Dwell at TDC?

[ne57]

When going to a longer connecting rod, piston dwell at TDC increases which increases torque as I've read. I'm not sure why but that's not my questiom right now. Question is what are the practical consequences of increased dwell. Will this make the motor more or less prone to detonation which might have an impact on compression ratio choice and/or spark advance?

In this case the motor is an old 392 hemi which MAYBE I'll offset stroke(therefore the longer rods). I posed this at a hemi board but didn't get a sure fire answer so maybe with a bigger membership here someone might know at least in theory. The motor will eventually sit in a 66 Dart.

Many thanks

 

[moper]

That's going to be a big shoehorn .

If you think of TDC as the time the piston and rod and crank journal are all in a stright line, then you're not seeing the whole picture. That's because the crank and rod are turning. So the piston reaches TDC a few degrees before the crank and big end of the rod do. It also means the crank and rod are past TDC before the piston begins to move down. The longer the "levers" you have in a given amount of space (connecting rod and crank throw in this case) the longer the dwell at TDC. What this means is there is more time in terms of milliseconds for the mixture to be ignited and begin to expand prior to the piston being able to be moved. If you've tried, you'll note you cannot push a piston down when the crank throw and rod are straight at 90° to the crank centerline. In most naturally aspirated gasoline burning cars, a longer rod will give you more low end off idle and a shorter rod gives you more top end power. That's incredibly generalized. But hopefully you get the idea. The problem comes if you are running pump fuel and a lot of ignition timing. If it happens too early, the pressures in the chamber get too high while the rod is still basically "straight" and the bearings and mains get the crap beat out of them, the chamber temps get much hotter, and the propensity for detonation and pre-ignition go way up. Personally, I design and build for the fuel type I need to run and as much stroke as the person will let me with the longest rod and a piston combo I can get that is no shorter than 1.45". The benefits of stroke are easilly recognized. The benefits of rod angle and length are much harder to "see" or feel and in most street engines are not that important.

 

[ne57]

Thanks, I kinda had the gut feeling that more dwell might increase the likelihood of detonation, but didn't know it as a fact.

So if I stroked it with a 7.100 rod(stock length is 6.951") with a chevy journal would you say the safe thing would be to ease back on the CR? On the other hand I'm wondering about keeping the stock stroke and bumping the CR to 10.5:1, which the hemi guys seem OK with for the street. Need custom pistons no matter what because there are no shelf forged pistons for a .060" overbore, which I'm stuck with.

Thanks again

 

[moper]

Well, there's increasing the likelihood, and makign it detonate. I dont think simply changing the stroke will make it ping. But it might make it more likely. So that means you be more careful with tuning and parts prep. Most bad things that one can do to an engine have things that can offset that "bad". Like running quench in a wedge engine with a closed chamber head. Or runnign aluminum heads to help reduce the tendency. For what you propose very little is changing. You are running a longer rod. Not "stroking" the engine. I'd make sure the pistons are decent quality and dress the sharp edges. Have the heads' chambers cc'd and smoothed out. Run a cam that doesnt build a ton of cylinder pressure (no higher than a measured185psi) and pay attention to how to set up ignition curves and carburetors. You should be fine at a true 10.5:1 with attention to detail.