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Discussion Starter #1
Proof…with low timing, lean doesn’t hurt anything except power. I made a bone-head mistake this weekend and all that I did was run…slow! About a month or so ago, I did a back-to-back test with E100 vs C23. Well, to make a long story short, I left the C23 nitrous tune in the nitrous plate. First stage was 85N/63F, second stage was 73N/53F. This was the tune that was in the car on Friday night…using E100. Made a pass on 1 system and was about .1-.15 slower that I expected the combo to run. Plugs were good and clean (always clean with ethanol on 1 pass) A/F showed lean, but it had recalibrated after recently shutting the motor off, so I thought that it may have just been reading wrong. I then recalibrated again, and decided to make another pass (this time on 2 systems) with no changes to the tune. Got the timeslip and could not understand why the car didn’t pick up virtually anything. Downloaded the wideband data and it was leaner than the previous pass (like 18:1 a/f ratio!) Suddenly, it dawned on me…I don’t remember changing nitrous pills from the C23 tune. Pulled the plugs…some of them didn’t even have the coating burned off the strap. Pulled the carb, changed pills in the plate to the E100 tune, went back out and picked up .07 on 1 system. Next day, went back to make a shakedown to get into a nitrous small block race…turned on 2nd system at .1 and broke the input shaft on the gear change. Pulled plugs and this is what they look like after 2.4 seconds on 2 systems with ethanol.



On another note…with this motor, I did a little experiment. The theory is that the reason the passenger bank is always more timing sensitive is due to piston rock and a tightening quench issue as compared to the driver bank. Well, with that in mind, I decided to use a thicker head gasket on the passenger bank vs the driver bank (.060 vs .040). In theory, the heat should move from the passenger bank to the driver bank. Well, attached you will notice that cyl 6 and 7 were the hottest cylinders after 3 nitrous passes on these plugs. Of course, two of the passes were with very lean a/f’s…so I’m anxious to see what they look like with “normal” a/f’s.

 

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Discussion Starter #5
Randy, explain to me the pass. side bank "tightening". You got my attention.
This is, of course, all theory…but, I will explain it the best that I can the way that I understand it. It may not really be what’s going on, but it makes sense to me.
If looking at the engine from the front, the engine rotates clockwise. Rotation in that fashion causes the pistons on the driver bank to be kicked so that the outside edge of the piston is slightly higher than the “lifter valley” side of the piston. The pressure from the rotating crankshaft is pushing it in that fashion, and as the mixture is compressed and ignited, combustion pressure starts to push down on that same side of the piston, somewhat counter-acting the crankshaft pressure. As the piston rolls over top-dead-center, IF the piston decides to rock back the other way, it would be after it’s already traveling away from TDC. So, in effect, the piston never rocks towards the “quench pad” (or area of the piston/head that is flat, or the upper/lifter valley side of the piston).
Conversely, as the engine rotates clockwise, the pistons on the passenger bank are kicked so that the inside edge or lifter valley side of the piston is slightly higher than the outside edge of the piston. The pressure from the crankshaft is pushing it in that fashion, and as the mixture is compressed and ignited, combustion pressure is starting to push down on the opposite side of the piston, further enhancing the “piston rock” that is already there. That also means that since the lifter valley side of the piston is higher, it puts a tighter squeeze on the quench pad area of the piston. This creates more heat, and effectively makes the combustion area smaller, promoting faster flame travel, and a smaller area to be combusted. Combine all of this, and you have a more efficient cylinder that is prone to get hotter due to the tight quench.
This may not be the “correct” explanation, but this is how I look at it. I’m certainly no expert however. Maybe some experts will chime in and correct me or give another perspective. I also attached a pic to depict what I explained…and I’m no artist either…lol! It is drawn to slightly “excessive” specs, but maybe it will help you “see” what I’m saying:D
Also, depending on how short the skirt is, and how tight the piston clearances are, 1 or 2 thou of rock could mean 4 or 5 thou at the top of the piston in the quench area. I’m sure that as the skirts get “beat in” on an abused piston, it can certainly take a perfect tune-up and run it into detonation, with no changes to the tune.

