Milling the head

[toki]

I know when you mill your head it provides a smoother contact surface between the head and block, but i hear it can also raise/lower compression quite a bit, like .5:1. On a KA24E exactly how much compression would be added for milling it say 1/1000, as apposed to say 8/1000? I am not sure yet what I am doing with this motor, it does need a rebuild though regardless of NA or turbo in it's future. So the pistons I will be choosing will come into play very soon, and I will need to factor in the amount of comp added from the milling.

[toki]

anyone?

[Wakkolio]

If i remember correctly milling your head will slightly increase compression.

[fritts]

You will not be able to take .001 off and actually have a clean surface. It will probably take .008 or 8/1000 in for the top to clean up. So I would look for the compression increas at .008 rather than .001 in.

[Unnatural]

Milling the head will definitely increase compression. To what extent depends on how much material must be removed to acheive a flat surface. There many calculations needed to figure out the resulting compression (and unfortunetly I can't remember them all currently). You need to know: combustion chamber volume, head gasket thickness, deck height/volume above the piston, volume of piston dish/valve reliefs, etc..... It gets pretty complex. I would recommend contacting your machine shop and asking them to estimate the amount the head typically needs to be milled and the resulting compression ratio. Or if you can get a copy of Desktop Dyno 2000 there is a compression ratio calculator function, but you'll have to know all the previously mentioned figures as well.

[DAEDALUS]

If you can find out the stroke and stock compression ratio, we can calculate the effect of milling any reasonable thickness. It isn't that complex, since milling the head only changes the volume of a (roughly) cylindrical shape--none of the goofy volumes change. As far as minimum milling thickness, I don't see why .008 should be the minimum. First of all, I believe that is the max the factory allows per the FSM. Secondly, I know rotors can be machined in increments of .001-.003", and they're made of steel, which is much harder than al.Doing a *rough* calc, just estimating numbers, it appears that machining .008" off the head will increase CR by perhaps .2 psi over stock.

[toki]

Ok well I talked to a guy at Larry's Auto and Marine and he said it would take atleast 30/1000 to 60/1000 to change it noticibly, like .5:1. They do much much custom engine work, so I take thier word for it.

[DAEDALUS]

There is absolutely nothing subjective about the math involved or the methods by which CR is calculated. Although I made some assumptions on exact dimensions, my methods are fairly good. If a guy at Larry's can explain exactly how, why and to what extent milling a head changes CR, then my hat's off to him. But unless he knew what your stroke and stock CR are, then I'll bet he's pulling numbers out of his, er, pocket. I have the decency to put my numbers through an abacus before spitting them out. I know he's only guessing because he gave you a range of .030-.060 to reach a given increase. One bound is double the other!

[toki]

heh oki, I will get you the number captain math, but this shop also builds custom engines, like 8second mustang engines all day long. There are a limited number of feasable bore/strokes for a 2.4L 4cyl, so his guess may have been more educated than not.

[toki]

I know bore is 89mm stock, but will be upped to about 89.5mm with new pistons. Stroke is...*googleing*..96mm source #1...aaaand *googleing more* sources #2 and #3 sayyy 96mm! hooray ok.

89.5mm Bore and 96mm Stroke.

Calculate me!

[fritts]

Cleaning up the block will not need .030-.060 in that sounds like BS if you block was clocked that amount you would be having all kinds of problems with sealing. I have a hard time believing .001-.003 will clean it up but if you get lucky and they tram the block in carefully you might... be able to do it.

[DAEDALUS]

Can I assume a stock CR of 8.6? Your guy is right on. About .025" required to increase CR by .5psi, perhaps a bit more (.030"?) to compensate for tapering. I had assumed a stroke of 5" (127mm) originally. Shaving .008" would get you to about 8.76:1 CR.

[Unnatural]

Those numbers don't sound far off. But to get an accurate CR you do have to know much more than the bore and stroke figures. There are more calcuations that you must do to get it exact. But as far as a guess...that'll work. One thing though...Isn't stock CR of the KA24E 9.0:1? Or no?

