finally got in the 13's but......

[darktone]

well the underdrive pulley's and bumping the timing got me a 13.973 and a 13.972. I changed the secondary spring from a yellow to a purple and ran a 13.891 and 13.884. I still have something going on bog/hesitation wise a few feet off the start so I think I can gain a bit more yet. Anyway's on the last and quickest run I deleloped a miss so I packed things up and left but the miss was bad enough I stopped and pulled the plugs. One plug had the ground strap bent and almost touching the electrode. I put in a spare plug and it seemed just fine. What could have done this?

[dave5711]

Did you do any plug reading at the track?

Reason I ask, is usually the only thing that'll bend the electrode is a foreign object hitting it. +
BUT I have theorized (long story I won't get into) that if the mixes were way off, and the ground strap glowing as a result, it may get bent on a predetonation, or who knows when.

[darktone]

Dave, those are my thoughts pretty much too. I have not checked the plugs in a while but I think I will put an A/F guage on it. You know it seemed to run too good when I was on the run so it must have happened after I let off. I have not checked compression yet but it sounds and feels good.

[bobr]

What's your combination? I've had some pretty close piston to head combos do that. I just used an extra plug washer to cure the problem. -Bob

[darktone]

I thought about that too Bob. 74 360- stock heads with 2.02/1.60 valves,116 pistons, block has been decked and the heads shaved. Comp cams xe268. I thought about a valve hitting a piston and getting pushed into the plug but I think I would have had a few more noticable problems if that was the case.

[thatwasfunny]

Can u shine a light into the plug hole with piston almost all the way up and c any pitting or a very clean new looking piston top?
What carb, what jetting, what timing, and what cranking psi?

[darktone]

holley 770 with stock jets, 35 degree's total timing, I have never really checked cranking psi but on paper it worked outer to about 185psi. I was just getting to the point of playing with carb jets when this problem occurred. After thinking things over I bet the piston must have done it. It really must have barely hit it as I can see daylight between the ground strap and electrode. Also I can't really see any marks on the strap where it made contact with anything. Maybe this got bent bent days before and the daylight I could see was enough to still spark? When I pulled the plug it was not wet so it was firing somewhat. Well to turn a negative into a positive this could have costed me a liitle time on my ET- maybe I really should have ran 13.7.

[thatwasfunny]

Right on.
Say which cylinder?
Is your compression on paper counting for cam bleed off?
On the bog you had before you might wanna play with the shooter/pump cam,
does it go then stumble or stumble then go?

[darktone]

#4 it was the second plug I checked- my heart was racing as I checked the rest. Being a mechanic I have got used to seeing stuff like this when timing belts break but it not that bad when it's someone elses car and I make money fixing it. When I changed the secondary spring it seemed to get a little better- maybe changing to the next heavier spring will help- my plan was to try this on my next pass. I wanted to play with pump cams too- I was thing of trying one that would work better with foot braking the coverter to 3,000 like I have been launching. A 60 foot time below 2.0 would be great as of now I am in the 2.02-2.05 range.

[bobr]

"After thinking things over I bet the piston must have done it."


Either get some spark plug indexing washers or just use another plug washer. You are probably close on all of them. The rod will stretch a little at high RPM or if the crankcase temp gets a little warm causing the interference. Are these extended reach plugs? If so, try a non-extended reach plug. -Bob

[darktone]

I was thinking of indexing the plugs. What would be the best position to put the ground strap? Straight up or sideway's? I think I am lucky only one hit and that it was a very light hit that appears to have done no damage. Now I am kind of anxious to get back to the strip. What about cutting back the strap?

[thatwasfunny]

I just switch plugs around till they index correctly and sometimes buy a couple extra to make this work.
What I do first is get a marker and draw a line on the porcelain that is 180 from the ground strap [in line w/ the open side] and have the 'line' end up pointing at the exhaust valve when tightened.[on sb position ends up being like 10-2 o clock]

That's 1 way.

[darktone]

Originally Posted by thatwasfunny

I just switch plugs around till they index correctly and sometimes buy a couple extra to make this work.
What I do first is get a marker and draw a line on the porcelain that is 180 from the ground strap [in line w/ the open side] and have the 'line' end up pointing at the exhaust valve when tightened.[on sb position ends up being like 10-2 o clock]

That's 1 way.

that would put the ground strap closest to the piston right? That is how you want to do it for the best flame travel? But I am trying to the most clearance I can get.

