What is the best way to check for my installed height that I need.
I am running a low lift cam with alot of duration. A stock diameter spring with stock retainers etc. and a K-750 valve spring.

I may try a behive to bracket race but I need a stock diameter set-up to fit within rules....I am losing power around 5800...graph goes up and down, i'm thinking the springs are going not set-up properly for the camshaft

Any help with this would be great.....I really need to find out how to make this work with a stock diameter spring. Also stock retainers and locks and stock rockers have to be used

 

You set the installed height to get the desired seat pressure. If the spring was selected correctly, setting the correct open pressure will also provide the right open pressure.

Rich

 

I don't know what seat pres. I need. My problem is I'm losing control of the valve around 6K....I will check in the morning but I think i'm running around 135# now closed I do not know open....I know of a few others using the same spring, I can't get any info on setting them up.

I'm pretty sure I need to lower my installed hight but how far can I go?
And should I set up the exaust and intake up the same? if so do the exaust first correct?

 

The K750 springs have a seat pressure of 130lbs @ 1.700'' installed height. If installed at that height, they'll have an open pressure of 395lbs @ 1.150''. These springs are more than enough for your "low lift" cam.

I'm not sure what the factory installed height is for your valve springs but there's also a few more things about valve springs than just the installed height. The spring pocket is only designed for specific diameter springs. If a larger spring can be installed in the pocket, it's wise to use a locater to keep it centered. Depending what your "low lift" spec is, it's possible that the underside of the spring retainer hits the top of the valve guide if the cam has too much lift. Those K750 springs are rated at .610 lift at the recommended install height. If installed at a lower installed height, it will increase the seat pressure and reduce the amount of lift available because the springs will get coil bind.

Using a spring that has an install height anything other than a factory install height (normally higher) means the valve train geometry needs to be checked. Longer pushrods are used to correct the geometry. Longer valves can also be installed to allow a bigger, taller valve spring. My exhaust valves are .100" longer than stock.

Lastly is the rocker. A stock, stamped steel rocker can only provide so much lift on the valve until the slot in the rocker hits the rocker stud. A long slot rocker or any full roller rocker will fix this problem.

Losing power like you mentioned does sound like a valve float problem. Valve float is when the valve is slammed closed. If the seat pressure isn't high enough, the valve will bounce off the seat and bleed pressure off. A long duration cam will have more aggressive ramps which can cause this. That's why roller cams always have valve springs with higher spring pressures. What springs does the cam card recommend? A cast cam is limited to roughly what is recommended. A billet cam can have much higher spring pressures than recommended without the risk of damaging the cam. Too high an open pressure can break a cast cam.

 

The camshaft is stock lift...There is No cam card I was recommended the K-750 springs tho. I will run a dial indicator on it and see what the cam lift is but it is stock lift per NHRA/IHRA stock spec.... I'm not 100% but this cam may have .525 on both sides checked at the retainer...I am running a 7.3" pr now....I will also check that but i think they are good
I am running a long slot crane stock replacement rocker.....
What if I just jack up the hieght until I get it right under bind with .100 or so clearance will this work o.k.?

BTW billet cam

 

Spring installed height has nothing to do with rocker geometry...that is determined by stem installed height, which if you do not sink the VJ or put in longer valves - does not change. I also have never seen a cam break from too much pressure, I have though seen too much pressure cause a break-through of the hardening on the lobes which looks like little craters.

Stock installed height is 1.780" so if they are installed at that height, you are only at about 90-100lbs on the seat, and 320-360lbs open depending on your lift. The spring rate on those is too high at 482lb/in, and you need to raise the seat pressure, but the open will be too high at that point. You should shoot for 130-145 on the seat and 300-340 open.

What cam? Lift?

 

An Isky 235-D spring will work - 130# @1.750" and 320#@.530" Put a .030" shim under the locator and you will be good.

 

This is a custom pc. Stock lift. stock eliminator cam

I have no specs on it.

 

295-D would actually be a better fit - 135#@1.775" 325#@.525" - no shim needed.

 

o.k. the isky look good and I understand what you are telling me.....damn they are almost $300, any idea how long they will last?

Will the 295 isky work with the stock retainers for sure? I have seen some stuff says it will some say it won't....I do need the stock retainer Or an aftermarket direct replacement of the same weight

 

What if its a real aggresive cam? I'm guessing it is if its for a stock eliminator class. It might need more seat pressure than that. Just thinking outloud...

 

That's the problem with not seeing the true numbers on the cam, he could stick a .030" shim under there and bring it up to 150/350. I am a little worried about the stock retainer strength, can you use an isky retainer if it is the same weight or heavier?? That would be my choice, and the moly retainer will weigh more.

I will try a 235/295 spring in a stock LT1 retainer tomorrow - should fit.

 

you ever find out if the retainer would work? I'm going to degree the cam in tommorow, I haven't bought springs yet...just wondering

 

Mike, ask whoever made you the cam what pressure you need to run. Or is it that one you picked up locally?

Bret

 

yeah its the one I got from a fellow racer....I wanted to try it out and see if I like the set-up He ran the K-750 and told me to run them.