I bought a set of aluminum gt40 heads for my 53 ford with a 71 mustang motor, my question is do I need to change the valve springs to a lower pressure to work with the flat tappet cam because there built for a roller motor I currently have a stock cam in the car not these are the specs on the springs in the heads I bought Closed spring pressure is 145# @ 1.800" 358# @ 1.175" Sent from my SM-G955U using The H.A.M.B. mobile app
That is way to much spring pressure for a mild F/T cam. Something around 90-95 closed 220-230 open would be in the ballpark. I dont run that much pressure in my solid cam .600 lift BBF.
Your cam company can give you the exact specs for the springs required for your stick. Why chance it with something so inexpensive as a set of springs? Compared to wiped cam damage.
What is the cam? You are about 50/60# at both ends. I believe you can get keepers that are made to let the retainers sit at 1.850 or 1.900.. So see what your spring pressure is at 1.850 and 1.900, might be as easy as changing a set of keepers.
So, you bought some high performance heads but you plan on running the stock cam? Why not plan ahead and put a higher performance cam in now......instead of later? And if you do , why not use a roller cam ?
Good point but I can think of several reasons. I can't remember if a small base circle cam is a requirement or could use large base circle with proper lifters. I have an old world small base circle chevy cam and it is puny/scary looking..
Why not buy the proper valve springs for a stock cam, which he says he has and be done with it. Maybe a $100.
The higher seat pressure doesn't hurt anything on the flat tappet cam....cams don't wear out on the base circle, they wear out over the nose, so it is the open pressure that you have to be concerned with. Now, we can work with some simple math....you know seat pressure of 145 lbs @ 1.800" ht, and an open pressure spec of 358 lbs @ 1.175 ht....which gives you 213 lbs gained in .625" of lift range(1.800"-1.175"=.625"), which makes the spring rate 340.8 lbs/in....lets call it 341 lbs/in. Working with that, let's say you have a cam with .450" valve lift, we can figure out the actual open pressure over the nose of the cam lobe. .450"of lift x 341 lbs/in = 153 lbs gained in .450" of lift(1.350" ht). Add the seat pressure of 145 lbs to this and we get an open pressure @ .450" of lift = 298 lbs @ 1.350. For every .010" difference in lift, add or subtract 3.41 lbs. .425" lift + 290.5 lbs open pressure. .400" lift = 282 lbs open pressure. An already broken in stock flat tappet cam will be just fine at those pressures. No sense spending money you don't have to, save the money for springs to go on a different cam if you find you need to change them later. Sure, it's more spring than actually needed for a stock cam....but it isn't go to just wipe one out because of it. SBF and 351W Ford have plenty of cam to connecting rod/crankshaft clearance....they don't need small base circle cams, not even in stroker builds.
Needed.my stock heads rebuilt picked these up from a friend who just had them rebuilt for 500 bucks it's over kill for a stock motor but the price was right, I think I'm going to throw a small cam in and try to take advantage of my top end a little bit Sent from my SM-G955U using The H.A.M.B. mobile app
You only need small base circle cam if converting the older block to roller cam with factory lifters. You can use regular base circle cam (5.0) with aftermarket lifters like Crane's or Comp's link bar hydro rollers. If you're staying flat tappet, disregard all that above. Ask around with 5.0 mustang guys, everyone seems to have a spare cam laying around.
Here is what I consider to be important. I'm sure others will have different opinions. When you install a new flat tappet cam it is very important not to "overspring" the cam with too much pressure. Many builders recommend using light duty springs to break the cam in and then switching to the heavier springs after the breakin period. There are additives which help with break in, but for the most part, the "break in" is where you have too cross your fingers and hope you didn't screw the cam up. Even after "break in", you will be causing a lot more friction between the cam lobe and the lifter because of the higher pressure. Yes it will work for a while, and maybe a long while. It still isn't the best way to procede, so its best to try to do it once and be done with it. A roller cam and a set of roller lifters will cost you more money up front. You basically have no worries about wiping a lobe out with a roller cam and the springs you have. Thats a lot of peace of mind, and you will get much better performance from the new head/engine combo. Don't go overboard looking at the maximum HP numbers a cam can produce. For street driving you want a cam that gives good driveability. You need to start by determining how much travel the coil springs have before they reach the point of "coil bind".......when the springs bottom out. Once you know how much the rocker arm can push the spring down before it binds, subtract about .060 from that for some safety margin. Then work backward from that to determine how much lift you can possibly use on a cam and still not reach coil bind. You will need to know the ratio of your rocker arm. Simple example: Say cam has .500 lift and it is pushing a rocker arm with a 1.5 ratio. 1.5x.500= .750 Your spring needs to be able to move .750 and have an additional .060 before it binds up. You also need to be sure that the valves do not hit the pistons with the additional lift. Cam manufacturers (and maybe your friend) should be able to give you some idea what will work......but check it anyway. Last point: Don't shoot for ultimate hp at high rpms, get a cam that has good torque and a wide power band for street driving and you will like it a lot better than a peaky cam.
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