Date: Thu, 19 Dec 2002 13:43:06 -0600
From: "Steven Kan" (skan@ticnet.com)
I think there're a few reasons why rear rotor breaks more than front.
These are the theory that most people came up over the years in here and I truely think that heat is what really causes the rear to have lower compression and detonation is what breaks the apex seals at the rear (w/o the knock sensor). When I placed the J&S to the rear, I break front apex seals....
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Date: Thu, 20 Jul 2000 13:58:07 -0400 (EDT)
From: William T Wilson (fluffy@snurgle.org)
> What exactly makes an apex seal spring go flat?
Time, mostly; they just sag over time. Normally, apex seal springs don't matter too much except when starting the engine since they are helped by the rotation of the rotor at higher RPM.
> And how would one warp an apex seal?
You wouldn't. They are either broken or working, they don't 'warp'.
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Date: Thu, 20 Jul 2000 17:31:31 -0400
From: Felix Miata (mrmazda@atlantic.net)
> > What exactly makes an apex seal spring go flat? > > Time, mostly; they just sag over time. Normally, apex seal springs don't > matter too much except when starting the engine since they are helped by > the rotation of the rotor at higher RPM.
Wrong answer. Heat. Excessive heat. Like any spring, excessive heat while in a compressed state will collapse it.
> > And how would one warp an apex seal? > > You wouldn't. They are either broken or working, they don't 'warp'.
Another wrong answer. With a stock 3 mm two piece seal if you get them too hot by running hard and lean prior to full break-in and full warm up, the center can pull down toward the spring, leaving a gap between it and the rotor housing in the middle while the ends, where the spring pushes, will remain in contact. When it happens, it you can keep the engine running and/or start it in the first place, it will leave a layer of sticky black/brown goo stuck to the center of the trochoid surface due to the seal centers not making contact. If a mild case, you can drive it semi-normally, avoiding use of full throttle and high load, a couple of thousand miles to reconform them to the housing(s).
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Dale Clark (dclark@matheselectric.com)
Date: September 27, 2000
It is not uncommon for a motor (at least a TurboII motor) to have great compression and make great power when it's on the verge of dying. I've even compression tested motors just *days* before they went.
If you've ever gone through a motor, you'll know that the rotors are typically caked with black carbon. This raises the static compression ratio, and provides potenial hot spots for detonation to occur.
Is it a good idea to have a compression test? To some extent. For a novice buyer, it will tell you that the apex seals are sound and the motor is relatively tight, or if it *is* a blown motor, it will tell you as such. A trained eye/ear can see the telltale signs of a blown motor (idle tone, sound during cranking, etc.), but that will most likely not be obvious to a novice.
I have seen, though, a '90 TurboII that just flat out had *low* compression - no broken apex seals, just worn or stuck seals. The "put-put-put-put" sound when starting the motor was very faint, and it would flood frequently. While flooded, the motor sounded like an electric motor spinning - "hmmmmmmmmm", no compression pulses.
But, more common than not with old TurboII's, the car will seem to run great, if not really fast, just before they let go. This is on stock/near-stock cars that were relatively healthy (fuel filter changed, fresh plugs, no clogged cats, etc.)
Unfortunately, that's just the nature of the rotary. Pound on a piston engine, and you can slowly get lower compression, loss of power, etc., whereas a rotary just tends to go when it's time - more of a binary operation than a failure curve.
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From: Steve Cirian (steve@ScuderiaCiriani.com)
Date: September 27, 2000
>FYI, on that 93 Im looking to buy, both Mazda Masters and the local Mazda >dealership service dept have now told me that checking compression is >pointless. They say compression will stay high until the engine dies.
This does not seem to be the case. I got several messages from people saying that the apex seals or the rotor housings will wear, the springs could go bad, and will indeed show low compression. This opinion ran about 6 - 2 in favor of worn seals causing low compression, and one of the dissenting opinions was based on a race engine with softer seals.
I would say that a compression test is NOT pointless.
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Date: Fri, 26 Jun 1998 11:10:58 +0000
From: "David Lane" (dlane@peabody.jhu.edu)
Symptoms for a bad rotor--apex seal:
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See the compression test page for more info on the procedure David describes. --Steve
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