the shop manual (94) sets standard apex seal to groove clearance at .002 to .0039 w max .0059.



i assume that is for 2 mm apex seals. i was unable to find a value for 3mm in the manual nor is there one on bruce turrentine's video.



i am tracking down the reason for 65- 75 psi in the rear rotor of a 180 mile newly built motor ( the third this year) and would appreciate the 3 mm clearance value. my side seal clearances are sloppy too at .005 to .008.



how would sloppy apex seal clearance, if i do have it, effect the motor?



thanks,



howard coleman

 

Howard, the clearances are just bout the same for the 3mm seals, from my 12a sheet, .0020(in) to .0035(in) with a limit of .0060(in)..

From my own experience, starting goes to **** with wide clearances there, especially hot starting, low end goes for **** as well, the power seems to pick up with rpm when enough combustion pressure gets under the seal to start loading it against the housing, but thats a double edged problem as the hot gases roast the springs and they go flat, which makes starting worse, and leads to shitty compression values, .005 to to .008 is getting huge on the side seals as well, I start mine off at .001 and change them if they measure out more than .004 ..

The 3mm debate never goes away, I personally think its crap, I also think the reason aftermarket 3mm motors seem to take more boost and thus more abuse is that they have inherently poorer sealing, its one thing to throw 20 psi of boost to a motor with 3% leak down and one with 30% leak down, which one will be more likely to detonate, the one with better chamber sealing and high combustion pressures which 9 times out of 10 is the 2mm, throwing 20 psi of boost at sloppy motor with bad sealing is like running a 6:1 motor...I read alot of posts where people are pounding a motor with more than 20 psi on 92 octane, thats the equivalent compression ratio and combustion pressure of 22:1 at sea level, those with piston motor experience know that , that can't happen, unless the gas is higher than 93 octane,you are at some crazy altitude, or you have 50% leak down...Maxt

 

thanks max,



my apexseal/groove clearance is an amazingly sloppy 8 thou. i checked another set of 3mm rotors i have that have 40,000 miles on them and they are approx 2 thou so i figured mine are way off...



howard coleman

 

Out of curiosity , what brand of seal was it?...

 

mazda 2 piece 3mm solid corner seal

 

after building me a motor (#1) that had poor sealing from the getgo and ate the sideseals in 1680 miles my engine builder said the cornerseal pockets on my rotors were sloppy, that being the reason for sideseal failure



he located a set of rotors and vouched for their condition after measuring them. that motor (#2) ran 196 miles and ate the side seals. i opened the rear rotor up and replaced the sideseals which ran another 200 miles. i took the motor back to him and he decided that the ports were opening too early and eating the side seals so he replaced the secondary housings w close to stock ports just to be conservative. the last swing at the ball (#3) ran 184 miles before losing compression in the rear rotor.



at that point i decided i'd better get seriously envolved, a decision in retrospect, that was way overdue. i opened motor #3 up today and have only so far looked at the rear rotor... my guess is that a combination of 8 thou apex seal clearance and between 5 to 8 thou sideseal clearance is the problem.



i am quite relieved to be building my own motors going forward.



all these motor issues limited me to only 2100 miles of evaluating my twin t04 setup this summer... i will be logging twin egts and turbo manifold pressure when i get back on the road in march.



i plan to build 3 motors in january and may do one of them as a bridge but would have to seriously upsize my turbine housings for that porting. the manifold pressure log should speed up the process.



howard coleman

 

It would be equivalent to 22:1 CR if you had no intercooler and your turbo compressor was as inefficient at compressing as your rotor...



My 3mm seal motor had apex seal clearance at .0015 all the way around.

 

How do you figure that, even with a turbo that is 100% efficient is still wouldn't change the fact that you cant do that on pump gas, with a well sealed motor, in fact the more efficient the turbo is, the worse the theoretical compression ratio would be, as the intake would be denser, thus the combustion pressures higher...Maxt

 

The air intake temp is lower the higher the adiabatic efficiency of the compressor. The turbo is a much more efficient compressor than the rotor and has the aid of the intercooler.



Take my set-up that ran for ~9,000 miles boost creeping between 20-24psi on 91 octane as an example. My peak AITs were ~40 Celsius at 20 psi and I had 8:5 CR rotors. If you do the calculations I bet you would find the combustion chamber temp at full rotor compression for this set-up was significantly lower than if I had a naturally aspirated engine at 22:1 CR.



I don't think 20psi boost on pump gas is a good idea, but I also don't think the only reason it works is the engine "leaking" compression somewhere.



Anyways, sorry to help take this post off topic.

 

The efficiency of the turbo and the intercooler has really nothing to do with it, in fact it makes it worse, as the denser the air , the higher the combsution pressures, which in turn would either increase blow by in a sick motor, or actually tend to increase detonation, in a well sealed motor intercooling is a bell curve effect in terms of detonation, in terms of fuel quality and octane, anyone that has drive a turbocharged car on pump gasoline at -20 will confirm this. Pump gas has its definite limits, at a give pressure and temperature it will self ingnite, you can run the motor at 8:1 with no spark advance to get 20 psi, but you make more power at 10 psi with mixtures at 11.5:1 and real spark curve. From running a sick motor compared to good one, I am pretty much convinced the only reason 20 psi on pump gas works is the sealing or lack thereof on a motor. There are numerous effective compression calculators on the web, punch in 9:1 and 20 psi, the combustion pressures are in the diesel engine range...Maxt