[bobr]

Indexing is a waste of time insofar as making power. I've tried it on a dyno with absolutely no gain. Just put an extra washer under each plug to get it out of the piston's way. Once again, are you using an extended reach plug? If you are just changing to a non-extended reach plug will most likely solve your interference problem. -Bob

[darktone]

Bob I am just using the stock rn14yc. Should I go with a colder plug? I just did a compression test and they are all between 182-188 with the hole with the problem being 186. The compression seemed to up about 1psi per cylinder- I am guessing this is from getting a little more oil in each cylinder. I put the piston in the problem whole all the way up and tried to measure clearance with a small screw driver and it looks like I have a 1/4" between the ground strap and piston. Things could not expand this far? Looking at the piston top it looks like it is peppered with little shiney spots around the plug area- detonation? Thing this warrants a tear down? Or just try it a while as is?

[thatwasfunny]

Sounds like detonation to me.
Either get better gas or back the timing to 30-32 and give it a try.
Can u post a pic of the spark plug for us?
What brand what #/heat range.
Tear down? not yet. Sounds like u caught it early.

BTW Are these mag heads? or open chamber?

[darktone]

I can get a picture tonight or if someone can post it for me I can send an email adress I can send a picture with my phone. They are Champion 405 plugs rn14yc's. These are just the stock 74 360 heads with 2.02/1.60 valves and some home porting and .014 shaved. I was going to mix 50/50 with 110 this weekend but they wanted $10 a gallon locally. I guess I'll back off the timing unless I run race gas. What would a better choice for plugs be?

[donsa]

y = extended

could try RN 14 C .

[darktone]

I measured and I have more than a 1/4" clearance between the plug and piston so I don't think it's possible the plug hit the piston. I am leaning towards detonation as the cause. Forgot the plug on the workbench but I'll try to get a picture tomorrow.

[bobr]

Plug heat range is TOO HOT. This alone may or may not be causing your problem. I would try a RN9YC. If you have a quarter inch clearance you have plenty. I'm now leaning toward detonation or just a plug that is the wrong heat range. For insurance I would get some 108 octane racing gas and mix one gallon to 3 gallons premium or retard your timing about 5 degrees. Retarding your timing might cost you power so get the racing gas AND CHANGE YOUR PLUGS TO THE NUMBER I RECOMMENDED. -Bob

[darktone]

thanks Bob. I will use the plugs you reccomend. I guess I was a little naive in not realizing I was on the verge of destroying my engine because of detonation.

[crisser]

182-188 is getting a little high for pump piss. I try to run 170-175 max on pump gas with iron heads. That xe cam is a fast fill and it`s kinda short on duration. I too think the motor is detonating. Try the plugs that Bob stated first. Then, all the usuall temp reducing things next. If that doesn`t work, try retarding the cam 4 degrees. If that doesn`t work, might want to step up to the xe274 or try another non xe cam.

[darktone]

Yeah- I'll try those plugs,use some race gas at the strip and set my timing back on the street.

[bobr]

Originally Posted by darktone

Yeah- I'll try those plugs,use some race gas at the strip and set my timing back on the street.

Perfect! Let us know of any performance improvements. I think they are forthcoming. -Bob

[darktone]

I was wondering if I could get some better times when I correct this problem. Seems to reason I should. I am completely happy with my my times last week and have pretty much met my goal of mid 13's- another tenth or two and I am there.

[crisser]

Originally Posted by darktone

I was wondering if I could get some better times when I correct this problem. Seems to reason I should. I am completely happy with my my times last week and have pretty much met my goal of mid 13's- another tenth or two and I am there.

Absolutely. Detonation kills engines and the time on the board.

[aarracer]

Maybe reduce the maximum advance, and stop getting little aluminum balls on the plugs, and increase the mid range torque to go faster?

[darktone]

Originally Posted by aarracer

Maybe reduce the maximum advance, and stop getting little aluminum balls on the plugs, and increase the mid range torque to go faster?

what can I do to increase mid-range torque? I am pretty happy with my basic combination and don't want to crack the engine open. Like I say a mid 13 was my goal.

[bobr]

Originally Posted by darktone

what can I do to increase mid-range torque? I am pretty happy with my basic combination and don't want to crack the engine open. Like I say a mid 13 was my goal.

Get your tuneup correct and you will be right where you want to be. -Bob

[aarracer]

Originally Posted by darktone

what can I do to increase mid-range torque? I am pretty happy with my basic combination and don't want to crack the engine open. Like I say a mid 13 was my goal.

darktone

There is no way you can get your tuneup right without doing a running tune on a dyno (well maybe with 50 - 100 passes and unchanging conditions,lol).
To the nay sayers -you're better than the rest of us, maybe, lol.

I would get over to someone's chassis dyno and get the air/fuel ratio corrected to somewhere under 14-1, possibly as high as 12.5 - 1, (yours sounds like it may be quite lean). Then set the timing to where your torque curve shows best in the low to mid range 3000 - 4000 rpm.

I found that moving my timing back a few degrees from 39 back to 35 at 2000 rpm increased my midrange torque by 25 lb.ft.

You can get a baseline on the first pull and then start the tuneup.

I believe you will be pleasantly surprised.