Vortec L31 cylinder head
(→Vortec L31 5.7L head ID) |
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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." | 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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| + | They not only out flow but also out burn the older heads. The L31 Vortec used two castings 12558062 which you have uses induction hardened exhaust valve seats for unleaded fuel. The other casting is 10239906 which uses hard seat inserts on the exhaust side otherwise these two castings are the same. | ||
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| + | All of these came with self guiding rockers on press-in studs, the conversion to screw in studs would still require self guiding rockers unless there are sheetmetal guides under the studs in which case they can use the older unguided rocker. | ||
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| + | The most common issue with these heads is clearance of the bottom of the spring retainer to the top of the guide oil seal. The guides of this head are pretty high when the stem seal is fitted this restricts max lift to .460 to .470 inch. The most common way around this is to tool the guide lower to gain sufficient clearance. The valve springs are 1.25 inch in diameter which is too small for very radical cams. The safest way to deal with this is the use of beehive or conical type valve springs as the with the raised intake port enlarging the spring seat can easily cut into the port roof. The beehive or conical spring use a smaller diameter retainer which also provides some relief with the guide/oil seal to retainer clearance. | ||
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| + | Keep in mind that these heads were for a roller cammed engine that uses a shorter 7.2 inch push rod, when they are used with a flat tappet cammed engine you need to go back to those longer push rods. The average kid at the parts store probably doesn't know that so if he looks these up by year or type of head the short push rod is what he'll find, so you have to know that the push rod lenght goes with the type cam not the type head. | ||
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| + | The valves of this head are the same 1.94 by 1.5 Chevy is so fond of. Depending on cam and compression these heads are a 20 to 50 horsepower bolt on. They have very good midlift flow but run flat over .5 inch so going that much or beyond with the lift is of little value without porting them. They very much respond to a 30 degree back cut on the valve's seat, both intake and exhaust, this is very effective on a long duration cam that lifts from .45 to .5 inch at the valve. A split duration cam is also of value as the exhaust flow didn't improve compared to the intake so that side can use a little extra blow down time moreso than extra lift. | ||
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| + | The chamber burns fast compared to previous heads, you'll find its happiest with total spark lead of 34 to 36 degrees. The chamber is very detonation resistant compared to earlier chambers many guys run 9.5 or a little more with a squish/quench clearance of .040 to .035 inch. Flat tops or D dish pistons work best compared to the factory round dish pistons because those have too much of their squish/quench surface contribution too far from the head to be effective. | ||
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| + | Have them checked for cracks, they don't like to be overheated, if these are used heads and have been overheated the chances of cracking is super high. They also like 4 corner coolant return, there are kits that make this quite easy to install. A pair of fittings on the rear of the manifold over the coolant return opening of the head that can be connected to the intake's forward return before the thermostat. Or they can be plumbed into the heater/bypass circuit. This prevents coolant vapor bubbles from collecting above the rear cylinders causing them to run hotter than desired. This is almost as good for power as the 4/7 timing swap without the cost of the cam, your valves and piston on number 7 will thank you." | ||
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