Chamber Volume -- determining

I'm trying to determine the Wonderbus's compression ratio. It has a 1776 engine with 90.5mm bore, 69mm stroke. The deck height is 2.5mm. All I need is the volume of the combustion chamber. I could pour water into it and then pour it off into a measuring cup or something, but I bet someone has already determined it. Anyone know?

Reply to
Mike Rocket J. Squirrel Elliot
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I think there are differences in the heads and you need to measure yours. I've seen Jan use a syringe with measurement marks to=20 measure the exact amount of water inserted into the chamber. Better, I have pictures ;)

Reply to
Olli Lammi

...exactly. You need to measure your specific chambers. Easy way is this. Trim a CD to fit in the spigot hole. Go to farm supply store and get a large syringe. put a light coating of grease on bacck edge of cd to provide seal and insert it in head. Fill head with a light oil so you don't rust anything with water =-) tap cd as necessary to get out air bubbles and noit how many cc's of fluid it took to fill. Do both chambers or all four if you have removed both heads and average the results.

...Gareth

Reply to
Gary Tateosian

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Yeah, you could do that.

It wouldn't work, but you could do it :-)

Your deck heights are NOT all the same. Neither are your chamber volumes.

Chamber volume varies according to the casting, the spigot depth, the protrusion of the valves and their shape, and the type of spark plug being used.

For your present purpose measuring chamber volume to the nearest cubic centimenter will be good enough. But the method you've described fails to take the meniscus into account, which not only gives you an erroneous answer, the error is going the wrong direction, telling you the chamber is larger than it really is. Do the calculations and the result will be a CR that is smaller than it really is.

The proper method -- as described in numerous manuals -- is to put a sealing plate into the spigot-bore so as to put a 'lid' on the combustion chamber. With the head carefully leveled, you then fill the chamber with liquid through a hole in the 'lid.' SOP is to perform the measurement a multiple number of times for each chamber, throw out the high & low and average the remainder. Although the task may be done with something as crude as a turkey baster, if you're building a quality engine you would probably use a laboratory-grade burette graduated to .1cc.

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Once you've determined the volume for each cylinder, you use the SMALLEST for your CR calculation.

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In a properly built engine the displacement of the cylinders will match to within .5cc or less. Herez why:

Combustion generates heat and that heat causes the expansion of the gases in the cylinder (most of which is nitrogen, but there it is). The expansion of those gases is what pushes the piston down the bore and causes the crankshaft to rotate.

In a properly built engine all of the cylinders will generate IDENTICAL amounts of push.. or as close to identical as possible. Because when they don't, any DIFFERENCE in the push results in a loss of torque equal to the product of the difference times the number of cylinders.

In effect, imbalanced cylinders work against each other. Before the engine can produce any usable power it must first overcome any internal losses. Bottom line is that a casually assembled engine can throw away as much as 25% of its potential power.

But rather than show the newbies how to build efficient engines -- of any size

-- the instant experts have an entire grab-bag of cures including bigger jugs, more carbs and so forth. Always at extra cost, of course.

The owner of a sloppily built 1834 running Kadrons and a trick exhaust never believes it when he gets blown off by a stock engine.

Try to explain the exponential losses due to mass and volumetric imbalance and they'll usually say, "Oh yeah? Well, if that's true how come I've never seen it in any of the magazines?"

Which is a pretty good question, when you think about it :-)

-Bob Hoover

PS -- The surprising performance of crate engines from the Puebla plant (typically around 70hp for juicers, slightly less for those running solids) appears to be due to the fact VW de Mexico uses the same standards of balance for their air cooled engine as they do for the Golf and Pissant engines, which are typically an order of magnitude better than the archaic standards VW of Germany carried over from the 1930's (ie, up to TEN GRAMS difference across a set of rods... as compared to +/- 0.2g. Or +/- 1cc for volume vs +/- 6cc for the old standards)

Same amount of fuel, of course. The only difference is that now it's producing more usable power.

Reply to
Veeduber

This is all getting very scientific.

I like that. :-)

Reply to
Mike Rocket J. Squirrel Elliot

"Gary Tateosian" wrote

... and prior to filling the chamber with said oil, don't spend 10 minutes getting your heads all nice and level .... only to realize that you forgot to put the spark plugs in. :-\ A "friend" of mine did that once. ;-)

-- Scott

Reply to
Scott H

Um -- how does temperature affect CR?

[snip]

I've picked up those VW magazines a couple of times while trying to educate myself about my engine. What a waste. I know next to nothing, and yet I could spend a year reading them and not know much more.

They seem to be written for guys that move their lips when they read.

Another way of putting it: I could read one of those magazine cover-to-cover in the time it takes for me to complete one full session in the smallest room of my house. And I do my business pretty quickly.

Reply to
Mike Rocket J. Squirrel Elliot

As someone I once worked with was fond of saying "It isn't what you don't know that will hurt you. It's the stuff you DO know that's wrong."

Max

Reply to
Max Welton

On Fri, 28 May 2004 01:48:02 GMT, "Scott H" ran around screaming and yelling:

those silly friends people keep... JT

Reply to
Joey Tribiani

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Are you trying to be obtuse?

