What is "shaving the head"?

[ken cool]

Hope this helps in clearing the confusion.

Oh yes it does!

[Nirvana]

Thanks a lot Praful, i am getting a little idea about this shaving head thing.
Please help us with the source of this diagram, seems to be a informative doc.

[Bibhu]

Thank you for clarifying.

NOW comes my next volly of questions.

If I do this on a 4 stroke say the 200, say I shave the head by 1.5 mm, will the valves hit the piston?

Will there be any problem adjusting the timing chain?

Is port polishing necessary while doing a head shave?

[bobbykick]

I guess it looks something like this !!!

[MotorBreath!]

I guess it looks something like this !!!

OMG OMG is that u.. lolz what a timing to post that pic.

Btw, I had the same doubt.. thanx Mr. Tripathy for the diagram and clarification

[aargee]

Bumping an old thread to seek some clarification on increasing engine compression...

Part A
I understand the basics on how to increase a compression ratio in a cylinder, however I DO NOT understand how the calculation is done, like, how much of metal must be removed inorder to increase the CR from 9.1 to 9.5 etc. Its greatly appreciated, if some experts can throw some light on this front.

Part B
How about the engine's heat, I hope that should also increase considerably when the compression ratio is increased isn't it? So how is that handled on an air cooled engine?

[aargee]

Part A
I understand the basics on how to increase a compression ratio in a cylinder, however I DO NOT understand how the calculation is done, like, how much of metal must be removed inorder to increase the CR from 9.1 to 9.5 etc. Its greatly appreciated, if some experts can throw some light on this front.

Ignore Part A, a friend of mine helped me with http://www.csgnetwork.com/compcalc.html

I still appreciate an answer for Part B

[Old Fox]

Ignore Part A, a friend of mine helped me with http://www.csgnetwork.com/compcalc.html

I still appreciate an answer for Part B

Heat build-up arising out of increased compression ratios are well nigh impossible to estimate accurately. All engines are usually designed with a certain excess capacity in terms of their ability to reject excess heat. if the increase in compression ratio is within reasonable limits (which being ~ 10% or so), then rich mixture, low ambient temperatures and high octane fuel will probably allow the engine to perform without overheating. Beyond the 'safe thermal margins', an oil-cooler would probably help. But usually the fin area needs to be increased to increase the available surface area for heat rejection.

In a nutshell, its a territory that lacks predictable algorithms and so borders on what they colloquially call a 'black art' .

[aargee]

Heat build-up arising...increase the available surface area for heat rejection.

Beautiful; Tks for the explanation OF sir; now...moving a step ahead further...

This is on aircooled engines. How about liquid cooled engines? I can increase the compression ratio, say around 20% & use strong coolants with additives so that they can absorb the heat quickly & effectively isn't it? Just for an understanding, say I increase the compression ratio from 10.5 to 12.5 while the permissable limit being 11.5, so, if I use better coolant, can I still keep the heat under control?

Ofcourse, I know there's a limitation with the metals, valve clearance area etc, but, for a moment, lets keep them aside & think only on the heat perspective; OR, are there any more factors to be considered?

[Old Fox]

Beautiful; Tks for the explanation OF sir; now...moving a step ahead further...

This is on aircooled engines. How about liquid cooled engines? I can increase the compression ratio, say around 20% & use strong coolants with additives so that they can absorb the heat quickly & effectively isn't it? Just for an understanding, say I increase the compression ratio from 10.5 to 12.5 while the permissable limit being 11.5, so, if I use better coolant, can I still keep the heat under control?

Ofcourse, I know there's a limitation with the metals, valve clearance area etc, but, for a moment, lets keep them aside & think only on the heat perspective; OR, are there any more factors to be considered?

Liquid cooled engines are a lot more forgiving in this respect, having a larger heat rejection cushion. But then above a certain percentage increase in compression ratio, limitations of fuel octane number, cam profiles and mechanical limits of the crankshaft start dictating terms for engine integrity instead of excess heat.

Assuming for a moment, as you say, that all other factors can be disregarded, yes you can go to twice the increase in compression ratio compared to what you'd have managed with an air-cooled engine

[aargee]

Thanks OF sir, that helps me understand very well. Appreciate your answers

[psr]

use strong coolants with additives so that they can absorb the heat quickly & effectively isn't it? Just for an understanding, say I increase the compression ratio from 10.5 to 12.5 while the permissable limit being 11.5, so, if I use better coolant, can I still keep the heat under control?

Ofcourse, I know there's a limitation with the metals, valve clearance area etc, but, for a moment, lets keep them aside & think only on the heat perspective; OR, are there any more factors to be considered?

Coolant inhibits quicker distribution of heat....more the coolant faster the water and radiator will become hot....The coolant prevents freezing of water in extreme cold condition,and also functions as a lubricant.
For temperature above 20C , 25 to 30 percent coolant of volume is enough.
Unless the engine is designed for higher compression ,conversion to higher compression will lead to detonation and destruction of piston,con rod,and bore...along with associated components.
Like OLD FOX said 10 to 15% is the absolute max in compression enhancement.
Best coolant is Water

[jd666]

there was someone talking about shaving 1.5mm from the head. Its not as simple as that.

You calculate tdc and bdc after opening the head. ( top dead centre and bottom dead centre ).

Then you extend the valves fully to open position by rotating the overhead cam, and measuring that distance ( since the valves protude into the combustion chamber. A over shaved head, will make the piston collide with the valves. So you need to keep that margin first.

Generally shaving should be between 0.1-0.3mm. Other alternate, without spoiling your bike's head, is to experiment with a thinner head gasket ( you can simply grind it down on sandpaper and make sure you do it evenly. ). Or copy the gasket cut on thinner material and use that.

When you shave the head, you reduce the size of the combustion chamber at TDC ( lets just say the piston is more closer to the roof of the dome of the combustion chamber ), resulting in a higher compression ratio between BDC and TDC ( the distance from BDC also reduces, but then a reduction of 0.1mm from say 100mm and a reduction of 0.1mm from 10mm, the difference will be felt in the 10mm space where its a higher fraction. )

Dunno if the last part made sense. lemme know.

[aargee]

Very much, very much @JD. It all made lot of sense as how should one go about increasing the compression ratio. I cannot think of any other practical method & you chipped in at a right time

[aargee]

Assuming the valves don't hit the head & it becomes perfectly possible to increase the compression ratio from say from 11.0 to 13.0, wouldn't the torque improve? What happens to the engine life? I assume the life of the engine will not long last, is my assumption right? Is there a way to calculate by how much percent the life of the engine will come down?

There's also an interesting observation, not sure by how much, but atleast I was surprised. Here's the tech spec of Ducati GT1000 & Ducati 796 Monster. I know both of them are air cooled machines, but never expected their compression ratio in excess of 9.5 & I was little surprised as to understand how this is even possible. Even on single cylinder engines the compression ratio's are not more than 9.5 or 10, but these have in excess of 10. How is this even possible? So do these type of engine's also require premium or high octane fuel? Pls enlighten me. Tks.

Edit - Oh!!! and one more important stuff - MILEAGE How does this factor affected when the compression ratio is increased. I assume again, it almost remains unchanged, but still would like to get this discussed. Tks.