Ok, now that I kept the throttle wide open, I have new (and higher) results to report. I am told by an expert piston engine builder that overlap will definitely lower the compression #s a bit. Regardless, here are the results:

Front: 7.2/7.1/7.1
Rear: 7.0/7.2/7.2

This checker actually seems very good---I cranked 5 times on each rotor, and all readings were within +/- 0.1 of each other.

I kinda figured that the old #s werent quite right, because of the way my beast starts, drives, and rips down the highway

 

what were the cranking rpm's?

 

259-260. With the correction that may drop it a bit, but that's pretty damn close to 250.

 

Ah, sounds much better I was *almost* worried for you after reading your first results Still want to go for a ride in that m-fer someday

 

I still have to disagree that overlap causes compression change. First, by experience...out of all the stockport, streetport, and the few halfbridge engines I've built, I see no change in compression readings except that which is explainable by varying rotorhousing quality...the determining factor.

And just think about it, as it relates to a rotary. You're measurement of compression at the plugholes has nothing to do with the exhaust/intake overlap on the other side of the engine. Compression is basically just a measure of an engine's ability to pump air, so as long as the intake ports are open, the larger depth and duration of those ports at the low cranking speed of 250rpm don't make diddly to how much air it can suck in and compress. The only way I'd buy that story is if the intake timing were extended so far that the port was still open by the time the other tip of the rotor were at the trailing plughole.

 

That makes a lot of sense Kevin. Perhaps the expert piston engine builder doesnt know **** about rotaries

 

say on a bridge port, the whole time the smaller port is open, so is the exhaust. that means all that air is just going back out the exhaust port. so there is that much less in the combustion chamber at compression

when they are overlapping less, there is a much stronger pull on the incoming air, meaning more air comes in, which = more compression

 

There are no"pulses" to speak of at such low rpm's. IT is turning so slowly that flow characteristics don't come into play.

Regardless of the theory behind it, my experience in practice hasn't showed any significant change in compression expectancies of ported motors.

 

So Kev, what is easiest way to interpret the #s, in your experience? Ie, strong motor, average motor, weak motor, etc.

 

^
exactly what i was thinking.


i'm not even sure why this is an argument... the compression side is totally opposite the intake/exhaust side.


anything below 6.5 (~95psi) is getting weak. 7.0 (~100psi) is ideal, 8.0 (~114psi) is awesome, 8.5 (120psi) is perfect

 

would be interesting to see how compression changes over the break-in period

 

while we're at it.. whats your vacuum reading and how are those interpreted?

 

WOW! Much better results! Good to see that throttle was the problem.

 

Compression results above 100psi are good, above 110 strong, above 115 great, above 120 excellent...as good as it gets. I have never seen an engine **tested within proper perameters** exceed 125psi.

Vacuum readings CAN be affected greatly by porting, and also by compression. For instance...a stock port motor making perfect compression might pull 18inhg at idle. Keep in mind idle rpm and throttle plate setting affect vacuum as well....you can make an engine pull more vacuum by bumping idle up a hair to 1100. Anyway, a weak stockport engine with low compression might only pull 15" of vacuum...the chambers arent sealing as well, arent creating as much vacuum (think of it as anti-compression at that stage of the engine's rotation, still generated by the sealing ability of the internals) and will continue to drop slightly with compression.

A streetport motor usually pulls 2-5" less vacuum at idle than a stock port motor of equivalent health...depends on the shape and size of the porting, and how much overlap there is designed in. Overlap will reduce vacuum, not compression. halfbridges and bridges even more so, because there is more overlap...the vacuum being formed is partially satisfied by the left over exhaust gas still present when the intake port opens (defined as overlap). I see about 8-10" of vacum at 1100 idle with my half bridge. But I also tested 115-118psi last week on my halfbridge with 600 hard miles of breakin/seating since the build. This supports my past experience that porting doesnt have a large impact on compression.

IF you take that reasoning a step further, this is why heavily ported engines (even large streetports) idle rough and smoke at idle. The overlap makes for a dirty intake charge at low rpm, causing misfiring and rough idle, and a rich condition due to the lack of fresh air to mate with the injected volume of fuel. There is less vacuum because of the overlap, and on FD injection systems this means the map sensor sees a higher absolute pressure, making it think you're applying some throttle and it richens the mix even more than it otherwise would have been.

 

kevin,

seems you prefer "aggressive" break-in? what is your general procedure?

I have one used housing (about 8k miles on it, exc shape) and one new one, will I see a significant diff in compression?

 

My breakin depends on the use of the engine as well as the parts used. For standard used housing, atkins seal customer motors, 1-2k mild breakin is preferrable, and compression and low end suffer a bit to begin with. For a new housing motor breakin is 500 easy miles and 500 slightly harder miles, and then it's done all it will ever do.

For a motor I build for myself, let it run a half hour in the driveway, drive about 10 miles up the road to be sure nothing is out of whack, and then let 'er rip. Mainly because hey, if I break my own ****, I can fix it quickly and cheaply so it doesnt matter. Plus it gives me a chance to evaluate stuff that I normally couldnt/wouldnt on a paying customer motor.

An 8k mile used housing is just as good as a new in box one...both should make 120psi or more after a 1k mile breakin.

 

Kevin I know you have a ton of experiance, but I'm having a hard time agreeing with you about the overlap not lowering compression. Sure compression is measured at the plug area but still air has to get there first to be compressed. If your engine has overlap, then some air will be drawn out the exhaust port before it closes, this in turn should lower the amount of air that gets drawn in to be compressed at the plug area. Less air should lower the compression numbers.

 

I have a heavily streetported motor with lots of (probably too much) overlap. It is the only motor I have ever ported, so I had to start somewhere, lol.

at 900 rpm, I am pulling about -300 to -340 on my pfc commander, and show 10 to 12 inches of mercury on my defi gauge. These are pretty low #s for a stock ported motor, but I always knew the motor was fine, as these compression #s verified. 7.2 bar is 104 psi......if I had used new rotor housings the #s would have been quite a bit higher.

Rich

 

On a rotary engine porting doesn't affect the compression numbers at all. I have tested/built all types of these motors over the last 10+ years and can verify this. As a matter of fact, I just did a 3rd gen bridgeport that had readings of all mid 8s even at my elevation ( vacuum readings of around 8-10 @ 1300rpms ). Not to say that low 7s are bad or anything and actually your pulses look nice and even so not to worry. Other than a bad build there are only a couple of things that affect compression numbers:

RPM @ 250 - every 50 rpms in either direction and you will lose about 0.75 kg/cm2
A cold motor will read about 0.4 kg/cm2 higher than a hot motor
Elevation - every 2000 ft you lose about 0.50 kg/cm2

Anyone that says different is just blowing smoke and have never actually gotten any results ( which unfortunately is alot of people on here ).

My digital tester is about 6 years old now and I haven't had any problems at all with it....knocks on wood!!!

 

I get about 9-10 inches of vacuum at idle/ 900 RPM. Idle is solid but I've got a pretty big street port

 

 

great info, thanks

Rich

 

My engine pulls 16 in Hg at idle, so by the info provided that is good correct?

 

16 is normal for a stock port motor with no leaks, but on the lower end of what I'd expect. I'd say if you check compression you are in the 90-105psi range.

 

GoodfellaFD3S: clear out your PM box