Tried searching, wasnt even sure what to search for...


I have heard/been told that fresh rebuilds do not have high compression until they are broken in some. How long roughly would I need to run the engine before I can get my high number? I have ran it for about 3 hours, mostly idling and it has almost 10 miles on it. I cant drive it much around town yet as my alignment is all messed up since I replaced all my steering parts (idler, pinion, tie rods).

When I checked it, I unplugged the fuel pump fuse and let the carb run out of gas. I then used the trailing holes to check compression. I have ~100 psi on front and ~95 on rear. Engine was at operating temp when checked. The front rotor (first one checked) spiked up to 125 when it started from residual gas.

Thats as much info as I can give. Mods in sig, etc, etc.

TIA
-IanS

 

Your not really going to find much info cause most don't do a compression check right after a rebuild.

I personally did on one occasion a couple years back. I did an experimental mod on my rotors and forgot to reclearance my side seals to corner seal clearance. When I put the engine back together and tried to fire it up, it wouldn't start. I did a compression check and saw 55psi front and rear. Needless to say, that wasn't enough compression to start the engine. I had to put some atf to get it to run. Even then it ran like ****. That's when I remembered the experimental mod I did to the rotors. I forgot to re-check my side seal to corner seal clearance after doing the mod. When I broke the engine back down to remeasured the clearance, I was shocked to find that I was in the .014 range. Hell no wonder it wouldn't run. I grabbed a different set of rotors off my shelf and rearranged the side seals all around till I got the ideal .002-003 clearance. I put the engine back together and did a quick compression check b4 the 1st start-up and saw 95psi front and rear. I was really happy as I didn't really have any start up lube on the rotor housings so I knew I was making good compression. The engine fired right up without any issues. Keep in mind this engine had the rock hard RA apex seals and I reused housings (one of which had the apex seal damage from me previously blowing the engine).

Overall this experiment has really taught me the importance of that side seal to corner seal clearance. You can have bad questionable housings and still make good compression if that side/corner seal clearance is good.

Ians what did you clearance yours too? What kind of apex seals? New or used housings? Use my past experience as a guide.

 

I cant remember what the clearance was but I remember everything was in spec. I reused side seals, and got new OEM apex seals, used the atkins rebuild kit A (soft seals, springs), and reused the oil control rings. Am using used, but in way better shape than what I had bearings, and rotor housings (aluminums). Also using a different eshaft. I didnt see anything wrong with it, but from the bearing wear, I was not really comfortable with using it. Basically, out of the 5 engines I had tore down, I used the best stuff and made 1 (although I had to buy aluminum housings).

 

I was just reading the FAQs on Pineapple Racing's site. They say that the compression is changing all the time on engines with new seals. I suppose thats why people dont check the compression until they get quite a few miles on them (1000 or so).

 

Update: Checked compression again and both front and rear are equal now with 110 PSI, so it is looking better every day. I now have 220 miles on it and it starts better along with idling by itself after only running at 2K for about a minute. Before it had to be warmed up all the way and even then it still might act weird. I can feel a slight throttle response difference also.

 

there are many opinions to breakin but the one i like the most is the compression check every 100mi and once you see your compression even out your motor is escentially broke in. i have 400 miles on me fresh rebuild and it has gotten better and better running over this time i would love to check my compression every hundred or so but i have not yet purchased an aftermarket mazda style compression checker to get an accurate reading, i dont think the piston compression checker bounce method would serve accurate enough for this.

z

 

Yes, in my engine building experience I +1 that method :-)

It is always a wise idea to know what your compression is at the start, if you suddenly have a problem 400 miles later and do a compression check it doesn't tell you much other than if it is in spec or not.

For instance lets say you do a rebuild and after 200 miles of break in it is running good, then suddenly after 400 miles it becomes hard to start and is sluggish under load. You do a compression check and it is 100 psi front and back so you overlook the possable problem and think the problem is something else.

Now lets say you did do a compression check before you ever started it after the rebuild and you had 95 psi across the board, after 200 miles of break in you did another compression check and got 120 psi on both rotors. Now after 400 miles you don't like the way it's running, you do another compression check and it is reading 100 psi now you know something isn't right.

 

I will check it again then and keep an eye on it and make sure all is good. What you say does make perfect sense, and I also wonder why no one else has mentioned this as a good way to do it. I did quite a bit of searching one day (more than 5 hours) on this subject and I only saw maybe a couple other people mention checking it often for the first few hundred miles.