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Also considered to be on the edge of failing.
I must admit I was expecting much higher numbers for a new engine out of the crate.
Idk if it being mishandled has anything to do with it.
I've heard it's possible for the numbers to go up after break in, but I can't find any evidence of that.
not really, if it doesn’t already have 120+ psi it never will
which is what a new REW 13B engine should have, a Renesis should be 130+ psi but they rarely are unless hand-blueprinted
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the thing is, if it’s low from new it doesn’t mean it will have a short life, it may be that way for a long time until the seal springs begin to weaken
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JB, I think you asked me about my rebuilt engine's compression, I see why now. I'll grab my car on my lunch break and check comp after work. I didn't bother testing before because I'm lazy. I've had no issues with hot starts, low RPM performance, or misfires. I know its not numbers but it "sounds" like at least decent compression when cranking, and the engine stops spinning very quickly when shutting ignition off. Could still be on the low side, but I'll get some numbers later today.
My concerns in the numbers are the differences between faces. If they were slightly.low and even that would lend me to believe things will improve...
The differences are big enough that there is a seal/seals that are loosing lots of compression. That won't get better as it is a mechanical seal issue... not a bedding in problem
Ok, comp test done: Numbers are low. I do barely have 2k miles on the engine and already one track day.
1-Rotor 1
2-Rotor 1 corrected @250rpm
3-Rotor 2
4-Rotor 2 corrected @250rpm(apologies for the upside down photo) R
Also considered to be on the edge of failing.
I must admit I was expecting much higher numbers for a new engine out of the crate.
Idk if it being mishandled has anything to do with it.
I've heard it's possible for the numbers to go up after break in, but I can't find any evidence of that.
7= 100psi, not 78.
Also, I saw max compression in 450 miles on my own rebuild, which I broke in very easy since I was not 100% confident in my process ( I did some different things) or abilities.
200 miles is early though. Check it later. I bet you see near 110 psi. I would also ask how you measured it?
Last edited by kevink0000; 01-02-2023 at 11:37 PM.
Also, I saw max compression in 450 miles on my own rebuild, which I broke in very easy since I was not 100% confident in my process ( I did some different things) or abilities.
200 miles is early though. Check it later. I bet you see near 110 psi. I would also ask how you measured it?
Ok, comp test done: Numbers are low. I do barely have 2k miles on the engine and already one track day.
1-Rotor 1
2-Rotor 1 corrected @250rpm
3-Rotor 2
4-Rotor 2 corrected @250rpm(apologies for the upside down photo)
R
Awesome, thank you for the results! Looks like that's in the high 80-mid90s psi range, Was this a hot compression test?
On Facebook there's a guy with 600,000 mile RX-8. He's on his fourth engine which happens to be a Pettit engine with 122,000 miles. Unfortunately he couldn't provide any compression numbers but I'll try reaching back out to him for any history.
My concerns in the numbers are the differences between faces. If they were slightly.low and even that would lend me to believe things will improve...
The differences are big enough that there is a seal/seals that are loosing lots of compression. That won't get better as it is a mechanical seal issue... not a bedding in problem
I thought I read somewhere there's a 20psi window, so if the rear rotor is 100 and the front rotor is 75 that engine would fail because there's a 25psi difference. On my engine the front rotor face with low 70s makes me wonder if that's why the RPMs will drop down to 740-750 randomly.
I had Cam put an engine in my car several years back. As the old engine had 181K miles on it we went with a Mazda rebuild engine and rebuilt it instead. I did a compression test at 100K. I don't remember the exact numbers but it did meet Mazda replacement criteria. I have had no issues with it so far. This is also the first engine I have used premix in.
I thought I read somewhere there's a 20psi window, so if the rear rotor is 100 and the front rotor is 75 that engine would fail because there's a 25psi difference. On my engine the front rotor face with low 70s makes me wonder if that's why the RPMs will drop down to 740-750 randomly.
I noticed mine dropping yesterday when I drove it to get tires.
Plates today from the DMV then I'll start driving it regularly, weather permitting.
Awesome, thank you for the results! Looks like that's in the high 80-mid90s psi range, Was this a hot compression test?
On Facebook there's a guy with 600,000 mile RX-8. He's on his fourth engine which happens to be a Pettit engine with 122,000 miles. Unfortunately he couldn't provide any compression numbers but I'll try reaching back out to him for any history.
