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Maybe Bad News: Maybe I just have PTSD, but I might have crap compression numbers. Let me explain and then see what you smart people here think...
I borrowed a buddies Rotary Compression Tester so I could test my engine compression prior to leaving for the Dyno. Plugged it into rear rotor and it showed 23 23 23 corrected. No way that's right considering my car fires right up and I drove it the day before with zero issues. I replaced the battery in the tester, followed the reset/calibration steps, same issue. I called Bryan and he said his tester did that too a while back and the sending unit was likely bad so he got a new one and it was reading right. So I'm thinking bad tester. No time to do it with a piston motor so off I went to dyno. Nelson would have preferred compression numbers given what happened last time but seeing at this rebuild (only new seals) had ~300 miles on it, compression likely should be OK.
This session was less about tuning and more about finishing off the previous tune (since we only swapped seals) and validating the data was still good. It was also the hottest damn day of the year. It was 111 with the heat index (highest in the country on Monday). In the shop, the dyno was showing 95 degrees with about 60% humidity. Literally the worst day to make power. We did 6 runs in increments to get up to ~7k rpm. The dyno's "corrected" numbers at 6,900 RPM were 365hp and 297tq which track almost identical to my good dyno session from last year. Aside from making some changes to boost control, Nelson said the data looked good. One thing we noticed is the car was getting harder to start. It would take 8-10 seconds to start, sometimes needing half throttle. We did about 10 start, stop, change some settings, start again exercises but that didn't help much. This had Nelson a little concerned given our experience last time. I think I have a reason for this (more on that later). The shop was about to close and the dyno operator got a family emergency phone call and had to leave. Nelson said we were pretty much done and that without compression numbers he didn't wanna do any more pulls to get from 7k-8.5k. He said the tune was pretty much done and that compression numbers and some street data logs for things like downshift response were things he could tune later without a dyno but he said this tune was good otherwise. I fired the car up, drove her 40 miles home.
When I got home, I used a piston tester and did some tests. Cold was about 35 psi on 3 even pulses on Rear Rotor. At this point, I'm kinda freaking out. I pulled out the Rotary Tester again to see if it would work and I still got 20 20 20 on what it showed at 197 rpm starting which seems low. I jumped my battery to my daily driver and ran the test again with the rotary tester just to get RPM reading and it was showing 239rpm. I think my hard start issue on the dyno was my battery not having the juice for all that start and stop troubleshooting. It's a Odyssey PC925 AGM that is 330CCA and 900 PHCA. Not a huge battery.
I also realized I didn't pull the Schrader valve from my piston tester. I ran the same test again and was getting ~58-60psi warm on rear rotor and was about 60psi cold. Front rotor is a real bitch to get to because of how my harness and dual oil cooler plumbing was done. I was able to test front rotor via trailing this morning cold and got 60psi on 3 even pulses. I wasn't even going to attempt warm as my hand would burn trying to get work in that right space to get the plug out and piston tester in. At this point, I think a few things:
1. The piston tester in terms of psi accuracy is mediocre at best
2. I probably have no issues. The odds of me having 6 bad seals and same "bad" numbers across 2 rotors has to be slim
3. My battery doesn't have enough juice for repeated start and stop or even enough for a good start
I also know I have the 1.2KW starter (reman). I just ordered the 2KW high power one from Banzai this morning. I'm hoping having more power in the starter will mean the car starts better as well. I may also look into replacing the battery. I know there is debate on CCA vs PHCA as a metric for sizing a battery. Maybe my battery can't power the 2KW any better than 1.2KW but I'm hoping the starter will just have more kick and frankly, it's likely a needed upgrade anyways.
The Rotary Compression testers are sold out so I can't even order one of those. So...am I crazy? Is my car OK? Should I be worried? Bryan is on vacation so I'll sync with him when he gets back for sanity as well....
Attached is a photo of my dyno results and the non-screenshot is my good one from last year for comparison.
First off, you are still pretty early in the break-in process, the engine will keep building compression. The compression at 2000+ RPM is probably what it will be but the low speed compression is what takes time to build up.
A piston tester IMHO is only good for showing if you have even compression pulses. You can use it to spot a broken apex seal or a stuck side seal, that's about it. To know the relative health of a rotary engine you have to use a rotary tester and have the numbers corrected to 250 RPM.
Rotary engines by their nature have bad low-RPM compression. The long combustion chamber and lack of centrifugal force to press seals outwards contribute. If you have a weak battery, weak starter, or combination of the two you will have problems. My RX-8 when I got it had compression numbers in the 3's (kg/cm2) and the only way I could get it to start was when it was stone cold with a 15v jump pack and the upgraded starter. Once warm it would not start. Broken side seals will do that .
