Counter Weighted Crankshaft vs Non Counter Weighted

After the Superior XP-400/382 buyback I'm now looking at getting an IO375 engine for my RV8 with the fastback conversion. Among my many options, I have the choice of whether to get a counterweighted or non counterweighted crankshaft. I'm also looking at putting on a 74HRT composite propellor and also 9.7:1 high compression pistons. I do understand that the issue of vibration has a huge number of variables and is quite dynamic and decidedly not simple however is anyone out there able to shed some light on some information that may be able to help me make a decision? I am not an engineer so if replies are kept relatively simple, I would appreciate it. My building supervisor down (who is an engineer) here has the same engine (375, high compression with a 200RV prop) in his RV8 and it's generally as smooth as silk. The sales guy at Aerosport seems to think that as long as you stay under 9.0:1 compression, the non counterweighted crankshaft would be OK. Any thoughts on this issue would be appreciated.

You might be over-thinking this.
I started by building a Turbocharged Mazda Rotary so I could out-do any Lycoming. I certainly did that, with an airspeed that indicated over 220 HP.
However, living with this engine was not so enjoyable for me.
I settled on an IVO-360 out of a wrecked Brantley helicopter. An 8.25:1 parallel valve vertical engine with a solid crank that restricted me to a fixed pitch prop. it was cheap, and only had 167 SMOH. I prefer the conical engine mount (I fly IAC aerobatics in it sometimes)
I went with the Superior front induction sump, Airflow Performance fuel injection and a Catto 3 blade prop. Wood core props don't have RPM restrictions.
It's been simple & awesome. I know more HP and a constant speed prop give a shorter take off roll and steeper climb, but I stopped flying at full throttle all the time. Now I enjoy the simplicity and reliability. Cross country flight is optimized, and I espouse the 'lighter is better' concept. No interior except the seat cushions, light weight starter & alternator etc.
I understand the counterweight crankshaft added several pound to the weight of the engine.
Just a thought...

downundarv8builder said:

After the Superior XP-400/382 buyback I'm now looking at getting an IO375 engine for my RV8 with the fastback conversion. Among my many options, I have the choice of whether to get a counterweighted or non counterweighted crankshaft. I'm also looking at putting on a 74HRT composite propellor and also 9.7:1 high compression pistons. I do understand that the issue of vibration has a huge number of variables and is quite dynamic and decidedly not simple however is anyone out there able to shed some light on some information that may be able to help me make a decision? I am not an engineer so if replies are kept relatively simple, I would appreciate it. My building supervisor down (who is an engineer) here has the same engine (375, high compression with a 200RV prop) in his RV8 and it's generally as smooth as silk. The sales guy at Aerosport seems to think that as long as you stay under 9.0:1 compression, the non counterweighted crankshaft would be OK. Any thoughts on this issue would be appreciated.

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You didn't mention ignition timing, which will also be a factor.

Re your friend's 375, vibration as perceived from the cockpit is a very poor indicator....near useless for higher order torsionals.

I'm aware of a non-pendulum 360/74RT combination which, with high compression and a non-jumpered P-mag, broke the blade root ferrules. I'm talking missing chunks, cracks, and fretting. The owner noticed it throwing some grease and had it torn down. Good thing he did.

You should pay close attention if the sales guy at Aerosport says not more than 9:1 for a bare crank.

As a side note, I really don't understand the fascination with hot-rodding Lycomings and putting them in cross country airplanes. Acro, and racing, sure, it makes sense. The rest of us spend something like 5% of our time in climb, 5% in descent, and 90% in cruise. Trading lifespan and reliability for a little better climb rate during 5% of the flying doesn't make a lot of sense, in particular given the cold hard fact that a high percentage of builders can't cool the additional power for any useful extended period.

My 2 Cents

I have an IO-375 bought from Aerosport in my RV-7. Mine is the (nominally) 8:1 compression version. At that time, there was no "counterweighted" crank available for this engine. Hartzell vibration tested their older prop on this variant back some years before I bought mine (2010) and found that this combo had basically the same characteristics as a similarly configured 360. It's the main reason I felt fairly comfortable purchasing my 375. Even back then though, it was recommended that you use a composite prop if you went with the 9.6:1 compression variant. The 9.6:1 configuration was not tested if I recall.

I have about 420 hours on my 375 now. It's been a good, trouble free engine (so far) but I will have to say I have been a bit underwhelmed by it's apparent power output. At the time I bought mine, Aerosport was advertising my variant as producing 195 HP, although I believe this was an estimate rather than a number from dyno testing. I believe the Titan Engines version of my configuration (which they advertise as a 370) is rated as 185 HP. This number is more consistent with the observed performance of my airplane.

DanH said:

SNIP
As a side note, I really don't understand the fascination with hot-rodding Lycomings and putting them in cross country airplanes. Acro, and racing, sure, it makes sense. The rest of us spend something like 5% of our time in climb, 5% in descent, and 90% in cruise. Trading lifespan and reliability for a little better climb rate during 5% of the flying doesn't make a lot of sense, in particular given the cold hard fact that a high percentage of builders can't cool the additional power for any useful extended period.

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Dan - exactly correct for the vast majority of builders. Do what you can to spread such common sense.

Carl

As a side note, I really don't understand the fascination with hot-rodding Lycomings and putting them in cross country airplanes. Acro, and racing, sure, it makes sense. The rest of us spend something like 5% of our time in climb, 5% in descent, and 90% in cruise. Trading lifespan and reliability for a little better climb rate during 5% of the flying doesn't make a lot of sense, in particular given the cold hard fact that a high percentage of builders can't cool the additional power for any useful extended period.

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That is what I was thinking when I ordered 8:1 CR for my Titan 370. Also might be able to burn mogas.5

9 to 1

I completely agree with Dan. There is no reason to put more than 9 to 1 pistons in your RV. Higher CR will stress more than just the crankshaft and propeller. Your starter and battery will also be stressed to the max.

You will be happy with the performance of a stock 180hp Lycoming or clone and
you and your passenger will be safer too!

DanH said:

I really don't understand the fascination with hot-rodding Lycomings and putting them in cross country airplanes.
.... Trading lifespan and reliability for a little better climb rate during 5% of the flying doesn't make a lot of sense,.....

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Hmmmm.....

Now you're making me think.

After owning a Bonanza with an IO-550, rarely flown anywhere near gross weight, I've come to really like gobs of horsepower.

Though I'm nowhere near ready to get an engine, I'm already considering the options.
Maybe 180 hp is enough in an airplane as light as an RV-7.
Lots of time to think, and lots of rivets to buck before making a decision.