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Old 11-12-2008, 08:32 PM
GMR GMR is offline
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valve spring pressure

The springs in my heads are 115 lbs @ 1.70 and 300 lbs @ 1.25 and are supposedly good for .510 lift.

The cam I'm looking at says it needs 107 lbs @ 1.8 and 308 lbs at 1.35 and its lift is .470.

How do I know what my springs would exert at 1.8 and 1.35 (I don't know the spring rate)? Do I just assume they are fine because they are advertised as good for .510 lift?

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Old 11-12-2008, 09:12 PM
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I would ask the different manufacturers you are considering product from.
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Old 11-12-2008, 10:21 PM
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Deducting 1.25 from 1.70 yields valve lift of 0.450"
Deducting 115 from 300 yields pressure change of 185 lbs.
Dividing 185 by 0.450 yields spring rate of 411 lbs/inch

Deducting 1.35 from 1.8 yields valve lift of 0.450"
Deducting 107 from 308 yields pressure change of 201 lbs.
Dividing 201 by 0.450 yields spring rate of 467 lbs/inch

With your 411 lb. springs installed as is, the new cam @ 0.470" valve lift would go over the nose at 308 lbs.....
115 lbs installed plus 193 lbs gained from lift = 308
(193 is found by multiplying .470 times your 411 lb/inch spring rate)

With the other spring installed at 1.800", the cam would go over the nose at 327 lbs. (107 installed and 220 gained in lift). (220 is found by multiplying .470 times 467 lb/inch spring rate).

Call the cam mfg and ask whether 19 lbs less than what they recommend would result in an uncontrolled or unstabilized valvetrain with the new cam. They are the only ones who can answer the question definitively.

The other thing to consider is that your springs aren't new and probably wouldn't test at 411 lbs/in. In fact, the chances of that are somewhere between zero and a very large negative number, as was so eloquently stated by old bogie on another thread in another situation.

Last edited by techinspector1; 11-12-2008 at 10:34 PM.
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Old 11-13-2008, 08:07 AM
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Thanks to both of you for your replies.

Techinspector1, thanks for that great explanation.
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Old 11-13-2008, 08:19 AM
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Quote:
Originally Posted by techinspector1
Deducting 1.25 from 1.70 yields valve lift of 0.450"
Deducting 115 from 300 yields pressure change of 185 lbs.
Dividing 185 by 0.450 yields spring rate of 411 lbs/inch

Deducting 1.35 from 1.8 yields valve lift of 0.450"
Deducting 107 from 308 yields pressure change of 201 lbs.
Dividing 201 by 0.450 yields spring rate of 467 lbs/inch

With your 411 lb. springs installed as is, the new cam @ 0.470" valve lift would go over the nose at 308 lbs.....
115 lbs installed plus 193 lbs gained from lift = 308
(193 is found by multiplying .470 times your 411 lb/inch spring rate)

With the other spring installed at 1.800", the cam would go over the nose at 327 lbs. (107 installed and 220 gained in lift). (220 is found by multiplying .470 times 467 lb/inch spring rate).

Call the cam mfg and ask whether 19 lbs less than what they recommend would result in an uncontrolled or unstabilized valvetrain with the new cam. They are the only ones who can answer the question definitively.

The other thing to consider is that your springs aren't new and probably wouldn't test at 411 lbs/in. In fact, the chances of that are somewhere between zero and a very large negative number, as was so eloquently stated by old bogie on another thread in another situation.

Tech, you have a way of making things easy to understand for the novice, that is something that I'm sure a lot of people on here appreciate. I have trouble remembering at times that not everyone on here is a engine fanatic and you still make sense of all of it for the new guys. Its a great way to introduce people into the hobby.
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Old 11-13-2008, 08:43 AM
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I prefer to get spring specs from eBay auction descriptions, from dyno only or one run top fuel cars... good deals too.....
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Old 11-13-2008, 08:51 AM
How fast is fast enough?
 
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Even with the amount of fatigue that a top fuel spring would have I still would hate to try and run that on a typical street chevy camshaft.

I know you said that in jest, but still, its crazy to even think about.
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Old 11-13-2008, 08:52 AM
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If you plan on doing more then one set of heads - the right tools make a big difference and allow you be a lot more accurate with what you are trying to achieve.

You can buy a spring height gauge for around $45 - the tells you your installed height on each valve. Also you can buy the spring force rater for around $55 - this goes with your spring in a vice, you have to manually measure the spring height for each rating, but it will get the job done.

The kickbutt way to do it is with a good spring rater (fortunately I got a screaming deal on the digital Intercomp) - but they are really expensive (around $800).

The thing to remember is that not all of your installed heights are identical on a head and none of the springs have the same exact rate. Kinda have to measure everything you have and then start moving springs around to different valves and make everything as close as possible.
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Old 11-14-2008, 07:37 AM
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Thanks Koolaid
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