During some of Chris's "cranking" sessions, we decided to conduct a
compression check for grins and giggles.

Max compression was impressive, with 80-90 PSI noted. BUT. It was only
one chamber that did this on each rotor.

Rear Rotor had 80, 65,65 or so
Front had like 85,70,60 or so.

This is within book spec for differences, and this engine was assembled
over 18 mos ago (but kept oiled internally). Has never been run since
rebuild by us. To my knowledge, every rotor and seal was mic'd within
spec, and we were meticulous with assembly. We have made a point to coat
the inside of the intake or exhaust ports with MM or WD40 and manually
turn the engine by flywheel or prop. ALWAYS good sounding compression.
ALWAYS oil mist from the ports. All the apex seals are compressible.
visual inspection reveals no issues.

Did anyone else ever do a cold compression check prior to first start?
Should I not worry about these numbers unless they persist after we've
started the engine and blown/burned out the pickling?

Dave

My first rotary race car was an RX-2 with a junk yard engine. It had no compression at all and would only start if it was hot, or spun up with 24 volts or towed behind our van.

 

Then I broke the code on cold starting a rotary. One hot cup of coffee and motor oil squirted into each bore of the carb. Big cloud of smoke and the onlookers loved it. But it worked and race we did.

 

You cannot tell anything until the engine is broken in a minimum of 2 hours of 2,000 RPM or better. And, compression is checked hot, with the ignition disabled and the throttle wide open. The side seals will have too much clearance cold to do much sealing.

 

It is probably just fine. No worries.

 

Here is a thing I did on the "Nopistons list" for basic thinking on the rotary.

 

TUNING,

Remember that the crank in a rotary is turning 3 times as fast as the rotors. So each crank revolution gets you two power strokes in a two rotor engine.

Three situations control timing.

(A) timing is advanced to get highest cylinder pressure about 50 degrees after TDC. This is a function of the mechanical layout of the engine, to produce best fuel economy at low throttle settings, and best power at high throttle settings.

(B) In boosted engines, the controller will pull off advance whenever the Lambda sensor detects a detonation event. 

(C) under spooldown from speed (throttle closed, high manifold pressure (vacuum)) the controller may advance the timing up to 40 degrees. There is no load and the sparse mixture is difficult to light, you could shut off the fuel during this time but the cat temps would drop.

To get to the highest cylinder pressure at the correct crank angle the controller will move the timing all over the place. Because the burn rate changes for temperature and engine speed and throttle position.

You can imagine an over full chamber from high boost, and high temp from high outside air temp plus the boost heat, and that charge is going to burn very fast. So ignition advance may only be 10 degrees BTDC, Before Top Dead Center. So all operations may be between 10 degrees and 40 degrees.

These advance numbers may seem small for the lower numbers, but remember that the rotor is turning 1/3 as fast as the crank, and the timing is measured at the crank. So, even at high temps and high boost, the timing at the rotor might be 36 degrees BTDC (12 at the crank) so the rotary can operate at very high advance numbers (at the rotor), compared to a piston engine.

In a piston engine, the crank must turn 2 times to complete the 4 strokes of the Otto cycle. TDC is always at the top of the cylinder. BDC is always at the bottom of the cylinder. So, the first TDC is with both valves closed and a fuel air mixture compressed into a small space above the piston. Burns real fast. Not much chamber volume to pull heat out of the burning mixture, highest cylinder pressure needs to be about 18 degrees after TDC.

And you know the rest, another TDC with both intake and exhaust valves open a little during overlap and so on.

The rotary must turn the crank 3 times to complete the 4 strokes of the Otto cycle.

The rotary has two TDCs and two BDCs just like a piston engine. However, the two TDCs are in different places, and the two BDCs are in different places. This is difficult to grasp at first, but is basic to understanding this engine. And these events are in 4 locations in the engine.

TDC. This is known to all as it is the basis for ignition timing events. The rotor has a face against the plugs.
The fuel air mixture in the volume of the combustion chamber is at its smallest, the bore centers of the corner seals are on a line 90 degrees the pan mating surface. The plugs have fired and the mixture is burning.

BDC On the power stroke. The bores of the corner seals are parallel to the pan parting line. And this is an easy one because it is at the bottom of the engine. Chamber volume is the largest. Exhaust port is being exposed.

TDC Overlap. The rotor face is now on the exhaust side of the engine. The bores of the corner seals are on a line 90 degrees to the pan parting line. Chamber volume is the smallest. Intake and exhaust ports may be connected (overlap) just as a piston engine.

BDC intake stroke. At the top of the engine, the bores of the corner seals is again parallel with the pan parting line. Chamber volume is the largest. Chamber contains some volume of fuel air mixture.

And now you know.

If you tune with EGT, best power will be just rich of the highest EGT. So you start off well rich of best power and lean to peak EGT. So as soon as EGT starts down you have just passed best power. Sadly, best power will be just 30 to 50 degrees before peak EGT, and that may be too hot for the apex seals to last very long.

My 12A bridge ports always run below 1600 degrees, with best power at 1580 degrees. A/F will be about 12:1 at this EGT. That figure is not the best power the engine can produce. There is another 3 or 4 HP in the engine, but it can be very painful to go after it. For short bursts in a low gear for a second or so, it may be fine in an NA engine. Less so in a turbo engine. For a long run at top speed stay at least 100 degrees rich of peak EGT. EGT is tuned with fuel not timing. Although timing can change it. Timing is for pressure at a crank angle as above. Some builders tune for 1700 degrees, and that is fine with good oil cooling and stock seals.

I run carbon seals and ceramic seals. Ceramics will take anything temp wise.

There is your start on tuning.


Lynn E. Hanover