For one rotor run the motor hot and do a compression test as fast as you can after running the car. Repeat the test as the engine cools down about every minute. Heat the engine up again and repeat for the other rotor.
This may show whether a corner seal or rotor sideseal face is binding as you will have a jump in compression as the engine cools. Measuring smaller time intervals may help. I noticed this on one of my rebuilds when I tried the Atkins single piece corner seals and they appeared to be prone to this issue.
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IMO I think it is unreasonable to try and get 140k miles out of your engine. Unless you plan on buying entirely all new parts. Running the apex seals and housings down as far as possible will only cause more damage to the apex slots of the rotors.
The main wear points on the original 185000 mile engine that failed in my 2007 blue car, were the apex seals and the housings. The rotor slots in the undamaged rotor had wear, but only a bit more than .001" from new,(not wear limit) spec. It could have been re-used. The side seal clearance was immense, (.025"+), but hard to tell how much was wear or loose fit spec'd by Mazda in the era the engine was built. Supposedly they were leaving the factory with .010"+ clearances as new at that time.
The engine had huge wear on the housing faces, and the apex seal sides, and tips. Remarkably little wear anywhere else. Not a premix engine, either. The "cocking" of the apex seal in the rotor groove was much, much more a factor of apex seal side wear than rotor groove wear.
But as I said at the time I took it apart, the shallow rotor groove is the Achilles Heel of this engine in my opinion, for long life. Most people emphasize the various known side seal issues, but from what I saw, the apex seals and housings get brutalized in normal use.
The eshaft, stat gears/bearings, and front iron from that engine are in a current home rebuild that is running now, and has 8.3-8.6 compression with used housings and rotors, but all new Mazda seals.
Last edited by kevink0000; 01-10-2023 at 08:44 AM.
used rotors continuing to wear at an accelerated rate is possibly not being considered
assuming you we t to Pettit in the first place for the obvious reasons; all I know for the final time is that the guy who built the best and most powerful Renesis engines for the Mazda Formula Pro race series and also with Mazda directly on it told me not to reuse rotors or rotor housings; the exception is rotors modified for deeper RX7 apex seals and especially if Iannetti ceramic apex seals/springs are used then they can last for several rebuilds. Otherwise believe whatever you want and listen to whoever you want. If all you care about is that it runs and mediocre performance is acceptable then by all means, do so.
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All that is fine. Drummond is your chief reference we all know that by now. I am relating my experience, and I find that the absolutism (Drummonds) you continually state is not the case every time with these engines, just as absolutism was never the case with other engines I have rebuilt, and others I know, both for a living and as a hobby. The Renesis is not a mystery, it is a machine, like any other.
Also, it must be stated, a full race builder's priorities are very much different than what a street builder will emphasize. Which includes raw $$ concerns, and cost/benefit. An engine build that employs used parts is not automatically junk as you like to imply. If you can get 80-90% (or more to be frank) of the performance and 70%, 80% or more of the lifespan, why wouldn't you consider doing a build at 50%, 40% or 30% of the cost of a Drummond build? Especially for a street car. And that calculation is not different for a rotary or piston rebuild.
Those decisions get made, successfully, all over the world, for piston and rotary engines every day. But people do not come here to let us all know about them.
Like I've said before there are a lot of builders and smart rotary guys who never darken the door of these forums. They just quietly build in their shops and will not come here to debate the likes of you or I. So we jaw back and forth about this and that, and the real world is well beyond what is on the RX8 Club.
Last edited by kevink0000; 01-11-2023 at 02:37 PM.
Regarding engine builders. I contacted Pettit, Chips and Rotary Resurrection about commissioning a s2 'race' engine last year and one of the estimates was nearly $20k usd. I was planning to have it balanced / blueprinted and use Iannetti seals, etc. I'm just another street driven rx8, don't turn a wrench much anymore and a firm believer in the 'you get what you pay for' philosophy. To me when selecting a professional for services, I consider technique, experience, reputation, etc as well as one's own creativity and integrity (which is often overlooked but possibly most important imo). I think the whole experience depends on what your perceived expectation of the ideal outcome is.
I agree that new housings are almost non-negiotable for a rotary rebuild and the rotors themselves are a special case by case basis imo. You figure it's not too much more investment for the new housings when rebuilding an engine so might as well replace them; part of the 'fix it right the first time' mantra imo. And a tale as old as time, there is no replacement for first hand experience.
01-02-2023, 05:54 AM
