Compression ratio computation

[ronrapp]

My IO-360 has a data plate on it which says it's 195 hp, but does not list the compression ratio. I read the "Piston/compression/fuel combinations for O-360 and O-320" sticky post but did not see any combination that resulted in 195 hp.

Can anyone help me with the compression ratio? I see 9.7:1 on threads sometimes, and am guessing I'm somewhere in that range, but I don't know the math.

I'm interested in that data because sometimes I look at CS props and things and often find compression ratio limitations.

For what it's worth, I contacted Aircraft Cylinders & Engines to ask if they could provide me any information on the engine, since I didn't build the plane, and the reply was simply "No."

--Ron

 

[RV6_flyer]

Sounds like 195 HP out of a parallel valve 360 Lycoming engine.

My guess is that you have 10:1 pistons. I know others that have 10:1 pistons and make claims of over 200 HP but they are also running over 2,700 RPM.

About a decade ago, Kitplanes had an article about Efficient Engines that sorta points to 3 to 5 HP per cylinder with an increase of 10:1 compression from the stock 8.5:1.

NFS and Combustion Technologies both make EXPERIMENTAL pistons for our experimental aircraft engines.

 

[rmarshall234]

Type Certificate 1E10 shows the Lyc IO-360 C1A engine as being 200 HP at 2700 rpm at Sea Level with a compression ratio of: 8.7:1

Not sure why your data plate says 195 hp but I would assume you have a 8.7 to 1 CR.

If you actually have the higher 9.7:1 CR you would be putting out more than 200 hp.

 

[Bicyclops]

That's not a Lycoming data plate for sure. If it really does have 10:1 CR, most engine builders would spec the timing at 20 not 25 degrees BTDC. If all your CHTs are under control probably moot.

Ed

 

[blaplante]

A lot we don't know. Stock camshaft or not? Ported cylinders or not?
With ONLY compression ratio you need a lot of increase to get that much gain. But in combination with porting and camshaft, it could be much less CR change.

Unfortunately you'll need to treat it like it needs 100 octane, because we just don't know.

 

[ronrapp]

Sounds like 195 HP out of a parallel valve 360 Lycoming engine.

It is definitely a parallel valve 360.

Regarding the cylinders, the original engine log entry after overhaul says nitrided steel cylinders with 9.5:1 pistons were installed. See attached image.

However, there's also an entry from 2011 that says four new AEL65102 cylinders were installed at that time. (My A&P checked and AD 2009-26-12 does not apply). It doesn't say whether the CR is the same, however.

So there are a lot of questions about the engine that I cannot answer. I wish Aircraft Cylinders & Engines was willing to help.

--Ron

 

[ronrapp]

That's not a Lycoming data plate for sure. If it really does have 10:1 CR, most engine builders would spec the timing at 20 not 25 degrees BTDC. If all your CHTs are under control probably moot.

Ed

Thankfully, CHTs are not a major problem with this engine. I have my CHT alarm set for 400 degrees and even on hot days don't usually see any cylinder hit that mark.

To be fair, I have a fixed pitch prop so I'm not pulling anywhere near 100% power.

--Ron

 

[ronrapp]

A lot we don't know. Stock camshaft or not? Ported cylinders or not?
With ONLY compression ratio you need a lot of increase to get that much gain. But in combination with porting and camshaft, it could be much less CR change.

Unfortunately you'll need to treat it like it needs 100 octane, because we just don't know.

Yes, that's another one of my questions: would auto fuel ever work with this engine? With the advent of G100UL it may not be an issue, but I'd like to at least know if, in a pinch, I'd be able to use it or if it would be risking detonation.

I bought the plane from Reid-Hillview in NorCal, where 100LL was no longer available. I had enough fuel to make another airport without an issue, but it did put the question into my head pretty early on.

--Ron

 

[rmarshall234]

Swept volume

You can measure the swept volume and compute the CR from it. Haven’t done it myself but essentially:

Bring the piston to TDC and fill with marvel mystery oil thru the top spark plug hole. Bring the piston to BDC and fill with MMO. Measure the difference between the two numbers as a ratio. Drain the harmless MMO from the bottom plug hole and go fly.

Maybe there’s a IF1 tech inspector or someone more familiar that can chime in.

 

[rmarshall234]

Yes, that's another one of my questions: would auto fuel ever work with this engine?

--Ron

I believe if you change your CR to 8.5:1 or less and run 91 octane car gas the answer is yes. Lycoming Service Instruction 1070AB is the document you want to reference regarding using auto fuels. Good luck.

 

[lr172]

You can measure the swept volume and compute the CR from it. Haven’t done it myself but essentially:

Bring the piston to TDC and fill with marvel mystery oil thru the top spark plug hole. Bring the piston to BDC and fill with MMO. Measure the difference between the two numbers as a ratio. Drain the harmless MMO from the bottom plug hole and go fly.

Maybe there’s a IF1 tech inspector or someone more familiar that can chime in.

It only needs to be done at TDC, the rest is math, assuming you have the bore and stroke ((cyl vol @ TDC+ displacement (square of (bore*.5)*3.1414)*stroke)) : cyl vol @ tdc). However, this approach will not work unless cyl is upside down and valves are well sealed. That is thin liquid and will run through the rings. The likelihood of error in that method is quite high if done by a novice.

OP: cylinders are most always sold with pistons, so there is a pretty good chance that the CR is now stock.

 

[BillL]

Find a Whistler . . . .

