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Chevy Small Block Part 12 - PT. 12: Rockin' Ratio

Testing Different Exhaust Rocker Arm Ratios

By Mike Petralia

So many outstanding improvements in hot rodding can happen by accident. Take rocker arm ratios for instance. Lately, various unique rocker arm ratio combinations have been attributed to power gains. This has especially been brought to light after some of the top engines from the Popular Hot Rodding Engine Master's Challenge finished in the top 10 using very low ratio 1.3:1 ratio exhaust rocker arms. The thinking behind the power is that duration helps exhaust, but lift has little effect. In other words, the longer you can hold the exhaust valve open, the more time the burnt gasses have to escape the cylinder. But if you lift the valve further off the seat, that does little to help make power.

What we wanted to try was reducing the exhaust rocker arm ratio on Danger Mouse until we saw either a loss or gain in horsepower. Well, we didn't get very far in our tests and the results were not exciting. But, it did teach us two things.

One: If your combination works as well or better than expected, you may not always be wise to mess with it. Danger Mouse has spent a lot of time on the dyno and the combination has been excruciatingly scienced-out. Meaning that there's little, if any, extra power in the current combination. It'd take a major parts swap, like heads or camshaft, to see any big improvements in power.

Two: Just because the parts worked on someone else's engine, does not mean they'll work on yours. We never were able to get down to the 1.3:1 ratio exhaust rockers because they caused the pushrods to hit the heads. Since we didn't have time to remove the heads and enlarge the pushrod holes, our test was done after the 1.6:1 intake/1.5:1 exhaust combination was tested. And it didn't produce any noticeable difference over running 1.6:1 rockers on all the valves.

When it was all done and we tallied the results, the averages told the tale. Nothing happened. There was a very slight gain in peak power with the 1.6/1.5 combo, but we're only talking about 3 ft-lb of torque and 2 hp. The bottom line is that, while it's still a good idea to try new parts in the search for more power, don't run out and buy the next best bolt-on you read about just because someone else made more power with it. Instead, try to borrow one for testing and see if it's the magic part for you.

Dyno Testing Part 12
Danger Mouse specs for Part 12: 355 cid, 8.5:1 cr, 4.030-bore, 3.48-stroke, 5.7-inch rods

Test 24: TFS aluminum heads (Summit Racing PN TFS-30400013-CNC, 72cc chambers, 195cc runners, 2.02/1.60 valves), Edelbrock Victor EFI manifold (PN 29785), F.A.S.T. EFI system with Accufab throttle body and 30 lb-hr injectors at 55 psi Comp Cams Xtreme Energy EFI prototype hydraulic roller camshaft (281/288 adv duration, 230/236 duration at .050, .544/.555 lift with1.6 rockers, 113 LS) straight up. Comp Cams 1.6:1 Pro Magnum roller rockers, 36 degrees total advance.

COMP Cams offers its Pro Magnum roller rocker arms for small-block Chevys in both 1.5 and 1.6 ratios. Previously, we had found the most average power using 1.6 rockers on all valves. This month we tried 1.5's on the exhaust only and got little in return.




Our main goal for this whole test was never realized. We wanted to run COMP's 1.3:1 (PN 1012-16) special break-in rockers on the exhaust to see if the reduction in lift would make more power as some claimed. But, in order to decrease the ratio, the rocker arm's pushrod cup must be moved away from the trunion centerline. This in effect caused the pushrod to hit the cylinder head (note top of pushrod in photo). Had we not checked for this interference and ran the motor, power would have been down tremendously and damage could have resulted. Always check for proper pushrod clearance when installing new rocker arms or changing ratios.

Test: 25 Swapped COMP Cams 1.5:1 Pro Magnum roller rockers on exhaust only

  Test 24 Test 25
RPM TQ HP TQ HP
2600 347 172 352 174
2800 346 184 348 186
3000 351 200 351 200
3200 367 224 369 224
3400 370 240 370 240
3600 367 252 366 251
3800 363 262 361 261
4000 366 278 366 279
4200 372 298 374 299
4400 383 321 387 324
4600 400 350 398 349
4800 404 369 402 368
5000 400 380 399 380
5200 406 402 402 398
5400 406 417 410 422
5600 406 432 407 433
5800 405 447 402 444
6000 394 450 395 452
Max 407 451 410 453
Avg 381 315 381 316

We're always looking for new ideas. Do you have a better one for Danger Mouse? Send your test suggestions to:

Super Chevy MagazineAttn: Danger Mouse720 Hundley WayPlacentia, CA 92870Or Email: mike.petralia@primedia.com

SOURCES
MSD Ignition
El Paso
TX
9-15/-857-5200
msdignition.com
Accufab
1516 E. Francis St.
Ontario
CA  91761
909-930-1751
www.accufabracing.com
SX Performance
7700 Gravois
St. Louis
MO   63123
COMP Cams
Summit Racing Equipment
P.O. Box 909
Akron
OH  44309
Edelbrock
Dept. 5.0
2700 California St.
Torrance
CA  90503
310-781-2222
www.edelbrock.com
Russell Performance Products
225 Fentress Blvd.
Daytona Beach
FL  32114
904-253-8980
www.russellperformance.com
FUEL, AIR, SPARK, TECHNOLOGY (F.A.S.T.)
151 Industrial Dr., Dept. SC
Ashland, MS 38603
Westech Performance Group
11098 Venture Dr., Unit C
Mira Loma
CA  91752
9-09/-685-4767
www.westechperformance.com
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By Mike Petralia
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