First let me share some of my philosophy about M-block performance enhancement, just so you’ll understand where I’m coming from.
I won’t give any recommendations for building an M-block racing engine. I know a few guys who’ve done it, but I don’t really know that much about building racing or marine engines. If you’re serious about building for off-road-only applications, feel free to drop me a line so I can hook you up with the right people.
My interest lies mostly in building a reasonably powerful engine that runs well in a truck with:
Impeccable driveability and street manners
As close to bullet-proof reliability as possible
My suggestions for performance enhancements are based on these criteria.
In a medium-to-heavy car, like the vehicles that Ford originally equipped with the M-block 400, I believe these same characteristics that I want in a truck engine would work equally well.
A major issue for many M-block owners is emissions legality. This is particularly true in California, but there are many other locales with some kind of emissions testing.
Many performance enhancements for the M-block are not technically emissions-legal, and the legality of some enhancements varies from one locale to another.
I strongly recommend that you do the research necessary to understand all the details of the regulations that apply to your vehicle in your locale. That's the only way to know exactly what you can and can't do to improve your engine's performance.
I hesitate to suggest any enhancement that might compromise driveability, reliability, or longevity. However, in some cases there are trade-offs, and I strive to describe those as clearly as possible. If you are ever in doubt, you should undertake your own research and make an educated decision for yourself.
Now for some serious advice: Don’t make the mistake of falling in love with some hi-po widget and blowing major cash on it without even knowing if you can use it on your engine. The coolest mondo-hot-rod trick part might be totally useless on your engine when you build it to perform the way you want.
Worse yet, if you insist on using that trick part on your engine, it might produce a lot different performance than what you really want, and it might even render your engine impossible to use on the street or impossible run on pump gas. And after all the money you spent on it, that would be a serious bummer.
General performance considerations
Rebuilding an engine for higher performance is a complex project that requires careful planning to achieve the desired results. Start by answering a few basic questions about your engine:
What do I want to use this engine for?
Is it for heavy towing and hauling in a truck, or street cruising in a car?
Is it for drag racing, or rock crawling?
What degree of power enhancement do I want?
Do I want a mild improvement over stock, or a major improvement?
How much longevity or reliability am I willing to trade off for power?
Do I want the maximum possible power at any cost?
How much can I afford to spend on this engine?
Is cost no object?
Is there a strict budget that will force me to make hard choices?
Your answers to these questions will set the general direction and parameters for your engine project. No matter how you answered these questions, you’ll have more questions to answer and more details to plan as you move forward with your project.
M-block performance issues
When planning to build an M-block 400 with better-than-stock performance, you have to address two major deficiencies in the OEM specification:
Even though these items represent two specific areas for improvement, don’t forget that an engine is really a complete system — one in which all of the parts must work together. And don’t make the mistake of thinking you can just “bolt on” more performance.
If you improve the camshaft and compression, you should also improve the induction and exhaust systems to derive the maximum benefit from the camshaft and compression changes.
On the other hand, if you have a limited budget and you want to upgrade your engine one step at a time, I strongly recommend that you start with the internal components (i.e., pistons and camshaft), and then add the induction and exhaust enhancements later, when you have the money for them.
Why? Because it’s a lot easier to change the intake manifold than it is to change the pistons.
If the budget is limited, I would always prioritize internal modifications over external modifications. If you get the M-block’s compression up and cam it for power, the engine will perform substantially better than stock, even with stock manifolds and the OEM 2V carburetor. Later on, after you’ve saved up enough for a good intake manifold, 4V carburetor, and headers, it will really come alive.
If you don’t take care of the low compression while you can (with the engine disassembled for a rebuild), it will never live up to its real power potential, no matter what intake/carb and headers you put on it.
Factory camshaft profiles
Most M-block engines were manufactured with very mild camshafts, suitable mainly for leisurely performance in a large, heavy passenger car or station wagon.
The following table lists M-block OEM camshaft specs: