tpi1986

I have a wierd problem for you guys. No not that kind of problem.

I fired my race motor last night and it ran fine for a moment. Then it began to miss and sputter. I check the wires etc. Finally I pulled out the plugs and the gap was closed on nearly all of them. Those plugs were autolite. We had a new set of Champion plugs, put them in and the same thing happened. The heads call for a .460 reach 14mm thread tapered seat. Exactally what both sets of plugs were. I called Jegs and they told me it sounded like the Compression closed the gap. There wasn't any damage to the plugs other than the gap was closed. Not even a mark on them that I remember. The CR is 12:8.1, the cam is a 260 @ 50 with a .680 lift. So I imagine that it is building up some serious cyl pressure. But I have never heard of compression closing the gap. Has anyone heard of this???

bogus

damn... I have never heard of that.

I would get a bore scope and see if any signs of the damage to the pistons.

Did you use a thinner then specified head gasket?

You may want to index the plug, too. just to be sure.

tpi1986

No the head gasket is correct. I did not index the plugs. Since the plugs were not fubared I figured that the pistons would be okay. But at this point anything is possible. BTW I forgot to mention this is a 454 +30.

Any help is appreciated.

bogus

big block? gee...

I donno what to say. I have never heard of this happening due to compression.

VAHROOM

I have friends that ran big blocks with 14:1 compression that really abused the engines. The plugs on those motors never had the plug gap closed. Even turbo and SC motors I have seen with all that boost running don't close gaps. I would look for other causes.

tpi1986

UPdate

I spoke with a rep from jegs, he told me that the plugs were closing due to high compression Sounded very strange to me. I posted this info and CFI-EFI responded that was Ridiculous. He was correct. I then called Champion Spark Plugs and they told me that when the BBC with my heads CR is 12.5 or more that I have to index the plugs to clear the dome of the piston.

BTW that is the same recommendation that I received from the forum. Lets try it and find out. Thanks bogus and CFI. It is times like this I question my interest in cars.

Thanks

Later

aboatguy

I think the techs are feeding you some BS. How can compression close your spark gap. There is pressure through out the cylinder I would bet that you have mechanical interference. Can you measure the distance from your piston (at TDC) through the spark plug hole with a vernier caliper or other device.

Mike

JAKE

Compression closing the plug gaps is BULL!

I believe your pistons are hitting the plugs causing the gaps to close up. That's bad news, especially if it's happening at idle. As the RPMs climb the interference worsens because of the stretch that occurs.

BBs with knuckles on the domes are know from that. I use to have to grind a fire slot in the piston dome for the needed spark plug clearance.

Didn't you clay the domes during a trial assembly to see what clearances you have?

You can try indexing the plugs to position the ground strap 180 degrees from the piston dome.

You can try grinding down the ground straps on the plugs so that the tip only extends to the center of the electrode, not beyond it.

You can try running different plugs, same heat range but without the projected tip; that's if you're running PTs now. Sometimes just that change can give you the clearance you need.

There are retracted tip plugs available too, but they may cost a little power.

The only other options I see would be a much thicker head gasket, but depending on how much clearance you need, even going with a .060 might not be enough.

Final resolution would be to have the piston domes slotted. If you decide to go that route, you should first clay everything to see just how much needs to be removed. You'll have to check with your machinist to see if re-balancing will be required.

Hope this helps.

Jake

bogus

what about a thin washer on the plug?

ALLT4

This might sound unorthodox, but if this were happening to me I'd grossly open the gap, turn the motor over by hand if possible. Pull the plug and have a look see at what's going on. Something’s telling me you could come up with some math to determine the amount of clearance you'll need but it's escaping me. Probably because I'm lazy.

airdeano

im suprised the champion rep didnt offer a colder plug. the colder plug
has the projected tip retracted closer to the plug, away from the
piston dome.
if youll look at a race plug the tip is bearly visible above the thread
base.
using a thicker washer will pull the plug away from the dome, but will
raise the heat range of the plug.
its not compression, its interference from the piston dome making the
compression....
airdeano

jfb

Cylinder compression can't do anything to the position of the ground arm of the spark plugs. The entire surface of the plug ground arm is under the cylinder compression and therefore there is no net force to move the arm. What is happening is the piston is striking the plug arm which applies a closing force to the plug arm and reduces the plug gap.

OneRedLT4

Saves me a bunch of typing.
Aside from the automotive application, sticking a sparkplug in a compression chamber will never cause the electrode to fold over due to extreme compression. Something had to be coming up and it.

tpi1986

Update. It LIVES IT LIVES! I indexed the new plugs last night. Kind of a pain in the axx but not to big of a deal. When I hit the switch it came to life. Sounded great! The smell of spent Cam 2 fuel filled the air.

I don't know what that goof at jegs was on, but I don't want any part of it. I have said it before, I love the forum. There are so many members that really try to help. It is often surprising to me just how hard everyone tries to help and the honest nature of so many members to find an answer.

I hate FORDS 2

bugus, jake, CFI-EFI, airdeano and anyone else that I forgot's handle, I really appreciate you taking the time to help.

This project is far from done, but now that it can move inder its own power (No more pushing it ) and sounds soooo good it is very inspiring. Thanks again.

cv67

Congrats!Now...sound clip?

JAKE

That's what's used to index the plugs. The kit comes with an assortment of copper (I believe) washers. You just try different ones until the ground strap stops where you want it when the plug is tight.

You just mark the plug with a marker (I use finger nail polish for most of my "marking" chores). That way you'll know where the strap is positioned.

Jake

JAKE

YEA!!!!!!!!!

Jake

BigC4

Dang that seems like some whack info from Jegs and Summit. I have to agree with the guy up above stating that all of the pressure surrounding the plug parts is equal "hydrostatic" I think, so there is no movement due to high pressure. Sounds like the piston is touching the plug.

LT4BUD

Indexing a plug to prevent it from hitting a piston dome sure sounds like living on the edge to me!!!!!!

But if that is what the experts say, so be it....learned something new..

JAKE

Don't know if I consider myself an "expert", but his problem is common with Big Blocks running knuckles. In addition to notching the domes, BBs call for a few other little tricks to get the most out them; things the SB doesn't require.

The combustion chambers are so big on the big port open chamber heads (in the area of 119 ccs before decking) that in order to get decent compression for racing, the pistons require pretty good sized domes. Notching the domes for plug clearance is a common practice.

I'm just glad to see that indexing the plugs took care of it; at least so far. At high RPMs, the pistons may kiss the plugs and they may still close up. I'm hoping for the best though.

Jake