The V8 Owners Forum -

ok my engine has the standard high comp 9.35 heads and at the moment im getting 10.5 bar from all of 8 cylenders.....

from what i already no from V8's the more compresion you have the more power you get (in lame terms that is) and usualy to increase the CR you have to buy different piston to increase it or buy some stupidly exspesive prices for some racing heads......

what i was thinking was, what about if i had the heads skimed so the combution chamber is smaller which would increase CR???????

i know ill have to be careful on how much to take off as the valves might foul, but do you think this is possible?????


now for a bit more complex thinking...... if this is acheived then what will happen about pinking!!!! will i have to retard the timing, or will i have to adjust the fueling to add more fuel to try and stop it from pinking......


how much would have to be skimmed off the heads to increase the CR, as the combutuion chamber isnt square its difficult to work out the volume of it before and after, it isnt realy ciurcular neither so its going to be very difficult to work out.... think this one is for the clever champies out there!!!!

Hi
the simplest and most acurate way find the chamber volume is use the method shown here http://www.merkurxr4ti.com/chambervolume.html .
With regard CR some is good more is better aproach does not work, it is a compromise, as you already have hinted at.
With 95 to 97 octane petrol on a fairly old chamber design like the rover with the head made of ally and a standard cam you can take 10:1 maybe 10.5:1 compression go higher and it will detonate (as you say) you can retard the ignition but that means you are now not giving the engine enough time to fully burn the fuel air mixture before the exhaust valve opens, and you loose power.
Before you start skimming, try a tin gasket, it should give you about .5 extra CR, the actual amount is on several posts on here (you will need to shim your rocker posts).
If you then want to go further get all your chambers exactly the same size, compression heights matched, direct port fuel injection and convert to trigger wheel based ignition and go for an extra .5 on the CR, that way you may gain power and not just hole pistons.
The real steel or wild cat heads would probably allow an extra .25 to.5 CR over this as they are a better design but to work well you would need to change to flat top or "reverse crown" pistons.
have fun
Mike

The other point about skimming heads a lot (in addition to having to shim rocker pedestals) is that you'll need to skim the inlet manifold too, and make a narrower valley gasket.

Yes you would need to adjust the timing.

Chris.

As above really and as Kiwicar said if you need to back off the timing then its a waste of time as what you gain you will lose out on again. So either allow for running better fuel or octane booster or really don't go above 9.75:1. I dont know the age of your 3.9 but you will probably find unless its the very last of them or the gaskets have been changed you already have tin gaskets. A point to note on compression as well is all the above is talking about static compression. If you put in a big cam with a lot of overlap then you can up your static compression by a lot as you loose a lot through the valve overlap. If your using a fairly small standardish cam then be careful as you will run into problems.

Here is a link to a website that has some calculation programs. One of them allows you to put in some info about your engine along with the new compression ratio, it will then estimate the change in BHP.

I haven't got a clue if these calculators are any good, but its worth a look.

Pete.

http://www.bgsoflex.com/auto.html