Compression Ratio Calcs

[mike.smith]

I'm rebuilding my AV 2.1. This time I decided to measured the actual CC's of the heads, and so have been calculating the barrel shims necessary to get the 8.5:1 ratio for av gas. When I built the engine the first time I had 2 shims on each barrel: a .045 and a .060 (that's what they measure with a micrometer), which is what I calculated using the AeroVee manual's chart. I always seemed to be under powered compared to other AV's and now I'm thinking my compression was too low.

I did the calculations tonight on my #1 cylinder after measuring the deck height and using the actual cc's of that head. I plugged everything in here:

http://www.csgnetwork.com/compcalc.html

I converted all measurements to metric, and used:

Bore = 92mm
Stroke = 82mm
Head Gasket Bore = 92mm
Compressed Head Gasket = 1.0 mm (* see below)
Combustion chamber volume = 59.2 cc
Piston Dome volume = 0 (flat top pistons)
Deck clearance = .89mm (** see below)

* Compressed head gasket: when I measured the thickness of my previous head gasket it was about 33% thinner, at 1.0 mm (a .060" gasket = 1.5mm).

** Deck clearance: without a barrel shim the deck height was -0.61 mm (.61 mm above the top of the cylinder). If I add a .060" (1.5mm) shim, then 1.5mm - 0.61mm = 0.89mm

Using all these numbers, I get a compression ratio of 8.595:1, which is about right.

If I add on another .045" (1.15 mm) shim (like I had originally) I get a deck height of 2.04 mm, and that would make the compression ratio only 7.86:1. My original pistons had a deck height of -0.04 (1.016mm), so adding a 1.5mm and 1.15mm shim would have been a deck height of 1.634, and my compression ratio was 8.1:1, which is still a ways from 8.5:1.

Please let me know if anyone sees any errors in my calculations. It appears that I should only use a single .060" (1.5mm) barrel shim.

Thanks,

[Gripdana]

According to the Areovee manual for AV gas they call for an 8.0:1 compression ratio. Does going .5 over recommended make a difference in performance or wear?

[SonexN76ET]

Mike,

While you have your engine apart calculating your compression ratio, you may also want to lap your valves in an effort to improve performance. I lapped mine the other day. I was surprised by the amount of carbon build up I had accumulated. I am hoping the valves will seal better and give me more power. I am putting things back together now.

It seems that you and I both experience many of the same issues. Just last week I took my cylinders off to try to recalculate my compression ratio as I had similar performance concerns.

Did you have any trouble getting your wrist pins out of the Pistons? Mine were stuck in pretty hard.

When I expressed concerns about performance Sonex suggested I try a different propeller. That may be my next experiment if the valve lapping doesn't do the trick.

Jake

[Darick]

Jake, what Prince prop are you running? Could you be more specific on the performance...climb or cruise?...what is your typical flying weight?
I bought a Prince p-tip because I read many good reviews, but am still 6 months or more from flying.

[gammaxy]

Mike,

I double checked your math and it seems correct, but I'm pretty sure you are aiming for the wrong compression ratio. Should be 8.0:1. Seems like your original settings were pretty close to correct. Why is your deck height so far outside Sonex's table now? Are you using different pistons?

I lapped mine the other day. I was surprised by the amount of carbon build up I had accumulated. I am hoping the valves will seal better and give me more power.

I don't really know, but intuitively expect you would need a pretty bad leak (you'd definitely be able to feel it when turning the propeller) before you'd notice a change in power. Lapping is probably a good idea anyway for longevity. I relapped an exhaust valve recently that I noticed poor compression on when turning it by hand, but never noticed an increase in power.

I believe moving from 7:1 to 8:1 compression ratio yields ~5% more power, so personally probably wouldn't be too concerned about being off by a tenth or so.

what Prince prop are you running?

I know this question was directed towards Jake, but I'll share my experience too.