[DAEDALUS]

To calculate a CR from scratch, yes, you need to know a lot of details of what makes up the volumes in the head. But to calculate the effect of milling on a given CR, they aren't necessary. Keep in mind that the volume that's being removed is an almost-perfect cylinder. You have the bore (not that you absolutely need it) and you know the thickness. Using the CR, bore and stroke, you can calculate the *exact* volume above the piston at TDC. Remove a known cylinder from this number, divide it back into the BDC volume, and you have a new CR. The only problem with this method is that the material being removed isn't an exact cylinder, but it is VERY close for most heads, probably to within 5%.

[Unnatural]

Agreed. I guess we are saying the same thing, ultimately. You're just doing a better job saying it.

[toki]

the compression ratio will be 10.5:1 before milling, would that make much difference?

[toki]

Also, this is probably stupid and something I should consider, but will 10.5-10.6 : 1 require me to use anything higher than 91 octane? If so will spiking every tank with some octance booster suffice? Or will I actually need to mix in 100+ octane gas?

[toki]

Cleaning up the block will not need .030-.060 in that sounds like BS if you block was clocked that amount you would be having all kinds of problems with sealing. I have a hard time believing .001-.003 will clean it up but if you get lucky and they tram the block in carefully you might... be able to do it.

He didn't say it would need 30-60 to clean it up, just that it would take that much to change it noticably like .5, as far as how much will actually be needed I suppose no one will know until I get it in there.

[DAEDALUS]

The higher the CR before the milling, the greater the net increase for a given milled thickness. To get a .5psi increase starting at 10.5:1, you would only have to shave off .016", maybe a bit more for tapering.Not sure about the knocking issue, but that is a pretty damn high CR. Octane booster will not help much. If you add a pint of 104 octane booster to 12 gallons of 93 octane gas, you'll end up with a tankfull of 93.11 octane gas.

[toki]

I thought octane booster did some crazy chemical reaction that changed the octane rateing of all the gas, not just supplemented with higher octane gas. I was reading the backs of the bottles the other day because I was bored and it said not to exede more than 10% of total fuel with the stuff, so it can't just be bottled gas.

[DAEDALUS]

No chemical conversion that I'm aware of. Bottled octane boosters are pretty much a load of horse crap. But then again, you would have to add 1.33 gallons octane booster to 12 gallons to make a 10% mix. End result would be 94.1 octane. Much cheaper to just buy toulene.

[toki]

Elaborate on this toulene you speak of.

[toki]

Toluene even?

[toki]

Ok after googleing I understand it is basically something you can safely add to your fuel mixture that has an octane rating of 118, as much as 30%? Although 10% is generally recommended.

1 gallon of 118 + 9 gallons of 91 = 93.7

3 gallons of 118 + 7 gallon of 91 = 99.1

Yeah...99.1 Octane sounds niiiiice

[DAEDALUS]

Yeah, I think you're right--toluene. It has 114 octane if I remember right (R+M)/2.

[toki]

114...118..whatever :P

[Unnatural]

You can use Toulene or Xylene...both of which are the main ingredients in most of the supposed octane boosters on the market. There is no magical chemical reaction. You end up paying $6-8 for like a 12 floz bottle of what is basically toulene or xylene (or a combination) that you can buy for about $10 gal. Mix it yourself and save money.

Xylene ~ 117 octaneToulene ~ 114 octane

Don't mix in more than 30% Xylene/Toulene with your gasoline and be sure to add some type of lube, such as a few oz's of Marvel Mystery Oil. This works pretty well and MUCH better than any octane booster you can buy. I used to run this mix occasionally in my Grand National-powered Trans Am with good results...running between 16-18psi of boost on a 9:1 CR with no knock.

[toki]

From what I read Xylene concentrations are already pretty high in pump gas?

[toki]

And do you happen to know the answer my question as to what CR will call for what Octane?

[Unnatural]

There's really no way to know what CR is going to require what octane...to a certain extent. I can tell you that 10.5:1 CR on 91-octane will most likely be pushing the outer limits however. There is more to an engines resistance to detonation than an engines compression ratio, although that does play a large part. Piston and head design play a big role too. For example, GM's LS1 V8, used in the Corvette and Camaro/Firebirds, runs a CR upwards of 10.5:1 and does require 91 octane or higher premium fuel. But GM was able to control detonation with an excellent combustion chamber design inside the aluminum heads. I'm going to guess that 10.5:1 on a KA will be close to the ragged edge on 91 octane. But that's only an educated guess.