On Fri, March 23, 2007 2:13 pm, Dennis Haverlah wrote:
> What grade - regular, premium or super - would be the recommended auto
> fuel for the RX-8 Renesis engine with RD 1C and normal intake system? I
> designed the fuel system for auto fuel and have been ground running it on
> regular fuel.
>
> Thanks,
> Dennis H.

 

While the immunity to detonation is not total even in the NA (normally aspirated) rotary, it is

far more difficult to detonate than is a piston engine. This has to do with the very large surface area of the combustion chamber, and its low temperature. Flame front travel is slow to the point that some fuel near each apex seal does not burn, and you get the typical high

unburned hydrocarbons and slightly lower fuel efficiency, compared to a piston engine.

 

The higher compression is used to instill more energy into the mixture and improve mileage and HP. The improved compression ratio is little more than the equivalent of  a few pounds of turbo boost, when the street kids and drifting addicts are using 15 pounds to push out close to 500 HP.

 

Not much danger of detonating anything normally, but you can tune a bit rich right off idle up to cruise RPM. Avoid rapid throttle changes from idle and just off idle.

 

Keep timing around 20 to 22 degrees BTDC. Run split under 10 degrees if any at all.  

 

Keep trigger wires well away from the plug side of the engine, to avoid cross talk with the controller from high voltage secondary wires. The engine can survive a few bouts of detonation. A cross fire between housings (fires a housing 180 degrees too soon) will take the engine right off the mount. Sheared dowels, broken rear irons the whole shooting match.

 

A big throttle increase from off idle, looks the same as a big timing increase to the engine. Piston or rotary. The stock controller will get a hit from the knock sensor and take out 2 degrees of timing before the next housing fires. And then 2 more degrees for each additional hit. This is a big help. But if that feature is not available then the operator just needs to learn to be gentle with throttle movement speed from low revs.

 

So effective cylinder filling at low revs can be close to or even above 100%, so the effective compression is above 10:1 is it not? (that 10:1 is calculated from known volumes) so as the revs come up and actual cylinder filling begins to decline, the effective compression ratio starts down. Head space stays the same while the cylinder is getting shorter.

 

Once you read this and get it in your head once it will be clear for ever. Ignition timing is based on only one piece of information. Were (in crankshaft degrees) do we have to light this particular charge in order to arrive at the highest cylinder pressure, at the best mechanical advantage crank angle, for highest torque. That would be about 18 to 20 degrees ATDC for piston engines and about 50 degrees ATDC for rotaries.

 

Another feature to get wedged firmly into the thinking is that the timing callouts are at the crank, and the rotor (piston) is operating in slow motion. (One third crank speed) so the 20 degree timing of a rotary is the same as a 60 degree timing for a piston engine. So smokey

Yanuk was always killing himself trying to get more dwell time (piston at or near TDC for a longer time) and the rotary does that every time you start it up. So, little advance is needed to get the ideal torque output. There will still be poor burning near the apex seals, but it is a function of quenching temperatures rather than ignition advance.

 

But wait, if any of the parameters of the charge change, then the highest pressure will be in a different place. Yes, it will be all over the place, and Mazda moves it around for reasons other than ideal torque output. Less obvious is the slower burn rate of over lean mixtures, and way over rich mixtures, and again by percent of cylinder filling (changing effective compression ratio). So on an RX-8  fuel map/ignition map you might see nearly 40 degrees of advance on closed throttle high vacuum high rpm, like spooling down from high speed.

 

You are slowing so there is no throttle opening to speak of and the idle mixture is very lean and difficult to light at all, so the big lead and improved agitation and burning. When the throttle is opened, and manifold pressure is low, timing may be pulled back to 18 and 5 so the low RPM and good cylinder filling does not detonate the engine. Once rpm are built up, and cylinder filling and effective compression ratio has dropped, timing again advances to maintain ideal torque generation. Simple..........................

 

How does this help you?

 

The rotary at 10:1 acts like a piston engine at about 9.5:1. Not such a big deal at all.

 

I race on 87 octane with no alcohol and 24 degrees of advance and 9.5:1 rotors. Timing is always 24 to 27 degrees.

 

We did race for years with timing at 20-22 degrees with just slightly higher EGT but little difference in power. That 20-22 degrees is plenty up to 9,000 RPM.

 

I used 1 ounce per gallon of Redline synthetic 2 cycle oil. That is nearly twice the amount you need to run for 6,500 RPM.

 

The driver has managed to detonated my race engine by running along at idle in first gear

(a 1.96:1 ratio, like a short third in a street car) and adding throttle with out slipping the clutch.  

 

But I run on as usual.

 

Lynn E. Hanover