Comments on compression video
 

13b NA streetport with S5 rotors. Engine was cold.

Comments from the experts?


Front http://www.youtube.com/watch?v=TyU3StdnJ9E

Rear http://www.youtube.com/watch?v=64CLx9lFdsU

What apex seals? Do you have hot start issues? How does it perform when running?

The low numbers would indicate the need for a new engine. But I've seen incredibly low cranking compression still perform well on the track.

Hurley 2mm race apex and Atkins solid corner seals. Don't really have hot start issues.

I have always felt power is pathetic, at least that's what 2 different Dynojets have said. Compared to other EP RX-7's (FC and FB) and RX-8's on track some can overtake me on a straight, I don't overtake them. I have managed 2nds out of this motor in National races wet and dry though.

It's got a brand new big money complete exhaust and I didn't get the gains I was expecting (did baseline before and after and had well known tuner tune with new exhaust) .

I am trying to determine if the reason is the motor is just a POS and not sealing, the porting is bad, whatever. The motor has never made good power and the builder and I had a falling out after the first time this motor saw the dyno. I should also mention it has ALWAYS spit up 1/2 qt during a race and it's got a 3/4" breather on the filler neck (side seal issue?)

Looks like you are saying the compression is low Chris in your opinion?

yes the compression numbers are definitely low especially for s5 rotors. The fact its about even front to back show that it is uniform problem and not a bad apex seal or something like wise. Depending on what porting is done to the engine could be an issue with the port actually coming in contact with the oil control ring area that would explain why you are getting oil pushed out of the motor. On a good motor even cold for s5 rotors you should see in the 95-120 ish compression numbers depending.

Yeah, it's definitely low. But cranking compression isn't always the whole story. I've built several engines with one piece carbon seals and they always have low cranking compression. However, the dynamic sealing at higher revs is good enough to make good power. So, if you had said you were using those seals and you were on par with your competitions power, the cranking compression numbers wouldn't surprise me.

From the description of power output and the fact that it's losing a good amount of oil, I'd say you definitely have a sealing issue. ROTARYCRAZY has a very good point about the port intruding into the oil seal path. Wouldn't cost much to tear it down, inspect it, and install all new seals. Properly fitting the side seals is where you're going to see the best improvements in sealing. Personal preference would be to ditch the solid corner seals as well. I've always preferred the OE corner seals, especially in a relatively low output engine like this. I'd also take a long, hard look at a 2-piece ceramic seal. They're expensive, but they're ideal for this type of engine. When you consider that they don't wear and can be reused in multiple engines and look at them as long term investment, the price begins to make more sense.

Was throttle open or closed when this test was done?

Most likely a combo of all or a few of these:

Too much of a "shine" to the plates and housings.
Too much clearence between side and corner seals.
Too much clearence in apex seal grooves.
Too much porting into the oil control track.
Crappy apex springs.

Educate yourself, open it up, fix it. You will save yourself a lot of headache. Or pay it done with a builder that has a proper resume.

our piston engine guy gave us a buildsheet, and it listed the clearance of the part before, the factory spec, the action taken, and the ending clearance.

doing that with a rotary is a really long spreadsheet, but it takes a lot of the guesswork/magic out.

Throttle was closed.

The last builder had a resume but I later found out also doesn't have a good reputation.

Before dumping the money into opening it up I want be 100% certain that is the only fix.

Most likelly so. If you are somewhat mechanically inclined its not all that hard to do it your self. I think o ring kit from atkins is 160 bucks or so if the apex seal are within spec you are ok to reuse them just make sure to lay them out to where they go back into the same place they came from

the compression tests look normal, the Rotor face should be around 30psi Per Face and a combined total of 90-100psi as said in the factory service manual

Its no more than 30 psi difference I'm not sure where are you getting the combinet total from.

There is a way to test it properly, those numbers are normal with a compression tester, you can do what i do

i remove the Valve on the end where you screw onto the spark plug hole and it will read Properly around 90ish psi per pulse, if you leave the valve in it will read lower at the numbers in your video shows, the valve can be remove with ease with a tire valve removeable tool

yeah sorry i reread that and i was going to edit it but it was too late, if you test it without holding the button open it will read about 90-100psi

Using that same kind of tester, how is it I manage to get 3 pulses over 90 psi, and with the button closed a build up of around 130... of all engines i release to a customer, usually before full break-in. (more then 50 miles but less then 200)..

but that's on stock seals/factory clearances... not sure how hurley seals stack up today's ALS and goopys.


I've also tested a motor with the same type of compression tester shown and got 60 psi on the two faces adjacent to a cracked apex seal. 30 Psi would tell me something is wrong.


this is with the check valve removed, and the pressure release open.

i got pulses of around 85 on my motor with the valve removed and button held and with it total build of around 110 on my S5, i got around 95-100K mileage on the motor now, now sure who rebuilt it but its lasted 7 years so far

#1, the throttle should be held open during testing. Closed can dramatically affect your results. Try re-testing with the throttle open. My EP motor reads 105-110 testing cold. It has ceramic seals. On the dyno it was showing 200 rwhp. I have made a few changes since then and I know my power is higher now, but haven't been back on the dyno to verify. My engine builder is Jesse Prather in Topeka, Ks.

2010 SCCA EP Hard Charger

I have heard about open/closed throttle and will retest with throttle open.

As to the "button" on the tester I have it taped down so it is "open". The schrader valve is still installed

this is exactly what i told him.. thank you

dont bother.. but....if your looking for higher compression #'s, puts lots of oil in the intake, hook up a battery charger so the motor will crank much faster and blow compressed air in the intake while cranking all at the same time.. you should get some good numbers then.. bad news is your motor is still kaput.. im sorry

Turns out the motor was junk. It never made power and now we know why.

It was a HACK job build by someone on this forum who claims to be an expert. Perhaps a thread with more details and pictures would spare a few others from wasting their money on him. He knows who he is and is welcome to contact me and discuss a fair resolution before I consider posting a long thread with pictures and more details.

Side seal gaps were way too big, after seeing .011" didn't even bother to measure the other two that were even larger.

The engine was built with the larger FD thrust bearings. That's great, but the front counter weight is from an FC. The bearing surface on the counter weight is not large enough in diameter for the FD bearings. About half of the FD bearing was riding off the bearing land and looks like it was actually making contact with the weight itself. Not good. He had cut down an end play spacer in an attempt to get the correct end play, but there should be no need for that if the parts are fit together correctly. The spacer he ended up with is much thinner than any of the OE spacers.

The porting, that speaks for itself.

This motor had less than 15 hours on it. What a waste.