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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,

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?

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

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.

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?

SonexN76ET wrote: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.

Darick wrote: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.

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

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

gammaxy wrote: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.

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...

SonexN76ET wrote: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 :-)