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RE: Engine Math...a little

Posted by JURB (A) on April 20, 2021 at 06:42:40

In Reply to: Engine Math...a little posted by pictureguy on April 19, 2021 at 16:20:41:

There is much to that which is beyond me, however...

"The IC Engine in newer cars has reached a very high degree of development. Lots of things being done today are a direct result of racing development and material science guys."

Just that was plenty for a wile. One of the main things has to do with the cam. Newer engines can have a higher theoretical compression ration which is countered by more cam overlap. What this means is the compression is not so high you need a diesel starter motor. The overlap does away with most of it so it cranks easy. Then you need a super high energy ignition to get the things to start, whivh they now have, since the 1980s.

However an effect of this cam overlap is a higher volumetric efficiency, sometimes over 100 at certain RPMs. Then the higher theoretical compression means something. It is like having 6:1 to start and up to 12:1 when you use passing gear.

The ECM since the 1980s in the good cars usually had fuel injection, even if throttle body it is better than a carb. Precise control over the mixture as well as ignition timing made a six run like an eight.

With the higher RPMs now roller cams are not the rare thing they used to be in the past.

"What makes it go is pressure on the piston. Measured, ususally in PSI, though a more advanced measure, BMEP, which is Brake Mean Effective Pressure.....and is a calculated value"

And the BMEP will be higher if the fuel burns more slowly. Premium generally does and if not the EGR makes it burn slower, thought without the added thrust. However an engineer told me that there is a way to tune an engine so that it will actually run better (faster) on lower octane gas because of it burning more quickly. And "tune" to a guy like this means "Well, let's just make that exhaust lobe a little bit more...". More than likely they ping their ass off.

"I'll deal ONLY with 'square' engines"

That takes all the fun out of it.

"Given equal pressures, the multi-cylinder engine will put out more power than the single."

That is because the peak PSI on the cylinders is in the sweet spot more. You take a huge cylinder, it is damn hard to get enough fuel in there to keep pushing the piston, and why bother as it gets to the bottom ? And actually that bottom there is usually a vacuum for a split second which can scavenge exhaust from other cylinders. There is an exhaust system called an "evacuator' system which uses two long pipes but connected at the end which uses this principle and also applies the inertia of the exhaust gases. The only production car (not that much production) to come with one was the Olds Toronado. Later, when you put in a dual exhaust, they got a kit with a bunch of temperature indicator tapes and you run the car, rad them and when the tapes are a different color meaning a little less heat is where you put the crossover in. Sure you lose the stereo sound of the exhaust but you gain power.

"Over 7000rpm by some tests of the day"

The olman had one that did 9,000. Had a Racer Brown roller cam, and back then it was not easy to put one in, the block had to be trued and squared or you would get cam walk, not desirable. He was a machinist and could do his own line boring etc. He also had the crank machined undersize maybe a thou or so, then had it hard chromed back up to size so it took the regular bearings.

If you exceeded 9,000, I mean like 9,0001RPM it would wipe out #1 main very time. The hard chrome protected the crank quite well but it was still a pain to get down there and change it. They tried everything. Chevies are fed oil form the back, so they also fed it from the front. No good. Nothing worked. He might be thinking about why in his grave, err urn.

If you notice the oil pumps these days, many are on the crank. For one that means they run at twice the RPM as the old ones that ran off the distributor. Bigger gears and all that. That is why you can start the car in zero degrees and just drive, no warming up needed.

You don't ant to deal with now if you don't even want to deal with oversquare or undersquare engines. They have g=had variable cam timing for a while now, and now they got dual, intake and exhaust controlled independently. And direct cylinder injection.

Talk about NASCAR contributing to the art ? Well it looks like the price caught on, take one of those high end engines and blow it to bits and see what it costs for a replacement. You wil see why they have no trouble seling those extended warranties, like Car Shield and Protect My Car etc.

A little shitcan engine can be like $7,000, the high end ones ? Twice, thrice that.

I think I want a Model T. Need an engine ? Well make one in the basement and carry it up the stairs and put it in. Or walk.

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