11 replies to this topic

[chunkyjh]

heya, got a 998 cooper 12g295 cylinder head that i want to put on my 998 mini. The block and pistons are all totally standard. Vizard says that before putting this head on the block i need to have it skimmed to raise the compression ratio to a minimum of 9:1. Does anyone know how much i need to have off? If i get the chambers reshaped will i need to have the head skimmed again to change the CR once more? Any help will be greatfully appreciated. One other thing, what are the benifits of double valve springs? Am i right in thinking they wouldn't be much use on a very mild road 998?

[icklemini]

heya, got a 998 cooper 12g295 cylinder head that i want to put on my 998 mini. The block and pistons are all totally standard. Vizard says that before putting this head on the block i need to have it skimmed to raise the compression ratio to a minimum of 9:1.

Yes - the 998 cooper engines used pistons with a raised D shape in the piston crown...
Depending on your 998 engine, you will have either flat top or dished pistons..

Does anyone know how much i need to have off? If i get the chambers reshaped will i need to have the head skimmed again to change the CR once more?

Its *roughly* 60 thou, assuming the cooper head you have is at standard. Reshaping the chambers will effect it.

You need to work out the CR properly, and measure the chamber volumes of the cooper head accordingly..
Be carefull that skimming a wedge of the head can lead to other problems, both with the head, valve train geometry, etc :\

One other thing, what are the benifits of double valve springs? Am i right in thinking they wouldn't be much use on a very mild road 998?

Not much - trick is finding the correct springs for the cam and intended RPM - too heavy (single or double) and it'll cause more harm than good...

HTH,
Dave

[dklawson]

You posted this question twice so be sure to check both threads for answers.

In addition to the information above, and since you have a copy of Vizard, I recommend that you start by looking closely at what you have and what you want to do. Don't shave the head until you can tell the machine shop exactly how much you want off of it.

Before anything else you need to CC the chambers to determine their volumes. While you're measuring, be sure to carefully measure your bore and stroke. If the pistons are dished, you'll need to CC the volume of the dish. Measure the head gasket thickness and it's bore. Don't forget to measure the distance from the top of the piston down to the first ring AND the OD of the piston. It all adds up and there is no substitute for measuring twice and cutting once.

I found a fair amount of difference between the chamber volumes on my 12G940 head. I suggest you study Vizard on measuring chamber volume, correct and balance the volumes as necessary, then calculate the remaining volume you NEED to get the CR you want. You'll have to do some creative math to figure out how much AREA the chamber has, and from there determine how much to shave off the head. Anything less is just a guess.

[Dan]

You can work out the area of the chamber at the face by smearing the head with some blue (engineers blue, a temporary dying compound used to check tolerances and clearances) or similar and taking an impression of it on graph paper. Or learn calculus.

[dklawson]

That's an excellent idea Dan. I wasn't sure what the best method would be.

That brings up a useless bit of trivia. At the turn of the last century the world was starting to make and use electric light bulbs and Thomas Edison was growing the R&D division of Edison Electric. They were starting to fill bulbs with inert gasses instead of vacuum. One group was assigned the task of calculating the volume inside a typical bulb. After working on the problem for weeks a young mathematician came up with the formula and very excitedly reported his results which were soon forwarded to Edison. Edison went to see the young man and thanked him for his efforts. He then said to the young man, let me show you how I find the volume of a bulb. He went to a sink where he filled an empty bulb with water. He then emptied the bulb into a graduated cylinder, read the volume in cc, and walked out of the room. Sometimes the obvious is right before our eyes and we don't see it.

Thanks again Dan.

[Pavel]

Isn't the accepted method using a sheet of perspex with a hole and a burette?

[Dan]

That's for finding the volume of the chamber, not the cross sectional area of the chamber. Once you know the volume you need to know the cross sectional area to calculate how much height to skim off the bottom to arrive at the volume you require.

[Pavel]

Oops forgot we're skimming heads here!
Incidentally dan, approx how much can you skim off safely before breaking into something that you shouldn't?

[Dan]

I have no idea but I suspect it's different for each casting type. That's why I said to Dogmatix the other day that he should ask a head specialist to have a look at his very skimmed 12G940. Our own resident head expert isn't around much any more (by which I mean Blue). Does anyone else do many heads?

[blacktulip]

just a thought but isnt there a way of using liquid at all to measure the chambers? and temperarily blocking the hole for the spark plug up.

[Dan]

Yes, that's what Pavel says above. You cover the chamber face with some glass (glass is more likely to be completely flat than perspex) leaving as small an opening as possible, screw in a plug (if you just plug the screw hole no allowance is made for the volume the plug takes up) and fill with liquid from a burette so you can accurately measure the volume. But that will only tell you the volume, not the area. As has been said.

[chunkyjh]

Thanks for the response guys. its a lot more complicated than the old wizard makes out. i didn't know that the 998's came with different pistons. I've got his how to modify... book is his other tuning the a series more in depth? is it a must have? Think i'll take it to a mini engineer and let them have a look. thanks again james