It sounds like a 320 "BUMPED" is a good tradeoff to 360 for a light FP RV4. Does anyone know of a fact based limit for compression ratio and still use auto fuel. let's talk in terms pump numbers like 87/89/91/93.
I have 8.7:1 on my IO360, running 91E10 for the last 60-ish hours. During my testing I tried hard to induce detonation just to find out where and if that would start, and only one time was I successful in doing so, and it was a case of beating on the engine hard to make it happen - not something you would typically do without knowing it.
If you have full engine monitoring and 8.5:1, you are not going to get it to detonate on 91E10 without at least a couple parameters being well outside the normal operating range, IMHO.
Lycoming detonation studies would seem to indicate otherwise with timing at 24 degrees BTDC.
I am wondering how you are measuring detonation? Are you able to hear it above the exhaust noise?
I have 8.7:1 on my IO360, running 91E10 for the last 60-ish hours. During my testing I tried hard to induce detonation just to find out where and if that would start, and only one time was I successful in doing so, and it was a case of beating on the engine hard to make it happen - not something you would typically do without knowing it.
So what were the operating conditions which would induce detonation? And that's a parallel valve IO-360?
(Note, there is some decent data for the angle valve, but I'd love to see a parallel valve detonation study.)
No, I was watching the CHT's like a hawk and saw two cylinders take off like a rocket, I made the logical assumption that I was seeing early detonation. I've got my timing set at 25 BTDC.
And yes, this is a parallel valve IO360, Bendix injection with balanced nozzles, Superior cold-air sump, James cowl and plenum, CS prop, one Slick Mag and one Lightspeed Plasma I.
And what is the fuel pump situation, are you using standard engine driven pump with electric pump in series?
I suspect new mechanical pumps do use ethanol compatible materials but no manufacturer will verify it.
What you have seen must have been very light detonation. I have seen one totally destroyed engine with a hole in one piston and some more with damaged pistons and connection rod bearings. It just took 2 seconds from the first indication of detonation until reaching the kill switch. This was on a well-equipped engine test bench. I would never ever experiment with detonation margins in the air.
With some programmable EIs like the CPI, you have the flexibility of easily varying ignition timing for octane used to get best performance possible.
This.
Also, EFI and knock sensing and the advancing engine management technology that is starting to migrate from the automotive world into the aviation world.
The new cars out there are often 10:1 or higher compression, new Lexus V8 is 12.3. Smart electronics make it possible--------that and more effective cooling ability.
When this thread started, I wondered if anyone would get into the cooling aspect of the increased compression-----guess I need to open that door.
Would you believe that some cars are now running 14 to 1 CRs on regular pump gas. My Suzuki bike is 12.8 to 1 and also happily runs on 87 octane with no knock sensors and port injection. It's all in the chamber designs, valve timing and computer controlled ignition to make it happen safely. We can do some of the same things with Lycomings at least on the ignition fronts with EIs.
Liquid cooling (better temperature control), variable valve timing, small(er) cylinder bore sizes, and better intake charge distribution (4 valve cylinders help here) are some of the things beyond knock sensors that help modern auto engines survive with such high compression ratios. For this reason, I urge caution to those who believe that the compression on a Lycoming can be taken up to automotive levels and still be run safely, especially on MoGas.
Skylor
RV-8
I don't think anyone here thinks they'll be running 14 to 1 or anything close to that in a Lycoming on Mogas. I earlier suggested 8.5 was a good target. Even then you won't be running 25 degrees of timing at SL WOT. The thing is, programmable EI will allow you to sacrifice only a little power at SL WOT while still giving you decent cruise power and economy at altitude. Fixed timing with mags won't allow you to do that as well.
I have operated the parallel valve IO-540 in the RV-10 with 93 octane NON-Ethanol car gas for about 2000 gallons with 9.0:1 compression ratio and electronic ignition varying from about 20 deg BTDC at full power (2700 RPM) to around 31 degrees timing at 2400 RPM full throttle and ~10,000 DA.