rs_1101

heres my theory. this is simple physics here.

lets start with the beginning, oil comes from an OMp on the front cover of the motor, where it is sprayed into the motor from there. now to start weve all heard of the brown streak around a rotor housing thats caused by oil not spreading evenly, b/c the oil lines to the rear rotor are longer, there is less pressure, and less oil distribution going to that rotor.

what happens from there? the center of the apex seal is spared from great stresses due to a generous coat of oil, while the sides of the tip seal take more wear.
now if you can imagine this, now the sides of the seal are shorter than the center, and the springs push down on this, because the sides are shorter than the center, it creates pressure. \_/ imagine, if you pushed down on the top of this shape, where is the stress? in the center. so thats my theory.

feel free to correct if im wrong, maybe i have the apex spring direction all wrong, but im sure this is something along the lines of what goes down.

OverDriven

I think in S4's its always the front rotor and S5's it tends to be the rear. I can tell you that my rear housing (87 TII) is in GREAT shape, and the front has a little bit of scarring around the trailing plug hole. The coolant seal also blew out on the front side housing.

-Joe

Dan H

I think there was a poll in the past. And a large majority blew rear rotors whether it was S4 or S5.

Tofuball

I'll throw in some of my experience (engines that have blown under my watch, as in, they worked, then they did not):

Erik's S4 TII: Blown rear rotor (Zero PSI!) (Thats where the fire was)
"Rocky" S4 GXL: Blown front oil seal
Noel's S5 TII: I THINK IIRC the front oil seal is blasted.

There are others, but I cant recall right now.

ajsuper7

i believe it is speculated that the rears go out because the coolent to the rears is hotter than when it reaches the front rotor. not sure if anyone has any true proof on why the rear goes out but that is what many people hypothisise

Dan H

I think you're right. I remember reading about something like that and how the rears run hotter.

300zx

Its also tends to go on the done force aswell. Both my motor are damaged in the same spot on the rear rotor and housing.

And the motor in my car is getting weak and on its way out the door. I am starting to build myself a cermet motor. The car is put away for the winter now but when i was puting it away it only started on one rotor and then i heard a weird sound and both rotors started to work i dont know lol. I am going to take it apart before it blows and see what it looks like inside before it goes.

KompressorLOgic

iTrader: (6)

my 86 the rear rotor failed, then on my 87 i notice compression is lower in the rear than the front.

Dak

In my car it was the rear that failed but I think my car was a victim of carbon lock. I did tear apart the series5 motor in my parts car and the front rotor went out on it. Had a couple of apex seals broke, but it seems the front is somewhat of a rarity.This engine did have alot of carbon build up in it as well.

scathcart

Spring pressure is next to nothing compared to the centrifugal forces generated by the spinning rotors. As the shape of the apex seal is relatively constant across its length, the force exerted on the seal will remain pretty much constant across the entire contact face of the seal... except when the seal transverses across the periphereal exhaust port, at which point the seal is supported solely along the sides exerting force and physically bending the seal like a bow as it crosses.
So I'd say you're definitely wrong on the "spring pressure" idea.

As for the oil pressure differential... If you look at how the MOP lines run, they are pretty much a constant length, differing at best by about one inch between the longest and the shortest line... so I'd say that debunks that theory.. Sorry.

Rx-7Doctor

iTrader: (13)

The rear rotor fails usually because of the deacceleration system, remember fuel
is supposed to cut off to the rear during deacceleration, if this is not happening over a long period of time, it washes out the seals. rx7doctor

RotaryEvolution

iTrader: (30)

most engines toss an apex seal under normal conditions that i have heard of/seen around here on the forums, ie. cruising down the freeway not hard on accel or just idling at a light or after shutting down and restarting...

this would rule out the OMP, because it isn't even feeding oil at that point. my speculation is as some stated that the the rear rotor and housing tends to go through the most heat up and cooling(expansion and contraction) because it is the furthest from the water pump and oiling. cruising, the engine is dropping the heat it generated to get up to speed, idling the engine is doing the same. when a car is shut off it generates the most heat because there is no cooling at all unless you chose to rig your e-fan to run when the car is shut off like some autos.


this is just my 2 cents though.

jacobcartmill

iTrader: (6)

i have blown the front rotor twice. in my NA and in my jspec TII engine. both s5's.

NZConvertible

Wrong. All injectors are cut during deceleration, not just the rear, and if your OMP is working correctly there will still be oil being injected into the chambers even when they're off.

Coolant flow is the most likely reason for the rear being the most common to fail, for the same reason Nissan straight sixes usually blow #6. Slight variations in the amount of coolant flow each rotor (or piston) receives means one will be running slightly hotter than the other(s), so that one will probably reach the threshold of detonation first. It's not a guarantee the rear will always blow, but the chances seem to be much higher.

jreynish

iTrader: (2)

If you think about it the rear housing will have the most amount of heat, it is sandwiched between the Tranny and the center housing the front rotor housing is reall only got the front cover, (allow much more radiant cooling) So I think it is because the coolant is more warm there due to the fact that it is simply more warm in that area of the motor because of the nature of the beast.