You can measure the swept volume and compute the CR from it. Haven’t done it myself but essentially:

Bring the piston to TDC and fill with marvel mystery oil thru the top spark plug hole. Bring the piston to BDC and fill with MMO. Measure the difference between the two numbers as a ratio. Drain the harmless MMO from the bottom plug hole and go fly.

Maybe there’s a IF1 tech inspector or someone more familiar that can chime in.

Pretty much have to measure to be sure of what you have. Especially if you want to experiment with fuels.

There is another method but not widely available - it is a whistler. It uses resonance of the volume to determine Cr. Used in racing because everyone wants an edge.


Edit; tool link

 

[ronrapp]

OP: cylinders are most always sold with pistons, so there is a pretty good chance that the CR is now stock.

I hadn't considered that possibility, but you're right: when the cylinders were swapped out for the ECI units, the compression ratio probably went back to 8:1 and nobody ever changed the data plate on the engine.

With ECI being out of business, I don't suppose anyone would be able to tell me about the cylinders by simply having the serial numbers, would they?

--Ron

 

[mahlon_r]

The other side of that coin, is that the installing mechanic realized that the engine had non- stock pistons installed previously and reused them only changing the actual cylinder. Does the replacement log entry call cylinder assemblies of just cylinders? Either way, the only way to be sure, of what you have, IMHO would be to pop a cylinder off and check the piston P/N.
Happy New Year,
Mahlon

 

[lr172]

I hadn't considered that possibility, but you're right: when the cylinders were swapped out for the ECI units, the compression ratio probably went back to 8:1 and nobody ever changed the data plate on the engine.

With ECI being out of business, I don't suppose anyone would be able to tell me about the cylinders by simply having the serial numbers, would they?

--Ron

ECI only made std CR pistons IIRC. Therefore, if that is correct, the only way yours are non std are that the PO used the old pistons with the new cylinders and that is a real possibility, as the pistons really don't get any wear in most cases. The top of the piston is usually engraved with a part number, but is likely now covered with deposits.

 

[ronrapp]

ECI only made std CR pistons IIRC. Therefore, if that is correct, the only way yours are non std are that the PO used the old pistons with the new cylinders and that is a real possibility, as the pistons really don't get any wear in most cases. The top of the piston is usually engraved with a part number, but is likely now covered with deposits.

Thanks for the info. I looked at the 2011 logbook entry again, the one where the cylinders were replaced. I'll attach an image of that page. It says the cylinders were replaced, but does not mention the pistons.

--Ron

 

[lr172]

Thanks for the info. I looked at the 2011 logbook entry again, the one where the cylinders were replaced. I'll attach an image of that page. It says the cylinders were replaced, but does not mention the pistons.

--Ron

Yeah, that is a total unknown. A new cyl assy implies a new piston, but that doesn't necessarily mean they used it and instead re-used the old piston to maintain the higher CR. That would not be considered poor workmanship, as the piston usually doesn't wear like the other cyl parts.

 

[Taltruda]

For what it's worth, I contacted Aircraft Cylinders & Engines to ask if they could provide me any information on the engine, since I didn't build the plane, and the reply was simply "No."

--Ron

That’s some BS there.. what does it matter if you built the plane or were the original customer of that engine.. they built it, you are the owner.. they should be able to tell you what it is!

 

[fl-mike]

Is 25 BTC a normal timing for HC?
My 195 HP IOX-370 with 9.6:1 is 20.

 

[ronrapp]

That’s some BS there.. what does it matter if you built the plane or were the original customer of that engine.. they built it, you are the owner.. they should be able to tell you what it is!

I agree. But I don't suppose I can compel them to provide the information. Funny though, that the logbook entry they provided says to contact them for further information about the engine...

--Ron

 

[RV10inOz]

You need to know.....Not guess, and not trust anything/anyone. I am soon to write an educationally summary on a trashed engine from a major builder who nameplates 25 degrees on an angle valve IO390. 600 hours later and the thing is toast.

Details matter. If you do not know for sure, you don't know at all.

If it is an angle valve 360, then it is 20 degree timing. If it is parallel valve and indeed 195HP then the CR needs to be 9.6:1 or more, so once again 20 degree timing.

CAUTION: If you are fitting an electronic ignition to this (or any engine) set it up with more like 18 degrees due to the faster response time of the EI. there is no lag with electronics.

Hope that helps.

 

[IowaRV9Dreamer]

If it is an angle valve 360, then it is 20 degree timing.

Is this correct? My Beech has an IO-360-A2B, which is a 200hp angle valve engine. Both the data sheet and the Lycoming TCDS list 25 degrees for timing, 20 degrees is listed as an alternate timing. I’ve been running at 25 for 9 years, 600 hours with no issues. CHTs are very cool.

TCDS available here from the FAA

 

[ronrapp]

If it is an angle valve 360, then it is 20 degree timing. If it is parallel valve and indeed 195HP then the CR needs to be 9.6:1 or more, so once again 20 degree timing.

CAUTION: If you are fitting an electronic ignition to this (or any engine) set it up with more like 18 degrees due to the faster response time of the EI. there is no lag with electronics.

Hope that helps.

My engine is definitely parallel valve. And it has been retrofitted with electronic ignition.

I believe it started life as an O-360-C1A and was converted to fuel injection at some point. The overhaul entry calls it an IO-360-C1A, however the engine serial number suffix is -36, which indicates a carbureted powerplant.

There are things about this engine I do not know and probably will not. The "experimental" thing cuts both ways I suppose. Sad to know that the knowledge is out there in some overhaul shop's records and they simply won't share it.

Anyway, sorry about your engine woes.

--Ron