Hi,
Didn\'t see this posted anywhere before so just thought I\'d ask your opinion on it..
Bought a street ported 13BT with just over 6000miles allegedly on it, finally fitted the Microtech and fired it up. Have been driving it for a while on the slightly modified basemap, but have still been having some problems with it starting and running smoothly once its left to sit after running (i.e. when the intake temps are at 60-65deg+). Doesn\'t seem to have any problems starting and running from hot if the intake air temp is lower than this. Anyway took it out, left it to sit idling for 10min, turned it off, then 5 min later struggled to start it, got it running on what sounded like one rotor and it had absolutely no power and struggled to stay running. Long story short I ended up at Hurley rotary getting a compression test and ended up with (fairly even) 4-5 reading on the all rotor faces.
What I wanted to know is, does having a ported motor effect these reading? I was also told that it has low comp. rotors from a series 4 in it, but I know that they still have a MUCH higher reading standard.
I rang Powergains (who apparently were responsible for the rebuild) and they said that this is normal, with a bridgeported motor they recently rebuilt only having 3-4 readings.
It just seems strange if this is the case why I have found NO mention of this anywhere? None of the compression test guides anywhere mention this.. I couldn\'t even find a single post on here after looking for 20 min mentioning it.
Well unless I\'m being really dense the compression cycle takes place on the other side of the housing to the ports so the size and nature of the ports should have no bearing on the compression reading.
Yes a ported motor will have lower compression at cranking speed.
Intake if it\'s bridgeported I wouldn\'t expect it to be smooth anyway, but if the starting problems are specific to 65+ intake temps, you may have to have your map looked at might be as simple as removing more fuel at this intake temp range.
rgds
Brian
Yep Brain is right, big street ported and Bridgeported do give lower readings at 250 rpm test speed the big ports dont create enough vacuum/air speed to compress enough air to give a high reading.
Big Porting needs more rpm hence why the power band is moved upwards
Brett, I thnik we have to imagine that the ports are no longer optimized for low speed intake. Therefore there will be less air taken in at cranking speed. You are right of course, the actual compression phase is far away from the port closing.
Yeah, got it now - it was early when I answered! lol! :wave
Presumably, an alternative to adding more fuel at those temps could be to retard the ignition a few degrees?
Cheers for the replies, yep will check the map for air temp timing changes, already looked at the air temp fuel compensation and tried adding and taking fuel away to no avail (it runs lean anyway in those 60Deg+ conditions).
Still the compression thing is an interesting one. 4-5 readings do scare me slightly for a street port but Hurley said at least they were consistent, even suggested that it might be caused by excessive temperature making the apex seal springs weaker when warm, therefore stopping the apex seals from making proper contact with the housings.
Would be interested to hear a tuner/builders point of view? (Dragon/Ross? Carl Hayward?)
You can mail me.
Regards
Carl
That compression at 4. something or 5. something is very low.
A street port wouldn\'t make that much affect on the engine compression to give that low a reading. The last street ported engine I done that I know had a compression test was reading in the 7\'s after a few thousand miles and this was on good condition used housings.
Best liaise with the builder about your concerns, if there is no warranty with, not much you can do really taht won\'t cost you in some shape or form.
rgds
Brian
Can\'t see it making any difference,wether a stock port or a bridge,on the secondaries that is,because they are shut when cranking it.Therefore you should have 8 bar compression if you are wanting real performance.