I have a Prince P-Tip 54x46. I flew in a closed circuit at an air race at WOT and averaged 135mph at 3200rpm. On good smooth days it will go about 140. At ~1100 lbs on a 90 degree day I climb around 300-350fpm. At ~950lbs I get ~650fpm. I'd really like another 100fpm or so at gross.

This chart from Sonex makes me feel like I'm leaving a little under 5% of my power on the table running at 3200rpm:



I think there's a good chance my 54x46 is pitched a little too course and 54x44 would be a better fit. I don't know if anyone has tried 54x42, but I'm guessing it might be a pretty good climb prop.

[Darick]

Mine is a 55x44.
Oops, I meant 54x44.

[SonexN76ET]

My prop is a 54x44 Prince P tip. I asked Prince to slightly bias it for cruise. As such I see performance numbers similar to those Chris reported. I have been mostly concerned with low climb rates and rapid speed bleed off while doing steep turns. I decided to lap my valves after having my EGT numbers gain a wider temperature difference. One side was 1150 and the other stayed around 1300 at full throttle.

Jake

[mike.smith]

Mike,

I double checked your math and it seems correct, but I'm pretty sure you are aiming for the wrong compression ratio. Should be 8.0:1. Seems like your original settings were pretty close to correct. Why is your deck height so far outside Sonex's table now? Are you using different pistons?

Yes indeedy! That's what I get for doing math at 2:00 in the morning! I don't know why I had 8.5:1 in my head, but of course that's wrong. It should be 8.0:1. And yes, I have new cylinders/pistons. I had to replace the Nickasils that mushroomed at the top and did other bad things. So here is the correct math for my heads/cylinders/pistons:

=============================

* Compressed head gasket: when I measured the thickness of my previous head gasket it was about 33% thinner, at 1.0 mm (a .060" gasket = 1.5mm).

Cylinder #1

Bore = 92mm
Stroke = 82mm
Head Gasket Bore = 92mm
Compressed Head Gasket = 1.0 mm (* see above)
Combustion chamber volume = 59.2 cc
Piston Dome volume = 0 (flat top pistons)
Deck clearance = 1.64mm (** see below)
COMPRESSION RATIO = 8.1:1
** Deck clearance: without a barrel shim the deck height was -.61 mm (.61 mm above the top of the cylinder). If I add a .060" (1.5mm) shim, and a .030" (.75mm) shim then => 2.25mm - 0.61mm = 1.64mm

=========

Cylinder #2

Combustion chamber volume = 59.0 cc
Deck clearance = 1.72mm (** see below)
COMPRESSION RATIO = 8.07:1
** Deck clearance: without a barrel shim the deck height was -.53 mm. If I add a .060" (1.5mm) shim, and a .030" (.75mm) shim then => 2.25mm - 0.53mm = 1.72mm

=========

Cylinder #3

Same numbers as #1 cylinder

=========

Cylinder #4
(This cylinder has a chamber volume and deck height much different than the others, so the shims need to be different to get a similar compression ratio)

Combustion chamber volume = 62.2 cc
Deck clearance = 1.19mm (** see below)
COMPRESSION RATIO = 8.1:1
** Deck clearance: without a barrel shim the deck height was -.71 mm. If I add a .045" (1.15mm) shim, and a .030" (.75mm) shim then => 1.9mm - 0.71mm = 1.19mm

=====================

I know 8.1 is more than the 8.0, but if I bump up to .040 shims from the .030, then my compression ratios drop below 8.0.

[rizzz]

For what it's worth, I think you can set your compression ratio to 8.1 or higher if you want to for use with Avgas, at least according to the Great Plains manual, here's a scan of the compression ratio calculation page in the manual:

Off course given yours is an AeroVee, probably best to follow the AeroVee instructions...

[mike.smith]

Mike,

While you have your engine apart calculating your compression ratio, you may also want to lap your valves in an effort to improve performance.

I recently had new valves put in, and that was part of the process :-)