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basic question on compression ratio

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prkaye

Well Known Member
Is compression ratio a function of the cylinder, of the piston or both? For a given cylinder, is it possible to achieve a different compression ratio by installing a different piston?
I ask only because i'm trying to understand something that my engine repair shop said. I always assumed that a given cylinder part number would only support one kind of piston and have a fixed compression ratio. Is this assumption incorrect?
 
When I was working with Aerosport Power to spec out the engine for my RV-10, I changed the pistons to achieve a slight bump in compression (9:1). I am sure that there is an upper limit of how high you can increase compression by changing the piston, but it definately is possible.
 
UnPossible said:
When I was working with Aerosport Power to spec out the engine for my RV-10, I changed the pistons to achieve a slight bump in compression (9:1). I am sure that there is an upper limit of how high you can increase compression by changing the piston, but it definately is possible.
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Ok, thanks! This helps me understand something i was recently told. 17 years after starting to build my RV-9A (and 13 years after first fligth) I'm still learning things! My recent early piston-ring failure has ended-up being very expensive (exposing other problems) but has been an incredible learning opportunity!
Next stop, Pennsylviania for the Lycoming engine rebuild course :)
 
prkaye said:
Is compression ratio a function of the cylinder, of the piston or both? For a given cylinder, is it possible to achieve a different compression ratio by installing a different piston?
I ask only because i'm trying to understand something that my engine repair shop said. I always assumed that a given cylinder part number would only support one kind of piston and have a fixed compression ratio. Is this assumption incorrect?
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Yes, your assumption is incorrect. You can change the pistons to achieve different ratios for the same cylinder bore.
 
Compression ratio is the volume of the cylinder with the piston at bottom dead center divided by the volume of the cylinder with the piston at top dead center. So yes, a piston with a taller crown (top of piston to centerline of the piston pin) will give a higher compression ratio. Whether such a piston exists is another matter.
 
This info was originally submitted by Mahlon Russell:

Quote:
7:1 compression ratio, in an O-360, is had by installing 75413 or equivalent pistons and it is generally accepted to burn mogas with 87 octane and above with that compression ratio. Take off Hp is in the range of 168 with those pistons.

8.5:1 is the standard compression ratio for the O-360 and it is attained by use of 75089 or equivalent pistons. It is generally accepted to run mogas of 91 octane and above with this compression ratio. Rated power is 180 HP with those pistons

9:1 compression is attained by use of the LW-15357 or equivalent, in the O-360, and it is generally accepted that you should use 100LL r above with this engine. Normally see around 185 HP at take off, with those pistons.

The 320 and the 360 have the same bore, only the stroke is longer on the 360 to get the extra displacement. So you use the same pistons to get the same compression ratios for the 320 as with the 360.

7:1 compression ratio, in an O-320, is had by installing 75413 or equivalent pistons and it is generally accepted to burn mogas with 87 octane and above with that compression ratio. Take off Hp is in the range of 150 with those pistons.

8.5:1 is the high compression ratio for the O-320 and it is attained by use of 75089 or equivalent pistons. It is generally accepted to run mogas of 91 octane and above with this compression ratio. Rated power is 160 HP with those pistons

9:1 compression is attained by use of the LW-15357 or equivalent, in the O-320, and it is generally accepted that you should use 100LL above with this engine. You should see around 164 HP at take off, with those pistons.


__________________
 
Pistons

Non certified pistons are also available for the 320 and 360. 10/1 is most common and the racers use even higher compression.
Elimination of lead from fuel is a future question mark.
 
Compression ratio is just the ratio of the cylinder volume when the piston is at the very ‘bottom’ (‘outboard’ for our horizontal engines) divided by the cylinder volume when the piston is at the very ‘top’. The ‘right’ way to increase the compression ratio is to increase the throw of the piston. But that means a new crankshaft, very costly. If you just swap in a taller piston, you decrease the volume at the bottom of the stroke (so you cannot put in quite as much fuel/air as before) just a little bit, but you decrease the ‘top’ volume by a significant percentage. In an ideal four stroke engine the efficiency depends only on the compression ratio. So in most cases, the increase in efficiency more than makes up for the small decrease in the fuel flow, and you end up with more power, and definitely lower fuel flows. The down side is that both the density and temperature of the fuel air mixture go up as you increase the compression ratio, and at some point even the highest available octane gas is not enough to prevent detonation from setting in, and rapidly destroying your engine.
 
Thanks Bob! Another really nice educational summary. i was tempted to downgrade my engine to 150hp, figuring that the lower compression would be less stress on the engine and longer engine life, but i worried about resale. The ability to run on 87 octane mogas seems somewhat neutralized by the fact that the only ethanol-free gas available here is 91-octane.
 
prkaye said:
The ability to run on 87 octane mogas seems somewhat neutralized by the fact that the only ethanol-free gas available here is 91-octane.
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I don’t understand this comment. If your engine is approved for 87 car gas (which is about 83 motor octane, the aviation standard) then it is safe to run higher octane 91 auto gas. Just don’t go lower than the minimum needed octane.
 
what i mean is, whats' the point in having an engine that can run on 87-octane, if the engine needs ethanol free, and that is only available in 91-octane. It means the ethanol-free requirement means that i can't actually realize the cost savings of running at the lower octane.
 
That’s certainly true. But here in the States the difference between 87 and 91 car gas is 20 cents/gal, while the difference between auto gas and avgas is like $2/gal.
 
I'm paying about $3.70 a gallon here for 93E10, helps a lot with the fuel equation.
 
