Hi guys, My son has a 94 RX7 see pic... Anyway, it was recently rebuilt
and dynoed at 387hp but after it warmed up they were seeing a drop off
in compression... So, they tore the engine back down and did three
things... 1st they verified the oil metering was ok... It was. Then
they verified all the clearances which led them to think that the 3mm
apex seals were sticking because the clearances were on the "Tight"
side.
Here was the reasoning... They thought that the rotors may have been
machined wrong and when the engine heats up that the rotor is expanding
just a little and pinching the seals not allowing them to slid properly
and thus allowing some blow by. Well they re-Machined the rotors put the
engine back together and guess what... It Ran fine until it heated up
then lost compression and quit. So it is coming back out again for
another tear down... Has anyone ever seen this before or have any ideas?
Oh, also the reason it was originally rebuilt was because it lost
compression... New rotor housings, seals, bearings etc... The only Idea
I have now is to have them replace the Rotors. Help!
Mike C.
When the rotor expands due to heat the slots get slightly larger.
The side seals could be sticking. There is an end clearance spec on the
side seals.
3 mm apex seals means they are after market apex seals. That means after
market apex seal springs. The seal slots run very very hot so the apex
seal spring heat treat if any could be going away. Check the spring
height before and after. It is also a good idea to check the spring rate
but that is much harder to do. There are tools out there for checking
the spring rate on valve springs but they are not much good for apex
seal springs. The material the springs are made from is critical.
Another possibility is stock apex seals have a crown. Place them belly
to belly and you can see it. Check your after market apex seals for
crown. If no crown you have bad apex seals. The crown handles heat
distortion in the rotor housing when it warms up.
http://www.rotaryeng.net/seals.html
Mix 2 cycle oil with the fuel. One ounce per gallon of gas.
What brand of apex seals are these?
--
Paul Lamar ...No rotor no motor.
Thanks for all the advice Paul... I'm not sure what brand seals they are
but I'll check and let ya know. I will pass these off to the shop that
has been working this and if anyone comes up with anything else to check
it would be greatly appreciated... These guys have never seen this
before and understandably they are getting tires and frustrated with
putting it in and taking it out.
Mike C.
Um, 3mm seals are still available from the factory along with the
springs,
84-85 GSL-SE.
If they are aftermarket steel seals they probably got hot and turned
into
bananas like they all do, even the factory ones if pushed too hard
or run
lean. Maybe the engine is lean or it's still a problem with the apex
slots
that was not corrected? Are the corner seals Factory? I have seen
aftermarket corner seals that the opening is too small and does not
provide
enough clearance to the apex seal and actually gets tighter as you
measure
deeper into the slot. Combine that with a groove that is not aligned
right
and you've got problems. What is the apex seal to groove clearance? You
don't say exactly.
It is not the crown you are looking for in fact it's quite the opposite.
What you are looking for is the seal to be concave along its radius
(banana), this will cause loss of compression which sometimes recovers
(factory seals) when the engine cools down.
Happy trails,
Sven
Let us keep in mind that if its a 94, its an FD twin turbo with 255
HP stock. And it has been to 387 HP. If the rotors were machined for
3MM seals, were the rotors ruined once already? Was the machining
done by a known rotary shop like Mazdatrix or Racing Beat? If not,
we might guess a corner seal binding up from an off centerline cut,
or similar malady. Rather than keep repeating this expensive and
heart breaking pattern again, how about a set of FC single turbo
rotors new with 3MM slots from the factory. Lighten the rotors to
match the FD rotors, or, run the early counterweights, and if you
can afford anything after all of that get the whole thing balanced
by a known rotary balancer.
If just driving around is not going to be enough, and this thing is
going to the dyno races every weekend, then ceramic seals are a must.
Either Mazda one piece or aftermarket one piece or two piece. Measure
the hot side of the housings and compare that to the cold side to see if
it may have overheated already and ruined the housings. They can look
great and be paper weights.
Higher HP requires that at least the secondary injectors and maybe both
injectors be able to deliver much more fuel and stay in their mid range
of duty cycle. So, look around and see what everyone else is using and
try a set of those. An after market controller with knock sensing is
required. As the boost comes up, the effective compression ratio goes up
as well. So the ignition advance must be pulled back towards zero.
A timing split starts looking like detonation at high boost. Close up
the split and eventually just go to zero split. If your controller still
sees knock hits shut off the trailing plugs. Mount the MSDs on opposite
sides of the car. Run the sensor wires for leading and trailing in a
piece of dash 4 teflon lined stainless braid with the braid grounded at
both ends. Same thing for coil primary wires. Alone in their own dash 4
braided line and grounded. Keep the coils away from each other, and use
the shortest possible inductive plug wires. I don't know if the Mazda
coils can stand up to a MSD, so be sure they are listed before hooking
them up.
No wires to the controller can be run near any high voltage wires. Use
only shielded wires inside the engine compartment. One out of phase
ignition hit and the engine is gone.
Pour in the fuel to keep the EGTs low. For 500 HP you need twice the
fuel delivery of the stock engine. You will be out of turbo before that.
So see what the big HP engines have mounted to go higher. Put in the
biggest intercooler available and the biggest oil cooler. Spray water on
both of them. Share what you learn.
Lynn E. Hanover
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