Dwell

[speedy]

A month or so ago we talked about hihger compresion and a longer stroke to run on pump gas.Well this months mopar muscle FEB 2000 did a article on strokers and said the better the rod ratio .the longer the piston will dwell at TDC TO complete the combustion cycle.NOW i figure the reason we use 9.5 compresion on pump gas is because the lower 93 octain is max to fully burn in the combustion cycle .NOW say you build a 451 or the /6 long stroke as far as that go's' you would have more dwell time at TDC TO USE 87 octain or less with 9.5 compresion .OK NOW the ECU controls the dwell but it is not adjustible .SO what's your opinion on this subject guy's??

[340king]

There are two types of dwell. The piston dwell refered to in the article is the time measured in either degrees of rotation or actual time that the piston does not move up or down at TDC. The ECU dwell you talked about is the ignition coil cycling time expressed in degrees of crankshaft rotation.

They both have an effect on what you can run for octane and not ping/detonate. You have to be careful when comparing octanes, as the knock resistance of one 93 octane fuel is not always the same as another. This is becasue of the method of determining the rating. The pump displays the formula R+M/2. This is the research method and measured or motor method added together and averaged.

The research method compares the chemical makeup of the gasoline compared to isooctane. Isooctane is rated at 100. Chemical departures from this "ideal" fuel are given credit by adding or subtracting from the 100 octane rating of isooctane.

The measured method uses a variable compression engine to measure knock resistance. The one I helped work on was from Waukesha. You actually change the compression ratio of the engine while it is running until it starts to ping. The compression is measured from a depth type guage on the sliding cylinder and an octane rating is determined.

The equation R+M/2 was developed at Shell Oil Co. by Bill Groves, one of the original Shell Answer Men. I had the honor to study under Bill Groves and work with the variable compression engine one time.

There are many factors that affect how a certain octane fuel will perform inside the combustion chamber of a four stroke engine. The piston dwell is rather insignificant in this mix in my opinion. The shape of the chamber, heat retention and others have a much larger effect on knock resitance than does piston dwell.

[Christopher]

340King,You know your stuff!!Very informative answer,and that variable compression engine must have been fun to work with.

[340king]

Yes Christopher it was fun to work with. I was one of Prof. Groves pets, or at least respected enough to be asked if I wanted to dissassemble the engine, inspect it, replace a broken part and reassemble it. I jumped at the chance.

The engine was a work of art. It had external pushrods and valve train. There was a cool fulcrum and lever setup to keep the valve lash correct. The engine had a set of rings between the outer cyl. wall and the top sleeve/cyl. head assembly. This allowed the upper portion to slide up and down, varying the compression.

When we finished assembling the engine we fired it up. Then we set out to make the thing ping. It was really cool watching the cylinder slide down and the valve train still work. We cranked it down until we heard the audible ping, and then just released it.

I need to finish up one thing though. I apologize for leaving it out, but I had to go to a big meeting. The reason two different fuels with the same octane rating under R+M/2 could work differently, is if you had one with a R rating of 104, a M rating of say 84, the average would be 94. You could also have a fuel with a R of 99 and a M of 89, the average would still be 94, but the actual performance of the second one could be better. The R+M/2 was devised to keep poor performing fuels from being advertized improperly.

[speedy]

340KING vary good !!i was wondering about your formula R+M/2 is R for refined gas and the M for methinal and ethinal or what else is used.PLUS did the stroke have much effect on pinging ?ONe more thing i'm a little confused about is using different temp.thermostates' hotter or colder what's better for performance for street use.I got a million Questions on this but i don't wont to be a pest.

[DodgeMan]

I run a 160 degree thermostat in my 440, but it doesn't see winter duty. This lets the engine oil get up to temp. Cold oil doesn't lubricate well. The 160 helped keep cylinder head temp down there by reducing knock. Through years of racing my Charger with the 440 I found it likes warm heads and a cold intake. I've had more than one person laugh as I change the iced down hand towels I wrap around the intake to keep it as cool as possible, but it works. A note to the wise, don't use "good" hand towels if you want to have a good night with the wife.

[Speedy#1]

i didn't know you could get a thermastat in 160*.i'll have to try that.