Topic: how do u determine compression rating?

[the technological J]

sorry if thats not what its called..but....i was thinking of doing some hemi heads on my 73... in thoughts of hondaman he had to mill the block 8 thou to bring it back... so how do u determine this?

[mystic_1]

Well, you could measure the capacity of the combustion chambers, before and after, in cubic centimeters.

Then measure the bores in the block and use the formula for volume of a cylinder (pi * r^2 * h) in order to determine how much you need to take off, in order to offset the volume you added to the chambers.

mystic_1

[the technological J]

Well, you could measure the capacity of the combustion chambers, before and after, in cubic centimeters.

Then measure the bores in the block and use the formula for volume of a cylinder (pi * r^2 * h) in order to determine how much you need to take off, in order to offset the volume you added to the chambers.

wow how do i measure what i removed from the head?

[the technological J]

Well, you could measure the capacity of the combustion chambers, before and after, in cubic centimeters.

Then measure the bores in the block and use the formula for volume of a cylinder (pi * r^2 * h) in order to determine how much you need to take off, in order to offset the volume you added to the chambers.

wow how do i measure what i removed from the head?

if worse comes to worse is it better to error on the side of too much compression?

[mystic_1]

Well, you could measure the capacity of the combustion chambers, before and after, in cubic centimeters.

Then measure the bores in the block and use the formula for volume of a cylinder (pi * r^2 * h) in order to determine how much you need to take off, in order to offset the volume you added to the chambers.

wow how do i measure what i removed from the head?

Disclaimer:  I've never done this.

I think the process involves inverting the head and, with both valves closed, filling up the combustion chamber with fluid, measuring as you put it in.  Then after you mod the head, repeat the process and compute the difference.

Google may yield more information.

mystic_1

[TwoTired]

Compression ratio relates to the volume of air at BDC, to the volume of air at TDC for any given cylinder.

You need to know two numbers; the piston swept volume (this is the engine displacement per cylinder) and the combustion chamber volume when piston is at TDC.  This is the "raw" CR.

"Effective" CR has to do with valve timing.  If, for example, the intake valve is left open while the piston rises, or the exhaust valve is opened before the piston reaches TDC.  These events reduce the effective CR.

As compression ratio increases, the risk of detonation of the volatile air fuel mixture also increases.  Compressing a gas generates heat, that coupled with residual heat from the previous power cycle, can combine to ignite/detonate the entire air fuel mixture before the piston reaches TDC, without requiring a spark event.  This sort of thing bends rods, holes pistons, and generally does very bad things to engine reliability.
In normal combustion, the spark ignites the fuel and there is a flame front travel away from the spark, and the combustion expands much slower while pushing down on the piston crown.  Ideally, the combustion/gas expansion event occurs over the entire time duration of the power stroke.
To compensate for higher compression, a higher octane fuel rating is required.  Higher octanes require more heat to spontaneously ignite the fuel and can avert detonation (to a point).

More compression generally relates to more power output available from the engine.  Higher peak power.  The reason being that more oxygen molecules are then mashed into a smaller volume which can be paired with more fuel molecules, which generate higher pressures on the piston during the power cycle.  The engine cooling system must also remove increased waste energy in the form of heat.  And, everything in the power train is also stressed beyond what manufacturing engineers allotted to endure.  Further, increasing compression ratio places higher stress on wrist pins main and rod bearing journals at all operating speeds, which generally increases wear and reduces service life between overhauls.

Then there is the ignition to consider.  As chamber pressures rise the arc between spark plug electrodes requires a higher voltage to initiate the spark event.  Go high enough with compression, and the "reserve" potential designed into the the stock ignition begins to reach it's limit, and misfires can occur.

These discussion points are often termed "engineering trade offs" as very few "gains" are obtained without some "cost".

[the technological J]

would a hemi head be benifitial if the same compression ratio was maintained?

[TwoTired]

Could be... For some aspect of the engine performance envelope.
Goals?

But, what you are asking is kinda like.  "Is a beer stein beneficial for drinking beer?"

[the technological J]

Could be... For some aspect of the engine performance envelope.
Goals?