All that said, the upgraded starter is a solid plan. If you have a relocated battery I would make sure it's as good as it can be, 90% of relocated batteries have electrical problems. You should be seeing 12.4-12.5v at the battery before cranking and should have 220rpm cranking speed to get a good solid start.
@DaleClark so what I think you're saying is my engine is probably fine and just needs some more miles? I'm doing a charity track day event for ALS Research at the end of the month (if anyone wants to make a tax deductible donation, message me.) and I was planning to take the FD. Not sure Ill be able to add more than a 200 miles between now and then though.
My Haltech gauge shows 13.1V before I start the car when it's fully charged. At the alternator, it shows 13.8V. I never looked at the cranking RPM, but I guess I can check my Haltech gauge as my OEM tach is def off a few hundred RPM.
Yep. You really can't gauge compression on a new engine until about 1000 miles or so.
Try and get as many street miles as you can on it with varying load and RPM.
I'd probably start with the upgraded starter then start looking at battery and wiring if you are having low cranking speed.
Dale
Yeh. I'm wondering if the battery is just too small on the CCA/PHCA rating? I ran all new 1 gauge welding cable for my power from the rear bins to the OEM battery harness/connector in the engine bay.
Yep. You really can't gauge compression on a new engine until about 1000 miles or so.
Dale
Assuming I can put another 200 miles on it in the next two weeks (500 since rebuild), do you think taking it to the track is a bad idea? I know there are lots of schools of thought about proper break in. The motor, minus the seals, would have over 1000 miles. But since the new seals went in, I realistically could get to 500 miles at best before July 1. I don't commute for work so I have to find time to literally just go drive around for an hour or two which is easier said than done with the kids...
The break in is done before going to the dyno to push it hard. Its too late to think about break in imo. You only can hope thats what its going on. But if it starts after a drive , it should be ok.
The break in is done before going to the dyno to push it hard. Its too late to think about break in imo. You only can hope thats what its going on. But if it starts after a drive , it should be ok.
We did 6 pulls in 4th incrementally to 7k rpm. You can see in photo where each pull ended. Honestly if the dyno session broke anything, I’d be pretty irritated because during a regular driving, I’ll occasionally push varying rpm’s probably to 6.5krpm anyway. That’s not even driving it hard.
We tested my buddies FD and his tester (which is the one I borrowed) came back at 81 77 76 warm. He said a few months ago it was testing in the 100's. We're both now hoping the tester is just off by 20-30psi. If that is true, then my car is more like 80-90psi which for a street port I think is good.
I have been messaging the guy at Rotary Compression Tester and he asked us to send the tester back so he can test it and fix it (if needed). He said he's only seen a handful that read low over the thousands he's sold but at least it has happened. For my sake, he hopes his tester is busted...
I've very seldom had luck with compression testers. Piston ones or rotary specific. My rotary tester read very low, and read lower with the engine warm than it did when it was cold. I have the digital rotary tester that most people have, and also the other rotary tester you hook up to a computer and shows the pulses, but that one has a lot of setup steps so I don't usually use it. I've been more successful getting accurate results with that one though.
Assuming I can put another 200 miles on it in the next two weeks (500 since rebuild), do you think taking it to the track is a bad idea? I know there are lots of schools of thought about proper break in. The motor, minus the seals, would have over 1000 miles. But since the new seals went in, I realistically could get to 500 miles at best before July 1. I don't commute for work so I have to find time to literally just go drive around for an hour or two which is easier said than done with the kids...
Hello! My quick contribution - a major point of the break in period is ensure that the seals are "seated" properly. This occurs over the 1,000 mile period, with compression rising as the seals properly seat themselves.
IMO - yes, it is a bad idea to track the car until this is completed. But you already dyno'd the car without doing this, so you can decide. When I am on track, it's difficult for me to not hit boost and run the car a bit. This is what I would avoid until your seals are properly broken in. Going up and down in the RPM band I don't think is a problem, but the boost part is.
And those compression numbers are low. I doubt they will stay that low once the seals are broken in, but know that a street ported engine should be higher. For comparison, my 3-rotor has a medium street port and 3 MM seals (will always read lower on compression) and it still shows above 100 PSI.
For reference, this is a video of a warm start on my car. It sounds like the general consensus is to not track it. I could turn the boost down, but to @David Hayes point, I can't NOT be boosting on the track. Bryan gets back from vacation Monday so I'll give him another call to get his thoughts now that we have some compression numbers (accuracy is still TBD). Driving the on the street, I probably am mostly in the 3-7psi boost range and occasionally bump higher but its hard to get to double digit boost on the street without being obnoxiously loud or unsafe.
