June 2010 Chevy High Performance Letters - Performance Q & A

Tom Truty May 5, 2010 0 Comment(s)
View Full Gallery

Early Chevy Power
Q: My '51 Chevy Sedan Delivery holds a '68 large-journal 327. It currently dynos at 211 hp and 241 lb-ft at the rear wheels. The engine is 0.040-inch over, 9.5:1, 882 heads with 1.94/1.50-inch intake/exhaust valves, with a Crane 204/214-duration cam, 0.469 lift, a dual-plane manifold, an Edelbrock 750-cfm flowing through Sanderson 15 /8-inch block huggers flowing into 21/4-inch tubes through 24-inch MagnaFlows, and HEI for spark. I'd like to boost the horsepower and torque about 100 at the rear wheels and wonder what you would recommend. For instance, how much power would come from a set of 13/4-inch equal-length headers flowing into 21/2-inch tubes, and will the larger-diameter exhaust gain horsepower but cost torque? I've been looking at Edelbrock upper engine kits (cam, heads) and they seem promising, but you may think another combination is an equal alternative. If I want the most accurate reflection of horsepower at the rear wheels, should I be looking for absolute loss figures for each component of the drivetrain, or stick with a generally accepted percentage? Is there a significant difference in rating on different kinds of chassis dynos?

I know my car doesn't fit your mag's usual profile so it may not be column-worthy. I hope I still might receive a response.
Mick Toney
Tucson, AZ

A: Any Chevy-powered vehicle is worthy of our advice. You have a very cool cruiser. My dad is in the process of restifying a '51 Sedan Delivery, installing a MagnaCharger-blown LS1, a B&M TH700-R4 trans, and a Speedway Engineering Quick Change rearend. He is about three-quarters of the way through the project. It's going to be one nice ride when he's done. Let's check out your 327.

Adding 100 hp and torque to your baseline is boosting the power of your base engine over 30 percent! That's quite a jump and will change the personality of your current engine quite a bit. Driveline losses are in the 18 percent range based on many factors: manual trans versus automatic, differential type, and engine accessories. We commonly use 18 percent as an average power loss. I've run engines on the engine dyno, then stuffed them into their cars and run them on a chassis dyno. I've seen as little as 14 percent loss with a manual-trans '70s muscle car, and as high as 25 percent on an F-brand small-block with automatic trans and a 9-inch diff! That one is the highest I've seen.

The best thing I can say about dynos is that you don't race dynos. Chassis dyno power measurements vary from brand to brand, and from cell-or lack thereof-that they are mounted in. You can usually compare data from a SuperFlow to a SuperFlow or a Dynojet to a Dynojet, but I don't like to compare across brands. Their correction factors vary in the ways they manipulate the data. The safest way is to compare all changes on the same dyno.

The header change you're mentioning will add a few ponies, but as you stated you'll give up some torque with the 13/4-inch tubes. The 327 is totally happy with 15/8-inch primary pipes. If you wish to go to full-length headers we'd stick with 15/8-inch primaries. A set of full-length headers over your block huggers will give you around the same horsepower but should boost the torque by about 10 to 15 lb-ft from the tuned length of the long-tubes.

Let's get back to your original question. Taking 18 percent loss into your numbers, your little 327 is producing approximately 250 hp and 284 lb-ft of torque. To kick the power up to 350 hp is going to take a set of aluminum heads, a larger camshaft, and the supporting inlet manifold. Edelbrock produces some very nice top end packages for small-blocks. This all goes back to the way you drive the car and what personality you want it to have. Most street rods are mellow cruisers that you want to enjoy the drive. We can easily give you 50 hp and torque gain without killing the driveability.


Connect With Us

Get Latest News and Articles. Newsletter Sign Up

subscribe to the magazine

get digital get print