Author Topic: Head skimming for higher compression?  (Read 2233 times)

guest1590

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Head skimming for higher compression?
« on: December 27, 2012, 09:01:58 PM »
Has anyone got experience on skimming the head on a single, Rotax 560, what are the pro's and con's? this is purely for road racing so more power is needed, bigger valves, head flow and huge carb fitted already, I think higher comp is the way to go, I've got a tuning shop willing to do the job but wouldn't mind a bit of unbiased opinion before diving in!
many thanks in advance,
 canolman

themoudie

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Re: Head skimming for higher compression?
« Reply #1 on: December 27, 2012, 10:15:05 PM »
Aye canolman,
Where are you?
What sort of racing are you planning on doing?
More importantly, "How big are your coffers?"
Most importantly, "How bad do you want to race?"
Some straight answers, to these sorts of questions, would help.
If I may say so, the title is not where you need to be looking to go, to start with.
RELIABILITY!
To win, you have to finish the race.
'The Curmudgeon'

guest564

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Re: Head skimming for higher compression?
« Reply #2 on: December 27, 2012, 11:16:33 PM »
I would go with a higher compression piston before considering skimming the head. Do you have any way of correcting the valve timing?

JOOLZ

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Re: Head skimming for higher compression?
« Reply #3 on: December 28, 2012, 10:25:41 AM »
You will need to have a slotted cam sprocket to dial in the correct timing when you skim the head. You change a huge amount of variables when you skim a head, cam timing, valve to valve pocket clearance, squish clearance and many other things, and you would have to remachine the piston anyway to regain your valve pocket clearances, so it is far easier just to buy a high compression piston, and make any necessary clearance adjustments to that, and you will generally find it to be of better quality, and have a thicker crown than the standard piston.

johnr

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Re: Head skimming for higher compression?
« Reply #4 on: December 28, 2012, 01:03:56 PM »
im offering no advice but this. if you wan more power, then you need an understanding of what every part of the tuning process does. if you want to skim your head, then undersand why people skim heads. ditto big bore, slttred sprockets, big carbs, exhausts etc. that way when you make a decision, you know why. tuning isnt like painting by numbers, you dont go through a list that guarantees more performance. its quite possible to spend thousands making an engine less powerful and slower and less reliable. if you dont understand the principles of performance tuning, i suggest that you learn first, before you part with any brass. your tuner should have explained this to you before he offered to do anything for you. were it my money, there is more that i would have done before i resorted to skimming the head.

Steve H

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Re: Head skimming for higher compression?
« Reply #5 on: December 28, 2012, 03:31:42 PM »
My guess is the controlling factor on how much you can skim will be the squish clearance. Yamaha singles tend to stretch the stock conrod a high revs so you need a larger than desirable squish to stop the piston hitting the head.
High compression pistons tend to have a high domed centre which isnt so good for combustion as it tends to divide the combustion chamber into two sections.
Power is a function of torque and engine speed, so fitting a camshaft which pushes peak torque further up the rev-range is a another way of increasing power. You then need to work out how to keep the engine in one piece at the higher revs. Am I right in thinking its possible to get different stroke cranks for the Rotax, if so a shorter stroke crank may help it spin higher but retain some of the reliability.   
 

guest1590

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Re: Head skimming for higher compression?
« Reply #6 on: December 28, 2012, 08:46:00 PM »
Many thanks for your replies;
I think I should point out that I have been racing for over ten years, mainly japanese 4's, but have been 'developing' the Rotax just to be different!
A high compression piston was an idea but no-one appears to make one for this engine and as Steve H says will probably divide the chamber, (twin plugs?), the stroke is around 81mm which is the longer throw of the singles so a shorter stroke, 79.?, from the 500cc, is a possibility.
At the moment we have kept peak power and torque to around 6600 rpm which seems to be fairly stress free for this motor, we achieved this by lengthening the intake and using a 45mm carb.

The other idea I've had would be to fit thinner copper head and base gaskets to increase compression, the same problems of timing and valve clearance will still occur but no metal will have been removed which can't be replaced.
Another major concern would be the increased engine temp that high compression will create, this being an aircooled motor.
johnr, I'm intrigued, if it was your money what would you do?
many thanks

johnr

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Re: Head skimming for higher compression?
« Reply #7 on: December 28, 2012, 10:45:58 PM »
well rotax themselves do a piston giving higher compression and just short of 596cc, slightly more power and torque, but theyre up in the high 11s comp ratio wise and assuming that youve a kick starter, might be a pain to start on a hot day with a hot bike if youve stalled it or dropped it. first  thing i would have done is go throgh the vast range of rotax engines to see what they have on the shelf.

themoudie

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Re: Head skimming for higher compression?
« Reply #8 on: December 28, 2012, 11:24:35 PM »
Aye canolman,

Just read through the replies and your further posting, so now we know where we stand.  :)

Please go and have a read of this little lot from the USofA. I have no idea if they are still racing/in business and I have no personal axe to grind. I have just enjoyed the website as a bit of 'Thumper porn',  ::) for want of a better description, for a few years now. I think many others will enjoy a wee read! ;)

BareBones

All the best for your future building and tweaking and if you get out on track, please drop a post on here, you might find support turns up!

My regards, Bill.

guest1590

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Re: Head skimming for higher compression?
« Reply #9 on: December 29, 2012, 05:23:23 AM »
Yes, I have thought of upping the cc but would mean having to open up the crankcases to except the larger barrel, a job I was quoted for a couple of years back....with parts, £1000+. Spare engines or parts are rare.
Barebones racing, yes I've been in touch and hope they can help but import duty and vat!!!
Many thanks

guest564

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Re: Head skimming for higher compression?
« Reply #10 on: December 29, 2012, 09:59:03 PM »
What gears are you running? If you have road or offroad gearing then you will need to keep the power band reasonably broad and that may mean increasing the cc as you push peak power higher. Opening up the crankcases shouldn't be that expensive unless they are stripping and rebuilding the entire engine.
Back when I was racing singles there was lots of support for the Rotax, I'm surprised that there isn't much about now.

guest1590

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Re: Head skimming for higher compression?
« Reply #11 on: December 30, 2012, 12:00:02 PM »
Barebones don't do anything for the 560, only option again would be to bore it out, I'm trying to avoid this as I'll technically be then in the open class with most clubs....I could of course cheat....
At the moment the motor is in the same state of tune as the Woods Racing short track engines not slow by any means! The only thing that stopped development was a rule change over there
My tuner was  well impressed with the motor, especially the torque but if you race then it's never enough!
I'll get her stripped down and have a long chat with the dyno boys see what we can squeeze out of a mid-eighties engine!!
Anyway thanks again

guest564

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Re: Head skimming for higher compression?
« Reply #12 on: December 31, 2012, 11:29:25 PM »
That sounds like a pretty well tuned engine as it is, what is housed in?

guest1590

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Re: Head skimming for higher compression?
« Reply #13 on: January 01, 2013, 02:54:51 PM »
RGV, just had a message from Jay at Barebones and he says it's ok to leave out the base gasket and adjust the timing to suit....hmmm, interesting

andy230

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Re: Head skimming for higher compression?
« Reply #14 on: January 07, 2013, 08:05:48 PM »
« Last Edit: January 07, 2013, 08:58:07 PM by andy230 »