shaving the head down

[severous01]

i was wondering if anyone knew exactly how much material i can take off the head surface without destroying pistons and valves...

i see all this talk about using camry or prius pistons to increase compression and all this other hype about turbos and stuff...but it seems way easier to me to increase compression, get a custom tune for it and start buying premium or midgrade for the performance boost by taking out space between the head and piston. i had an old 4cyl mustang that i warped the head on and i had to shave it...long story short i boosted compression ratio due to wrong gasket used in repair and i had to run midgrade to keep from pinging...the car was a 4cyl and would keep up with the 5.0's all day long, and i got better mileage knockin around 43 average in a 93 sports car.

any way...goin from 8:1 to 10:1 would net almost 20 hp theoretically and would be much better than all these clips n stuff i see of superchargers and turbos. and it would be much more reliable.

tighter squeeze, bigger bang. there's no other cheaper option for more power (from what i see any way)

[ka0sx]

going from 8 to 10 is not that drastic. should be fine at 12to1 i would be getting worried about the pistons.

[Mouse]

Maybe this clip from an article i have found can help....granted it's for the 1.6 in the miata but the general concept applies....another thing to take into consideration is that the 1.6 from the miata likes boost without having to change any of the internals since the engine is from the 323 GTX....

"6) Shaving the head. Compression = power. Shaving the head .010" will increase the compression ratio about a 1/4 point. I dyno'd this and it was good for about 4 HP and 4 ft lbs of low rpm torque. It is very noticeable across the entire rpm range. (I don't know what the shaving limit is but I have not heard of anyone going beyond .025". FYI, the shop manual limit is .008" for 1.6 and .004" for the 1.8). As long as you have the head off you might as well R&R the valve seats too. Cost about $250 if you remove and put it back on yourself.


UPDATE 6/23/98 Recently I have talked to a couple of people who have installed and tuned adjustable camshaft gears. They both report that 6-8 degrees retard produces the best high rpm power. This makes perfect sense to me since I never understood why I made so much more power with just a simple head shave. You see, shaving the head retards the cam timing too. A .020" shave will net you abotu a degrees of timing retard in addition to raising the compression ratio (BTW, each cam gear tooth is equal to 12 degrees, that is why everyone who had their belts off one tooth lost so much power)"

Article was sourced from this website....http://www.solomiata.com/index.php

[jkuchta]

This car has 10.5:1 compression as stock, and has shown a propensity to knock under mild boost, so I don;t think the engine could handle much more compression without a major tune.

[severous01]

well ur talkin boost vs natural. ur talkin atmospheres vs psi. sure u can convert 1 atmosphere to 14.4 psi which it is...but i have no idea how to convert compression to psi.

also...for pinging...a boosted engine or an engine with any 'power adder' has more of a chance to ping than any natural engine...for the most part because of heat of the air entering the engine...hence intercoolers.

10.5:1 is high from the factory for any gas engine and i think that i wouldnt be able to get more than about 11.5:1 tops and it's not worth it unless im doing a head swap any way.



so......thanks for the input and i appreciate it. another thing is...i'd have to take off sever hundred thousandths to get the comp up and if the book says no more than .008....and i'm wanting .200....there's a huge diff there. but then again, maybe they dont want u takin more off than .008 due to compression increase and reliability/warranty issues

[jkuchta]

There also might be valve clearance issues.

[ka0sx]

1ATM is 14.7 at sea level
running boost just pre compresses the air, increasing the internal system pressure. 8psi boost is the equivilant of atm. Psi + 8 so at sea level 14.7 + 8 = 22.7psi
the comparison of boost vs compression ratio is
at atmosphearic 14.7 you have the stock compression ratio of 10.5 to 1 as mentioned above. so running this boost without change in compression would put us at an equivalent of 16.24 : 1 compression. Running this at a constant is streight up bad. even at 5psi boost its still the equivalent of 14.07:1 compression. while the engine may be able to handle this for short periods of time at a time, prolonged running of the engine at this high of a compression would begin to overheat and burn through the pistons.

motorcycles run on compression as high as 15:1

some higher performance cars run 13:1 such as BMW

1955-72 a lot of cars ran a ratio of 12.5:1 as a common practice.

also, increasing your compression ratio by shaving the head decreases your engine volume. so you would no longer have a 1.5L (1496cc)engine but something closer to 1.4L.


running higher compression while it may yield minutely higher gas mileage and slightly more power. you are going to be running premium without a choice.

[rob323]

If you touch the head, block or head gasket thickness and you will need to look into adjustable cam gears to get the cam timing back right.

[dallas]

Valve to piston clearance can become an issue as well as the timing will change. It can be done but would take some knowlwdge.

[severous01]

rob and dallas.....that's what the belt tensioner is for. it takes up the slack in the belt so that 1 turn of the cam is 2 turns of the crank.

chaos: ur engine volume is derived from pistons and crank movement, not head and valve clearance. it's derived from the block deck height and piston radius/diameter and crank radius/diameter. not the distance from the head to the block or volume of air in the 'chamber'.

any way....either way u look at it im not takin that much off the head. not for 20 hp any way

[dallas]

Yes and no ! the tensioner will take care of the slack chain, but the timing from cam to crank will change. You will be lucky to get 4 hp on this engine with a decent shave.


rob and dallas.....that's what the belt tensioner is for. it takes up the slack in the belt so that 1 turn of the cam is 2 turns of the crank.

chaos: ur engine volume is derived from pistons and crank movement, not head and valve clearance. it's derived from the block deck height and piston radius/diameter and crank radius/diameter. not the distance from the head to the block or volume of air in the 'chamber'.

any way....either way u look at it im not takin that much off the head. not for 20 hp any way

[cleong]

I've encountered an previous generation Vios with a 1NZ-FE that was running a 13:1 compression ratio. It made an estimated 135hp at the crank (dynoed at wheel axles), with reinforced internals, and a piggyback computer. It had good torque, but fuel economy suffered - it ran figures similar to a WRX.

[rob323]

rob and dallas.....that's what the belt tensioner is for. it takes up the slack in the belt so that 1 turn of the cam is 2 turns of the crank.

If only it were that simple.
If you change the distance between the crank shaft and the cam shaft (which is what you will be doing if get the block decked or the head shaved, and don't compensate by using a non standard head gasket thickness), it will put your cam timing out. I'm sure there are plenty of diagrams on the net to show you how this happens if you want to look into it further.

[06silveryaris]

I have shaved many cylinder heads since I worked at a machine shop. Most heads have small markers indicating the limit before timing is affected. They are usually small circles on the intake end and on the exhaust end. They are usually not close to the chambers more like on the outer sides. You will see minimal gains by doing this, the only way to get a significant increase in compresion is by shaving a lot off but then it affects timing. Here it is very comon for people to shave .040 and is claimed to make good gain, which were noticable in the buttdyno. I shaved .160 on a hyundai excel that I owned, the thing revved like a sportbike but I had heating issues. Worked with the timing and all fixed out the increase was OK but not great. IMO got with higher comp pistons that is were there a real gain. The risk of clearance is not my major concern just damaging a perfectly good head is when the gain is so little.