Agreed, which is why i'm glad that at 8.5:1 i can run 91 octane which is available here in ethanol-free. What i mean is that a lower-compression that would allow 87 octane is useless to me because ethanol free is not available in that low-grade fuel here.
 
scsmith said:
Carl, it appears that this Lycoming SI predates the approval of G100UL last fall.
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Looks like Lycoming hasn't updated the SI yet to add G100UL, or at least add the new 'specification' that GAMI may have defined for their fuel (but I think GAMI is considering it proprietary). Lycoming may be evaluating it currently or will add it once it becomes available.

The Swift Fuel UL94 apparently meets the ASTM D7547 Unleaded Avgas specification listed in the SI.
 
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Looking back, I find it interesting that, when pursuing my PP license, I was able to rent a VFR C-150, wet, for about what some airports are now charging for a gallon of Avgas.
 
Others have given the compression formula . . . Typically the chamber volume at TDC is reduced by material added to the top of the piston. In the Lycoming case, it is across the entire face of the piston so no bumps etc are needed.

This is typically an allowance designed into the engine, at least for some range. In automotive crown shapes may become necessary to reduce the compressed volume as there is squish around the perimeter and a close piston/head clearance that should not be reduced, or with thought and possible experimentation.

Personally, lowering the Cr would be less desirable (lower combustion efficiency) vs a timing change to accommodate lower octane fuels. My IO360 M1B has standard compression and Lyc said it will accept "automotive" fuel. Nice to have a backup option ;). Maybe yours would too.
 
Car and motorcycle engines are running upwards of 12:1 running 87 by using knock censors and pulling timing.
 
swjohnsey said:
Car and motorcycle engines are running upwards of 12:1 running 87 by using knock censors and pulling timing.
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Those engines also have variable valve timing. YOu cannot compare static CR's across engines. The actual rate of compression cannot be determined without knowing the angle at which the intake valve closes and every designer puts it in different places based upon their overall design and goals. Typical range for IVC is 50-60* ABDC. In modern cars, the IVC is moving around a good bit to optimize varying loads and RPMs. Providing this guidance for use on Lyc engines is just dangerous. 12:1 CR on a Lyc is like BEGGING for trouble, let alone trying it with 87 octane.
 
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swjohnsey said:
Car and motorcycle engines are running upwards of 12:1 running 87 by using knock censors and pulling timing.
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High rev’ing, small bore, one example being water cooled. Those are very different design boundaries versus our machines. Most (d@mn near all?) of the commercial applications that you mention are more about mitigation than prevention
 
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Stock Lycoming 10:1 Piston

jrs14855 said:
Non certified pistons are also available for the 320 and 360. 10/1 is most common and the racers use even higher compression.
Elimination of lead from fuel is a future question mark.
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What Jim says is true. However, a number of Lycoming helicopter engines have 10 : 1 compression ratios [and 1,000 hour TBOs] Per Lycoming SSP-110-2, page 13 the HIO-360-D1A has 10 : 1 compression ratio.

Lycoming Piston Part Number LW-11487
 
Mel said:
The Lycoming is a very noisy engine. I believe knock sensors have been tried to no avail.
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Dan has posted Dyno run data from the FAA that included detonation data, so it must be possible to measure. Though I suspect it is not as easy as putting a piezo sensor on the block. From what I have read, each engine requires a lot of experimentation to figure out the correct frequency to look for when identifying detonation.
 
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lr172 said:
Those engines also have variable valve timing. YOu cannot compare static CR's across engines. The actual rate of compression cannot be determined without knowing the angle at which the intake valve closes and every designer puts it in different places based upon their overall design and goals. Typical range for IVC is 50-60* ABDC. In modern cars, the IVC is moving around a good bit to optimize varying loads and RPMs. Providing this guidance for use on Lyc engines is just dangerous. 12:1 CR on a Lyc is like BEGGING for trouble, let alone trying it with 87 octane.
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The compression ratios published are all computed ratios. Some have variable valve timing snd some don't. All are liquid cooled with makes like easier. All have electronic fuel injection and knock sensors that allow for timing to be retarded to prevent detonation and pre-ignition. Some normally aspirated engines running on the street are getting 3hp/cubic inch. Aircraft engines are lucky to get 1/2hp/cubic inch. One giant factor in RPM. Engines getting 3hp/ci are turning 15,000 rpm.
 
Trying to compare a modern automotive engine with a tractor engine (Lycoming) is a waste of time. The only thing that is sort of similar is that they are both reciprocating, gasoline burning engines.
 
They compare favorably with air cooled VW and Porsche engines . . . except for the low rpm. Porsche used knock sensors in high hp opposed 6s both turbocharged and normally aspirated. Folks like Dave Anders are getting impressive hp using modern technology, electronic fi and variable timing. How much horsepower do you figure it takes to push a RV-4 to 250 mph? He is running something based on a Lycoming IO-360.
 
swjohnsey said:
The compression ratios published are all computed ratios. Some have variable valve timing snd some don't. All are liquid cooled with makes like easier. All have electronic fuel injection and knock sensors that allow for timing to be retarded to prevent detonation and pre-ignition. Some normally aspirated engines running on the street are getting 3hp/cubic inch. Aircraft engines are lucky to get 1/2hp/cubic inch. One giant factor in RPM. Engines getting 3hp/ci are turning 15,000 rpm.
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I struggle to see 12:1 actual compression running on 87 octane, no matter how much timing is taken out. I set up 11:1 CR on a chevy big block (496) in a corvette. HOWEVER, it was a very aggressive cam and therefore the effective CR was down around 9:1. Then again I do not follow modern hot rod techniques.
 
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