Strange premixing observation
 

The subject is a 4-port engine with GSL-SE rotor housings and Atkins 2mm apex seals. Bridge ported on injection.

I normally have the idle set to around 1000-1100 for driving in traffic. I also normally premix whatever 2-cycle oil I can find at gas stations (usually Speedway brand, sometimes Pennzoil) at the rate of 8 ounces/5 gallons, which is about 1:80.

Yesterday, I was out and about after work and had to fuel up. The quart of oil I thought was in the car wasn't there and the station didn't have any. After much consideration, I bought a bottle of straight 30 (they have THIS but no 2-cycle?) figuring that the OEM system used engine oil anyway, and dirty oil at that.

Premixed this engine oil more heavily than the 2-cycle stuff. 8 gallons of fuel, and I'd put in over half of the bottle, so say 20 ounces. Call it 1:50.

Get back on the road, drive about a half-hour, get off the highway, and I find that my idle speed has gone UP to 1600.

Normally I'd say that the extra and/or different oil has shown a marked change in compression or reduction in friction, but I also remember my friend's 12A that started idling at 2000rpm when it blew a rotor...

Either way, I'm going to do a compression test today on a cold engine to see what it is. Last compression test I did was on Monday, and it was 100-100-100 front and 90-80-80 rear.

damn , well let us know how that turns out.

105-105-105 front, 95-90-90 rear.

So, huh, I dunno. Is it the more oil or the different oil? I feel leery about burning this much oil intentionally lest great rivers of carbon ensue on the rotors.

For certain, this is something I will continue to experiment with.

Could it be that the heavier concentration of oil in the fuel caused a leaner overall miixture, thus affecting idle speed?

Something I'd thought about, but unlikely. The oil is only displacing a small amount of fuel, not enough to make a significant air/fuel ratio change. At 1:80 then every cc of fuel is 98.75% gasoline, whereas at 1:50 it is 98%. I don't think MegaSquirt can even control fuel to that fine of a degree, given that I'm idling at something like 2ms pulsewidths and it can only control to the tenth of a millisecond.

Plus, I'm already running it as lean as it will consistently burn, and oil can burn too so it's not even displacing that much in terms of burnability. Heck, normal variances in ethanol content probably make a larger difference.

I have no idea what the actual air/fuel ratio at idle IS given all of the raw oxygen faking the wideband. Brap brap brap swings it wildly around the 15:1 range :)

The butt dyno says it feels more powerful, but the butt dyno is also a lying liar that lies, so who knows? The fuel will be long gone by the time I can make it to a dragstrip, and the sorry state of my transmission (plus what I suspect is a bent clutch fork. Again.) means that meaningful data is unlikely to be had anyway.

Subscribed. All the years ive been into rotary i never put much thought into oil lubrication effecting the afr. Learned something new today. Thanks for putting this out there.

This is very interesting to me as it brings back a memory from my 2 stroke racing kart days. Just like PeeJay, I was caught short one time and had to use 30 weight dinosaur oil in my kart at 50 to 1 instead of the normal Bel Ray (hot oil at that time). The kart was substantially quicker and responded better to throttle inputs but smoked like a mosquito killer truck. Pulled the motor down after the weekend and it was in great condition with no bore wear, but some traces of carbon.

We spoke to PCR Engineers (engine manufacturer) about this and they agreed that regular motor oil actually works better than synthetics in that it provides slightly better lubrication, but doesn't burn as cleanly which leads to available air/fuel mixture changes at idle and WOT, plus more carbon on the piston head in the quench area as that doesn't burn off cleanly.

The team I was racing for then performed a test where we set the carb up using our standard Bel Ray and then tested the effects of different oils as the mix oil. We ended up using the original Castrol Castor Bean Oil as it gave us the quickest laptimes and smoked the least, but it would settle in the gas very quickly and tended to gum up in the Tilletson carb pump chamber if you left it in the tank. It also gave us a bit more flexibility on carb burping as it ran cleaner when you picked back up on the throttle.

LOng story short, I believe that there is a substantial impact on performance due to the type of oil you use for pre-mixing and I always "try" to use Redline 2 Stroke Racing or Castrol 2 Stroke Racing oils in my racecar. Might be emotional, but the car always seems to work better and foul plugs less.

