Ok, I am still confused as to why I need to record the throttle position. It's arbitrary at best and these rochesters aren't exacting enough for very much precision. Explain in detail why throttle position (per test) matters. I wish I had a protractor type control, but the very best I can do is measure how far out of the socket the cable is with a tape measure. Is this still a good idea to you?
Rochester or Jack Benny does not matter. You want to run each test at the same RPM's.
Changing plugs from .035 to .040 on SI or going to HEI can cause idle speed to change. To be testing for SFC the idle speed must be 650 RPM for each test.
So the idle speed screw may have to be adjusted to have the idle speed back to 650. For that 650 is the constant makes the results valid.
Yes you can measure throttle cable length changes. You can also count the number of turns of the idle speed adjusting screw needed to return the idle back to 650 RPM. Most likely in fractional turns 1/16, 1/8, 1/4, etc.
You can mark the pivoting centerline on the arm that the throttle cable is attached to with a magic marker. Then if you can mount a $.50 plastic protractor in front of that arm. As the throttle plate gets moved you will be able to see how much it moved in degrees and whether it was opened or closed.
Now when you are doing the all of the 30 MPH and 55 MPH (both SI and HEI) tests having the engine running at the correct RPM's will not be enough for say if the SI .040 SP gap is needing less throttle to hit the correct RPM and if HEI still needs still less throttle opening the tachometer will not be able to tell you this.
You do not just want to see that SFC dropped. You want to see a reason such as less throttle was needed and that is why SFC decreased.
You NEVER EVER gap an HEI to .035. Its way too little gap and is totally not recommended. It would be like gapping a stock ignition to .025. It makes no sense whatsoever. If I don't understand it the way you do, explain?
Never gap a HEI to .035 for it is not recommended? You are not setting up a pre HEI engine to run on HEI for use on the road.
You are testing how ignition system changes effect SFC. You want to see how SI compares to HEI. Well when you do a test you only have one variable at a time to get valid results.
So when you test SI with stock SP gap of .035 the first test has to be just running the HEI with the stock plug and gap.
Your test results will be valid because you have followed basic testing logic limiting your test to one variable. Then you have a valid 2nd base result to build further tests on.
Now with SFC between SI and HEI with stock gap you then can add a new variable such as HEI with the .050 SP gap. Then again this test's results will be valid because you had a base with one added variable.
THIS PARTICULAR HEI is totally different than anything else out there because of its 'small body' nature. The stock distributor cap, rotor and wires are used and there is very little room in that cap for the extra high voltage electrical impulses to fire without causing a crossfire condition. I know all this because its been fully explained to me by the guru. He says there is no worries of any of that with this system if it's set up correctly.
I do not know how important that space needed by a larger dist cap for I remember or think I remember that there were some GM HEI systems that had the smaller cap because the coil was not integral (GM's words to say the coil was separate from the dist cap) to the cap. Though your Expert probably knows more then me.
So what I CAN do is get a different set of plugs with larger gap characteristics and try them at .060 for starters. I am a little leary of getting too carried away with gap since that fire has to go somewhere, and if it doesn't jump the gap it can go back down the plug wires and do damage to something.
If I was doing the test I would be not feel the need to go past .060. The secondary voltage always wants to get to ground.
Back in the 80's the large HEI distributor's had a large round rotor that cover most of the HEI workings. When a SP wire would go bad that voltage not being able to jump across the plug would try to find a way to ground out to the distributor housing. As the secondary voltage would repeatedly flash over the rotor it would eventually weaken the plastic and burn a hole through the rotor and the engine would stop running due to loss of spark. New rotor and the car would start right up. However if the mechanic knew why the hole was burned in the rotor he would mostly find a bad plug or wire and sell a tune up with new plugs and wire set so there would be no come backs.
Can you recommend a spark plug that is the right threads and correct for wider gaps?
I can not recommend a specific spark plug. You need to get a AC Delco book or Champion book and find a plug with the correct reach, as close as possible to the same heat range that is designed to have a .060 gap.
Care must be taken to make sure that there will be clearance between whatever plug you put in and the top of the piston so they do not hit each other.
The codgers I am talking about are adamant that any engine from that era (1941 to 1962) runs better with .016 and .040 respectively. They say forget about the book, just do it and watch the difference it makes. This is a good time to see if they know something I don't. That part is just out of curiosity.
Well when the points were set wider on the earlier engines at .022 the time spent closed was less which resulted in less dwell time/coil saturation time then when gap was changed to .016. So it would appear that the later cars had more dwell time so they should of had a stronger spark. So the later cars then would of been able to work better with the .040 SP gap then the earlier cars did.
Though based on what I know those old codgers were right. Though we don't know what else GM changed besides the point gap on those later models and why they went with a smaller point gap.
How did you compute the 46.7 rpm for every 1 mph? That would be interesting to put in the article.
GM Heritage Center. For your year truck they give the stat that for every 46.7 RPM's your engine does will yield 1 MPH. I think the Power Glide was 44.something. So 46.7 x 30 = 1,401 RPM when doing 30 MPH.
Deve, this much writing is too hard for my brain. If you like PM me and I'll give you my cell. Jeffrey
