David and Goliath Part 1 --- Building the 400 vs. a stroker
May 3, 2014
What other way can we describe the often-overlooked 400 when many are going with the larger stroker engines out there today? Yes, we build those and make very good power, but for most, a 400 can, and will, make MORE THAN ENOUGH power. We like the 400! In fact, we build more 400's than we do strokers. Well, they ARE less expensive and in this economy, perhaps that is the driving factor! One thing we notice about many customers is that they would LOVE to get around 400-450hp out of their 400 and the set of 6x heads they just purchased. The only problem there is the overly large combustion chamber leading to a low static compression ratio. Let's cut to the chase: 400's NEED COMPRESSION! Well, EVERY engine will enjoy an increase in compression but some more than others. The larger the engine, the less efficient it can be and STILL put you back in the seat due to the sheer displacement. Smaller engines must be more efficient and use more gearing to approach equal output at the contact patch (the tire) until their higher rpm benefits are realized. We know smaller engines can rev more freely and make their power upstairs. It seems somewhat funny to consider the 400 a "smaller" engine, but when MANY of the 400's out there are being stroked to 4.5", then yes, anymore ONLY 400 cubic inches IS small, and we aren't even discussing the monster stroker engines from Chevy, Ford, and Mopar! Rod length Pontiac designed the 389 to have a tall deck and a long rod. I can only assume piston side-loading was considered to be a real concern. The 3.75" stroke engines using 6.625" rods have a 1.77 rod-to-stroke ratio, whereas, the 4" stroke engines have a 1.66, and the 455 has a 1.57. Typical stroker combinations use a 1.6:1 rod ratio (4.25") all the way down to a 1.48(4.5" stroke)! I believe for a typical street/strip engine, the 400 actually has TOO LONG of a connecting rod! What does a higher rod ratio have to do with anything? When the piston approaches top dead center, we have a "squish" effect, where the air/fuel charged is forced into the combustion chamber by the approaching piston and cylinder head decks. The speed at which they approach each other will affect the speed of the mixture motion. With a longer connecting rod, the velocities near tdc are lower than those with a shorter rod creating LESS mixture motion. But since the piston is moving slower after tdc as well, there may be a more complete burn of the charge. AFTER the charge ignites and the piston is near tdc, but very slowly moving down the bore, the effect of the cooler cylinder head and piston decks being very near each other, has a "quenching" effect. If those surfaces are too far away from each other, then they will heat soak and that may cause detonation. How does this affect intake port sizing? The higher rod ratio tames the instantaneous piston velocities approaching and leaving TDC, therefore, airspeeds whithin the port are likewise affected, meaning, for a given cross sectional port area (especially small ones like our cast iron factory heads), the instantaneous intake port velocities are lessened and "averaged" over the entire stroke, so to speak. Too high of a port velocity causing turbulence is now delayed, allowing a higher piston speed. PISTON SPEED THAT is what the intake port sees, NOT rpm! The consideration of what piston speed means to port size and cam timing is what I believe many stroker combos are missing. I won't say where I believe some of those combinations are going wrong but you will see some of our strokers in the near future making even more streetable power! How can we use the rod ratio and relatively short stroke to our advantage? For a low rpm combination, we need to keep the port smaller. Pretty simple proposition for our cast iron heads! For aftermarket heads, the 1st consideration would be the Edelbrock D-port head with its smaller intake port. Cam timing would be kept conservative with "enough" lift to allow the heads to breathe. Lobe Separation Angle (LSA) and Intake Centerline (I/C) should NOT be chosen along the same lines as the 455 or other stroker builds. That's all I will say about that right now. For enthusiasts wanting more rpm with the smaller E-head ports and you don't mind a slight tradeoff of low rpm torque for top end power, simply change the cam for a longer duration (specifically, a later intake closing) and more lift. Make sure you have enough compression if you go that route. More peak torque can be achieved with a tighter LSA (more overlap). Advancing a larger cam should be considered if you are wanting to skew your power band lower while still desiring a good torque number. A higher stall speed convertor and/or paying more attention to the valvetrain is of more importance at this point. If exhaust port configuration is of no concern and you want more power, I would go with the Edelbrock RPM (round exhaust port) cylinder heads. Their combustion chambers aren't as efficient as the D-port offerings, but for most applications, these earlier generation cylinder heads will more than hold their own in power production! Go with the 72cc castings because low compression, aluminum heads, and the Pontiac 400 JUST... DON'T... MIX! Is it possible the higher rod ratio and a lower static compression ratio, cause less mixture motion and combustion efficiency to where it is just a total DOG of a combination? I believe so. Cam Timing Many enthusiasts are aware a small 230 degree flat tappet hydraulic camshaft in the .500" lift range with a set of unported E-heads providing at least 10:1 compression will support 500hp in a 461 Pontiac. Now, when it comes to the 400, MANY enthusiast believe it takes significantly more camshaft to make the same power. I believe that may be because the 400 makes its power at a higher rpm. Let's take a look at that... It is KNOWN that a given camshaft duration requirement goes up as the cylinder head needs more "time" to feed the bigger cylinder (In reality, the larger cylinder needs a bigger "curtain area" with which to breathe, but increasing valve size or substantially increasing lift is usually much more expensive than just installing a longer duration cam). So... why is the Pontiac 400 the ONLY engine that "needs" more duration than its bigger displacement counterpart according to some enthusiasts? The answer is.... IT DOESN'T, and it's not the camshaft that is the problem. MORE TO COME.....
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