Author Topic: Skimming  (Read 1497 times)

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ypsilon_lancar

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Skimming
« on: November 25, 2011, 07:02:35 PM »
Hi,
Sorry if this has been asked before, i tried searching but couldnt see the answer i was looking for!  :$

My engine is out at the moment and with an enthusiastic mini engineer! (who seemed more excited about my 126 than the tuned cooper S that also came in  ;D )

Is it worth having the head skimmed by 1mm or not? I dont know what benefits it will bring but he has said they can do it if i want.  He is porting the head whilst its out as well.

Cheers!!

smallcox

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Re: Skimming
« Reply #1 on: November 27, 2011, 09:54:11 PM »
Hi ypsilon_lancar,

The guy doing the cyclinder head work should be able to advise you on this as there are pros and cons in raising the C/R.

The air cooled engines from my experiance don't like high C/R's so I would advise if the camshaft is not being changed 8.6:1 would be my max. From the driving point of view this will feel more tourqie in the lower revs and therefor improve mpg slightly. Any higher than this and you will feel the engine has a less refined fill to it.
Also increasing the C/R to much, you will require to retard the timing and this will loose power that you was hoping to gain.

Smallcox :)
air cooled, gt15 garret turbo with intercooler, 14x9" wheels with 225/40/14 toyos, front arb and disc brakes

DEYAN IYI

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    • Bulgarian Polski FIAT 126p Fanclub
  • Location: Plovdiv/Bulgaria
Re: Skimming
« Reply #2 on: November 28, 2011, 02:47:57 PM »
Hi!

According to the polish experience 1mm skimming (on a standard motor with the new type of combustion chamber - 650E) is not dangerous for its life. http://www.126fan.pl/www/glowica.htm

They don't comment if it will require gas 98 and colder plugs, but have it in mind when doing the final adjustment.

Regards!


fightingtorque

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Re: Skimming
« Reply #3 on: December 01, 2011, 01:24:43 AM »
Is it an air cooled or water cooled?

I raised the CR on mine, because the head face needed a skim anyway as the engine had eaten something metal (long story).

I cant remember how much by.  I was targetting increasing the CR by half a ratio, its always been my approach to do this in any tuning project.  I went through the proper process of measuring the existing chamber volume using a burette, calculating the required change in chamber volume and then buretting it again and measuring the amount to be skimmed, slightly fiddly but your porting man should have done this 100 times in the past and know what he is up to.

I think the result was either 0.5mm or 1mm, i seem to remember a roundish number.

It gave a really noticeable extra shove of torque at lower revs, I didnt feel so much change at the top end but at the time i was getting into marginal fuelling and marginal air flow up there because of the fuel injection system i was using.  I did notice it got close to the detonation limit which hadnt been the case before.   Therefore I would suggest that on a bis, half a ratio is ok (i.e. if it was 8.6 before it can be taken to 9.1).  I cant remember the standard figure off hand though.  I do not think it would be good to increase by a whole ratio.

fightingtorque

ypsilon_lancar

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Re: Skimming
« Reply #4 on: December 11, 2011, 09:16:28 PM »
Thanks for the replies everyone  :)

Its an Aircooled 650E model. 

I couldn't imagine what it would be like if it was less refined at speed, i took it on the M62 once and quickly got back off again! Stick the the B roads!!

Still not sure what to do with it, the engineers are keen to increase the CC with VW pistons etc. Its been a bit slow going recently due to the purchase of a Fiat 900T Amigo  ;D

Think i might just get it cleaned up, ported and back in for now. I really need it for winter in case the heavy snow makes a return this year! Come summer I hope to have both it and the Amigo stripped and painted in matching colours  8)

 

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