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Technical Articles | |
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351M/400 Performance Build Up. by Paul Eschenbach. (Revised 10-23/02) |
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There are four basic factors with the 351M/400 that need to be addressed to accomplish a true high performance buildup. They are:
By far, the best thing you can do to your engine is to raise the compression ratio. All of the 351M/400s came with 8.0 to 8.4 to 1 compression ratios (except for the 1971 400, which had a 9.1:1 CR). While even a 9.1:1 compression ratio pales in comparison to the muscle car area of the '50s, '60s & early '70s, that's about all you can feasibly squeeze out of a 351M/400 motor on the 92 or 93 Octane gasoline available today. If you go much higher than say 9.5:1, then you will have problems with pre-detonation (also called "pinging") w/o the use of octane boosters. Raising an engine's CR will give it the following benefits:
So what's the best way to raise your engine's compression? Well, there isn't an easy answer to this question and it also depends on how much money you have to spend. Having said that, there are three ways to increase your engine's CR. They are:
Shaving (milling or decking) the heads
Cylinder heads tend to warp slightly over time. A quality cylinder head rebuild will include a procedure where a small amount of material (around 0.010") is milled, or shaved, off of the bottom of the heads so that they will lay perfectly flat on the engine block. Shaving 0.010" will increase the compression ratio by approximately 2.3%. So, if your stock 400M has 8.5 to 1 CR, it will now have approximately 8.7 to 1 CR. About the most material you can reasonably remove from your stock 351M/400 heads is 0.025", which would increase your CR by approximately 5.75% which would give a 400 a CR of 8.98875.
Installing a different set of heads
Since the 351M/400 engines are in the 335 engine series, every single cylinder head from any engine in the 355 family will all interchange with these engines. That leaves at least 6 distinctly different types of cylinder heads that will work on the 78 & 79 351M/400. Below is a chart that list some of the 335 series heads along with each head's CC (combustion chamber volume) that can be used on the 351M/400.
As you can see from the chart above, the best head you can install on the 351M/400 is the Australian closed-chamber 302 2V head. Just by bolting on this head, a 400 will develop 10.75 CR and a 351M will develop 10.12 CR. However, these heads are EXTREMELY rare and you will have to pay a hefty price if you can find a pair.
The 70-74 351C 2V heads will give your engine a slight increase in compression, however, the small increase in CR is not worth swapping the heads, in my opinion. You can get nearly the same increase in compression by milling your stock heads the standard 0.010" when you have them rebuilt. However, if you were to buy set of 351C heads and have them milled by 0.025", then you can raise the CR on a your engine by nearly 3/4 of one full point!
The 71-74 351C 4V heads, like the 2V heads, will not give your engine enough increase in CR to warrant changing your heads. However, the 70-71 CJ (Cobra Jet) 4V heads will give your engine a considerable increase in CR. Unfortunately, the 351C 4V heads are not a good choice for a low RPM truck engine. The 4V heads have much larger intake and exhaust ports (2.19" intake & 1.71" exhaust vs. 2.04" intake & 1.66" exhaust) than the 2V heads. These heads were designed for VERY HIGH RPM and do not produce good low-end torque. The Bronco is much too heavy to make use of these heads.
Installing a set of high CR Pistons
Another way to increase your CR is to install a set of high CR pistons. Unfortunately, there are not may choices available for the 351M/400 engines. One option is to have a custom set of pistons made for your engine. There are many companies, such as J & E and Keith Black, that will make a custom set for your engine in any CR you want, but be prepared to pay a minimum of $700.00 for a set of eight and to wait for 4 to 6 weeks for them to be made. However, there are a couple of other much less expensive options available for both the 351M and the 400.
The next best thing you can do for your engine is to install a better camshaft and valve train. The 78 & 79 351M/400 used a retarded camshaft and timing chain to meet stringent pollution standards (see Dispelling the Myths & Untruths surrounding the 351M/400) from the factory. The stock cam and timing chain are, for lack of better words, pure unadulterated garbage. Fortunately, there are many aftermarket companies that offer high-performance cams & timing chains designed for the 351C and the 351M/400. I recommend using one of the newer dual-profile or roller camshafts available from companies such as Crane Cams and Comp Cams. They are very expensive, but you would not believe the benefits you will receive from your engine with one of the newer types of cams available today. However, ANY quality aftermarket camshaft kit will be a SIGNIFICANT improvement over the stock cam and timing chain.
I used the Comp Cams 255 DEH Dual Energy cam kit on my engine. This cam has a useable RPM range of 1000 to 5500 RPM, gets good gas mileage, and makes a great RV or towing cam. I also recommend buying the K-kit (P/N K32-206-3) which includes lifters, matched valve springs, valve spring retainers, valve locks, valve stems seals, dual-roller timing chain set, assembly lube, decals, and instructions. No matter which manufacture you choose to use, make sure you buy the matched valve train kit with your cam. Don't try to mix and match parts!
Another good choice is the cam that was used on a 79 400 in an article entitled "Down & Dirty" that appeared in the September 1998 issue of Hot Rod Magazine. They used the Comp Cams X-Treme Energy #268 camshaft. This cam is a little more aggressive than the one I chose with a useable RPM range of 2,000 to 5,200. Hot Rod claims that this engine put out an impressive 382 HP and 456 ft-lbs of torque! As soon as I have my engine run on a dyno, I'll post the results in this article.