Eric

I think it is more of coincidence, something else is likely going on. I haven't really been premixng much recently because my engine burns oil I add it sometimes when I fill up and not really any specific ammount, my idle is unaffected by this.

Your going to lower the octane level with adding that much oil. Also straight 30 is of a higher viscosity than 2 cycle. Equal amounts of both should show higher compression with the 30 vs 2 cycle in back to back test. Hell Im curious as to what cooking oil will do in an external tank to feed the omp. :)

The idle "stabilized" at 1400. The blue smoke show when running cold is annoying, though, so I'm going to move on a different tack.

Kevin, the thing that gets me is that compression went up by 5-10psi.

Knocked off a couple tenths and gains a MPH. Best run 13.75 @ 101.27 with a 13.77 @ 101.24 backup. That's making my shifts at 9500 and going through the traps at ~8800 in 3rd. Not bad for a quiet exhaust, one pair of 680cc injectors, and a way overweight car.

My last run had a 1.90 60' time (as opposed to my normal 2.1s) but I borked both shifts and it came out as a depressing 14.2 at pathetic MPH.

The premix sauce was a combined 1:50 using 12 ounces of 30-weight (the rest of the bottle) and 6 ounces of MMO as a cleaner/scavenger in eight gallons of fuel. Yeah I was making runs with over a half tank in the car :)

Now, how to get this pig in the 12s without adding more power or cheating with race rubber.

Hmm this sounds like a nice cheap power adder for certain occasions. ;)

filling those gaps! Also I should comp test my engine, its been about a year... good reminder!

Maybe I should try playing with some straight 30W for premix. So far all ive done since ive been premixing was Amsoil Saber Synthetic premix at ~1-1.5oz /gallon (gets kinda pricey @ $12-14/qt bottle)

Yeah anyone else notice the jump in price on 2 stroke oil recently (last several months) or am I the only one?

Must be a Michigan thing. It's still $5/quart at Speedway like it's been for the past year or two.

Used to be that, when heading up your way, I'd wait until I got to Adrian before adding fuel to my car because fuel prices were cheaper in MI.

July... holy crap, dude. What was $3.60 or so here was $4.10-4.20 up there. And then the people on 23 decided to play rubbernecker instead of driving 85mph like I've come to expect from Michiganders...

nope. same thing here in Central FL. :(

I thought of this thread recently while I was reading Norbye's The Wankel Engine.

pg 41 (last page on chapter One "Design" subheading "Sealing"

*notes* are interruptions by me in an attempt to provide context.

Toyo Kogyo *Mazda* decided against a metal-to-metal interface *first Mazda production engines, carbon compound apex seals*, for which lubrication would be a vital requirement. They adopted a specially developed carbon compound as a new material for the apex seal because the carbon has self-lubricating properties and the presence of an oil film is not absolutely necessary. However, because the lubricant in the internal combustion engine also serves as a gas sealing agent, it became necessary to study the lubrication of the apex seal in terms of gas sealing efficiency in addition to the problem of friction and wear.
NSU pointed out that carbon seals must be at least 5 mm. (0.197 inch) thick to have sufficient strength. *note NSU refused to employ a sophisticated timing advance/retard system and instead decided on steel apex seals to withstand the minor knock in normal use* Metallic apex seals need be only 1.6mm. (0.063 inch) thick to be effective. Tests showed that the presence of a lubricant still had a favorable effect on the gas sealing performance, although the carbon apex seals formed a geometrically linear contact with the working surface.
There are three methods of supplying lubricating oil to the apex seals of the Wankel engine: one is to mix oil with the fuel mixture (as was done in early tests); the second is automatic metering of lubricating oil from the rotor side; and the third is an independent feed system by which oil is introduced into the intake ports. The oil-fuel mixture method would have worked if the necessary amount of oil had not been in constant proportion to the fuel flow. This method has been rejected as unsuitable for modern automobile engines. The second method also was rejected, because it is almost impossible to accurately control the amount of oil leakage from the side surfaces of the rotor under all engine operating conditions. Independent feed systems have been successfully developed, however, and they supply the necessary amount of oil to the intake port according to the engine operating conditions.

Anyways, the jist of it is that the rotary needs "lubrication" for the apex seals to seal properly. I am sure the worse the sealing capability, the more the lubrication aids the gas sealing of the apex seals.