Dyno numbers - cabin fever

[Duke W.]

Okay, it's been raining out here in "sunny Southern California" for three days now. It's supposed to clear up tomorrow and Thursday, then rain again this weekend, and this isn't even an El Nino year.

So, I decided to bide my time today by plotting my lift crank angle data for the 30-30 and LT-1 cams and run some comparisons on my digital dyno, otherwise known as the Dyno 2000 simulator.

I had to make some educated estimates for the Duntov and Comp Cams Extreme Energy 268 because I don't have detailed data for them and can't pick out the exact valve open .006"/.010" on the opening/closing, respectively. These points usually provide the best results on my simulator for an asymmetrical profile as both the 30-30 and LT-1 are.

The long block is my '63 SHP with nicely pocket ported heads and a very precise three angle valve job. Real CR is 10.75:1. It's .030" over and the AFB flows 550 CFM at 1.5" Hg. The engine is mounted to my dyno with short exhaust pipes going through generous sized mufflers. Of course, the production exhaust manifolds are installed. Observed numbers are corrected to 29.92" Hg. 68 degree F dry air.

Actual installed numbers in the car will be a bit less since it will have a fan and alternator and a bit more restictive exhaust system. The actual numbers for your similar engine will vary depending on your installation and level of blueprinting, but all other things being equal the percent differences will probably hold.

I've provided three data points from the results - torque at 2000 revs, peak torque, and peak horsepower. For reference, the production engine shows 298 lb-ft at 2000. peak torque of 342 lb-ft at 4000 and 308HP at 5500.

Duntov - 302 lb-ft@2000, 359 lb-ft@4000, 346HP@5500-6000

30-30 - 267 lb-ft@2000 344 lb-ft@4500-5000, 351HP@6000

LT-1 - 306 lb-ft@2000 365 lb-ft@4500, 356HP@6000

So if you were rebuilding a mechanical lifter SHF/FI SB, which cam would you chose. (Hint: Fifteen years of Chevrolet Engineering R and D paid off, but they took a step backwards before they went forward.)

For comparison I tried my long block with a Comp Cams Extreme Energy XE 268-10, which seems to be a popular replacement for high performance rebuilds. This cam has very aggressive action and will require stiffer than production valve springs, but it pays off across the board.

EX268H-10 - 321 lb-ft@2000, 372 lb-ft@ 4000 364 HP@6000

Using more conservative acceleration more typical of a factory high performance cam yields numbers very close to the Duntov

309 lb-ft @2000, 356 lb-ft@4000, 342 HP@6000

The factory mechanical lifter cams all have soft action for longevity and both the LT-1 cam and EX268H-10 feature a bit more exhaust duration, which is effective when running the production type manifolds and a muffler equipped exhaust system.

Duke

[John C.]

Re: Dyno numbers - cabin fever

Were you using the stock aluminum intake?

[Duke W.]

Inlet manifold

The software only allows the selection of the type of manifold architecture such as a single plane manifold, or dual plane, or other choices such as individual runner, which models Webers. I used a dual plane manifold, but the program cannot distinguish between the design used on my SHP engine versus the later LT-1 manifold.

The usefulness of these runs was basically to compare the torque curves of the various camshafts because that was the only variable.

Duke

[Chris Burbage]

Re: Dyno numbers - cabin fever

Would you mind running your virtual dyno with a mythical stock 396 motor and compare the stock cam vs ?? (you pick em). Looking to freshen up my motor and thought that a cam change would be fun.

Thanks,

Chris

[Duke W.]

Re: Dyno numbers - cabin fever

Sorry, Chris. I don't have sufficiently accurate cam data for your BB to do a meaningful comparison. The numbers published for the mechanical lifter SHP BB cam in the Chev. Power manual and other sources are measured at some undefined amount of lifter rise and there's no way I can convert their data with any accuracy to an actual valve lift figure of .006"/.010".

The data I used for the 30-30 and LT-1 cams was measured off of very accurate lift-crankangle diagrams that I took for the LT-1 and Bill Clupper provided for the 30-30. The published specs for the Duntov cam are based on the end of the clearance ramps, and I was able to make a reasonable estimate for the seat to seat timing using .006"/.006" actual valve lift, which works for a symmetrical lobe like the Duntov.

BTW, in calculating the actual.006/.010" valve lift points for the 30-30 and LT-1 I used a rocker ratio of 1.37, which is what I measured on my engine at the end of the clearance ramp. The ratio at max lift is 1.44. The common ratios that we are familiar with - 1.5 for SBs and 1.7 for BBs are just approximations. The actual ratio varies with valve lift because the fulcrum point changes throughout the motion range, and unless a lot of head and deck machining is done, the max ratio at full lift will probably never reach 1.5/1.7, and it will average less throughout the range with the minimun values at low lift.

The lower than advertised real rocker ratio is why I recommend setting the valve to the recommended clearances multiplyed by 1.37/1.50, except the .008" inlet clearance for 327s with the Duntov cam. This cam is manufacturered with a nominal ramp height of .008" which would imply .012" clearance with a 1.5 rocker, but Chevrolet tightened up the spec for the 327 to give a bit more effective duration, so .008" clearance will always insure that the valve is set back down on the seat while the cam is on the closing ramp. Earlier Duntov-equiped 283s are speced at .012" inlet clearance.

Duke