Compression Ratio vs Manifold Pressure

Been contemplating this idea but I'm sure there's a reason I haven't seen it done:

Any thoughts about going a tad high with compression ratio, but then limiting manifold pressure accordingly- say for example mid 20's.

Would that give "full" power availability to a higher altitude or are there other considerations involved? cooling, efficiency, etc...

This makes me think of turboprop flying in the sense that you may need to watch the guages as you push the throttle forward at high power settings, but other than this workload item(unless an electronic throttle limiter is added), can this combo partially emulate normalization?

There is no such thing as a free lunch comes to mind...

Why?

Why would you want to limit power at lower altitudes?
Most show-offs operate near the runway.

what would be the performance penalty

PilotjohnS said:

Why would you want to limit power at lower altitudes?

Click to expand...

I guess the way I meant it was that, "Can an increased compression ratio compensate for that reduced manifold pressure at low altitude?"

But now that reduced MP would be protected well into the climb instead of having it start evaporating immediately into the climb...

I suspect just like the warbirds limiting MP to compensate for running 100LL instead of 130 fuel, you could do the same.

My neighbor Bill Harrelson has 10 to 1 pistons in his Lancair IV (the one that flew over both North and South Poles and did a 38 hour run from Guam to Jacksonville Florida). He has those way high pistons to gain engine efficiency, not power (other that the very few 380 gallons of gas takeoff runs). Every other takeoff he limits MP to gain detonation margin.

The problem with all this is data. For example if you run an IO-390 engine on Swift 94UL, how much do you need to limit MP on takeoff to have assurance you have adequate detonation margin under all conditions? It would be great for Lycoming to do these data runs (and perhaps they have) but I suspect the lawyers would go nuts if they ever suggested using any fuel other than 100LL or the not yet existing 100UL in these engines.

Carl

MP

Tall_Order said:

Been contemplating this idea but I'm sure there's a reason I haven't seen it done:

Any thoughts about going a tad high with compression ratio, but then limiting manifold pressure accordingly- say for example mid 20's.

Would that give "full" power availability to a higher altitude or are there other considerations involved? cooling, efficiency, etc...

This makes me think of turboprop flying in the sense that you may need to watch the guages as you push the throttle forward at high power settings, but other than this workload item(unless an electronic throttle limiter is added), can this combo partially emulate normalization?

There is no such thing as a free lunch comes to mind...

Click to expand...

Be careful with this assumption. The injection or carburetor system is set up to provide higher fuel flow at full throttle settings to prevent detonation, Increasing the CR requires even more fuel (at the same octane) to prevent detonation. Backing off on the throttle on a higher than designed CR ratio could actually CAUSE detonation depending on the specific combination of components in the equation.

Good idea!

Tall_Order said:

Been contemplating this idea but I'm sure there's a reason I haven't seen it done:

Any thoughts about going a tad high with compression ratio, but then limiting manifold pressure accordingly- say for example mid 20's.

Would that give "full" power availability to a higher altitude or are there other considerations involved? cooling, efficiency, etc...

This makes me think of turboprop flying in the sense that you may need to watch the guages as you push the throttle forward at high power settings, but other than this workload item(unless an electronic throttle limiter is added), can this combo partially emulate normalization?

There is no such thing as a free lunch comes to mind...

Click to expand...

Sure, that is a valid way of controlling the cylinder pressures, which is the main issue causing detonation and other life effecting engine issues. You can even calculate the pressures to give yourself some documented guidelines, which would be a good idea if you are going to be pushing limits. Automotive does it via valve timing. The big catch, is on a not getting the throttle correct on a hot day. Another manual.mental limit to be applied to work (as you stated).

Maybe an EFIS limit or red line?? JIC

I have considered this as well for future "improvement".

Using one of the new engine controllers like the EFII or the SDS, you can program in spark timing versus manifold pressure. If you want to run high compression pistons for good efficiency at altitude, just dial in a lot of timing retard as manifold pressure starts climbing above about 24". This will limit peak cylinder pressure which is, as noted above, a key contributor to detonation. By the time the flame front is fully involved in the combustion chamber the piston is further down and dropping and the peak pressure will be considerably lower.

In my IO360 with 8.7 compression, I'm running 25 base timing below 24" map (burning 91E10) and it applies 1/2/3/4 more degrees of retard as the MAP rises from 24 to 30".

I had thought about this exact thing. I have heard 12:1 pistons exist, so maybe limit manifold pressure to 24, and be very efficient. At the end of the day, we're gaining slight efficiency, but at the cost of much more complex operating technique needed.

When do we need full power from our engines? Take-off and go-arounds. Where should we be looking and putting our focus during take-off or go-around? OUTSIDE, and the manifold pressure gauge is inside. I'd hate to see someone swerve and hit a runway light, damaging their airplane because the we're focusing on the MP gauge during take-off. Would quickly negate any potential fuel savings.

The comparison has been drawn to turbine engines. They're often operated with 2 pilots. 1 pikot focusing on flying the plane while the other focusing on setting power. Also, turbines are very tolerant of over-torque. I'll pick the King Air 200 because it's commonly operated single pilot. Normal takeoff torque is 2230ft-lbs. The transient torque limit is 2750ft-lbs. That's a lot of oops factor. I suspect a piston engine controlling detonation with manifold pressure doesn't have a 23% buffer zone.

Thanks

Thanks for the input, all. It's great that everyone sees things like this from different angles. I'm undecided ATM, but there's lots of food for thought...
Cheers

MOST CARBURETORS AND FUEL INJECTION HAVE AN ENRICHMENT IN THE LAST 1/8 OF THROTTLE TRAVEL. TAKING OFF WITH LESS THAN FULL THROTTLE MAY CAUSE A LEAN CONDITION, AND WITH MODIFIED ENGINES, THE POTENTIAL FOR PRE IGNITION AND DETONATION. GRAB A 172 OR CHEROKEE, TAKE OFF FULL THROTTLE AND NOTE THE EGT'S. THEN REDUCE POWER, AND NOTE THE EGT RISE ALONG WITH THE CHT'S. WE NEED FUEL FOR COOLING AT THE LOWER AIRSPEEDS, UNTIL THERE IS ENOUGH COOLING AIR OVER THE FINS TO DO THE JOB!

DAR GARY