Machining and effects on compression ratio - 4AG.

[Drifty Midship]

I'm trying to estimate my final C/R after getting my block and head machined in order to help me choose the correct pistons.
My starting point is a 4AGE bigport which did about 275k.

I'm trying to get an understanding of which surfaces will be machined, how much they will take off.

I'm sure I will need the head mating surface re-done but no idea how much they will take. Can anyone provide a rough figure?
Also should the block deck surface be machined also... how much do they take...

With my rough sums based on them taking .5mm off both the block and head and using 8.9C/R pistons and Toyota 1.2mm HG plus .5mm O/S cyl bores I get a combustion chamber size of 39.89cc and cyl bore of 401.69cc which equates to C/R of approx 10:1.
This seems a huge difference from the 8.9 c/r pistons just from loosing 1mm from between head and block...
I've messed up somewhere i'm sure... can someone else have a go at running the sums and see what they get.

Else... does anyone have the figures for combustion chamber volumes with 8.9 pistons.

Cheers.

[DeftAnesthetik]

luckily enough the 4age is engineered in way that makes it easy to figure out changes...... ive been playing around for the same reasons as you. at any rate with standard bore on the block you can figure roughly for every MM you lose in material of the head/block mating surface to gain approximately 1 full point of compression. so your figuring is correct. you should have 9.9:1 compression. with 0.5mm overbore it will be a little less than a full point, i dont have the formula's in front of me but they are out there.

to my knowledge the stock bigport pistons are 9.4:1 not 8.9, the 8.9 were in later spec, MAP based GZE's

its hard to say how much a machine shop will take off the mating surfaces, generally they will take as little as possible to make it flat, which of course varies. if you tell them you want a specific amount shaved they will do that.

personally i would only have them remove the minimum amount and then use the proper head gasket thickness and pistons to achieve my desired compression ratio

[oldeskewltoy]

Did you include the combustion chamber in total volume???

bore stroke, piston volume(+ or -) and combustion chamber add up to total volume?

[Sam_Q]

ok i worked it out:

stock total chamber size: 50.29cc

the bore increase doesnt affect the chamber size which unless the head is modified is 39cc

with the bigger bore it becomes 9:1

with 0.5mm off the block (bad idea!!!) it becomes 9.4:1
instead of this just use a 0.8mm trd gasket and shave your head and extra +0.1mm to compensate. Otherwise your squish height becomes too tight for comfort.

with 0.5mm off the head again it becomes 9.9:1



DeftAnesthetik: at higher compression ratios your rule becomes less and less accurate.

[Sam_Q]

oh also i forgot to post that I wrote a difficult to read guide on all this:

http://www.ae86drivingclub.com.au/fa...ression-ratios

[oldeskewltoy]

Sam....... I'm NO expert, I DON'T know the math..... BUT, I've recently learned you CAN'T just use bore and stroke and chambers...

First, you CAN'T assume all volumes to be 39cc... there is a substantial amount of differences.... some say as much as 3cc! Some of it has to do with valve placement (depth) in the head originally.

There is at least one area not included in your calculations... the area between the piston and cylinder wall above the first ring........ Loynings tells me this can equal as much as 1 full cc

These are small things I'm learning from Loynings Engine Service as they do the machine work on my engine.

If you remember my porting page.... MY final volume before going to Loynings was 37cc.... The head will be cut so that final chamber volume will be a scant 33 cc!!!

Loynings did some measuring on my head/block/piston, and OEM head gasket combination, and before machine work the results would be between 9.8 and 9.9 to 1.... with my 37cc chambers

The block is being decked .007"(.18mm), the head is being cut down for 33 cc chamber volumes. That will bring my static cr to 10.7 to 1... with the stock head gasket and the 10 to 1 CR pistons..... at least that is how they have calculated it......

[Sam_Q]

ok first up DeftAnesthetik was right the ratio is meant to be 9.4:1 not 8.9:1 so all my theoretical calculations would only work for 4agze pistons in the idea world.

oldeskewltoy: somehow I missed your post before. Going from the start I calculate the chamber size which is the combination of the head chamber+piston dish/-piston dome+ gasket displacement+head to bore(new to me!)+spark plug volume,etc.... If the compression ratio is right and the bore/stroke is right then the chamber size can be worked out accurately because it automaticly allows for these changes. However as you are findout out its not a perfect world and there are variences. I in my calculations am assuming that the quoted CR is right when in the real world theres a bit of varience. Which I really should be commenting about when I make posts like this. Does that make sense?

BTW you do sound the expert to me, hehehehee

[jeffro ra28]

oldskewl has got the right idea.

As the piston diameter is smallest above the top ring there is space in here you need to add into the math. Also head gasket fire ring diameter. If you want to get extremely picky you should also add into the eqaution your bearing clearance and small end bush clearance which could change deck height on an operating not static engine by .001-.005" depending on clearances.

Also, the fluid that you use in your CC gauge has alot to do with obtaining the proper measurement. Using water to CC your engines is a no no!!!!!! Something with a specific gravity close to kerosene is needed. Most good engine shops will just use kerosene. It also flows much better and has alot less surface tension then water making it easier to elimante air bubbles, minimise miniscus tendencies and overall acheive a more accurate reading. Perhaps ask a lab technician why CCing with kerosene rather then water is more relavant to the figure we are trying to acheive because of its density etc.

Like it all matters in the end anyway As it is all going to change when the engine heats up expands and clearance etc change. There is no real "right" compression ratio anyhow. To me its just a way of making sure all the cylinders are equal.

[Drifty Midship]

Thanks for the replies... all this info has gotten a bit much for me to deal with.
I should have mentioned this motor is gonna be FI. That is why I stated the 8.9 pistons in my measurements.
Basically, I'm just trying to work out if I can safely get the 8.9 pistons with teh machine work and keep at an acceptable level to run about 12psi thru a 205 w2a cooler.
Bah... I'll just do it... tell the machinist to be gentle....!

[jeffro ra28]

you will be absolutely fine providing your head and block are in good condition meaning the machinist can take minimal size cuts.

[Sam_Q]

yeah I have no argument there, my point was though even if innacurate all I was doing was using the factory quoted C.R which if correct means it has already allowed for all these things.

[oldeskewltoy]

oldskewl has got the right idea.

Like it all matters in the end anyway As it is all going to change when the engine heats up expands and clearance etc change. There is no real "right" compression ratio anyhow. To me its just a way of making sure all the cylinders are equal.

Hehehehehe... Loynings Engine Services.... besides using a torque plate.... they bring the block up to operating temps BEFORE boring and honing! That way the final bore IS accurate @ operating temp.........

[jeffro ra28]

yes and do they measure the piston skirts at operating temp aswell?

[Sam_Q]

a bit off topic but what do you guys think of using a torque plate? I cant find an engine shop down here that has one and they assure me they never have used one and havent ever had a problem.

[oldeskewltoy]

a bit off topic but what do you guys think of using a torque plate?

I've had engines built/rebuilt without torque plate use. They all ran well... I've had engines built/rebuilt with torque plate use... they all ran well too...