 

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I also did a lean move in the spring trying some new larger noids. Short story is we thought we where rich due to the dark fuel ring. Added more air and it looked no better and slowed .05. I was confused. (maybe to track???) Added more air and slowed more with a darker ring....WTF is going on. Answer.....ALL AIR NO FUEL = NO fire. (cold chamber)
Started all over after thinking about things......Went back to square 1 w/ 2lbs more fuel on each kit and wla......now we could start tuning it in again...
(Also........not one thing was damage but ALSO: i ALWAYS keep timing out when trying something new until we feel we are ready for it.

Nice to see another story 10.5 like mine.....live & learn.
 

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It was a god race this weekend but i was expecting more cars.Sorry about the input shaft,been there done that.Dont beat yourself up about making mistakes we all do it.
 

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Great thread 10.5 !
 

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We'll send you to art school next year! LOL, i get the idea though. I have done a bone-head dry-spray before too. Left the nitrous pump unhooked on accident, made a couple passes that were about 2 tenths slower than normal, and it didnt burn either. Didnt nip a plug or anything, just really CLEAN.
 

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Randy, I think you are dead-on with the piston "rock". A guy brought a BBC by the shop today, and I was checking it out, and lo and behold the pistons had been kissing the head. Now,the passenger bank was touching on the top side(tighter quench) and the drivers side was kissing on the bottom of the pistons(smaller quench area). I'd believe this to solidify the "theory" to "actuality". Now, what are we gonna do about it?
 

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Discussion Starter #12
When softening heads for nitrous, have a "passenger softened" head and a "driver softened" head? Add ~10 thou more to the thicker quench pad area of the passenger bank, add ~10 thou more to the smaller quench area of the driver bank?
 

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Randy, I think you are dead-on with the piston "rock". A guy brought a BBC by the shop today, and I was checking it out, and lo and behold the pistons had been kissing the head. Now,the passenger bank was touching on the top side(tighter quench) and the drivers side was kissing on the bottom of the pistons(smaller quench area). I'd believe this to solidify the "theory" to "actuality". Now, what are we gonna do about it?
take 1 degree out of 2-4-6-8. JMO... if you have that problem.
 

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Trevor Stripling
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Strange as it may seem, my passenger side (2,4,6,8 ) is colder than the driver side (1,3,5,7) and I either have to remove fuel or add nitrous or add timing to get it clean. Goes totally against everything I have read but is reality in my motor. I have mostly raced a jet limited class this year and have jetted down the fuel to clean up 4,6,8 and added fuel to 7. See pics from last race: http://www.yellowbullet.com/forum/showpost.php?p=7114324&postcount=22

Trevor
 

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Strange as it may seem, my passenger side (2,4,6,8 ) is colder than the driver side (1,3,5,7) and I either have to remove fuel or add nitrous or add timing to get it clean. Goes totally against everything I have read but is reality in my motor. I have mostly raced a jet limited class this year and have jetted down the fuel to clean up 4,6,8 and added fuel to 7. See pics from last race: http://www.yellowbullet.com/forum/showpost.php?p=7114324&postcount=22

Trevor

I have seen the same with mine. My driver bank shows leaner as well...
 

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Our 638 is leaner on the passenger side.

OT: Jeff, that is a killer video...4.28 is getting it done! Good Job.
 

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Our 638 is leaner on the passenger side.

OT: Jeff, that is a killer video...4.28 is getting it done! Good Job.

Thanks. I'm planning on putting it into the 6.50's at Reynolds at our last division 2 race in October. 10.5 (who started this thread) was the camera man
 

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Jeff, forget a 3rd gun...I need a motor that makes the HP of your RM!
 
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