how to measure displacement?

[Brian Cameron]

I'm looking at an 81SC that the owner says was upgraded with 3.2L pistons and cylinders. However, there's no documents to confirm this, as he alleges he didn't get 'em from PO.

Is there some test that can be used to confirm the displacement?

Obviously I'm looking for a relatively noninvasive test as the current owner probably won't look favorably on me dismantling his engine.

thx/BC

[Adam]

Displacement is normally measured by the bore/stroke etc, but I doubt you'd be able to tell from the outside of the motor, unless the barrels have some specific outer markings or fin design.

Maybe Warren/John Walker/Tyson etc might know what to look for. The metal tinware on the barrels might make it hard to see though.

[Langers]

Could it be measured by placing one piston at BDC, pouring in oil or some other liquid, turn over the engine by hand, and measure the fluid as it comes out, then multiplying by six? (Followed of course by complete removal of the oil or other fluid) Woops just remembered the engine is in the car........perhaps some variation on this could work?

[cstreit]

Well a lot of racing org.'s do it by using a displacement pump. Essentially an adapter is screwed into a spark plug hole. This connects to a hose that has fluid or a marker in it.

I imagine you could make something like this...



And crank the engine over once to see how much fluid comes out...

[HawgRyder]

Chris has it right.
The tube connected to the spark plug hole goes into a graduated vertical tube....the valves are disabled (so no fluid can get through them)....the cylinder is filled and then cycled a couple of times...then the low and high readings are taken (using an alcohol based fluid...it vaporises easier).
You subtract the low from the high...you get the cc's for one cylinder.
A bonus here too....you also can figure out the compression ratio (approximate).
Bob

[Chuck Moreland]

Bob, how does that allow you to figure compression?

[cstreit]

Well I was thinking a bit differently.... that instead of filling the cylinder with fluid, the air coming out should approximate the volume. If the fluid in the tube was just under where it would spill out, you could at least get an approximation by turning over that cylinder once.

The flaws in my theory is that the valves aren't constantly closed... But if more than 499cc of fluid came out, you certainly have bigger than a 2993cc displacement (3.0L)

Might work, I dunno.

Maybe the PPI location can help?

[Jamie79SC]

Air compresses, screwing-up the measurment.

Alcohol does not.

[cstreit]

I doubt that it would compress enough to screw the difference between a 3.0L measurement and 3.2. It's almost 200cc of difference and since the spark plug is out, there's nothing except the weight of about 500cc of alcohol to compress it.... Sure, alcohol in the cyl would be better, but this way is easier and more likely to get the present owner to accept the test.

I wouldn't allow a perspective buyer pour alcohol down MY cylinders...

[Joe Bob]

More than a champagne glass is a waste....oops, this is motors....sorry...

My Bad....

[911pcars]

Perhaps a more accurate method would be to start with some sort of viscous liquid (ATF) in a graduated container connected to the spark plug opening. Start with the piston @TDC, end of compression. Open the spigot and allow fluid to enter the combustion chamber to fill. The amount of liquid drained is the volume of the comb. chamber + deck height. Record that amount.

Rotate the crankshaft so the piston is now at BTD and allow the cylinder to fill w/liquid. Measure and record the liquid remaining in the graduated container. The difference is the cylinder volume which should approximate either the 3.0 or 3.2 liter cylinder. You might be able to fabricated this setup with a flexible compression gauge hose, some adapters and a graduated container with a spigot at the bottom.

Don't forget to remove all liquid from the cylinder/combustion chamber. Run the engine to clear and change the oil (since it's not your engine... yet).

It seems like there should be a hi-tech method. I wonder if a boroscope or ultrasonic device (or equiv.) can measure internal dimensions.

Sherwood Lee
http://members.rennlist.org/911pcars

[Chuck Moreland]

Sherwood, with the engine installed the plug hole is not at the top of the head. Air would not readily escape the chamber above the plug hole.

The issue of air compressibility is a function of how rapidly the engine is rotating. Do it by hand and you'll get ~0 compression.

I think Chris' technique would work along with loosening the rockers to have the valves fully closed.

[911pcars]

".....with the engine installed the plug hole is not at the top of the head. Air would not readily escape the chamber above the plug hole."

Chuck,
Good observation. Let's assume we're going to compare approx. cylinder volumes and not attempt to accurately "cc" the combustion chambers and/or cylinders. With that in mind, as long as the air bubble trapped within remains constant, the method described should theoretically work. Having given this more thought, the method I describe will only work when comparing two vehicles; one with a 3.0 liter engine (reference) and the other with a suspected 3.2 (or otherwise).

Let's jack up one side of both cars (alot) equally to bring the spark plug opening a little closer to vertical. This might help a little to reduce the trapped air. With the same setup as described earlier, drain fluid into the cylinder at TDC and take a reading. The air that's trapped is there. Okay, let it be. Now rotate the crank so the piston is at BDC and take another reading. The trapped air bubble should remain constant at TDC vs BDC (why shouldn't it?). You still measure the difference in liquid between TDC and BDC. There's not enough head (pressure) difference to appreciably compress the bubble. When this procedure is repeated on both cars, one being a reference car, that should give you an indication of what car has a greater cylinder volume.

What I'm brain farting here is theoretical (and sounds like a lot of trouble doesn't it?) and it may not work out in real world cylinder spelunking (sp). Just a thought - maybe even a minor league one.

Sherwood

[TimT]

PCA uses some sort of gizmo to measure displacement, along the lines of what Chris describes, except that no fluid is used (except air which is a fluid)
This gizmo is accurate enough to have bumped a friend of mines car up a class.

[Brian Cameron]

Thx once again guys, you've given me some good ideas to go with, and I'll also see if the PPI wrench has some kind of nifty tool for this.

BTW...

Originally posted by cstreit
I wouldn't allow a perspective buyer pour alcohol down MY cylinders... [/B]

I would... if it were a 25yr old Macallan...

[HawgRyder]

Chuck...I compute the approximate compression by comparing the filled volume of the cylinder with the crushed volume.
It's not dead accurate..but will give a ballpark number if you know the rough volume of the cylinder head.
Does this not come out right?
Bob

[Chuck Moreland]

Actually you need to now the volume of the cylinder with the piston at TDC, not just the head. A domed piston protrudes into the head space.

If you happen to know the volume at TDC, yes I see how to compute CR. But the test doesn't measure that volume.

[HawgRyder]

Chuck...when you first fill the cylinder....the piston is at the top...the valves are closed...and you measure the amount of fluid going in at that time.
I know it's not dead accurate, but it's close.
It lets me check to see if I'm looking at a low or high compression engine before tearing it down.
We used to go through engines at the junk yard this way to weed out the grocery getters from the track possibles.
Bob