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I'm pulling the head off my 1.8 84' GTI in a couple weeks,and going to have it shaved before reinstalling. I see that .025 is the max for shaving,and don't want to go over the .5 max for compresion ratio and wreck the head for racing purposes.How much does the .025 bring it up?
I beleive you can shave the head more than .025, the GCR says
"Where a factory specification for original cylinder head thickness can be proven, a tolerance of .025 inch less than the service limit will be permitted."
According to the manual the mimimum for a JH head is 5.219" so if you take .025 from that, I beleive that is your minimum amount, so your head must not measure less than 5.194"
Hopefully someone can confirm that as I do not want to lead you or anyone else in the wrong direction.
I just rebuilt my motor and to make sure that the compression was legal I cc'ed the block and head and then calculated the compression ratio. I was a little under so I shaved my head to get me the 9.0 that I am allowed. If I remember correctly shaving .01" from the head will reduce the combustion chamber volume by one cc.
I use 5.200 for the size. Measure at all four corners!!
Dont foreget to take off the sharp edges around the chamber. I have found that if you blow a head gasket, only once did the head get put back on. Most of the time the head gets detonated and really pushes the head surface around, ruining the head. 32 * timing at 4000 and above.
Most VW blocks have the bolt holes pulled up some, clean this up, either deck mill or drag file, softly.
FWIW I have looked at many head gaskets and most are not round!!! They measure about 82 one way and 82,5 the other way. Check yours . The Felpro gasket is the roundest that I have seen. I have not measured the ABA tri metal gasket. The smallest , roundest will yield the highest compression, duh..
Customer cars that get race prepped at my shop often have 190# of compression, with total seal rings and the cam set straight up or ahead a touch. The valves are all new , and hand lapped. The seats are new or very good used, not ground . Any time you grind either the seat or valve, the chamber gets bigger and you lose compression and shroud the valve. I also "edge seal" the seat. That is, the valve and seat seal at the chamber edge of the seat by a very small angle difference. the triple gap Bosch plugs also help to fill up the chamber. I would assume that 200# on my guage is about 11.5 to one for a VW, with a legal cam.
If you measure the front five cars at most ITB races, they may have 200+ cranking compression. The Volvos had about 220, the BMW about 205.
YMMV , Mike Ogren
I am assuming you're talking about MKII VW's, not the 84 GTI. As I understand it, this car can only run 9.0 to 1 compression legally.Quote:
I use 5.200 for the size. Measure at all four corners!!
. . . . .
Customer cars that get race prepped at my shop often have 190# of compression, . . . . . I would assume that 200# on my guage is about 11.5 to one for a VW, with a legal cam.
If you measure the front five cars at most ITB races, they may have 200+ cranking compression. The Volvos had about 220, the BMW about 205.
YMMV , Mike Ogren
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Will 5.200 work for the JH engine (84 GTI) as well, to get it to 9.0?
The edge at the bottom of the 1.8 head combustion chamber probably helps homogenize the mixture. Thus, try not to remove that edge if you have to mill the head. The preferable way to get the CR up that allowable 1/2 point is to increase the bore by 0.040. If that alone doesn't do it, truing of the block surface may be better than cutting these particular heads, IMHO.
Look in your book for the size. I am pretty sure that they are the same,. solid or hyd. The pistons are the compression difference. The block can be decked about 020, but this really gets the squish band small. Any more than 020 and the block/head combo may detonate. The ABA gasket is thicker. I think that it fits. The squish band is mostly for hydrocarbon emissions, in theory. Rounding the ledge off is illegal,(More than the small radius that is allowed, by machinest stadards), IMHO.
But I believe that would be an illegal part (9.1.3.D.1.o - 2007 GCR pages 297/298).Quote:
The ABA gasket is thicker. I think that it fits.[/b]
Is that how they would be able to check your comp ratio? By doing a compresion check? Actual ratio and the amount that actualy gets squished (technical term :P ) will change according to valve overlap and other things.So by staying under the 190# (guesstimate) you would be ok as long as nothing illegal by ITB rules is done?
If I were to write a protest to check someones compression, the only acceptable method would be to remove the head and physically measure. Any non-stock or unallowed items would be apparent.
Real world; Buy the guy a few beers. and mention that his car was real fast. ( Compression equals sound. BTW)
Tell him that you want to look in the plug hole and measure the head size.
It is easy to do and quick, just pull the Valve cover and plug. Take your car apart also. If you have looked at a 10-1 piston and an 8-1 piston, than you can see the piston dish.
If he refuses , let him know that you will protest him next race.,( not this weekend )Be nice about it. peek at the cam while your there.
Most regions are not ready or have the tools to bust any engines apart. At least the SE doesnt, as a rule. The NE did a lot more and it makes for a lot better racing, IMHO.
SCCA Pro ,tubed all of the TA cars, before the races. That is a big syringe that measures the air intake and compression. If you were over and sure that you were legal, you had to take it apart and prove it.
Back in the Briggs Kart day, we ran a long sparkplug in the short hole, as the plug was not teched. The valves hit it some times tho. MM
PS. I take them apart when they get under 180# You will not run in the front with leaky valves and/or rings.
I agree with the philosphy if discussing before protesting. Just pointing out that a simple compression test # does not in any way indicate whether you will withstand a protest.