I'm still convinced those numbers on the tester are wrong. I forgot to add above that I rested mine with his tester after it stopped reading all 20's and it showed me at 51 56 55 and 49 50 45. Now the same tester was used to test my previous iRotary blown engine and it showed 55 59 48 and 48 51 24 and that car wouldn't start at all. I also don't think I'd make the power I did either at the dyno.
Doing the math, if I can drive 50 miles a day for the next 14 days, I can get to 1000 miles on this engine before the track day...
That doesn't sound like a blown engine. I think you need to put some miles on the car and continue to test the compression. I think you will see it increase over time.
I got my new high power starter from Banzai Racing installed. Cold starts are tad faster and I feel like warm starts are better for sure. Below are two videos. One is cold start from the car sitting over night. The warm start is after about a 38 mile (mostly highway drive). I came home, turned off the car, went inside for maybe 10 minute and came back out to start it. The speed of the starter def. sounds like other cars I've heard.
Thanks for the offer. We sent my buddies tester back to Rotary Compression Tester and he said he'd test is ASAP against his other ones and a Mazda to see if there is a discrepancy. He said he'd overnight it back to us once he fixes so hopefully in a week. If it will take any longer, I'll hit you up.
Yes it is good to break in an engine with new seals. However, a new engine, properly built, will not start off with compression in the 50's and then magically improve to 100's. If your newly rebuilt engine is not at least in the nineties, something is wrong. If it starts well, I'm betting it's just a bad sensor on the compression tester.
Jus heard back from Rotary Compression Tester. The sensor is off by about 20-22psi. That would put my car in the low 70's with the old starter. Going to retest when I get the tool back with my new starter to see where I land...
Hopefully the combination of starter and new sensor show you in the 90's. Anything lower and the engine isn't built right. Break in is to bed things in, not build 20-30 psi of compression.
Got the Rotary Compression Tester back and did another warm test. I'm getting 67 67 62 on my rear rotor. I didn't bother testing the front as I suspect it will be the same (it's also major PITA to get to with the way some of my plumbing is run). That being said, the car starts fine cold and starts pretty good hot so I don't know how to read this. If you didn't tell me compression was bad, I'd never know. Car idles and drives fine. Bryan at Rotorsports told me to put more miles on and test at 1000 and then we can discuss next steps. He stands by his motor.
Since there is always something, I drove the car today and a few times I pressed the clutch in and it got stuck/didnt return. I had to manually pull it with my foot. A few times when I had pressed in, I'd let it out ever so slightly and the car would stall or lurch forward because it was trying engage the clutch way too early. I suspect its the master/slave clutch cylinders. I didn't check the slave but the master looks like OEM so at 29 years old, I guess it could use replacing. Thankfully, the parts from RockAuto were like $91 shipped (total).
Here is a video of my warm start with my WAY FASTER starter:
IMO if the car starts up hot and cold, idles fine and drives fine, makes the expected power--- don't go nuts compression testing it.
Frankly I think the proliferation of all these testers and everyone testing their FDs that run great is not any kind of real advancement. Just leads to needless anxiety and sleepless nights for well-meaning rotorheads.
Also, a compression test is a test of the tightness or sealing of the engine internals. Apex seal to rotor housing and corner seal, side seal to side housing and corner seal. Over the years I've built many healthy streetported engines with aftermarket apex seals (usually RA super/black or RxParts) that tested 120+ psi per face after minimal runtime.
That sucks.. once those compression numbers are 60's it gets real hard to start the car after a long drive or hard pull and you fear that one day wont be enough to start the car.
Also, a compression test is a test of the tightness or sealing of the engine internals. Apex seal to rotor housing and corner seal, side seal to side housing and corner seal. Over the years I've built many healthy streetported engines with aftermarket apex seals (usually RA super/black or RxParts) that tested 120+ psi per face after minimal runtime.
I don't see how I can get a huge increase in compression after break in so if I’m at high 60s now, maybe I can get to 80s???
i just don’t get it. I need to check with Bryan but I think he reused the springs since they had maybe 1000 miles on them when the iRotary popped. I’m starting to wonder if those seals warped enough that the springs are no longer returning to full extension? My gut says those seals can’t be bad so does that mean the clearances are off? The housings are like new. The rotating assembly was sent off for balancing, engine is studded, etc. WTF during assembly would cause this?
also, if I wasn’t making a good seal, would I see more oil in my catch can?
06-15-22, 08:08 AM
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