Vortec L31 cylinder head
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| + | *The following information was posted by Bogie on a thread that had to do with cylinder heads and I found it so informative that I didn't want it to be lost, so I'm adding it to the bottom of this L31 article.... | ||
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| + | "The lope sound of the muscle car era is as obsolete as the tube radio. Those cams made more noise than power. If you want to run with the big dogs today, you've got to look for cams with a lot of lift for their duration and fairly long LSA's compared to the old stuff. Big problem today is the inexpensive flat tappet cam is as obsolete as the dodo bird. The flat tappet is very dependent upon low detergent, high zinc/phosphate (ZDDP) oils which except for expensive race only blends, are not to be found anymore. Today's oils are high on detergents and dispersants while low in zinc and phosphate additives. This is because the zincs and phosphates gunk up converters, costing OEM's money for warranty claims, so they've been outlawed. At the same time it is realized that a clean engine experiences less wear, therefore produces less pollution so with the help of 5 micron filters, the detergency is pumped way up. Now there are additives on the shelf to bump the zinc and phosphate levels up in ordinary off-the-shelf oils but these oils are high detergency oils that scrub off the zinc and phosphate coatings so pouring in more from an additive bottle has been proving to be somewhat ineffective. The OEMs went to roller cams in the period of 1985 to 1995 in anticipation of the change in oil chemistry resulting in wear problems between the lobes and flat tappet lifters. In my opinion the day of the flat tappet cam in the hands of the average hot rodder on the street are over. The loss of a flat tappet cam and the secondary damage from all the metal shavings circulating with the oil is so high that it just isn't worth the front end costs that were thought to be saved. So if you're building on a Vortec block from 1995 or later it is at least provisioned for a roller cam if a 95 and from 96 up it has a roller cam. The factory lifters can be used with a more aggressive roller as can the push rods and rockers. The Comp XR264HR has a good midrange with an rolling idle, the XR270HR pushes the power band up the rev range and has a solid stagger to the idle it needs a high stall converter and 3.50 or numerically-higher rear gears. | ||
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| + | The production L31 Vortec head is popular because it has a lot of bang for the buck an easy 20 horses to as much as 40 or 50 over previous factory heads with no other changes to the engine except the head swap. There are many heads that are the equal or better but some can get pretty costly. To really make recommendations would take knowing how much money you can put into your project. The factory production L31 is about the low end cost wise but needs work and parts to run with a hot cam. The guides need to be cut down, screw in rockers installed, the factory springs replaced and better rockers are an improvement. All this gets costly. An inexpensive aluminum head that's very effective is the Pro-Comp. But like the L31, if you’re going for a big cam, they'll need at least better springs. Either of these can be built for less than a grand. Best way is to buy them machined but without valves, springs, retainers and studs, then put in what you need for the cam you choose. | ||
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| + | The static compression ratio, thus piston crown shape and volumes, the head chamber and the cam's intake closing point all walk together if you want power. Modern tight chambers make power and lots of it even with a less than ideal port feeding it, whereas a poor chamber characterized by everything that predates the L31 style chamber for the Gen I block is ineffective regardless of what you do to the ports compared these new heads. Aluminum lets you push the compression ratio up at least one point over iron and takes 50 pounds off doing it. To get to the compression ratio need to start with the cam, the point where it seats the intake is all important as at that point the piston has risen in the bore and pushed inducted mixture back out. This must be compensated with more compression. This leads you to the concept of Dynamic Compression Ratio which is an adjustment to the Static Compression Ratio for the stroke used up to the point where the intake valve closes. You want a DCR of at least 8.00 to 8.50 for an iron head and 8.50.00 to 9.00 with an aluminum head, so this drives hard on all the volumes in and above the piston crown. Check this out to play with numbers http://www.kb-silvolite.com/calc.php?action=comp2 | ||
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| + | Flat top pistons will drive you to a larger chamber head, but as long as you stick to recent aftermarket offerings you should find a 72 to 76 cc chamber with all the configurational advantages of the Vortec L31 head. Don’t be fooled, these heads are much different from the early large chamber heads even though they may have similar volumes. The modern chamber pushes the spark plug well in toward the valves, it may favor the exhaust valve to get in even deeper, there will be a significant relief on the intake side between the valve and spark plug and there will be a beak that penetrates from the squish/quench step to between the valves. These are important refinements to getting power out of the chamber by burning everything that gets in, the older chambers threw a lot of fuel out unused or incompletely used which is the "why" for all the emissions gadgets on the engines of the 1970’s and 80’s. A big reason why that stuff is gone today is the new, highly efficient, fast burning chambers you see coming on the LT1 and LT4 of the early 1990’s and the L31 Vortec of the mid 90’s. Other brands did the same thing under different names at Ford it was the GT40 head for the Windsor, at Chrysler it was the Magnum head for their LA block." | ||
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| + | Bogie | ||
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