Last post on Aug 05, 2001 at 9:11 PM
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Dec 29, 2000 (12:42 am)
At risk of over-simplifying a complex issue, yes in some cases. Idle and part-throttle cruising require less octane because the cylinder pressures are not as high. At what point does cylinder pressure rise enough to make fuel quality an issue? That depends on many factors including the engine design itself. The recommendation for 11:1's is around 100 octane.
One of my engines is an 11:1 402. I've run this one off 92 w/octane booster and a few degrees of timing removed. It was ok for putting around but it's not the blend to be using if you plan on racing someone.
Dec 29, 2000 (7:54 am)
You dont' want to mess around with severe detonation. If you've ever taken an engine apart that has failed from detonation, it's not a pretty sight...it's really a violent event, and quite a surprise to see holes punched right through the tops of pistons, or cylinder walls shattered by the force of the mishap. Often you can just throw that block away.
Dec 29, 2000 (5:30 pm)
Cylinder head design does make a big difference. For instance, I found a big difference between the old style '66 Pontiac heads and the redesigned '67s. Same 10.75:1 CR but the later heads tolerate 92 octane a lot better.
I used to recurve advances with a Mr. Gasket kit that let you take advance out of the distributor and add a few more degrees at the crank. It really sharpened throttle response so I always ran at the hairy edge of audible pinging, but apparently the inaudible variety can get you too.
Dec 29, 2000 (9:36 pm)
What's really annoying is to have the problem only in 1 or 2 cylinders.
#70 of 75 that's interesting...
by MrShift@Edmunds HOST
Dec 30, 2000 (8:44 am)
What do you think causes that? Did someone perhaps not build the engine right...you'd think it would be pretty hard to give an individual cylinder higher compression than the others or more advance.
Dec 30, 2000 (1:36 pm)
One variation is that combustion chambers in mass-produced cylinder heads aren't all the same size. A combustion chamber that's on the small side of production tolerances will give you a higher combustion pressure than one on the large side. That's why when you blueprint an engine you "cc" the heads so each chamber has the same volume.
Sometimes a combustion chamber will have a piece of casting flash that heats up more quickly than the surrounding metal, causing pre-ignition or uneven combustion.
Another variation is air-fuel mixture. Some cylinders will run lean because of compromises in intake manifold design. Those are the ones that ping first.
Dec 30, 2000 (4:09 pm)
True ... and throw in an oil burning problem. I know of at least 1 block where the cooling passages are not equal and 1 cylinder can run hotter making it more prone.
Good point on intake manifolds. That's one of the areas the aftermarket manufacturers have evolved in over the years. On dry EFI systems it's a moot point. Speaking of EFI, new FI/Ignition systems like the FP sefi8lo allow you to take ignition timing out of just 1 cylinder.
#73 of 75 How to drive & water in the carb
Feb 06, 2001 (3:18 pm)
How to drive em with their motors beggin for leaded 100+ Octane? Well if you have an original with a good strong original motor a lot of the value is in the originality but the driveability is not. Solution..... Pull the original engine, detail it and put it on a stand in your rec room. Replace it with a modern monster motor built to today's specs and putting out more horses and a cleaner exhaust.
As for water down the carb... yes it does work but better yet do what some old time racers along with the US Army Air Corps did with some of the fighters in WWII...... Water injection. Its cheap, simple and not only helps avoid carbon build up but increases horsepower by providing a denser cooler air/fuel mix when you're running. You can get extra cute and rig it to work only at near or full throttle or triggered to actuate at a certain manifold pressure.
Aug 05, 2001 (7:09 pm)
When your old carbuerated engine hydraulics(seizes, blows, whatever you may call it) let me be the first to know so i can laugh at ya. Yeah it's good once in a while for blowin out carbon build-up but a water injection system is NOT even an "ok" idea. When you inject water it can almost double normal cylinder pressures and most likely either blow your rings or your head gaskets or in some cases even both. Water injection is only for those extremely experienced in engine building and performance...amatuers can very easily blow their engines.
#75 of 75 IN ADDITION TO MY PREVIOUS POST...
Aug 05, 2001 (9:11 pm)
Water injection would be streetable on very low compression engines with low cylinder pressures and high volumetric efficiency