Can I run with different pitches on twins?

When I had my 350's compression checked, one motor showed lower compression by about 10-15lbs accross all cylinders than its sister. The lower compression engine runs 2gph less efficient at cruise than the other when synched on rpms, and the difference moves up to 5gph when secondaries jump in. The stamps on the props are matched as far as pitch and diameter, but what would happen if I reduced the pitch on my less efficient motor. The rpms would increase on the less efficient motor so I could back down the throttles a little compared to the other one, thus synching up the fuel flow. Is that a bad idea?

I think so.

I don't see an issue with doing this, other than time and $. But then again, i don't see an issue with not having "sync on fuel flow" either.
I'm in a similar postion, my STB engine has two lower compression readings, the pistons are original 1976 vintage. I would not consider 'fixing' the prop. I AM considering a top end job, however!

Harmonics may be another factor.
We sync to help reduce harmonic vibrations as well as balance the prop thrust Port/Stbd.
What is you guys take on that thought?

It'll pull to one side, but it'll work.

Jeff

fastjeff said:

It'll pull to one side, but it'll work.
Jeff

Click to expand...

He indicates that he'll increase RPM on the "less efficient" engine. Should keep the hull straight, but is why I mentioned Harmonics!

In some boats you can hear or even feel if the props are not turning the same speed. In light twin aircraft, the effect is even more pronounced. In my boat, I can't tell at all if they are synced or not by feel, one of the reasons I have digital tachs. I run long distances with one engine only, i'll troll with either/both; I just don't see any issue with running non-synced engines. My engines are widely spaced and I will see adverse yaw during a high power acceleration, if they are not putting out the same power. At speed the rudders will mask any differential thrust setting.
My advise is to the fix the problem child engine when you can, and don't mess with the prop.

Like Dave noted, the sensory inputs from the harmonics is hull dependent. I've run a few where you couldn't tell if the engines were synced or not and I've also ran ones were you could have sworn you were on a vibration table when sync was lost.

I've been in multi-engine turbojets where sync between the engines was lost - a very stressful environment is a kind description.

I've venture Dave's last suggestion would be significantly cheaper than the original concept. I'd even venture a guess as to a likely source of the difference - full advance on the ignition timing.

Mine is more auditory and you can just hear that sinusoidal beat up to about 2500 rpm. After that, you're on your own or the gauge

..."I've been in multi-engine turbojets where sync between the engines was lost - a very stressful environment..."

The Luftwaffe didi that intentionally during the London Blitz to terrify the population. The crews didn't care for it!

Jeff