[DeTomaso] A geek engineer designs a camshaft
Daniel C Jones
daniel.c.jones2 at gmail.com
Tue May 18 23:08:36 EDT 2010
Recently, I've had the opportunity to design several cams using the lastest
version of Dynomation. For years I've had an old DOS based version of the
program which was far too slow to do any sort of serious optimization. The
newest Windows based version is a couple of orders of magnitude faster with a
built-in tool for optimization. The optimization loop performs around 9000
simulation iterations and stores the 10 best profiles. Initially, it performs
a coarse grid search so it doesn't get stuck on a local maxima then refines
the best candidates. With a decent processor, this takes around 3 hours using
a coarse RPM grid.
Dynomation has three simulation modes:
1. Wave Action
2. Filling-Emptying
3. Hybrid
The wave action simulation uses the measured geometry of the engine (entry,
exit and minimum cross-sectional areas, port lengths, header dimensions, etc.)
as well as the cylinder head flow data and cam specs. It models only open
headers and does not simulate the effects of mufflers. Also, the intake is
assumed to be a single plane (or independent runner) with equal length
runners. The fillying-emptying simulation is simpler but models the effects
of manifold plenum type and backpressure (mufflers and catalytic convertors).
However, the header and intake tracts are of optimal lengths for a specified
cross-sectional area. Finally, the hybrid simulation merges the wave action
and filling-emptying simulations. In comparing Vizard's cam selection
guidelines with the three modes, his cam guidelines appear to be closest for
the wave action simulation without restricted carb flow. Adding in the effects
of backpressure, limited induction (carb) flow, plenum effects etc. tend to
change the optimal answer.
In any case, I model the engine as accurately as I can and run the
optimization in hybrid mode. I've put together a simulation database and
measured all the intakes and cylinder heads we've used on the dyno 351C and
a few other engines. Based upon results so far, when given the correct data,
the simulation has been surprisingly accurate. For instance, it was within
6 HP of Mike McDougal's 393C, within 2 HP of my Rover 3.5L, within 2 HP of
Glen Hartog's 408C, within 10 HP of the 351C dyno mule and right on the money
on Mike Drew's 520+ HP 408C for most of the intakes tested. It does tend to
be optimistic on peak torque, though.
The iterator will optimize the cam for maximum HP or torque between an RPM
range or you can optimize the cam for best average area under the HP or torque
curve for a given RPM range. The user inputs are lifter type, RPM range,
ramp rate, maximum intake lift and maximum exhaust lift. Given these inputs,
the program will vary intake and exhaust duration and lobe separation angle.
First, I input several lobes from the family of lobes I'm interested in to
determine the ramp rate and the lift ranges. I pick the maximum lift based
upon lobe family, diameter of the valve (L/D) and the usage of the vehicle.
Then I determine whether the valve lift should be shorter on the intake, the
exhaust or the same. The first time I tried this, I ran three separate
optimizations but lately I've used a representative cam and vary the lift
as a test before running the optimization. In the cams I've designed so far,
the best performance has been with shorter lift but longer duration on the
exhaust side. Vizard suggested this may be the case in his article on cam
selection rules-of-thumb. I've also noticed that most of the Engine Masters
Competition engines used shorter lift but longer duration on the exhaust.
As an example, take the cam I designed for Orville Burg's 393C with Aussie
2V heads. In this case, a hydraulic roller lifter type was chosen with a
relatively mild ramp rate of 3.0. The cam specs were chosen to maximize
average horsepower between 4000 and 6000 RPM. Then I crossed the ideal intake
and exhaust specs with the Bullet lobe catalog and came up with a family of
lobes that were in the ballpark. The Bullet lobe catalog lists lobes by
lifter type, lifter diameter, nose shape, lobe symmetry, and RPM or Torque
lobe shape (ramp rates) and groups the lobes by a 3 letter code:
The first letter is either "C" for a conventional shaped nose
on the lobe, or "D" for a dwell nose. Dwell lobes are often
used when there is a lift rule, such as NHRA stock classes.
The second letter is either "R" (RPM) for lobes suited for higher
RPM applications or motors with numerically high rocker ratios,
or "T" (Torque) for lobes suited for lower RPM or motors with
numerically low rocker ratios.
The third letter is either "S" for symmetrical lobes (opening
and closing ramps the same), or "A" for asymmetrical lobes
(opening and closing ramps different). Asymmetrical lobes usually
have a slower closing rate to help prevent valve bounce.
