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Old 12-15-2010, 07:24 PM
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maximum valve lift?

I'm wondering how I might figure out how much lift my heads can physicially support? or if that even matters.. I'm looking at doing some upgrades in phases, and I'd like to do the cam swap fairly soon, as I suspect there are some issues with the cam I have now.. (very mild cam acts like a big cam, no low end torque. wants to rev like mad) I'm pretty sure it's installed straight-up but if I've got pull it apart to check. .might as well stuff a new cam in..

anyway.. I'm wondering how I can make sure the heads are going to be ok with the lift.. I realize stock heads aren't really going to flow well enough to make use of the bigger cam . .but I'm ok with that for the short term.

Valve springs are a non-issue provided I can find some springs that will also tolerate the lift w/o binding, and still fit the stock heads..

it's a basic stock 350 block, stock crank and rods.. I did upgrade to SpeedPro PowerForged flat-top pistons when I assembled it.. Stock GM heads. .they are something of an odd bird with the chamber size.

i'm flip/flopping on which cam I'm going to use. Lunati VooDoo 60102 or 60103.. the 103 is the larger one 227/233 @ .050 (I'm using a 214/224 right now)
my current lift is .442/.465 and I have no issues.. even with an accidental over-rev to 7200 (stupid shifter lol)
this voodoo cam is rated for .489/.504 lift..

so back to my initial question.. how can I make sure my heads are ok with .504 lift?

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Old 12-15-2010, 07:53 PM
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Quote:
Originally Posted by kc8oye
I'm wondering how I might figure out how much lift my heads can physicially support? or if that even matters.. I'm looking at doing some upgrades in phases, and I'd like to do the cam swap fairly soon, as I suspect there are some issues with the cam I have now.. (very mild cam acts like a big cam, no low end torque. wants to rev like mad) I'm pretty sure it's installed straight-up but if I've got pull it apart to check. .might as well stuff a new cam in..

anyway.. I'm wondering how I can make sure the heads are going to be ok with the lift.. I realize stock heads aren't really going to flow well enough to make use of the bigger cam . .but I'm ok with that for the short term.

Valve springs are a non-issue provided I can find some springs that will also tolerate the lift w/o binding, and still fit the stock heads..

it's a basic stock 350 block, stock crank and rods.. I did upgrade to SpeedPro PowerForged flat-top pistons when I assembled it.. Stock GM heads. .they are something of an odd bird with the chamber size.

i'm flip/flopping on which cam I'm going to use. Lunati VooDoo 60102 or 60103.. the 103 is the larger one 227/233 @ .050 (I'm using a 214/224 right now)
my current lift is .442/.465 and I have no issues.. even with an accidental over-rev to 7200 (stupid shifter lol)
this voodoo cam is rated for .489/.504 lift..

so back to my initial question.. how can I make sure my heads are ok with .504 lift?
Casting number for the heads would be pretty useful information, they're in the rocker box all you've got to do is pull the rocker cover and look for a reasonably long continuous series of numbers. Different castings take different lifts without modification. Once you know what you've got, planning can take place.

Actually low to moderate flowing heads get more from a long duration cam than more lift. There is a point for any and all ports where they max out, this is usually around .5 inch of lift. Getting more flow over that requires porting stock heads or purchasing some type of performance replacement head. If you look at most published port flows, this data is all over the web, you will see that the rate of gain in flow against lift tips asymptotic for most heads after .5 inch. Once a port is maxed, the only solution to more fill in the cylinder is a longer duration cam that holds that max flow for more time.

The compression that really does the engine's work is the Dynamic Compression Ratio which is a function of the Static Compression Ratio (that which you measure) adjusted for what looks like a loss of stroke length which occurs to the point in degrees where the cam closes the intake valve. The DCR is always less than the SCR. The SCR is raised to where the DCR gives a ratio in the very high 7 to ones to the lower 8 to 1's for a street engine. Since it's really the camshaft that dictates the compression ratio, piece mealing an engine together can result in some serious disappointments if not failures to function properly. The cam, pistons, and heads have to work together. DCR calculators are also all over the web, you will need to know in degrees when the valve is seated and the rod length going in. Rod length because in the trigonometry of an engine the working stroke length changes with the degrees of crankshaft rotation which for the same stroke but different length rods puts the piston in different places of the bore for the degrees the crank rotates.

Bogie
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Old 12-15-2010, 08:07 PM
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casting # is 3998920

They appear to be a 69cc head. This picture is a pic of my heads, pasted ontop of a scan out of one my chevy books. my head is virtually identical to the 69cc picture shown below it. This is the only info I've ever been able to locate about these heads. IIRC, it's a small valve head.

Rod length is stock chevy 350.

my only real concern here, is if I'm going to have mechanical problems trying to push my valves open to .504 on stock heads. Performance isn't a huge worry to me at this stage..the rest of my car is setup and ready for a big cam.

my static compression is in the high 9's. I don't know the exact number. (I know. I really dropped the ball here not making some measurements when I assembled the engine...
(Flat top pistons with 69cc heads) I know it pings like mad on anything less then 92 octane.

the cam is listed as 1800-6500 rpm so I expect my 2400 stall converter should be just about right. and I've got 3.55:1 gears.
(which is part of my problem, I run out of cam/rpm well before the speed traps with my current setup)

I run 27.01" tall street tires (255/60-15) and 26x10 drag slicks :>
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Old 12-15-2010, 08:13 PM
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If you are going to change springs to a set that will accomodate the lift you are going to run, the only other thing on the heads to check at this point is bottom of the spring retainer to top of valve guide/stem seal clearance.
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Old 12-15-2010, 08:18 PM
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so basically, I have to install the cam. set a pair of rockers open it up to full valve lift and try to get a feeler gauge or gauge pin in thru the spring to make sure it's ok?

i was hoping there was a slick trick to check it out before I bought the cam.. I guess I could use a valve spring compressor and a dial guage and just apply enough force to get to .504 "lift" on the valve..

i'm pretty sure my current springs won't take .504 lift.. IIRC they are rated for .470 or .480 lift.
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Old 12-15-2010, 08:29 PM
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Take the valvespring off the heads, reassemble the retainer and locks onto the valve, and measure the distance between the bottom of the retainer and the top of the guide or seal. Subtract your lift, and that is the clearance you have. You need about .050-.060" minimum between bottom of the retainer and the top of the guide or seal. If you don't have that much then the top of the guide must be machined down.
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Old 12-15-2010, 08:38 PM
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ok.. sounds like I'll be puling the heads off.. all though I suppose I could do it still on the engine. I changed all 16 springs before like that
i didn't have compressed air, so I fed a huge length of rubber vacuum line into the cylinder. .rotated the crank up to squeeze it and hold the valves shut while I swapped springs it worked pretty good.. just took a while.. made me whish I had just pulled the heads and spent the $$$ on head gaskets.. LOL
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