Compression ratio is is based on fixed, known, mechanical dimensions.

It's like asking "How does temperature affect a yard stick?"

The basic answer is that it has no effect, partly because the range of temperatures involved are too close together for the coefficient of expansion to cause any signficant change in dimension but more so because whatever the temperature and any dimensional change resulting from it, the RATIO of one set of dimensions to the other will remain the same.

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But if you were speaking of Effective Compression Ratio, which is the term applied to the amount of compression applied to whatever quantity of fuel/air charge happens to be in the cylinder, then you have to start reading up on Volumetric Efficiency.

In a normally aspirated engine the Effective Compression Ratio, normally shown as CR sub eff, is ALWAYS less than 100% and falls steadily as rpm increases. This gets into the physics of fluidynamics and lots of Who Shot John math that I couldn't explain even if you plugged my dick into a light socket for the simple reason that I don't know that much math.

But I do know that peak torque coencides with peak Volumetric Efficiency... and I've got a whole war-bag full of tricks that can improve VE for a given range of rpm. But that has nothing to do with slapping a cheap fix on a poorly built engine.

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But the reason for mentioning VE is that the fluidynamics of the fuel/air charge (gases are 'fluids' in that regard -- you gotta use whatever tools work and in this case it's fluid dynamics) ...includes ambient air temp as a factor when determining air density, and temp comes into play again with regard to the endothermic change-of-state that converts gasoline vapor into a gas, and then you have to go through another complete iteration because the physical properties of the fuel-air charge -- how well it turns a corner, how quickly it can accelerate (or decellerate) -- and what happens to its composition when it does -- are all effected to some degree by temperature.

But not your basic Compression Ratio. That's fixed, like your basic First Gear Ratio -- a mechanical quantity.

-Bob Hoover

Reply to
Veeduber

Well, no . . . I may be obtuse but it's not because I'm /trying/ to be obtuse. I'd rather be acute, but don't always achieve my goal. I'm endeavoring to get my head wrapped around all the relationships between all the parts.

Obviously (I see now, and would have seen if I had thought longer before posting) that if all the parts double in size due to heating, the ratio remains the same. In the case of the yardstick, if it is a steel one, its overall length /does/ change with temperature. But if the parts being measured are also steel they would change too, so the reading would remain the same.

My question had nothing to do with "Effective Compression Ratio" -- but I do thank you for giving me the opportunity to say, "Uh, yeah, THAT'S what I meant! Yeah, that's it."

[snip]

Hah. We'll show you, the Wonderbus and I, as we trundle down Coast Highway at speeds in excess of 40 mph leaving behind a cloud of oil and screws and nuts and stuff.

Reply to
Mike Rocket J. Squirrel Elliot

On 28 May 2004 05:08:04 GMT, snipped-for-privacy@aol.com (Veeduber) ran around screaming and yelling:

ouch...LOL JT

Reply to
Joey Tribiani

This thread might be a good opportunity to consider the virtues of the single-port head a Bus engine. I have read comments that suggest that an overbore ACVW engine with a single-port head has very good torque characteristics at a low-RPM. One might think that's a good thing for a Bus. Dunno myself, because I've never had one.

Reply to
jjs

And your friends initials are . . . S.H.?

Reply to
Mike Rocket J. Squirrel Elliot

No way of knowing how big YOUR head chambers are.

NEVER trust anyone else's estimate or even accurate data on *their* head to be the same as yours.

NEVER trust any given head CC reading even on new heads, measure them yourself.

And your method is wrong. (Sorry for being blunt honeypie, no time to sugar coat it :) )

Get a syringe, and measure it with that. Use water and an old CD that you cut with scissors to match the diameter of the hole in the head you are covering. Clean the head surface and smear vaseline on the edges, so the CD would seal well. Now you have a measuring plate with a fill hole in the middle.

Keep the head level and start filling the chamber through the center hole on the CD. Spark plug and valves must be in place for this. If the valves leak faster than you can measure, smear vaseline around the perimeter of the (closed) valve, to stop the leak until you have measured the chamber. Use only as much vaseline as absolutely necessary, this would be almost invisible. Any more will affect your CC reading.

A stock head cc would be ROUGHLY in the neighborhood of 50cc. (give or take 5-10cc, which is a lot) THIS VALUE is not enough for you to start making assumptions, go ahead and measure yours.

You are supposed to get a reading that's accurate within 1-2cc or so, for your purposes. (Good enough, given the performance level of your engine) Measure all of the chambers, or at the very least, one in both heads. The heads you have may not be identical (Builder error) in which case you need to bring them to same cc.

Email for further details.

A site with pics is under construction... sort of. But don't hold your breath.

Jan

Reply to
Jan Andersson

Darnit, you beat me to it :)

Jan

Reply to
Jan Andersson

Thanks Jan and Bob H. -- the concept and procedures are very clear. I don't have farm supply shops where I live (no farmers or ranchers) but McMaster-Carr sells syringes. I should be getting felt bob (hobs), a

10cc syringe tomorrow. Hubby has some hobby work this weekend. With Vaseline.
Reply to
Mike Rocket J. Squirrel Elliot

I didn't know they made light sockets that small :)

Jan

Reply to
Jan Andersson

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