Milling my head

[knut]

Guys,

First post, another Hong Kong rider (American expat). Previously MKI owner and now '98 AR and been lurking on the site for oh, 10 years now. Will properly introduce myself when I take some photos of the bike!

Anyway, I've searched the site for info on milling heads and there is a little bit. Am thinking of milling .3-.5mm. Will up to .5mm not alter the squish band too much and avoid detonation or other problems. Also, is anyone who's done it getting away with regular pump fuel or premium is a must?

Also, does anyone have specs of the Mugen modified head? How much is that milled?

Love the site, love 2 smokers, thanks in advance for any info.

[knut]

found my answer thanks, just not with the search engine.



Compression Ratios and Skimming

Postby knackeredMk1 » Sat Jan 08, 2011 6:56 pm
High compression engines are talked about all the time as high compression gives the fastest and most complete burn of the petrol/air mixture. However you cannot raise the compression ratio above a certain point or the fuel/air mixture will self ignite like a Diesel engine. This usually happens some time before the piston has reached Top Dead Center and causes catestrophic engine damage.

A quote for Eric Gorr 'Basic 2 Stroke Tuning' - 'Cylinder heads can be reshaped to change the power band. Generally speaking, a cylinder head with a small diameter and deep combustion chamber, and a wide squish band (60% of the bore area). Combined with a compression ratio of 9 to 1 is ideally suited for low to mid range power. A cylinder head with a wide shallow chamber and a narrow squish band (35-45% of bore area) and a compression ratio of 8 to 1, is ideally suited for high rpm power.'

From that it might be deduced that the desirable compression ratio for a CRM might be close to 9:1. However stock CRMs have different compression ratios - Mk1 = 6.2, Mk2/3 = 6.4, AR = 6.7. BUT with the exception of the AR all engines have the facility to be bored out when worn and without work done to the head at the same time the compression ratio will rise; Though apparently not to pre-ignition levels if not done as most people don't bother. Mugen heads raise AR compression ratios to 8:1.

Having read several articles about Japanese CRM tuning there seems to be a consensus that skimming (taking a thin layer of metal from the gasket surface) the head by 0.25-0.3mm adds significantly to the low end power without making the engine unreliable. 0.5mm has been done but only on Mk3 heads I'm not sure about reliability when you get to rebores.

Be aware that in raising the compression ratio it is prudent to use higher octane (RON rating) fuel. Higher octane rating does not mean more power but more resistance to pre-ignition. Be aware that a previous owner may have skimmed the head already . Also you are now making your engine work harder so it will wear more rapidly.

[knackeredMk1]

Knut - Be aware that someone may have skimmed the head already. I came across a Mk1 that had problems with preignition because someone had milled the head 'within an inch of its life'. Took a long time to work out what was going on.

Mk3 heads are, as far as I can work out, shaped exactly like an AR Mugen but with a deeper squish area. You can skim them by 0.5mm safely but use high octane petrol. The others I wouldn't recommend going more than 0.3mm and again use higher octane fuel.

[knut]

OK, question after all: Knackered, do I understand right the AR head has more squish clearance than some other models, ie can mill .5mm?

[knackeredMk1]

As far as I can tell the original AR head is very similar, if not identical to a Mk1, so I wouldn't go any further than 0.4mm absolute max. Would be happier with 0.2-0.3mm based on what I have seen done elsewhere.

BUT when it comes down to it I have limited personal experience so who am I to preach ...............

[knut]

cool, thanks for the prompt response. sounds like .3mm perhaps the go to be safe. will mull it over and report back if i proceed to give a performance review.