Spark plugs gap 68 has a supercharger

[1969shelbygt350]

spark plugs gap for 68 GT with supercharger!
I set my at 30.

Any recommendation!

Thx
Phuong

[gt350hr]

      50 years ago when I ran a Paxton on the street in my '66 GT350 , I found the BTF31 heavy duty plug at .028-.030 worked better than the normal .035. Yes this was with "old school" points but the increased cylinder pressure was happier with the smaller gap. In modern higher boost applications I have gone as low as .018. Far different than a non boosted gap which can range from .035-.045 with electronic assistance.
    Randy

[shelbydoug]

The term I've heard with superchargers is that the blower tends to "blow out" the flame. As a result you need to close down the gap.

The more the boost, the smaller the gap.

So far at 28psi on my Audi TT, I've left the gap stock, which was for 10-12psi. This is a bigger turbo with faster spooling rates.

The problem with this thing is it blows of the intake elbow which is a hose rather than a pipe. I haven't figured out how to fix that. It's a tight 90 with a 2" outlet to a 3" inlet. Nothing made aftermarket for it other then stock.

Once you go over about 12psi, you start to have these plumbing issues. I put up with it because 400hp out of a 1.8 liter 4 is pretty fun. Getting stuck along the roadside in the rain isn't though.

[pbf777]

The term I've heard with superchargers is that the blower tends to "blow out" the flame. As a result you need to close down the gap.

The more the boost, the smaller the gap.

     I understand the use of the term ("blowing out the flame") as bantered by the blower crowd, but I disagree as to the image presented, as the intake valve has been closed for a period before the spark is initiated at the plug. But rather do understand that as cylinder pressure is increased so is the resistance in the formation of the spark function; and also understand that the non-O.E.M. tuning executions generally exhibit an overly aggressively over-rich condition which may also contribute to "fuel-wash" of the plug further enhancing the alternate electrical path as apposed to the spark jumping the gap appropriately.   

The problem with this thing is it blows of the intake elbow which is a hose rather than a pipe. I haven't figured out how to fix that. It's a tight 90 with a 2" outlet to a 3" inlet. Nothing made aftermarket for it other then stock.

Once you go over about 12psi, you start to have these plumbing issues. I put up with it because 400hp out of a 1.8 liter 4 is pretty fun. Getting stuck along the roadside in the rain isn't though.

     Obviously I'm not present to witness the execution or installation of this elbow, and being such (both) does compound the "fix", but If I may suggest a link or strap (one may be sufficient or two may be required) of rigid material spanning the distance perhaps from one rigid structure to the other, thereby limiting the ability for them to expand away from one another due to the flexible connection, as this is often the practice for a cure.     

     And of coarse, easier said than done!     

     Scott.

[shelbydoug]

It's actually not the elbow that is causing the issue. Whatever would be used would still need a coupling. The clamps can't hold the connection dependably.

[gt350hr]

    Doug ,
       It probably happens when you roll off of the gas pedal . Boost is up and all of a sudden the throttle closes causing a big pressure back up.

     Boost pressure increases the resistance for the spark to "jump" the gap . Reducing the gap decreases the resistance allowing spark to happen when you need it. I have seen video studies of combustion ( with the "crystal" head single cylinder test engine) that relate to compression ratio flame propagation , boost influence fuel octane rating etc. One thing is consistent , the "big bang" doesn't occur BECAUSE of spark intensity as some ignition makers would like us to believe. The spark HAS to jump the gap to light the mixture which is on the verge of self ignition anyway. Higher spark energy ensures the gap will be jumped under more extreme ( combustion) conditions . To gain a "more complete" burn , multiple plugs would be a benefit. The multiple sparks are sparking in an area that has already fired (hard to visualize without actually seeing it). Where as an additional spark plug across from the original ( difficult to do on conventional "wedge" heads) would improve burn quality and burn time. People see "static displays" of various electronic ignitions that look like arc welders or a Tesla ball and are in awe. That same spark in a chamber at 150+psi and loaded with fuel vapor/raw fuel , doesn't look as impressive.
     Randy

