Hey, guys--

I'm building a hp 300 from 1975, with an '81 head. I was wondering why most hp builds go with the EFI plug? Can some one explain why I should go that route? Are we striving to get as close to a Hemi head as possible?

Thanks

 

are you talking about the spark plugs? a hemi head is named that way for the shape of the combustion chamber, and coorespondingly the positioning of the intake and exhaust valves. the combustion chamber is half a sphere, or hemispherical... a hemi... it has nothing to do with the plugs. there are a million different opinions on why to choose a spark plug, how to index them, how to gap, etc... i'm not sure what it is youre looking for?

 

Am I mistaken to believe that on a Hemi head, the spark plug screws straight down into the center of the combustion chamber?

And if that is the case, my question is if by using a longer plug, the EFI plug, one is trying to get as close to the center of the chamber as possible, thereby nearly dublicating the effectiveness of the Hemi head, or trying to get as close to it as possible. That is what I meant.

 

Not a scientific response but...I suspect it's for the same reason OEM went with the extended reach plug on the EFI engine much like some of us use the EFI exhaust manifolds as a poor man's upgraded exhaust system on a carbed engine...a wee bit more performance and/or efficiency.

 

well, for one thing the efi head had a different combustion chamber than the old carbed head. yes, on most hemi heads the spark plugs screwed straight down. in that case the spark plug was directly between the intake and exhaust valves. it was kind of a design necessity. be careful, because there is more to spark plugs than how far they reach into the chamber. be careful the plug doesn't hit the piston at TDC. also, the heat range of the plug is important to keep fouling down. you'd do well to index the plug in a manner that the open end of the plug is facing into the chamber, rather than shrouding the spark. a search on indexing spark plugs will discuss this better.

 

I was wondering if the guys with the older 300's--pre EFI--
could tell me which of the EFI plugs you are using, and what you gapping them at. I know the gap will vary from engine to engine, but wanted a starting point.

 

I think the model number for the EFI plug is probably pretty much the same regardless of the year. That is easy enough to check with a parts house though. I set the gap on mine at 50. If you feel like tinkering you can try anything from the stock gap up to 60 and see what works to your liking. They will tend to wear out a little faster with the wider gaps I hear.

 

i use autolite coppers for my 95 with the 300. as far as gapping, i cant remember off the top of my head...i did whatever it says factory specs. are..