Chevy Small Block Evolution - Performance Q&A

Kevin McClelland Feb 1, 2010 0 Comment(s)

What A Mutha
Q: My application is a 461ci big-block engine in a 3,200-pound '69 Vette with a 2,500-2,700 stall and a Turbo 400 coupled to a 3.35:1 differential. The engine configuration performance objective is for midrange torque and crisp throttle response with a torque peak at 4,200-4,500 rpm and peak operational speed of 6,000 rpm. Intended usage will be street, highway, and occasional road course action.

I'm running a complete Eagle rotating assembly with 10:1 pistons, Edelbrock Performer RPM heads drawing through an Edelbrock Performer RPM manifold, a Holley HP series 750-cfm carb, Hooker Super Comp headers coupled to a 2.5-inch cross-pipe system, and Walker Dynomax Ultraflow mufflers.

My cam selection is a Comp Cams XS-274-S; however, I have become intrigued by the claimed performance characteristics of the Comp Thumpr line. I am skeptical of the claims of low- to midrange torque in concert with good high-end power considering a) late intake valve closing, b) increased overlap, and c) less valve lift (relatively speaking). This tends to contradict, in many ways, conventional cam theory leading to my interest in your opinions of my original cam selection versus the Mutha/Big Mutha Thumpr cam selections relative to my performance objectives. Also, as I am sure you have picked up on, I am staying old-school with flat-tappet profiles. I look forward to your input and thank you in advance. Mac Kirkwood Livonia, MI

A: If we didn't have contradictions to the norm we'd never move forward. Things that wouldn't work 20 years ago because of cylinder head selection or manifold development may work very well these days. Things that work with the LS-based engines didn't work with either small-blocks or big.

We've always been big fans of a large duration spread between the inlet and exhaust lobe. The GM Performance camshafts in the ZZ4s and H.O. 454 and 502 have quite a spread. The ZZ4 has 13 degrees differential, and the H.O. big-blocks have 19 degrees. These camshafts were done almost 20 years ago, and they perform very well for their application. The Comp application of this theme is just extending the same logic. Most performance engines achieve peak cylinder pressure 12.5 to 15 degrees after top dead center (ATDC). On the exhaust side, after the combustion cycle, you have the next cylinder in the firing order at full cylinder pressure, approximately 105 degrees ATDC. That next cylinder is putting all its work into turning the crankshaft. This allows you to use an early exhaust valve opening to blow down the cylinder and really evacuate the previous cylinder. This works well when you have a marginal exhaust port and a restrictive exhaust system.

Comparing the mechanical Xtreme Energy camshaft to a hydraulic Mutha Thumpr is tough to do by the published camshaft specifications. Comp rates its hydraulics at 0.006-inch tappet lift, and its mechanicals at 0.015-inch. The larger number on the mechanical side is to compensate for the added valve lash associated with the mechanical camshaft. This differential of 0.006-inch of lobe lift makes a dramatic difference in duration numbers at this advertised point. With all this being said, the 287TH-7 Mutha Thumpr specs out at 235/249 at 0.050-inch tappet lift, 0.510/0.495-inch max lift, with a separation angle of 107, and is installed at 102. The XS274S Xtreme Energy cam comes in at 236/242 at 0.050, 0.568/0.578-inch max lift, is ground on 110 centers, and is installed at 106. One thing you must take into consideration with the mechanical is the loss of max lift from the lash. The lash point for the solid is 0.016 inch.

OK, down to the bottom line. Will the newly designed Mutha Thumpr be a better camshaft for your application over the mechanical Xtreme Energy camshaft? It's going to be a toss-up, with benefits both ways. The mechanical should give you a little more top-end engine speed; however, the XE lobes are very aggressive to get those lift numbers. The softer (less aggressive) hydraulic lobes may be better for the valvetrain stability upstairs. Again, we like the large spread on duration numbers, and the Mutha Thumpr is right in the sweet spot. Even with your Edelbrock cylinder heads, the exhaust port still can use some help. Leaning on some extra exhaust duration certainly won't hurt. Good luck and Thump on!


Won't Brake Me
Q: I have a '70 Chevelle with a 454-cid (bored 0.030-over to a 461 cid). I'm running a two-speed Powerglide trans with a transbrake. I was told if I ran the transbrake on the street it would get too hot. I'm running a 3,500 stall. I'm running a trans cooler and 4.56:1 gears. Any help would be appreciated. Thanks.
Randy Duggins
Midland, OH

A: Transmission brakes hold the vehicle and allow you to go to full power before releasing the fury of your big-block. The transbrake itself doesn't increase the transmission heat unless it's being used. The heat comes from the torque converter being held at full stall without any rotation of the input shaft. How transmission brakes work is they direct fluid pressure to First and Reverse gears at the same time, locking the transmission from rotating. When you release the transbrake, the Reverse gear is released and the vehicle is launched at whatever torque is being applied to the converter. Regular driving without using the transbrake will not increase the transmission temperature.

Now, studies have been done on 1,000hp/800-lb-ft big-blocks on the rate that the transmission fluid is heated during this full-stall event. The fluid temperature rises at a rate of 100 degrees F per second coming right out of the converter. This is why you don't want to be at full stall on the transbrake any longer than needed. Also, when you do a full-stall test to see what your converter will stall to, make sure you idle the car for 15-20 seconds afterward to flush out the hot fluid from the converter and through the cooler.

Enjoy those neck-snapping launches. Just remember, Johnny Law isn't going to be very happy with your antics. Keep the transbrake use to the strip.

Laguna Pretender
Q: I'm looking at purchasing a '76 Chevy El Camino with a Laguna nose. The powertrain is a 350ci with a four-speed. Did Chevy actually produce this car or could it be that someone added the nose years later? The owner is adamant that this is a factory car, but I have been unable to find anything on this one. If it is an actual car, could you tell me how rare it is? Any help would be greatly appreciated. You guys have a great magazine, and thanks a lot! Todd Dixon Philipsburg, PA


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