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G.M.C. Six Cylinder Performance
“G.M.C. Six-Cylinder Performance”

by Jeffrey Diamond

“G.M.C.” name for reference purposes only.
Not affiliated with General Motors.

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Chevrolet, G.M.C. & Buick Speed Manual
Chevrolet Inline Six-Cylinder Power Manual, by Leo Santucci
Chevrolet, G.M.C. & Buick Speed Manual
Chevrolet Inline Six-Cylinder Power Manual, by Leo Santucci

    This is





IBRARY’s comprehensive illustrated booklet on obtaining the best speed & performance from your 1939-63 G.M.C. 228, 236, 248, 256, 270 & 302 six cylinder in-line (L6) light truck engines. Your “obsolete” G.M.C. is capable of surprising speed & power, without loss of reliability. It’s the most complete literature you can buy to get the most from your motor. Before making any decisions about modifying (or even rebuilding) your motor, get the best help available, and

G.M.C. six cylinder engine

do it right the first time.
    This book is not a re-print of the firing order, options, number of cylinders, serial numbers, casting numbers, rated power, compression ratio, etc. It contains only information that is useful and relevant (no pictures of chrome accessories).
    Rather than merely summarize and repeat what you’ve already read by “California Bill”, Frank McGurk and Roger Huntington (which were written 50 years ago), etc. my purpose is explore, explain and detail modification, fine tuning, improvements and modernization using current parts and technology. Use it with your service manual for best results. Not an expert mechanic? You’ll still find many improvements you can do yourself with common hand tools.
    I guarantee that you’ll find information in this booklet you’ve never seen before, and which will help you plan and complete your project.
    Many of the comment and observations will also be of use to those with other older in-line OHV pushrod engines with siamese ports, including the Buick 248, 263 & 320 straight eight, Ford 223 L6, &c. although no specific recommendations are made.
    A booklet on older in-line side-valve (flathead) L6 engines with siamese ports, including Dodge & Plymouth 218 & 230, Chrysler & DeSoto 237, 251 & 265, Pontiac, Ford 226, Hudson 232, 262 & 308, Oldsmobile, Pontiac 239, Rambler 196, Studebaker, &c. will be available separately, click here for details: Six-Cylinder Sidevalve Performance.
    A booklet on the 1954-63 Chevrolet “stovebolt” 216, 235, & 261 L6 engines is available separately, click here for details: Chevrolet Performance.
    Consider this: if this booklet saves you from making even a single mistake or bad purchase, or gives you one new idea, doesn’t that make this purchase worth while? The price is a very small fraction of what you’ll invest in the motor - plan it more intelligently, and get better results.





IBRARY and the






APERS have been a source of performance literature since 1971. Click here for a brief bio on the author: Author of Chrysler Poly Performance. The advice and suggestions are not reprinted from other articles, magazines, or factory literature and are not available anywhere else, (although some of the data is generally available). Contents so far in the draft version are already 67 pages (over 45,000 words) stapled in paperback to save you printing costs, includes the following subjects, with over 30 black & white illustrations & diagrams, 12 data tables and a sample worksheet.
How these engines differ from the similar 1937-63 Chevrolet 216, 235 & 261 six-cylinder
      “stovebolt” engines, and Chevrolet & G.M.C. 1962-* 194, 230, 250 & 292” engines
How much power can I expect to get from my engine?
Siamese (shared) intake and exhaust ports: how do they affect the engine?
Valve train

; how the parts function, comparison between G.M.C. and Chevrolet engines.

; design, combustion chambers, port shapes and sizes, valves, dimensions, flaws.
Milled and filled? Ported and relieved?

; what does that mean?
Speed equipment

; aftermarket parts (both obsolete and current), cams, intake manifolds, carburetor
      choices, “split” exhaust manifolds

; what you can do yourself, including:
 »   Diagnosing and correcting multi-carburetor linkage problems (useful on V8 tri-power engines as well)
 »   Adapting an Eaton supercharger from a late model V6
 »   Aftermarket manifolds - why are there so many types? How are they different?
 »   Making your own multi-carburetor intake manifold
 »   How large should the carburetor be, and why?
 »   Planning a stroker crankshaft
 »   Alternate connecting rod choices
 »   Aftermarket 12-port heads, differences and advantages
 »   What is a “hybrid” head, and how to determine what must be done
 »   Improved piston design
 »   Calculating compression distance and ratio
 »   Up-grading the ignition and tuning the advance curve
 »   Choosing and installing big valves
 »   Modifying the head for better breathing
 »   Improving the original intake and exhaust manifolds
 »   Making or adapting headers
 »   Splitting the original cast-iron exhaust manifolds
 »   Fabricating a cast header out of common industrial supplies
 »   Analyze, compare, and select a new or re-ground high-performance cam
 »   Correcting the rocker arm geometry for hot cams, head milling, longer valves
 »   Improving the oil system
 »   How to predict a safe RPM limit
Data Tables

; showing both motor families, bore, stroke, rod length.

; listing suppliers of parts & services, etc. both mail and internet.
TRW Part Numbers

; lists many potentially useful obsolete numbers and applications.
Work Sheet sample

; to help you keep records and plan changes to your motor.
    Please note: this is not a coffee table book, it is intended to be a research tool for those who seriously want to improve their stovebolt motors. It is not a repair manual. If your car won’t start, spark plugs foul, low oil pressure &c. my booklet will not help you. It does not contain information on tune-ups, or explain how to remove, disassemble, clean or restore the engine. If you do not already have this information do not buy this booklet. If your engine is stock and you’re pleased with how it runs now, this booklet is not going to be of much use, although you might find it interesting.

    This book will be available approximately early 2013. Do not order, all payments will be refunded. Click here to receive notification when this book is available: Please notify me when the G.M.C. Performance booklet is available.

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