One other item you can upgrade in your valve train are the rocker arms. I used the Comp Cams 1.72 ratio Magnum Roller rocker arms in my engine (P/N 1411-16). You will have to convert your engine over to hardened pushrods, screw-in studs and guide plates to use these rocker arms. They are the perfect complement to a dual-profile or roller cam shaft. Roller rocker arms are more accurate, more efficient, and produce less friction than the stock rocker arms. Comp cams claims that these rocker arms can give your engine 15 to 30 additional horse power.
Installing dual exhaust and a high performance camshaft in your engine and leaving the stock 2V carburetor in place is like inhaling through your nose and exhaling out of your mouth. Just try it! It doesn't work very well, does it? Well, it doesn't work very well in your engine either. Fortunately, there are many aftermarket 4V intakes and 4V carburetors for the 351M/400 engines. You can even install an aftermarket fuel injection system like the ones available from Holley, however, they are very expensive and I'm not convinced that they are much better than carburetors.
Choosing an intake manifold is easy. I recommend a dual plane manifold such as the Edelbrock Performer 400 EGR (P/N 3771) or the Weiand Action +Plus (P/N 8010) or the new Weiand Action +Plus EGR (P/N 8013). The Edelbrock intake has a good carburetor mounting surface area and will help keep your Bronco emissions legal if you keep the EGR Valve. Both Weiand manifolds also have a good carburetor mounting surface area but the 8010 does not have a provision for an EGR valve. If you are not concerned with keeping your Bronco emissions legal, then go with the Weiand 8010 manifold.
Choosing the correct carburetor for your engine is a little more difficult. The key to proper carburetion is to choose a carburetor that correctly fits your application. There are many different carbs that will work on a 351M/400 engine. Listed below is a chart with the most common carbs available, the CFM (cubic feet / minute), the type of choke, the listed application, and the average retail price.
The formula below can help you determine the best size of carburetor to use with your engine.
I think the mistake people make the most when installing a carburetor is choosing a 750 CFM carburetor on an engine that clearly doesn't need that much carburetion. In the example above, even a fully modified 400 producing 100% VE would only need a 636 CFM carburetor at 5,500 RPM. Be that as it may, many engine builders recommend using a 750 CFM carburetor on a high-performance 400 CID engine. So, a 750 CFM carb should work fine on hi-output 400, however, on a 351M and on stock-to-mild 400s, a 600 to 650 CFM carb should meet the maximum CFM requirements of your engine.
I chose to use a Holley model 80457S on my high-performance 400. It has fair throttle response, gets better gas mileage than the factory 2V carb, and doesn't have any hesitation in it at all. Because my engine has been modified, I did have to make a few tuning changes to the carb to get it to work properly on my engine. The 80457S comes with #65 main jets and a #25 accelerator pump nozzle. Holley recommended using #66 or #67 main jets and a #28 accelerator pump nozzle. I went ahead and went with the #67 jets and the #28 nozzle. This combination works great! The Edelbrock Performer and the Carter AFB is also an excellent choice. From what I have observed from owners of both Holley and Edelbrock/Carter carbs it seems to be the general consensus that AFB-based carbs are a little better overall than the Holleys.
There is no doubt about it -- more cubic inches equals more hose power. Why do you think most people choose to swap in a 429/460? It's because they are simply bigger, more powerful engines. However, you can add more cubic inches (CID) to the 351M/400 (especially the 351M).
It is common practice when building an engine for more power to increase an engine's size (measured in Cubic Inch Displacement -- CID for short) by enlarging the engines bore and/or stroke. The bore of an engine is the diameter in inches of the piston (or cylinder bore) and the stroke is the distance measured in inches in which the piston travels up and down within the cylinder bore. The bore and stroke of the 400 is 4.0" x 4.0" and the bore and stroke of the 351M is 4.0" x 3.5". The formula for calculating engine displacement is:
Using this simple formula, you can calculate exactly how much larger your engine will be when you enlarge the bore or lengthen the stroke.
So, how can you increase your engine's CID? Well, if you read the article "Dispelling the Myths & Untruths surrounding the 351/400", then you already know that you can not increase the bore size of the 351M/400 more than 0.040 of an inch. If you plug this number into the formula above on a 351M engine you will see that you can enlarge it to 359 CID. (2.02² x 3.14 x 3.5 x 8 = 358.748738 CID). Also, there are no aftermarket crankshaft kits that I know of that will lengthen the stroke of the 351M/400. However, if you have a 351M then you are in luck because you can install a 400 crank shaft in your engine to make your engine a full 400 CID.
IMPORTANT: When installing a 400 crankshaft in a 351M, there a number of things that must be done and some things to keep in mind in order to ensure a proper installation.
So how many more cubic inches can you squeeze out a 351M using a 400 crank? With a .030 over bore the engine's final size will be 407.971408 CID! (or 408 CID).
Conclusion
If you stick with the guide lines I have given here in this article, I promise you that you will have an engine that will absolutely kick ass!!! In addition to the modifications I made to mine that I've listed above, I also performed the following things to the engine:
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How to Rebuild Ford Engines V8, Covers 351C/351M/400/429/460... How to Rebuild Ford Engines by Tom Monroe is by far the best and most comprehensive book ever written in regards to 351M/400 engines. The book covers nearly everything you ever wanted to know (and some things you didn't) about the M-Block engines. |
As I've said before in other articles in this site,
the 351M/400 is one of the most over-looked and under-rated engines Ford
ever built. However, with the proper components it can be made to be a
real screamer. I researched the high performance buildup of my engine for
months and I discovered how to make one of these engines capable of
blowing the doors off of just about anything else out there, all for less
than the price of a "crate motor".
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