Increasing Compression Ratio

[jeff88]

I can't seem to find any concrete answers, so I figured I would get advice from you guys.

I have an oil consumption issue in my Corolla and need to open up the top end, clean it up and re-drill the oil return holes (or buy new pistons, which will have the larger holes already). Another thread got me thinking about increasing the compression ratio by milling the head. I thought if/when I get around to taking the head off, that would be the time to do any modifications. The 1ZZ engine is stock 10:1 CR.

My issue is that I really know nothing about doing this mod. I have a few (lots of) questions:

1. How much does increasing from 10 to 11 increase fuel economy? I read on howstuffworks, that an increase from 8 to 9 can increase FE by 5 or 6%. Can anybody confirm this and would it be the same increase for a different CR start and end(10 to 11)?
2. How much would need to be milled for a one point increase? What is a "safe" amount to mill the head without having to switch out other parts? Can I shave enough off to have a 1.5 or 2 CR increase in a "safe" manner? What else needs to be swapped out, if anything to make this a "safe" mod? Also, how would I measure the CR after a mod?
3. Gas Savers has some mixed emotions (including members here, I think). One guy says a smaller bore engine can generally withstand higher compression better versus bigger bore, is this true? Is 79mm (my engine) a small bore?
4. The same guy mentions that reverse cooling (head first) is better, which makes sense, keeps the top end from over-heating. I'm not sure if mine does or doesn't, because I can't find the answer anywhere and my Haynes manual is vague. I attached a pic from my Haynes, which may help determine it.
5. From what I have read, I would need a higher octane fuel to reduce knock. But I have Variable Valve Timing, so would I need to increase to an 89 or 91 (versus 87) or will the VVT make up for any pinging issues? Also, CA (like most of the US) has ethanol in the gas. So if the octane rating is 87, is that before or after the ethanol is added? I would imagine that would make a difference with ethanol having a higher octane rating. Thought this was an interesting article: CR and Ethanol Blends

I know this is a long list of questions, but I wanted to consolidate the info into one spot, which may make it easier for me as well as anybody else. Thanks for any insight you can give me/us!

[H-Man]

11:1 is kind of high for an otto cycle engine on regular unleaded (it can be done, but it might not be easy.) You may not have the valve clearance to increase the compression ratio.

[trooper Tdiesel]

E10 with a 87 rating is with the ethanol

they blend 84~ octane gas with the ethanol making the 87 octane.

increasing comp from 8 to 9 will gain more mpg then going from 10 to 11 comp.
i found a chart on the net once and buy the time your up to 12 to 1 it was a lot less.
HAA found it SEE PHOTO.....

most heads can be shaved 0.040" but that's only going to get you about 0.5 compression or less.

the best way to know how much clearance there is, is to put a putty on top of the piston and put the head back on with the belt and crank over by hand, and pull head that will tell you how much space you really have to play with...

some times you can source flat top pistons from another engine that's in the same family that has a bigger chamber in the head and use the flat tops in combo with a head with a smaller chamber in the head to make for a comp boost.
or go the other way if you have flat top pistons all ready, and use a different head with smaller chambers.

or in some cases there are after market pistons that are extra tall to gane compression.

i don't know if the photo is right but it seems in line with my thinking at least.

[Occasionally6]

The engine manufacturer will specify how much material can safely be removed from the cylinder head. That will take into account valve to piston clearance and knock resistance. Often there will be a mark cast into the head for reference. You might use that as an initial guide.

They will also usually specify how much the combustion chamber volume is reduced per amount of material removed. Maybe ask at a shop that does the machining work what those values are. (I doubt it's in Haynes, but you never know...)

As a pretty close approximation of the latter, if you trace the outline of the chamber, at the head sealing surface, onto graph paper and count the squares you can determine the area of the chamber. Multiplying that by the amount you propose to machine off the head will give you close to the amount of volume change.

You will also have to include the thickness of the (compressed) head gasket in your compression ratio and clearance calculations.

When checking the clearance, take into account the effect of the VVT (check with the VVT in retarded and advanced positions) as that may alter how much is available. The rods will stretch with rpm and the valves may not exactly follow the cam (although they should) so there will need to be some (extra) clearance.

Combustion chamber volume may be measured (with the head off the engine) by sealing the valves with grease and measuring the volume of a fluid like kerosene it takes to fill the chamber. It is usual to use a pipette to measure the volume and place a sheet of glass or Perspex across the (levelled) head sealing surface so as to indicate clearly when the chamber is filled.

You might get an initial value by using the nominal compression ratio and swept volume (capacity) per cylinder and calculation to get the chamber volume.

The CR of factory engines do have to allow for such things as variable fuel quality, sketchy maintenance, oil burning with wear and Carbon build up in the chamber, so you can push it a little higher if you avoid those things. To be safe, you might just go to the limit of the factory service specs (as outlined above).

Machining the head or altering the head gasket thickness will alter (retard) the cam timing by bringing the cam sprocket closer to the crank while leaving the length of the belt on the tension side the same length.

An over bore (required with the new pistons?) will increase the swept volume and the CR even if the head is not changed.

CR is calculated by:

{Swept Volume (= bore^2 X pi/4 x stroke) + Head Gasket Volume (= bore^2 x pi/4 x gasket thickness) + Combustion Chamber Volume + (or minus) Piston Dish (or hump}/{Head Gasket Volume + Combustion Chamber Volume + (or minus) Piston Dish (or hump)}.

If you want to get really serious, software like Desk Top Dynos will allow you to build virtual engines. The basic program is reasonably priced. (Cheaper than rectifying something that doesn't work.)

[ksa8907]

I would highly suggest you find a reputable engine machine shop, or speed shop that can assist you in this mod. You may need/want higher octane fuel in the end. Likely you won't see more than 6-7% increase in efficiency, and that's the optimistic side. But, if you've got the head off you may as well.