For the 393C, I chose lobes that were either CRA or CRS. Ramp rates were
around 3.0 which is relatively mild, according to Dynomation. I chose several
intake lobes with greater lift and exhaust lobes with lesser lift and came up
with 18 different intake/exhaust lobe combinations that I ran back through
Dynomation. From this, two cams stood out from the rest. These two cams were
further tested with several different lobe separation angles until I came up
with the best:
288/296 (234/238) degrees duration, 0.593"/0.562", 109 LSA
overlap = 74 degrees overlap, 2.85 ramp rate (mild)
Using these lobes:
Intake: HR288/343 288 234 150 .3430 .593 CRA
Exhaust: HR296/325 296 238 148 .3250 .562 CRA
The simulation prediction assuming 800 CFM of carb flow is:
RPM HP Torque
2500 218 459
3000 263 461
3500 322 483
4000 381 501
4500 431 503
5000 467 490
5500 484 462
6000 480 420
6500 452 365
Peak HP = 484 @ 5500 RPM
Peak Torque = 503 ft-lbs PM
Average HP = 389 (over 2500 to 6500 RPM)
Average Torque = 460 (over 2500 to 6500 RPM)
For engines that I've modeled and had on the dyno, the peak HP has been
relatively close but the peak torque has been 20+ ft-lbs optimistic in the
lower RPM ranges. This is likely due to the fact the simulation assumes
perfect atomization and I'm running it in a mode that optimizes the timing at
each RPM and uses a perfectly flat air-to-fuel ratio. The program does have
a mode to use actual spark curves and air-fuel ratios. Also, the simulation
uses a single intake runner length. For engines like the Cleveland that have
long and short runners, I use an average. This tends to reduce the peak torque
and spread it out. Despite having an intake port that is down nearly 100 CFM
to the ported 4V's on Glen's 408C, the better optimized cam appears to give
the engine better power (though not as good as the optimized cam in Mike Drew's
408C). We'll see in a few weeks when we dyno Orville's engine.
Dan Jones
P.S. In no particular order, here are the lobe combinations I tested.
These assumed 110 LSA. A word about LSA. Vizard suggests that canted
valve heads need 2 degrees less LSA than inline heads. Since the
simulation does not model this, I subtract 2 degrees from the simulation
results for my cam recommendations.
1
HR288/345 288 226 143 .3450 .597 CRA
HR299/308 299 234 138 .3080 .533 CRA
Peak HP = 471 @ 5500 RPM
Peak Torque = 497 @ 4500 RPM
Average HP = 381 (over 2500 to 6500 RPM)
Average Torque = 453 (over 2500 to 6500 RPM)
2
HR288/345 288 226 143 .3450 .597 CRA
HR296/325 296 238 148 .3250 .562 CRA
Peak HP = 472 @ 5500 RPM
Peak Torque = 499 @ 4500 RPM
Average HP = 383 (over 2500 to 6500 RPM)
Average Torque = 454 (over 2500 to 6500 RPM)
3 *** 2nd Best ***
HR288/343 288 234 150 .3430 .593 CRA
HR296/325 296 238 148 .3250 .562 CRA
Peak HP = 474 @ 5500 RPM
Peak Torque = 499 @ 4500 RPM
Average HP = 383 (over 2500 to 6500 RPM)
Average Torque = 454 (over 2500 to 6500 RPM)
overlap = 72 degrees
ramp rate = 2.85
better under 5000 RPM
4
HR288/345 288 226 143 .3450 .597 CRA
HR306/319 306 239 145 .3190 .552 CRA
Peak HP = 460 @ 5500 RPM
Peak Torque = 485 @ 4500 RPM
Average HP = 373 (over 2500 to 6500 RPM)
Average Torque = 442 (over 2500 to 6500 RPM)
5
HR288/343 288 234 150 .3430 .593 CRA
HR299/320 299 243 153 .3200 .554 CRA
Peak HP = 473 @ 5500 RPM
Peak Torque = 497 @ 4500 RPM
Average HP = 382 (over 2500 to 6500 RPM)
Average Torque = 453 (over 2500 to 6500 RPM)
6 repeat of #3 ignore
HR288/343 288 234 150 .3430 .593 CRA
HR296/325 296 238 148 .3250 .562 CRA
Peak HP = 474 @ 5500 RPM
Peak Torque = 499 @ 4500 RPM
Average HP = 383 (over 2500 to 6500 RPM)
Average Torque = 454 (over 2500 to 6500 RPM)
7
HR288/343 288 234 150 .3430 .593 CRA
HR306/319 306 239 145 .3190 .552 CRA
Peak HP = 461 @ 5500 RPM
Peak Torque = 484 @ 4500 RPM
Average HP = 373 (over 2500 to 6500 RPM)
Average Torque = 442 (over 2500 to 6500 RPM)
8
HR289/329 289 233 148 .3290 .569 CRA
HR299/320 299 243 153 .3200 .554 CRA
Peak HP = 471 @ 5500 RPM
Peak Torque = 496 @ 4500 RPM
