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 Post subject: displacement
PostPosted: Mon Jan 18, 2010 1:38 pm 
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What exactly do the engine displacement numbers mean for a car (i.e 3.8L)? How is the displacement measured? Is it where the pistons are when the spark ignites the fuel or is it TDC etc.? Is there a way to increase displacement, say by adjusting timing and would that work to increase HP?


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 Post subject: Re: displacement
PostPosted: Mon Jan 18, 2010 1:44 pm 
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It's the volume of air an engine would move through one complete cycle (1 revolution for a 2 stroke engine, 2 revolutions for a 4 stroke engine). Displacement is measured by the bore and stroke, and number of cylinders. The only ways to change that are to have the cylinders bored out and get new pistons to match the new diameter, and/or change the crankshaft to one that has a different crank journal to rod journal relationship.

displacement = (bore/2)^2 * pi * stroke * # of cylinders

Now, you can gain hp by adjusting spark timing if it isn't already optimized, but it won't change the displacement.

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Last edited by J Wikoff on Mon Jan 18, 2010 1:49 pm, edited 2 times in total.

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 Post subject: Re: displacement
PostPosted: Mon Jan 18, 2010 9:53 pm 
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The volume of air would be measured in CFM. Displacement is the measurement of the engines internal cylinder size. It is usualy measured by filling a cylinder with a measured amount of oil and see just how much oil it takes to fill it, then multiply that measurement by the number of cylinders. It can be done mathematically too if you have all the measurements.
Where most people get confused is how the numbers are used. There are different scales when it comes to measurements, they all mean the same thing but are read in a different "language".
Example: The GM 3800
In liters, this engine measures 3.8 liters of displacement.
In Cubic Centimeters or CC's or Cm3 (centimeters cubed), the engine measures 3,800 cubic centimeters of displacement.
In Cubic Inches of Displacement or CID, the engine measures 232 cubic inches of displacement.


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 Post subject: Re: displacement
PostPosted: Mon Jan 18, 2010 10:04 pm 
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If you just fill the cylinder with oil, it would give you a number higher than the displacement, because you'd also be filling the combustion chamber (the space left when the piston is at TDC). You could fill it with oil at bottom dead center, then turn it up to top dead center, and then measure what was pushed out (displaced), and multiply that by the number of cylinders.

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2009 G8 GT - Sport Red Metallic, loaded, SOLO Axlebacks, Rotofab Intake, Tuned, autodim mirror, removed intake manifold cover, HSV GTS triple gauge pod, two tone red-hot shifter and HSV SuperSport steering wheel, GXP rear sway bar and diffuser, 3.45 diff and various Camaro suspension bits, LED Taillights


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 Post subject: Re: displacement
PostPosted: Mon Jan 18, 2010 10:08 pm 
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That is exactly right. And it is only one stroke measurement weather it is a 2 or 4 stroke engine.


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 Post subject: Re: displacement
PostPosted: Mon Jan 18, 2010 10:40 pm 
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"displacement = (bore/2)^2 * pi * stroke * # of cylinders"

That was what I was looking for. But is "stroke" the entire length of piston travel? This would seem to make Displacement a misleading number because some engines have longer stroke than others irregardless of fuel and air volume along with compression ratio . ??

J are you saying that the displacement no. which is calculated as you said directly proportional to the volume of air and fuel the piston draws?

If you have the same size bore and no. of cylinders on two different motors and 1 has a longer stroke than the other then the first has a larger displacement?

What about cylinder head internal shape and size?

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Note if you pull the injectors to obserbe fuel delivery...make sure you leave the plugs in and disconnect the ign. Believe me


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 Post subject: Re: displacement
PostPosted: Mon Jan 18, 2010 10:53 pm 
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Here’s the math for our engines:
Bore (3.8”) /2 =1.9”
1.9 squared = 3.61
3.61 x Pi = 11.34 (This is the area of the bore)
11.34 x stroke (3.4”) = 38.56 (This is the volume/displacement of each cylinder)
38.56 x 6 (# of cylinders) = 231.36 (volume/displacement of engine in cubic inches)

Stroke is the entire length of piston travel.
If two engines have the same bore & number of cylinders then the engine with the longer stroke has more displacement.
Cylinder head & shape (& piston shape) will effect compression ratio but not displacement.

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 Post subject: Re: displacement
PostPosted: Mon Jan 18, 2010 11:00 pm 
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Mechanical Mike wrote:
Here’s the math for our engines:
Bore (3.8”) /2 =1.9”
1.9 squared = 3.61
3.61 x Pi = 11.34 (This is the area of the bore)
11.34 x stroke (3.4”) = 38.56 (This is the volume/displacement of each cylinder)
38.56 x 6 (# of cylinders) = 231.36 (volume/displacement of engine in cubic inches)

Stroke is the entire length of piston travel.
If two engines have the same bore & number of cylinders then the engine with the longer stroke has more displacement.
Cylinder head & shape (& piston shape) will effect compression ratio but not displacement.


:booty:


So displacement is fixed. Timing, valve adjustment or machining down the cylinder head will NOT effect displacement...

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Note if you pull the injectors to obserbe fuel delivery...make sure you leave the plugs in and disconnect the ign. Believe me


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 Post subject: Re: displacement
PostPosted: Mon Jan 18, 2010 11:07 pm 
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But is "stroke" the entire length of piston travel? Yes.

This would seem to make Displacement a misleading number because some engines have longer stroke than others irregardless of fuel and air volume along with compression ratio . ?? I don't see why it's misleading.

J are you saying that the displacement no. which is calculated as you said directly proportional to the volume of air and fuel the piston draws? Yes. Although exact valve timing, and intake/exhaust restriction will affect the mass of air/fuel.

If you have the same size bore and no. of cylinders on two different motors and 1 has a longer stroke than the other then the first has a larger displacement? Yes.

What about cylinder head internal shape and size? They won't affect displacement.

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WHITE WHINE - 1992 SSE Supercharged 236.26 ci (.040 Over) 15.090 at 90.2 MPH on old engine w/ slipping trans & melted O2 sensor - Gen 3 M62 and matching TB, Gen 2 Pully, Zillamotorsports Ported LIM, YT 1.72 Roller Rockers, SII FPR & Injectors, Hypertech Thermomaster chip w/ 160 Thermo, TransGo Shift Kit, Infinity/Pioneer Speakers and 10" Subs, 1300 watts, 140 amp Alternator, Ricepipe CAI w/ heatshield, Pilot Angel Eye Foglights, Clear Corners, '02 17" Chrome Bent 5's, Magnaflow F-Body Muffler and Hi-flo Cat, Ceramic Coated Ported Exhaust Manifolds, Fan Override
2009 G8 GT - Sport Red Metallic, loaded, SOLO Axlebacks, Rotofab Intake, Tuned, autodim mirror, removed intake manifold cover, HSV GTS triple gauge pod, two tone red-hot shifter and HSV SuperSport steering wheel, GXP rear sway bar and diffuser, 3.45 diff and various Camaro suspension bits, LED Taillights


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 Post subject: Re: displacement
PostPosted: Mon Jan 18, 2010 11:10 pm 
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Displacement is fixed unless you change the bore size or change the stroke.

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 Post subject: Re: displacement
PostPosted: Mon Jan 18, 2010 11:17 pm 
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Yes. The only two things that can change displacement are bore and stroke. So changing pistons or crankshafts are the only two ways to change the displacement. Machining the head will increase compression ratio, but not engine size/displacement.


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