dynamic compression ideas

[Parkhurst]

Hmmm...I'm doing pretty much the same thing.

I designed my 383 SBC to run on 87 all week and 91-93 on weekends when I might get into the nitrous.

I also wanted to do it with a simple combination anyone could duplicate (it was for a magazine story).

I decided on 9.7:1 with flat-tops and 6-inch rods (part of Lunati's pre-balanced 383 recip. assy. kit), AFR 215s (CNC finished), Performer RPM Air Gap, MSD 6-AL, Comp solid roller (XR286/12-772-8, .576/.582, 248/254 @ .050, 110 deg. LSA), Calico coatings on chambers, ports, valves, springs, and bearings.

On Westech's dyno, it looked like this on 87, through mufflers:

RPM TQ HP

3500 416 277
3600 416 285
3700 424 299
3800 434 314
3900 448 333
4000 452 344
4100 467 364
4200 473 378
4300 478 392
4400 486 407
4500 488 418
4600 494 432
4700 496 444
4800 498 455
4900 500 466
5000 498 474
5100 495 481
5200 495 490
5300 493 497
5400 491 504
5500 487 510
5600 484 516
5700 481 523
5800 477 527
5900 473 531
6000 469 536
6100 464 539
6200 460 543
6300 454 545
6400 447 544

Needless to say, it works okay. Idle is rough, but the car has a stick so it's no big deal. Power comes on good at 2,800 or so, really winds up at 3,500 and I shift at 6,500. I will have quarter-mile times soon, but this is in a 3,700 lb wagon so its atypical for most. I'm adding a plate system soon, but I'm upgrading the entire fuel system to deal with it so it'll be a while before I have numbers with that in place.

With 35 degrees in by 2,300, no detonation. Naturally, nitrous timing will be different. I'm only adding 100-150 shot.

Again- this was the notoriously happy Westech cell, but the package sure works.

[putztastics]

E85 is $1.60 at the Cenex pump here in Bismarck, and it's 105 octane. So I'm going to play with it a bit...

[Joe Mendelis]

"Dynamic Compression" just seems like it doesn't work. I have never figured out what it is on any engine. I'm sure it works to get you in the ballpark for a certain range of engines. I just can't think that an engine with an intake closing at bdc with 8:1 will have the same tendency to detonate as one with an intake closing at 90* with 16:1. I would imagine that since the ratio of BMEP to peak pressure is so high with the 16:1 it would have more tendency to detonate. Maybe this is wrong?

[putztastics]

"Dynamic compression ratio" is just another instance of trying to figure out the theory behind the reality.

The engine Parkhurst built works in reality, does anyone think he's wondering what it's dynamic CR is?

Like Darin recently correcting the CFD graphics, they needed to be adjusted to match reality.

This a problem engineers sometimes have, seeing something working in reality and wondering how it works in theory.

They tried to keep the duration small , little overlap and as much valve velocity they could get away with on an endurance/street cam. They then kept bumping up the CR until it melted and backed off a notch. Its still trial and error is what I am trying to say.

Bravo! Common sense engineering.

[gas]

This all looks depressingly familiar.
Q: if dynamic compression is an accurate knock barometer for the entire RPM and load range, does this mean that an engine with moderate CR and late intake closing (DCR = will have lower power throughout its range than a motor with moderate CR and early intake closing (DCR = 9)?
A: if no, DCR does not predict octane requirement (despite what Mr. Kelley thinks).
If yes, we have all been wasting a lot of money, don'tch think?

you do fit your name boss....

http://web.camaross.com/forums/showthread.php?t=382484

Start at post 15 after you get thru post 1 and that's the best I can do.

Bret

After reading entire thread, got more out of the thread, when reading included all of originator's posts, than just reading your counter points beginning with post #15.

Gary

[MadBill]

Just chasing down some old topics and saw this. Very interesting exchange, a lot of worthwhile insights!
There is one big factor that was never mentioned however: Ignition lag time. It doesn't allow one to totally ignore the better filling at high RPM, but it goes a long way. When the required conditions (pressure, radiant heat, etc.) to detonate the end gases are met, there is a short, fixed delay before the bang. At low RPM, there is plenty of time, so you get a piston-cracking rattle. As the revs go up, turbulence greatly increases the flame speed and the window of opportunity until the regular flame front comes through goes way down. (Since lots of race engines use the same spark advance at 3,000 as at 6,000, obviously the burn is accomplished in half the time.) So detonation is progressively less likely as the RPM increases, thus the "above SCR" DCR at high RPM is much less of an issue than it would first appear.