i have seen a couple of the S4 rear irons crack in the vicinity of the oil filter dowel pin so i'm trying to see if anyone has come to any conclusive reasoning as to why they have been failing.

normally detonation will kill the apex seals, we know this already so why would detonation cause seals to blow at times and not others? well i figured we could try and figure it out with reasoning and understanding.

tonight my rear iron cracked with a rich AFR tune and modest timing map so i know it wasn't faulty tuning however it could have been the combination of boost and throttle. the boost levels i was running are ~18PSI on a 60:1 A/R hybrid S4 turbo with a large streetport running 91 octane gas.

i couldn't hear any audible detonation but who can hear anything over a rotary single exhaust anyways? well i mis-shifted into fifth and dumped it into third and hammered on the throttle but a little ways later i noticed a smoke cloud from the back of the car, and a little further the oil light flickered on so i pulled over and noticed the oil soaked engine bay and fat crack under the oil filter pedestal so i could only guess detonation due to the high boost level on pump gas but still i just can't help but wonder if there is another reason the irons are failing.

keep in mind this was on a LT8s standalone with the standard 2 bar MAP sensor so it was pushing its limits as it was but any other input so we can figure this out would be helpful.

 

Wiyhout knowing for sure, I would assume it could be due to thin casting, a little flex and the fact that the dowel, which is fixated in the rear iron, won't budge.

Could I be right?

 

pre-ignition

 

Yes it was faulty tuning... you went past your computer's ability to control fuel/spark at a minimum.

Are you saying it was 18psi by a gauge, or by a readout of what the computer is seeing?

 

I don't really have an answer but put me on the list of having cracked one. Now I run an s5 iron


Johny

 

one thing that i have been thinking about is the base timing on the LT8, the manual does not specify whether the values are on top of the leadings base timing or if it is based on TDC.

 

Dooood... Crazy. Both you an J-Rat in one day?!!

Do you have an EGT gauge? I'm seriously considering one now... I just got my FMIC installed on my hybrid setup and my Turbo II is ripping. I'm starting to get paranoid with just my piggy-backs and wideband.

BTW, I bet you'll find some cracked seals in their regardless.

 

Dosen't the LT8 have a knock sensor?

Hook it up - Suprises like this you don't need.

Pre-ignition - good guess.
Your plug tips were probably red hot when you buzzed it.

If you haven't already done it, cut some cooling grooves in the housing next to the plug hole for more coolant contact area.
And, switch to colder plugs.

 

I can see I'm getting too old for this high maintenance stuff.
I think I'll look for a stock RX-8.

 

Interesting.

 

Add me to the list. . lol It is odd that the iron would crack before the seals would break

 

I think it's a resonance problem.
The oil filter pedestal area was reinforced from 1989+, so Mazda knew it was a weak point.
Why would they reinforce it with stock power levels?
HMMMMM...


-Ted

 

sounds very plausible.

maybe they reinforced it at stock power levels because they were figuring there would be some cheap-o that wanted a Turbo Rx-7 but wanted to use 86 or 87 octane, too - all while beating the be-jesus out of the car ???

 

Not necessarily detonation, though that can kill them quickly. IT is simply twisting force that breaks them. To include hard launching and higher than stock torque output.

Think about it differently. Visualize each plate of the engine seperately instead of a whole block together. There are ONLY 4 small dowels to hold this whole thing square. The rear iron is solidly bolted to the trans and held by 2 locator dowels. When you launch, the engine is moving one way and the trans is trying to move the other (and the back plate with it). Obviously the loose sandwich design of the rotary allows for some housing-to-housing flex without extra dowels to hold them square.

When you apply enough power/shock/force, it twists more than it should and the weakest point cracks...the rear dowel hole.

This is why people spend hundreds on pinning race blocks, to prevent them from twisting upon each other.

 

http://www.rotaryresurrection.com/2n...ron_break.html

 

Correct, and usually due to firing the spark plug drastically too early due to poor tuning or an ignition misfire. The pressure applied to the portion of the rotor housing during the earlier part of the compression stroke forces it away from the iron housings laterally (deflection), carrying the dowel pin with it, putting pressure against the dowel lands.

Crack.

B

 

What do you guys think about the oversized stud kits that some companies sell, like mazda trix. Is installing oversized studs or dowlings a rotary engine better? I personally think that the oversized stud kits are stronger and would work better at keeping the engine inline and reduce cracking.
http://www.mazdatrix.com/studkit.htm

 

I'm very interested in the OS rotary engine stud kit also.

Who can do the machining required for this?

Ive heard from bryan @ BNR that the late 91 Turbo rear iron is just as strong as the FD rear iron, and has been strengthened beyond the S5 peice.

 

...

 

If you have the skills a drill press the right size ream and some measuring equipment you can do it yourself. Im gonna do it to my engine when i buy the studs.

 

Is this problem with the rear iron cracking just a problem with 2nd and 3rd gen engines. Or can it happen with any rotary engine? And who knows which is better the oversized stud kits or having an engine doweled?

 

I have a question about the Mazdatrix stud kit. I read thier info and it said to machine the tension rod holes to .2mm (.008") over the rod diameter. Doesn't that seem like a lot of clearance? The factory dowels are way tighter than that. Do they use other doweling methods besides the studs?

Which leads me to think maybe the different expansion rates of aluminium and cast iron, coupled with tight dowel clearance and the extra heat of a boosted engine (performance at that), might contribute to the failure. Maybe?

 

Mainly just with the 2nd gen turbos. The nonturbo rotaries don't usually make enough power to make it a concern, but it is possible (and has been done) on those models as well, when abused or modified enough.

It is also possible for 3rd gens, though not at all common. You have to be making quite a bit of power to crack those fat castings.

 

what exactly does this mean?

I read your link, and I find it interesting that you see multiple types of the S5 irons, could it be that some engines your seing where replaced where replaced under waranty and are mazda factor reman engines that where rebuilt with a newer part?

 

Most of the FC engines that I get are original or jspec. Most of the FD engines that I get are rebuild/remans.

I made that smiley because you basically repeated what was already said in the link I posted. Bryan *may* be correct in that only 91's got the fat back iron. Since I generally get cores after they are already out of the car, I can't say one way or another, only that most s5's that I see are not reinforced at all. I would feel pretty confident saying that all the mazda remans now come with the fat back iron...all that I have seen, anyway.