[pbf777]

     Other than in the application as mentioned  by Randy, of poor chamber/piston design perhaps creating dead zones of incomplete or only partial combustion (particularly of the F-heads, L- heads, T-heads, etc.), the use of multiple spark plug installations may have first appeared to overcome the simple unreliable ignition systems of early on (referring to that of something that qualifies as a spark plug vs. say a hot tube or other attempts at ignition), and also instances of dual systems consisting of battery ignition (with or without dynamo replenishment) for one plug set, and perhaps magneto for the other;  but also has been utilized in the responsibility of initiating multiple flame fronts within a single cylinder of larger bores (remember Henry's 999 was 1155 cu. in. in only four cylinders! and was a dual plug motor) so as to reduce the flame propagation time interval, perhaps better control of the period of peak combustion pressure, and thereby reducing the propensity for detonation that would be experienced otherwise.   

     Perhaps interesting, but ain't got a thing to do with your '68 GT!   

     Scott.

[shelbydoug]

There is a dual plug head for one of the 911 Porsches. I think it's the 935 which is the race version of the 930 which is a dual turbo 911 (depending on which version you have). I'm not sure why exactly the dual plug is better?

They're only about 450 under race prep. It was the 917's that were 600 something hp and they are flame throwers also.

My 84 930 was only rated at 285hp stock. I'm right at 400 now with this TT and it is blowing that elbow off under pressure, not stepping off.

The elbow and the intake are both machined to set the clamp against a "curb". I just threw a code for a lean intake. I have to check to see if I split something or the clamp is ready to slip off again.

There is something going on with the elbow under deceleration and we do think it is collapsing the  elbow. I need a camera under the hood on the thing.

The 935's are the cars that if you are behind them on the track, as the driver steps off going into the turn, throw about a six foot flame out of the exhaust. You don't want to get too close unless you don't mind singed eye brows.

The problem that I had with the Paxton was based upon engine rpm, it had to be limited to 6,000 rpm otherwise you would toast the fluid. Since it is automatic trans fluid, it would smell like a bad automatic transmission. After 3 rebuilds, it was time to move on to a different solution.

Now the blower in my SVT Contour, stripped the splines off of the input shaft on the blower. The solution was to "update" the blower to the tune of $2,500.

Supercharged cars just have abnormal issues.

I'd rather stay with my 2x4's and my Webers. Those don't snap crankshafts in half.


Incidentally, broken in half 289 cranks is the reason Ford revised the firing order on the 351's. How many broke is anyones guess but for Ford to take notice of an issue means more then just a few.

They can be a bunch of numbnuts. Consider the '68 428cj's running out of oil.

How long was the 428 in production before they noticed that they had a problem?


I'd consider going to a 351 firing order on a supercharged 289/302.

[pbf777]

The 935's are the cars that if you are behind them on the track, as the driver steps off going into the turn, throw about a six foot flame out of the exhaust. You don't want to get too close unless you don't mind singed eye brows.

     Although enrichment to the point of excess is inevitable due to the throttles transitioning from say W.O.T. to closed, as the fuel which has dropped out of the air stream and accumulated within the induction track flashes, as the manifold pressure abruptly drops; but also often a value defined as a "fuel over-run" sum is intentionally programed to further this effect as believe it or not, all that flame is actually the byproduct of a cooling function.         

     Scott.

[shelbydoug]

The 935's are the cars that if you are behind them on the track, as the driver steps off going into the turn, throw about a six foot flame out of the exhaust. You don't want to get too close unless you don't mind singed eye brows.

     Although enrichment to the point of excess is inevitable due to the throttles transitioning from say W.O.T. to closed, as the fuel which has dropped out of the air stream and accumulated within the induction track flashes, as the manifold pressure abruptly drops; but also often a value defined as a "fuel over-run" sum is intentionally programed to further this effect as believe it or not, all that flame is actually the byproduct of a cooling function.         

     Scott.

If you're close behind it, it ain't cool.

