help of a mechanic and had the block professionally honed and crank re-ground etc. Will see if they can help with that. Do you know what difference the dish makes to the compression ratio. Is it the bigger the dish the higher or lower the compression? Thanks for replies.

uyou use a dished piston to lower the compression ratio. What are you using the car for? If ur doing forced induction i believe u need a cr of about 8.1:1, for normal road use about 10.1:1 and race use i believe about 11-12.1:1

they are figures that ive come to believe from previous conversations but may well be wrong. Im sure some1 will be able to offer a more professional answer.

[samsfern]

the bigger the dish, the lower the compression ratio,

the smaller the dish, the higher the compression ratio


the camshaft manufacturer should be able to reccomend a compression ratio for the camshaft to be used. I think youll want 11-13:1 compression ratio for the kent 296

youll also need to use high octane good quality fuel such as bp ultimate or shell vpower for higher than standard compression ratios, to prevent detonation/pinking.

i run 12:5:1 on my 1330, thats my everyday driver, not had any problems, but have to use high octane fuel (shell v-power for me) and additive to raise the octane to 102, which does obviously cost more.

Edited by samsfern, 20 December 2010 - 06:28 PM.

[Turbo Phil]

You need to select the correct pistons to give the required compression ratio. Depending on the current volume of the head chambers, head gasket used & any volume above the piston at TDC. To get the best from the 296 you need to be looking at quite a high compression, 10.5-1+ though probably better a bit higher, though bear in mind you'll need higher octane fuel & maybe an octane booster to. I'm sure somebody else with a similar spec can advise here.
Have a read of this page, it should give you some help: HERE

Edited by Turbo Phil, 20 December 2010 - 06:32 PM.

[mini93]

if your going by design and alter your CR by skimming/opening the head i would have said the offset dish would be better, promoting better swirl...apparently flat top pistons dont help the swirl much at all

[minipower101]

I'll be using my Mini as a fast road weekend car. Just a bit of fun really, no track racing, but at the same time I would like it to be reliable lol.
Thanks for the info on working out the compression ratio, starting to make sense now
The head has been skimmed. Really helpful info everyone, Thanks.

Anyone out there with similiar spec?

[Cooperman]

The possible problem you have with deciding which pistons you should fit are that you may not have measured the distance from the top of the original pistons to the block deck before stripping it all down. If you have that dimension, rfom Omega's data sheets you can get the dimension of their pistons, compare the gudgeon pin centre to piston top and see how far down the bore the Omegas will be when assembled. From that you can decide how much needs to be machined off the block deck to bring the pistons right to the top and then work oput the compression ratio with the different pistons. My guess is that the flat-tops will be unsuitable, but whether the 7 cc or 11 cc dish is appropriate can only be done by calculation and measurement. With a 296 cam, for it to work properly the comp ratio is quite critical as 'samsfarn' has said above.
So, what you need to do now is to get the Omega piston dimensions and compare them to the original piston dimensions. Then assume that the original pistons came to 0.010" down from the deck and do the calculations on this basis. If you want to PM me on these dimensions I'll do what I can to help. You will need a vernier caliper and measure in inch dimensions. The other thing to do is to buy a 5 cc shrynge from Boots and measure the combustion chamber volume in a combustion chamber with valves and plugs fitted and the head absolutely level.
Before final assembly you will need to do a 'trial build'. Then the dimensions can be confirmed and either the block machined a bit more and/or the head stripped and skimmed or, if the comp ratio is then too high, the combustion chambers will need to be opened up a bit more by grinding out and polishing.
It really must be done properly if your engine and especially the very hot cam are to work well. I'm sorry if it sounds a bit complicated, but that is why builders of powerful engines don't do it cheaply if they do it properly.

[samsfern]

dont worry, its not as complicated as it sounds just a bit of measuring needed followed by some maths.

all the formulas and measurements needed are in vizards book. He explains it quite well.

Edited by samsfern, 20 December 2010 - 08:17 PM.

[minipower101]

Thanks alot, thats great. I best get measuring then I will keep you all posted, I'm sure I will get stuck along the way haha.

[Cooperman]

Thanks alot, thats great. I best get measuring then I will keep you all posted, I'm sure I will get stuck along the way haha.

We are all here to help.