J-Madd Posted September 14, 2007 Report Posted September 14, 2007 I was told by a guy who ported some Cubs of mine that I had signs of detonation in the exhaust ports. Before I tore it down, it had never "sounded" like it was detonating, the spark plugs looked fine, and it was running rich. The only thing I can think of is that the flywheel had a habit of trying to shear the key while kick starting it, and it would backfire while kicking, not while the engine was running. ...I'd loosen up the flywheel and re-tighten it and it would be fine. This only happened a couple times and I replaced the flywheel. It had probably a tank of gas or so ran through it since it last "detonated" while kick starting.....so what signs of detonation was he talking about?? Quote
Snopczynski Posted September 14, 2007 Report Posted September 14, 2007 Well the head and the piston would be messed up if it was detonating. What was your compression and what octane gas was in it? How far advanced was the timing? Quote
J-Madd Posted September 14, 2007 Author Report Posted September 14, 2007 Well the head and the piston would be messed up if it was detonating. What was your compression and what octane gas was in it? How far advanced was the timing? Head and pistons both fine. + 9 degrees timing (increased from +5 that I ran for a year), 160 psi, 108 octane. After I increased the timing I rode it and checked the plugs, then never looked back. I agree, detonation would jack up the pistons and domes, so I don't know what he saw in the exaust ports. Quote
J-Madd Posted September 15, 2007 Author Report Posted September 15, 2007 Well, I got the cylinders back today and the left exhaust flange (the outside of the port that the pipe slides over) is pitted slightly. Don't know WTF that means. Quote
jmd0346 Posted September 15, 2007 Report Posted September 15, 2007 Thats nothing to worry about. Id worry about a builder who told me that was detonation. Quote
RadarRacing Posted September 16, 2007 Report Posted September 16, 2007 Doesnt sound right to me either. I'm still curious as to why it would be pitted there. But not deto. Quote
J-Madd Posted September 18, 2007 Author Report Posted September 18, 2007 Thats nothing to worry about. Id worry about a builder who told me that was detonation. Yeah, I know. He kinda "owed" me some porting (long story), so I sent them out. The porting looks pretty good, however, and it runs very strong. And...I think the pitting could just be rough casting that the friction from the pipes has exposed. Quote
dajogejr Posted September 18, 2007 Report Posted September 18, 2007 I can tell you the head won't always show signs of deto... My 10 mil just deto'd a piece of the piston off, just above the ring land on the intake side. The head is 100% perfect.... I had a few top name builders tell me the same thing, deto...after looking at my pics. HOWEVER.... I also thought I was getting enough fuel, I wasn't. And I could hear anything abnormal, either... Quote
J-Madd Posted September 18, 2007 Author Report Posted September 18, 2007 I just had a friend melt/disintegrate a spark plug strap due to the powerjet vibrating tight, then detonating enough to loosen the plug/suck air. Both the head and dome were perfect. I thought that was odd. Quote
dajogejr Posted September 18, 2007 Report Posted September 18, 2007 When I first put my 10 mil together this year, I never thought to check the compression or dome CC. They ran fine all last year on my cub, on gas....never though to check it. Well...I should've. I wasted two trips to the dunes.... 16cc domes on alky near sea level is a bad idea...LOL. I melted a ground strap once, other than that...I popped two orings, and backed out a lot of plugs. The domes are still perfect...although I did have them re-cc'd to 20cc.... Those pistons were fine, too...until my last round a few weeks ago. Quote
J-Madd Posted September 18, 2007 Author Report Posted September 18, 2007 When I first put my 10 mil together this year, I never thought to check the compression or dome CC.They ran fine all last year on my cub, on gas....never though to check it. Well...I should've. I wasted two trips to the dunes.... 16cc domes on alky near sea level is a bad idea...LOL. I melted a ground strap once, other than that...I popped two orings, and backed out a lot of plugs. The domes are still perfect...although I did have them re-cc'd to 20cc.... Those pistons were fine, too...until my last round a few weeks ago. Off my original subject a little, but on those domes..........they guy that melted the strap has 17 cc domes (240+ psi) on his 4 mil stock cyl alky motor. We think that compression is a little too high for drag racing, even with alky...watcha think? Squish is 0.040'', timing at + 5, I think. Quote
dajogejr Posted September 18, 2007 Report Posted September 18, 2007 WAY too high... Guys around here run 160 to 180ish. Squish....that's another subject. There are a lot of factors involved in determining that. However...on my 4 mil and 10 mil, I run mid 40's...(.045 to .048 or so...) There's a real good thread on PS about squish, with several good builders and their input.... Quote
J-Madd Posted September 19, 2007 Author Report Posted September 19, 2007 WAY too high...Guys around here run 160 to 180ish. Squish....that's another subject. There are a lot of factors involved in determining that. However...on my 4 mil and 10 mil, I run mid 40's...(.045 to .048 or so...) There's a real good thread on PS about squish, with several good builders and their input.... Yeah, just read it today. :beer: Quote
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