But, what you are asking is kinda like.  "Is a beer stein beneficial for drinking beer?"

well since u put it that way of course and it makes u look cool.....
 i thought a hemi head added a little subtle power....... ive been looking into doing the little things that hondaman reccomended for a better ride and its my tooling around on engine... i thought the hemi would cause a better firing mist.... i dunno really dodge promos the crap outta hemi's so i thought there might be some benifit( im also just looking for the free upgrades)

[TwoTired]

The original Hemi had the intake valve on one side and the exhaust valve on the other side of a hemispherical combustion chamber.  The idea was that, at high velocity, the air mass could travel through the chamber more completely, allowing a more complete fresh new charge, with less remnants from the previous burn cycle.  The pistons were also domed so there would be more squish and less chamber volume at TDC, than the  combustion chamber would allow for flat top pistons.  (This is all from memory, btw)

Chrysler corp managed to trademark the "Hemi" moniker, back when they actually produced an engine with hemispherical combustion chambers.  However, the recent new engines sporting the trademark do not really have hemispherical combustion chambers.  Chrysler corp, it using the name to garner "brand loyalty" and marketing appeal as they own the trade mark.  There is no law that says the term must apply to actual combustion chamber configuration.  So, it yet another example of "buyer beware".

The old hemi made more power at high RPM (higher than the stock red lines) when the engine were pushed hard and a blower was attached relative to comparable displacement engines without the Hemi design.

If you intend to use up the engine faster by asking for more power than it has now, then the "hemi" tricks will be of benefit for you.

Note there is a difference between "more power" and "more efficiency" in a combustion chamber design (both are "performance factors").  ...Which is most likely one reason why the new Chrysler corp engines don't have a true hemi head design configuration.  The hemis were known for getting a more complete fresh charge, not necessarily a more complete burn of the charge inducted into the chamber.

The old hemi heads were pretty big and heavy, as I recall.
I stuffed a 392 Hemi into a 57 Plymouth in '64 or '65.  I can attest to it being quite brute, even with the meager tires available in "the day".
As long as you wanted to go in a straight line, few cars could keep up with it.  Just don't try to turn fast or stop quickly.  It was quite an impressive car, even with push button drive.  Sold it before going into the Navy, and the buyer killed himself in it in a (very) high speed crash.  It was the last "Hot Rod" I made and sold.

[paulages]

for what it's worth, i just de-shrouded the valves on a cb750 head i just ported, and to get it back to the same volume in the head side of the combustion chamber, we just cc'ed the volume of a stock head (22ccs), and the volume of the deshrouded head. rather than do a bunch of fancy math to calculate how much to deck the head, we just surfaced it... measured it... surfaced it... measured it, etc. in my case, it only took .003" off of the mating surface to return it to 22ccs, which happened to be how much was needed to remove some gouging in the surface. i wasn't exactly going for a "hemi" chamber, but rather just cleaning up the rough machining. the ports were REALLY opened up, so the de-shrouding should help with flow.

for what it's worth, that dirty red-headed stepchild, the cb650, has smooth hemi- combustion chambers stock.

[JohnCurW]

Compression ratio relates to the volume of air at BDC, to the volume of air at TDC for any given cylinder.

In normal combustion, the spark ignites the fuel and there is a flame front travel away from the spark, and the combustion expands much slower while pushing down on the piston crown.  Ideally, the combustion/gas expansion event occurs over the entire time duration of the power stroke.
To compensate for higher compression, a higher octane fuel rating is required.  Higher octanes require more heat to spontaneously ignite the fuel and can avert detonation (to a point).

I just graduated recently with a mechanical engineering degree, in my thermo class when we were discussing power cycles I was taught to model combustion in most internal combustion engines (exception of diesel) as occurring instantaneously with expansion following directly afterwords.  While I know this is a generalized model because combustion cannot happen instantaneously but it was stressed that it happens on a such negligible period of time that the assumption was 99% accurate.  Otherwise, everything you said in that post seemed to be exactly what I was in my lectures (and then some).

So I guess I'm not really trying to correct you as much as I am offering what little knowledge has been given to me and wanting to know which is more accurate.  Not that it even really matters as far as this thread is concerned, but I'm curious.

[TwoTired]

What did they teach about pre-ignition, spontaneous combustion, and detonation?

In your engine models, at what crankshaft position did ignition/combustion occur? 

Most IC engines have a spark advance, to light off the mixture while the piston is still rising.  Normally, the ignition event begins combustion, but the pressure doesn't really build until after the piston has passed TDC.   The faster the crank spins, the more appropriate spark advance becomes.

Was your engine modeling done with fuel injection?  Was it intake port based, or combustion chamber based?  Were you not compressing an air fuel mixture?
Did the modeling take in account the volatility of the fuel/air charge and the induction temps?

Oh, and congratulations on your graduation!

[mystic_1]

Is that supposed to answer his question, TT?

mystic_1

[mlinder]

lol

I've been keeping out of this, with the new things I've been learning about CR and engine pressure.
But yeah, simple CR is basically, volume of chamber at BDC divided by volume of chamber at TDC.
Pressure, however, is a completely different story, with some (seemingly arbitrary) variables.

