BBC CYL #2 Leanout

I managed to step on my unit this weekend and melted #2 plug,as well as blowing out the burst panel.
I am familiar with the problem of the BBC wanting extra fuel in #2 and I went too far without testing, but this left me with two questions that I had not been smart enough to ask in the past.
1. Why does it want more in #2? I have more than 1/8 of the total nozzle area at the port in #2 and it is not enough. Does that cyl actually get more air? If so, how? --or-- Does that cyl get more heat, or hotter air? If so, why?
2. This may sound stupid, as we all know that leanouts cause backfires....but what is the mechanism of it? How does the compressed air in the intake manifold get lit? If you do not have enough fuel to support combustion in the cylinder (too lean) where does the fire in the intake come from.

I realize that I may have some other problem that just coincidentally occurred causing the burst panel to go, like a piece of the (melted) spark plug or piston getting under the intake valve seat and allowing combustion to leak by(BOOM) but it is generally accepted that leanouts alone can cause this.

Any ideas? I need some salt for my humble pie

Kendrick

 

I think when it goes lean it produces hydrogen gas that expands back into the intake when the valve opens for the next cycle. Where it gets the ignition source from I can't recall may b from the cam overlap and the hot exhaust gas. I read it in how to make 5k hp on alc. By Bob Sazbo (SP?) I may be wrong it's been a while

 

I found that on pg 162 of Szabos book after reading your reply, thank you. There is a further reference (Adams, Tim G. SAE #84512) to the disassociated hydrogen (due to the higher temp caused by the lean condition) being the likely culprit as it would easily be ignited by something like a glowing spark plug electrode when the intake valve opened up.
Nice.

Kendrick

 

I ran a bbc with a conventional Chevy head with a 14-71 hi-helix ,#2 was always my problem child also,always had alot more fuel in it than others, since switched to a hemi with same blower and the problem is not as bad,but my next step might have been to take some compression out of #2 ?????????

 

U could set the blower back. I often thought about choking that port down in the intake ??

 

#2 gets more from its physical location in respect to the way the air and fuel is coming out of the blower.

Were any intake valves not sealing? As in a leanout for me usually gets the valves first and than you have an ignition source for lighting up the intake manifold.

 

#2

Get a PSI screw and number 2 will be the fattest in the engine. I think it's the intake and the way the air discharges from the roots. I think a setback blower would help.
My big chief is a lot better than conventional heads but the intake looks like it had some thought put into it.

 

Welcome to my world--try running two BBC's with your same type of setup. I don't have the answers why. I just know I fuel the heck out of #2 and I also use a B10 NGK instead of B9 plug (supposedly gives me another 150 or so degrees less heat). I am told that setbacks will help the issue. In one of the heads (Brodix BB2X), we even changed the configuration of the bowl to change the flame front a little. I have to say, that this year was our best as we did not take out any heads or head gaskets. We did pop a blower a couple of times (new rotors are expensive).. Supposedly, Gerardot is developing a new intake for BBC that will help to alleviate some of this situation. I know this doesn't answer any questions, but just know that we of blown BBC users put up with the situation as best we can!! lol JW

 

I had the same issue. I put a 12 inch snout on my blower which put the opening of the blower all the to the back of the intake. It completely solved the problem. My port nozzles are all within 3 numbers. EGT's are all within 100* down track.

 

The Indy manifold does a pretty darn good job in regards to the stated problem. As with anything it can be made better. I've been messing with 2 of them for several years and for the last 2 years been messing with an intake we made from scratch, 12" snout, etc. What manifold are you guys working with?

 

We have an old school Littlefield manifold with a conventional open bottom 1471 blower. I wanted to set it back some to begin with but we have to modify the body (23 T Altered). I was wondering if the set back was a good idea with the open bottom blower, but it is starting to sound like a better idea than melted parts!

I was wondering if running a higher percentage of fuel through the hat might drop the temps some, I'm already running #10 plugs.

If I am understanding things right it sounds like the manifold/blower combination is causing it to collect heat in that corner, as opposed to extra air. I wonder how I could measure that on my data recorder?

 

The easiest, quickest and cheapest thing you can do is make that port nozzle big enough to not melt parts.

Its not collecting heat, but instead its that blower/intake combo that is getting more air into that cylinder. I had some cragar, BDS, mooneyham style intakes and the worst was a spread of port nozzle numbers from about 25 to 70.

I think you would happier with a Indy style manifold with a short snout, but it will be taller and you will have to experiment through distribution again so like I said, fatten up that cylinder and go racing.

 

Went through this same deal with a roots and now with a screw. It’s the opposite with a screw but the same issue. Some cylinders get more air so they need more fuel so some cylinders make more power. Roots make more power on the front and the screw on the back in a BBC. Moving the blower or the discharge of the blower in the manifold will fix this issue and straighten out the distribution and the power each cylinder makes. Need to dyno the combination to see what works the best.

 

The Indy manifold is a good choice as is moving the blower back. Couldn't afford to do it last year--that happens when you break them 3 or 4 times a season! However, I have found that when you get really big on the nozzle, you lose fuel pressure (atomization) in that cylinder and the fuel might have a tendency to "dribble" in which in theory would make it leaner ( I think). So, this past season we had the manifold (Littlefield) drilled and tapped for another nozzle and ran two nozzles in the #2 runner. I was amazed at the numbers that I put in them to keep that cylinder cool. We went from one 84 or so (blew sh&t up) to two 65 plus (not sure of the numbers without looking at the data sheets). Miraculously, we did not burn pistons, scorch heads, blow head gaskets, etc. this summer. Realize, I tractor pull (minirods) so our runs are a little different than drag racing, but the punishment we put the motors through is no less extreme. If you can do it, get a new manifold or get the blower set back. That is the better option. JW

 

If your front cylinder nozzle jets are getting really big, you could put more fuel through the blower. On a smaller bottom opening blower, most of this fuel ends up in the front 4 cylinders anyway.