Compression ratio

[lutorm]

I've been learning as much as I can about aircooled VWs while rebuilding the Aerovee, and one thing that seems weird to me is that the manual specifies a 8.0:1 compression ratio when running 100LL. This seems very low. People regularly run 9.0 or 9.5 in cars with premium pump gas. Of course, the safe compression ratio depends on the cam in use and we also regularly run extended full throttle at relatively low RPM.

Maybe they just wanted to have real conservative detonation margins? But then the real weird thing is that they also specify 8.0 in the Turbo. If it's safe to run 8.0 at 40" MAP with the turbo (with no intercooler, no less), it should definitely be safe to go a lot higher on the CR when naturally aspirated.

Ignoring intake temp differences, cylinder pressure should scale roughly as MAP * CR^gamma where gamma is something like 1.2--1.4. So, if we can safely run 40*8.0^1.2 = 485"Hg cylinder pressure in the turbo, we should be able to run (485/30)^(1/1.2) = 10.2 naturally aspirated. (If we use gamma=1.4 instead, we get 9.8.)

Moreover, I just measured my combustion chamber volumes and concluded that the actual compression ratio is actually lower than what is indicated by the table in the Aerovee assembly manual. Those tables assume a 55cc chamber volume, but I'm getting it's more like 58cc, presumably because of the extra spark plug hole. This means that if you aim for 8.0:1 with that table, you actually get more like 7.7:1. (The cylinders also have about a 2cc spread in volume, so this also translates to a difference in compression ratio for the different cylinders of maybe 0.2. I'm in the process of grinding away material to even that out.)

Has anyone tried increasing their compression ratio?

[tljones42]

Actually, unless Sonex instructions have changed, 7:1 is specified for the turbo engine.

Tom Jones

[lutorm]

Actually, unless Sonex instructions have changed, 7:1 is specified for the turbo engine.

Tom Jones

I'm looking at http://www.aeroconversions.com/products/aerovee/turbo.html and it says 8.0:1 for avgas, as does the upgrade manual I downloaded maybe 6 months ago.

Perhaps the answer is: "For use with aviation gasoline 91/98 minimum grade conforming to ASTM D910."

ASTM D910 specifies a "Grade 91" avgas as well as 100 and 100LL, but it's not clear to me that this actually exists anywhere so that seems like a weird fuel to base your specs on.

[N190YX]

Recommend being cautious about considering increasing the compression ratio of an engine in an aircraft, which is required to run at 100% power some of the time and 65-75% power most of the time, unlike engines in vehicles. TCM Continental engines' highest compression ratio in a normally aspirated engine is 8.6 to 1 (in 260 horsepower versions of the IO-470) and these engines have ignition set at 20 degrees before top dead center. The 250 horsepower versions of the IO-470 engines have an 8 to 1 compression ratio and the ignition is set at 22 degrees before top dead center. Point being there are a lot of considerations to take into account when increasing the compression ratio of an aircraft engine.

[lutorm]

Yeah, it's a bit hard to tell with the airplane engines since they exist in so many variants that were certified for different fuels. I was just looking at the O-235 which exists with compressions between 6.5:1 and 9.75:1! (However, the ones with compression ratios at 7.0:1 and below were certified for 80/87 avgas, the ones with 8.0 and higher require 100LL.)

Moreover, the airplane engines have gigantic round combustion chambers without squish areas, and run at lower RPM than a VW, both of which lower the safe compression ratio.

I found this poll on CRs in VW airplane conversions: http://www.sonerai.net/smf/index.php?topic=1539.0. People were using everything between 7.5 and 9.5 on 100LL...

[tljones42]

Actually, unless Sonex instructions have changed, 7:1 is specified for the turbo engine.

Tom Jones

I'm looking at http://www.aeroconversions.com/products/aerovee/turbo.html and it says 8.0:1 for avgas, as does the upgrade manual I downloaded maybe 6 months ago.

Perhaps the answer is: "For use with aviation gasoline 91/98 minimum grade conforming to ASTM D910."

ASTM D910 specifies a "Grade 91" avgas as well as 100 and 100LL, but it's not clear to me that this actually exists anywhere so that seems like a weird fuel to base your specs on.

You are correct, I was wrong. However in reviewing my records I had an interesting exchange with Tech Support. I didn't have exactly the right mix of cylinder head spacers and the ones that I had were going to give me a slightly higher compression ratio than 7:1. In reply they told me that I should be fine if using 100LL and went on to say they had never tested auto gas in the turbo. That email was dated mid January 2016. Seems contradictory in retrospect to the section you pointed out.

Tom Jones