[JohnCurW]

Is that supposed to answer his question, TT?

mystic_1

Haha, actually it did,

It was really a question of "theory" over practical application I guess.

Thanks TT

[TwoTired]

Is that supposed to answer his question, TT?

mystic_1

Technically, John didn't ask a question.    
 But, he was curious about why his school work modeling was the way it was.  Can't really address that until I know the modeling details, or what the modeling was intended to accomplish.
If the model doesn't care when ignition occurs relative to crank position, it may assume it happens at or after TDC, therefore the entire pressure event occurs only when needed during the 4 stroke cycle.  That ain't the real world.  Our engines fire in advance of TDC.  If the combustion event were indeed instantaneous, there would be no pressure left during the actual power stroke.

Modeling a device can be simple, or it can very complex.  The models usually start simple, in order to learn simple functions.  Then the models get more detailed as they get closer to what an actual device/machine encounters/experiences.
You can make an operating model engine with the assumption that all the energy of combustion is expended during the power stroke of an engine.   But, there are quite literally millions of factors involved in simulating real engine operation.  Not all of these factors are required to be in place to understand many individual concepts.  The concepts are what the academia imparts.   These concepts are then later developed along with working skills and experience into a more complete understanding of actual operation.  Lots of minutia is not essential to understanding the grand scheme of things.  There aren't likely any iron filings in the computer engine model.  But, if those show up in the real engine, things just won't work out to plan.
As a parallel, you can learn to fly an airplane on a simulator.  You can transfer most of that skill into operation of a real airplane.  But, it is quite another thing to enter into a dogfight with an experienced pilot using only the skills gained in a simulator. (Unless, of course, it is the subject of a Hollywood movie.)  

Many theories apply directly and predictably to practical application.  The engine actually operates on a whole collection of proven theories.  But, in a computer environment, theories on metal coefficients of expansion, may not be needed to make predictions on a estimated output power of a engine.   Later, when you plan on making a million motors, you might want to add that theory into your simulation model, so all your built engines don't return as useless bits of metal pieces.

I am probably starting to ramble a bit... 

[TwoTired]

Look what I found on the internet, John...Check out page 392....

http://books.google.com/books?id=jcoZhwRr5UIC&pg=PA393&lpg=PA393&dq=internal+combustion+flame+front&source=bl&ots=6Wvl24NggS&sig=k5emYdfBOoAIS_F7HQS-jQbfnyA&hl=en&ei=3EUDS5KZCIf8tQP358i_Dg&sa=X&oi=book_result&ct=result&resnum=5&ved=0CCcQ6AEwBA#v=onepage&q=&f=false

[the technological J]

Look what I found on the internet, John...Check out page 392....

http://books.google.com/books?id=jcoZhwRr5UIC&pg=PA393&lpg=PA393&dq=internal+combustion+flame+front&source=bl&ots=6Wvl24NggS&sig=k5emYdfBOoAIS_F7HQS-jQbfnyA&hl=en&ei=3EUDS5KZCIf8tQP358i_Dg&sa=X&oi=book_result&ct=result&resnum=5&ved=0CCcQ6AEwBA#v=onepage&q=&f=false

you read all that the whole way to 392 in that format?

[TwoTired]

While I grew up, there was no internet, and no personal computers.  To learn things, you either had to have a knowledgeable mentor, read books, or spend a lot of money on experimentation.
I have always favored printed material that would teach me about things mechanical, electrical, physical (as in physics), as so much of our world now revolves around these things.  I started asking "why" at an early age and I never really grew out of that condition.  It was never enough to just have "it" work.  I had to know why and how it worked the way it did.  It often drove people crazy while I dismantled working devices (sometimes brand new), "just because".  I also rapidly became adept at reassembling devices so that no one could ever tell I took them apart.  (This is actually a separate skill/discipline in its own right.)  Later, when the device did break, my mental image of the internals and how it worked helped to make rapid repair possible.  There is also what I call "crossover knowledge" as many mechanisms and subsystems, though different, actually work in the same way or nearly so.  (The same principles that push coolant through an engine also apply to oiling systems, for example.  House waste water drain plumbing, behaves the same way as gas supply flow from an SOHC4 tank to carbs.  The list is endless. )

...but no, I didn't read this particular whole book I referenced for you.  Rather, I learned to use a search engine, and then skimmed to parts that looked informative/relevant and familiar from prior reading of other books.
Learning how to find information is just as important, if not more important than actually having the information in your head.  IMHO

...Steps down from the lectern...
   ;D;

[mystic_1]

Haha so in the end, after your long irrelevant explaination as to how you acquired superior skills, the bottom line is that you Googled it like any other regular smo.  Nice 

mystic_1