[gt350hr]

  Doug the Porsche is a "hemi" layout and the reason a dual plug will work. Ford did that on a 2300 Ranger engine that had a "twisted" valve layout. The dual plug "lights" the fuel at both sides of the chamber that in theory converges in the center and gives a more complete burn. Other engines use specially designed chambers to enhance the combustion process.  Ford played with "high and low" spark plug locations on the SOHC and Boss 429 there are "period" photos showing the different ( than production) locations.

     The Paxton unit was not designed for high rpm use . Slippage in the ball drive cause a tremendous amount of heat that cooked regular ATF. I found B&M trick shift ( lousy for an automatic) to work well with a bizzare cactus oil product called Alaskan Brand engine oil coolant. I rarely hurt drives at 7,500 and an unheard of 13psi boost.

You are right about the oil pan capacity of an FE. Guess what MANY guys do? Add a HIGH VOLUME oil pump for "insurance"  LMAO
     Randy

[shelbydoug]

  Doug the Porsche is a "hemi" layout and the reason a dual plug will work. Ford did that on a 2300 Ranger engine that had a "twisted" valve layout. The dual plug "lights" the fuel at both sides of the chamber that in theory converges in the center and gives a more complete burn. Other engines use specially designed chambers to enhance the combustion process.  Ford played with "high and low" spark plug locations on the SOHC and Boss 429 there are "period" photos showing the different ( than production) locations.

     The Paxton unit was not designed for high rpm use . Slippage in the ball drive cause a tremendous amount of heat that cooked regular ATF. I found B&M trick shift ( lousy for an automatic) to work well with a bizzare cactus oil product called Alaskan Brand engine oil coolant. I rarely hurt drives at 7,500 and an unheard of 13psi boost.

You are right about the oil pan capacity of an FE. Guess what MANY guys do? Add a HIGH VOLUME oil pump for "insurance"  LMAO
     Randy

Yes, the oil pan situation makes me nervous. The Ford fix was to add two more quarts and a windage tray to reduce the crank spinning in the oil with the stock oil pan.

But that was on the 69 CJ's and did nothing for a 67 GT500. Pumping the pan dry is pretty easy.

They made you think everything was alright by re-calibrating the dip stick. Real tricky? No, but cheap.


I remember being on the track at Utica in '78 and a 67 or 8 GT500 blew up in front of us at about 150 coming down the straight. It was kind of like a hand grenade going off except you had to drive through the smoke blind.

I think I need to face the reality that my 67 500 is a day two car, to accept that and put a good high capacity road race pan on it.

I know that I occasionally get overcome by another personality and can't control him. He hunkers down in the seat and his right foot goes crazy. I noticed in the rear view mirror that his canine teeth grow during this period of speed lust.


It's old news now but I remember Garlits talking about the limitations of the Hemi and it was about flame travel and spark plug location.

I can understand that with his 14:1 cr and domed pistons. But that engine was making 1000hp. The Porsches weren't.

Of course the availability of CHEAP 115 octane gas in the day helped the configuration. Not today.  As Dylan said, "It ain't me Babe, no, no, no, it ain't me".

[pbf777]

It's old news now but I remember Garlits talking about the limitations of the Hemi and it was about flame travel and spark plug location.

      Yes, of all its' popularity, the hemispherical combustion camber from the standpoint of providing ideal turbulence and geometric shape provided at the moment of ignition and for promotion of flame front propagation including duration, is less than the best. But this chamber design, particularly in a two valve configuration, does also lend itself to the dual plug adaption readily, which does aid in offsetting some of these drawbacks.

     And as witness to such, it is understood that the "Hemi" is also not the cleanest from an emissions standpoint, which left me surprised when Chrysler announced the current generation of Hemi's; but alas, they have proven to not be "true Hemi's" after all!     

     Scott.

[gt350hr]

    Ford engineers knew the problems from the SOHC days and tried to correct it on the BOSS 429 by adding quench pads, It helped but they also recut them to full hemi. The "current" kidney shaped chamber is the best around ( for NATURAL aspiration applications). Boosted applications follow different rules.
  Randy

[Bigfoot]

Very interesting thread. Like the ole dayz......