27 replies to this topic

[The Matt]

Right, I've been having a play with engine components and I am trying to work out the chamber capacity needed for my cylinder head.

I am using 74mm pistons, but they are flat tops (actually from an 1100cc crossflow Ford engine I think). The issue is that they are 2mm lower than normal mini pistons (gudgoen to crown height). This obviously means the piston will be lower down the cylinder, lowering the CR slightly (which I will get into later). What I need to know, is:

Does it cause any problems other than compression ratios if the pistons sit lower in the block?
Or do I need to get the block decked?
If so, is 2mm alot to ask from block decking?

So, the CR:
If I get the block decked, and effectively a deck volume of like 0cc I need chamber sizes in the region of 28cc to achieve a sensible compression ratio.

Is the easiest way to get a decent CR to leave the pistons/block as they are, then get a normal Stage 5 head from someone like Minispares (with a 24-25cc chamber or similar)?

This would be preferable cost wise (no block decking, off the shelf cylinder head etc).

[Bungle]

sorry i can be much help

but visards yellow book has a lot about working out CR

[kada1980]

I reckon your best bet would be to get a cylinder head with the ideal cc in it so you don't have to deck the block. 2mm sounds like quite a bit and once its gone you can't put it back.

[The Matt]

sorry i can be much help

but visards yellow book has a lot about working out CR

Thanks Bungle, to be honest I used Alexf2003's excel file in the FAQ to work out the CR etc.

It's more the principles of block skimming and whether I need to do it, that I guess I am asking!

Though it does have alot of questions in one post! lol

I reckon your best bet would be to get a cylinder head with the ideal cc in it so you don't have to deck the block. 2mm sounds like quite a bit and once its gone you can't put it back.

Well, if I don't deck the block, I can use a standardish head.

The other bonus to this is that a 25cc standard ish head from Minispares or similar would also work with my spare 1380cc block, crank and pistons. So it really would be best all round! I could retain all other ancillaries if I did have a problem with the 1400cc block.

[Leonard]

i think in an ideal situation, the pistons would be level with the deck at tdc. on a standard engine, deck height is about 40thou (so im told) which is about 1 mm. the block can easily have that removed i know for sure, which would leave 1mm on your engine, which is what the standard engine has. the best head shape (for performance) is one with shallow chambers. would your pistons take dishing?

[Leonard]

just looked on morspeeds site, theres an option for "surface block up to 0.150"" i work that out as 3.81mm, so 2mm should be ok.

[The Matt]

hmmm, just spoken to a well known engine builder who has actually done this spec build before and it wasn't really what I wanted to hear, but basically what you are saying is correct Leonard, you need the piston to be higher up in the block for performance, but that means you have to enlarge the chambers massively (up to like 30cc ish).

This in itself is gonna effect performance (something to do with 'squish area' or something). Which means the head spec and piston location is critical. I think it may be time to look into getting a dish machined into the pistons along with a block skim. If I get it skimmed and dish the pistons to lower the CR, it will mean I still have the option of using the 1380 block if it all goes horribly wrong!!!!

[Sprocket]

It ruins squish. A little is fine as this is how Rover altered the CR anyway but that much may be an issue. As for how much the block can be decked I dont know, but then you still have the issue of not enough cc above the piston. There is a topic somewhere that says something about sqish. Ill go find it

I thought you were considering having the pistons machined with a dish to over come this.

Oh and still waiting on my rings, doh


you up for a race at drag wars :wink:

[Bungle]

If you cut the top of the block off to match where the pistons are , arent the push rods then to long ?

[Sprocket]

Not quite the same in priciple, but the same in theory

http://www.theminifo...showtopic=20369

[The Matt]

I am still in a state of confusion to be honest! I don't know how well the pistons will dish, I have asked around a few engine workshops about getting it done, but they need to know the shape of the dish, not just the CC??????

I am not an engine builder by any means and am wondering if the 1400cc thing is a bit too experimental for my liking!

oooh and MS, as for a race at Drag Wars, yeah as long as it's finished! lol

[Sprocket]

If you cut the top of the block off to match where the pistons are , arent the push rods then to long ?

Yes. they have to be machined too. 998 rods are shorter by about 3/8th inch and may be a tittle too much too short

[The Matt]

Ok, cheers MS!

[Sprocket]

I am still in a state of confusion to be honest! I don't know how well the pistons will dish, I have asked around a few engine workshops about getting it done, but they need to know the shape of the dish, not just the CC??????

I am not an engine builder by any means and am wondering if the 1400cc thing is a bit too experimental for my liking!

oooh and MS, as for a race at Drag Wars, yeah as long as it's finished! lol

Just take them to any precision engineers rather than engine shops, they usualy have a different attitude. Doubt you would need any more than a coulpe of mm machined out. It doesnt need to be a dish as such but rather just a recess with radiused corner leaving 5mm around the perimeter at the standard compression hight of the piston.

Its also a little easier to do the maths and work out how much is to be removed.

To find the crown thickness, measure on the inside of the piston from the bottom of the skirt to the underside of the crown, then subtract that from the total piston hight ( bottom of skirt to top of crown)

I hate this new skin, its dam confusing

[The Matt]

Thanks for all the advice MS, I will re-ponder the dishing and work out the CR again I think it was a 6cc dish that I needed to be able to run a fairly off the shelf high spec head.

Is the shape of the dish really not that important then? Or is it just that ideally, it would be machined, tested, machined, tested and so on until the dish gave the right results.