Average HP = 380 (over 2500 to 6500 RPM)
Average Torque = 451 (over 2500 to 6500 RPM)
9
HR290/340 290 232 145 .3400 .588 CRA
HR306/319 306 239 145 .3190 .552 CRA
Peak HP = 462 @ 5500 RPM
Peak Torque = 485 @ 4500 RPM
Average HP = 373 (over 2500 to 6500 RPM)
Average Torque = 442 (over 2500 to 6500 RPM)
10
HR290/340 290 232 145 .3400 .588 CRA
HR296/325 296 238 148 .3250 .562 CRA
Peak HP = 472 @ 5500 RPM
Peak Torque = 497 @ 4500 RPM
Average HP = 381 (over 2500 to 6500 RPM)
Average Torque = 453 (over 2500 to 6500 RPM)
11
HR294/342 294 236 149 .3420 .592 CRA
HR296/325 296 238 148 .3250 .562 CRA
Peak HP = 473 @ 5500 RPM
Peak Torque = 494 @ 4500 RPM
Average HP = 380 (over 2500 to 6500 RPM)
Average Torque = 450 (over 2500 to 6500 RPM)
12 *** Best with 110 LSA ***
HR294/342 294 236 149 .3420 .592 CRA
HR299/320 299 243 153 .3200 .554 CRA
Peak HP = 482 @ 5500 RPM Peak Torque = 493 @ 4500 RPM
Average HP = 383 (over 2500 to 6500 RPM)
Average Torque = 452 (over 2500 to 6500 RPM)
overlap = 76.5 degrees
ramp rate = 2.76
better shifting at 6500
13
HR294/342 294 236 149 .3420 .592 CRA
HR306/319 306 239 145 .3190 .552 CRA
Peak HP = 469 @ 5500 RPM
Peak Torque = 489 @ 4500 RPM
Average HP = 375 (over 2500 to 6500 RPM)
Average Torque = 444 (over 2500 to 6500 RPM)
14
HR293/341 293 239 152 .3410 .590 CRS
HR299/320 299 243 153 .3200 .554 CRA
Peak HP = 474 @ 5500 RPM
Peak Torque = 493 @ 4500 RPM
Average HP = 380 (over 2500 to 6500 RPM)
Average Torque = 449 (over 2500 to 6500 RPM)
15
HR293/341 293 239 152 .3410 .590 CRS
HR306/319 306 239 145 .3190 .552 CRA
Peak HP = 468 @ 5500 RPM
Peak Torque = 487 @ 4500 RPM
Average HP = 375 (over 2500 to 6500 RPM)
Average Torque = 443 (over 2500 to 6500 RPM)
16
HR296/335 296 238 150 .3350 .580 CRA
HR299/320 299 243 153 .3200 .554 CRA
Peak HP = 473 @ 5500 RPM Peak Torque = 490 @ 4500 RPM
Average HP = 378 (over 2500 to 6500 RPM)
Average Torque = 447 (over 2500 to 6500 RPM)
17
HR296/335 296 238 150 .3350 .580 CRA
HR296/325 296 238 148 .3250 .562 CRA
Peak HP = 473 @ 5500 RPM
Peak Torque = 492 @ 4500 RPM
Average HP = 379 (over 2500 to 6500 RPM)
Average Torque = 448 (over 2500 to 6500 RPM)
18
HR296/335 296 238 150 .3350 .580 CRA
HR306/319 306 239 145 .3190 .552 CRA
Peak HP = 469 @ 5500 RPM
Peak Torque = 486 @ 4500 RPM
Average HP = 375 (over 2500 to 6500 RPM)
Average Torque = 442 (over 2500 to 6500 RPM)
Note that all cams peak at same RPM for HP and the
same RPM for torque. Still there are sizable differences
(460 vs 482 HP, 484 vs 499 ft-lbs).
Cams #12 and #3 are the best, so I varied the
lobe separation angle and re-ran the simulations.
12
LSA indicated = 110
HR294/342 294 236 149 .3420 .592 CRA
HR299/320 299 243 153 .3200 .554 CRA
Peak HP = 482 @ 5500 RPM
Peak Torque = 493 @ 4500 RPM
Average HP = 383 (over 2500 to 6500 RPM)
Average Torque = 452 (over 2500 to 6500 RPM)
overlap = 76.5 degrees
ramp rate = 2.76
better shifting at 6500
109 has lower peaks and average (78.5 deg overlap)
111 has lower peaks and average (74.5 deg overlap) but is
a bit better below 4500 RPM but not enough to justify.
110 is best but is equivalent to 108 degrees LSA
3 LSA indicated = 110
HR288/343 288 234 150 .3430 .593 CRA
HR296/325 296 238 148 .3250 .562 CRA
Peak HP = 474 @ 5500 RPM
Peak Torque = 499 @ 4500 RPM
Average HP = 383 (over 2500 to 6500 RPM)
Average Torque = 454 (over 2500 to 6500 RPM)
overlap = 72 degrees
ramp rate = 2.85
better under 5000 RPM
109 has lower peaks and average and is worse across board (74 deg overlap),
111 has higher peaks and average and is better across the board.
Peak HP = 484 @ 5500 RPM
Peak Torque = 503 @ 4500 RPM
Average HP = 389 (over 2500 to 6500 RPM)
Average Torque = 460 (over 2500 to 6500 RPM)
beats #12 across board and is overall best
111 degrees LSA is best but canted valves so is equivalent to 109 LSA.
Therefore the best cam specs are:
288/296 (234/238) degrees duration, 0.593"/0.562", 109 LSA
overlap = 74 degrees overlap, 2.85 ramp rate (mild)
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