FE-FC Holden Discussion Forum

I'm thinking about doing this to raise compression in my motor. Does anyone know how much to take off to achieve around a 9:1 ratio in a 138 grey?

I guess it depends on whether the head has been shaved in the past so is there some measurement to work this out?

Cheers,

Dave

Dave,
       With our head on the new motor, we had it shaved down to the thumb prints. we done this as our mate done the same thing to his motor with all his other mods. fully shaving it will be approx 9:7.1 (cant be sure till i have a talk to him)

the only problem was that you need to slightly file out the top holes for the sideplate, aswell as taking a little out of your top water pump holes.

It brings on a nicer idle to, expecially when youve got a big cam ;D ;D

Thanks Darcy,

sounds like you take off a fair bit if the sideplate and water-pump holes need adjusting.

I've heard of taking it down to the 'thumprint' before but can't visualise it. I haven't got a grey head handy to have a look at to work it so have you or anyone else got a pic to show what you mean?

Cheers,

Dave

Dave the thumb prints are a marking on the bottom of the head that indicates how much 'meat' is left on the head. ill go snap a pic ;)

EDIT: cant seem to get the pics loaded up off the cam.

I do believe that grey heads could handle 80thou shaved off, but this depends on if the head has been shaved before or not. if you take your head in, just say you want it shaved down to the thumb prints...they'll know what ya mean ;)

Dave,

Just a thought.  Have you looked into copper head gaskets or the like?

Could achieve some rise in compression?

Just an option.  I don't know too much about them.

I've been advised not to go past the thumbprint. There are easier ways to get more power than shaving too much off.
Colin

Thanks all. I do actually have a grey head handy, its just bolted to a grey block in my daily driver at the moment. Is the thumbprint somewhere I could see on an assembled motor? If not I'll have to wait for Darcy's pic- thanks mate but don't stress too much if the technolgy is giving you grief.

Col, I don't want anything too extreme. I've seen plenty of cracked grey heads and they hadn't even been shaved, ported etc. I just think that upping the comp will help bottom end torque at revs below those where better breathing will have much of an effect although I'll do those mods too.

Cheers,

Dave

hi dave i found shaving my grey head looks kool but is a bit cold in winter.the chicks dig it ;D ;D ;D thinking of getting a tattoo on the side of my grey head.... 8) sorry guys i couldnt resist ;D ;D

(http://i3.photobucket.com/albums/y80/pro_058fc_350/thumbprint.jpg)

quite self explanatory i think ;D usually shave em so that it just dissapears...if that makes sense  ::)

Ah yes, I remember seeing that. Thanks for the pic.

Cheers,

Dave

just wondering how expensive it is to deck the block instead of shaving extreme amount off head, as i have not done this for 20 years.decking the block has the added advantage of also making it square to the the bores in v8's so should help for even pressure in all chambers,surely this is money well spent in search for HP

its usually not all that expensive to deck a block, but im pretty sure Dave wasnt doing a full rebuild, but was only seeking a little more HP from his already running grey motor. 8)

Dave raising the compression ratio depends on what type of head you have in the first place (excluding if its shaved or not). The FX/FJ grey had a 6.5:1, the FE had a 6.8:1, the FC had a 7.0:1 while the FB/EK/EJ had a 7.25:1 ratio.

The difference in the FE / FC ratio while the capacity remained the same was a changing of the combustion chamber. I assume this was the difference between the FX/FJ and the E/C series. However, the difference between the FC and FB/EK/EJ was possible due to the different bore size. I am unsure of that one.

So just looking at the figures you need to shave off less if you have a FB/EK/EJ head.

The compression ratio is a ratio between the volume when the cylinder is at BDC and TDC. If you have got the head off the motor you can calculate the compression ratio by doing the following.

1. Calculate the volume of piston displacement. This will depend on what size motor you have and if its been bored out etc...
2. Bring pistion to TDC and using a dial indicator calculate the distance from the top of the piston to the top of the bore. Using this multiple that by pi and the radius of the bore squared (volume of a cylinder).
3. Turning head upside down ensuring valves are closed, stick an old head gasket to the head (ie: has already been compressed). Now the tricky part begins. Get a very accurate syringe (imperial if you have use imperial measurements above) and slowly put oil into the combustion chamber of the head until it just levels out at the top of the head gasket. You now know the volume of the combustion chamber of the head by how much oil you put into the head. (To do this very accurately you can use a piece of clear perspex with a hole in the middle where you squirt the oil in).
4.Add answer from 1,2 and 3.
5.Add answer from 2 and 3.

Final calculation is answer 4 : answer 5

This will give you a very good idea of the current ratio. From that you can work out roughly how much to shave off the head. The problem to get an accurate figure is that the combustion chamber of the head is not cylindrical.

I am in the process of doing some calculations for all standard bore sizes and heads at the moment converting them to metric. If you want my final calculations I will post them.

Rod

PS: The position of the valves can alter the ratio also. If they recede into the head (ie wear) over a period of time, the ratio will decrease every so slightly.

 You can go to 60thou past the thumb print.


Nick

Not sure if this helps anyone, but when I did my motor, I put an FJ head (shaved 40thou) on an EJ block and ended up with 7.5:1.

Cheers
Jason