cam lift vs coil bind

[DOTracer]

bbc drag race application.

How close do you all feel is "comfortable" to be from coil bind with respect to camshaft lift?

Are there other factors to take into account? Do not want to get into a situation where valvetrain reliability becomes a concern.

Application is factory iron castings with stock length valves. Installed height is 1.950" on a 1.550" diameter spring and that was using either +.050" keepers/retainers.

Two cam companies I have spoken with want to raise lift substatially similar in to how super stockers do to "crutch" a poor cylinder head. One company asked about going to a 1.625" spring, but with their taller installed heights, would require new valves which really adds to expense, etc. Not sure the cam swap is worth the expense if having to go this route.

The second company gave me a Crower spring part number which is a H11/Vasco "NASCAR" spring which he tells me can install at my 1.950" installed height and allow for the lift he's wanting to use.

Lets look at this particular Crower spring part number 68671.

1.950" = 263 lbs
640 #/in
coil bind at 1.110

1.950" - 1.110" = .840" theoretical clearance.

Cam grinder is recommending lift in the NET range of .760".

.840" - .760" = .080" coil bind clearance.

Is my math correct or do you add in the lash as your true net coil bind to lift clearance?

Wanting to improve performance, but not at the risk of possibly having valvetrain reliability issues.

If using the Crower 1.550" spring, I would reuse my current Comp titanium retainers as it looks like the steps would fit these springs OK. The valves are 2.25/1.88 11/32" stem stainless undercut stem valves. Engine speed would be limited to between 7200-7500 rpm redline. WIth the current cam I shift at 6500-6800, engine sees 7200 through thr traps...likely wont' raise the shift rpm much higher, if at all. If it see's more rpm it'll likely be through more trap rpm if it picks up performance by a measurable amount.

thanks for any help on this issue
Todd

[bill jones]

-One thing you need to consider is what you are going to do when the spring pressure drops off after you run the springs a few events.
-If you have to shim the springs or use .050" down locks after the springs take a set then you need to plan on that.
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-Typically I'd rather start out with about 40# too much pressure and let the spring pressure drop down to what I'd intend to race with after they have gone thru several hot and cold and stress cycles.
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-After the springs pressures take a set I have run springs to within about .040" to .050" from bind and this is measuring each and every spring raceready for honest lift at the valve and then checking each one for absolute coil bind.
-Another thing is the intakes seem to lose more pressure than the exhaust so you might want to consider switching the intakes with the exhaust after they have taken a pressure set.
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-Personally I wouldn't worry much about having valve train harmonics since you are well under 8000rpm even where you have the heavy steel valves in there.
-When you start getting to and above 8000rpm is when all the bad stuff really begins to happen.
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-I personally prefer to have about .100" from max lift to coil bind but spring people are always harping on choose a spring and run'm as close to coil bind as .050" so that you don't have that extra wire length in there to do the heeby geebies.

[dbusch]

what Bill said...

running .080-.100 to coil bind is better for the average guy who doesnt check each spring where they really coil bind, not just what the sheet of paper says. I admit i am guilty of that sometimes. I too, have run as close as .040" to coil bind (theoretically) and seemed to be no different (for spring life) to running .080"....

[Wolfplace]

Same deal. I have run a number of springs .040 - .050 from coil bind but,,
As was stated above, you need to check every spring as they can vary a bunch from what the card says.
Assuming coil bind when you are this close will ruin your day.

And yes it is net lift you are concerned with or what the spring actually sees.

[bill jones]

-A couple of things about checking absolute max lift to coil bind---particularly on BBC's is it takes some effort to make a tool that is sturdy enough to be able to pull the rockers from max lift to coil bind especially when the open pressures are 800 to 1200 pounds.
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-With stud mounted rockers and stud girdles the bridge that hooks under the pushrod end and then pushs on the roller tip end needs to be an arch to clear the stud girdles---and you need about 3 feet of handle length.
-The stress on that arch is huge and causes it to flex so you have to be real careful.
-If that arch flexs and slips off while you are pulling close to 1000# you crash from the release of all that pressure.
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-AND to try and test this with the engine on an engine stand is nearly impossible because it's easy to tip the engine and the stand over---and even if you have help to hold the stand in place it's still a pretty serious effort to check all 16 valves.
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-But if you really want to know you just do it and work around all those challenges of the project.

[DOTracer]

Thanks for the input fellas.

Couple quick questions.

When your saying testing the springs on the engine, I assume you are saying to run the motor over to peak the cam/rocker, then use a tool to further open the valve to coil bind and measuure this distance with a dial indicator on the retainer? Or are you stating something else here?

Also, does it or can it damage the spring in any way stacking the coils to coil bind during the mockup/testing like this?

Can't the springs simply be tested using the Rimac spring tester at the given installed and then coil bind distances? This could be a way to verify not only the seat/open pressures, but also the coil bind height?

Bill, if the stand is tipsy, then how might you recommend doing this testing. I know I don't have any sort of tool to open the rockers right now...could weld up something with some metal here I suppose.

I do have an engine cradle I made out of some 1" square tube...I guess the engine could be bolted to that to check the springs where it's sitting much lower to the ground and have someone hold it down?

I'm kinda hoping most of the measuring could be done on the Rimac when the springs are set up and avoid trying to measure on the engine...?

The one cam company supplies a Cam Dr. documantation, so if checking the springs to coil bind on the engine really needed if the data is verified with the Rimac tester? On the engine, wouldn't component/valvetrain deflection skew the results some, or are you looking for real world measurements taking such deflection into account? This is a standard stud mount rocker (Comp Pro Mags) and I would be adding a Crane stud girdle into the mix with this "new/higher lift grind". I'm sure even with the girdle you'll see some amount of deflection which would lessen lift and give slightly more room from max lift to coil bind, plus your lash as well?

thanks for the replies folks. I'm taking from this that this spring along with the proposed cam doesn't sound like it should be a valvetrain reliability issue in regards to coil bind, etc which is a bit of a relief to me. I'm used to mild combinations and stepping up like this does create a bit of concern to me, so I want to make sure I'm not getting into somthing that could be cause for concern other than your normal routine maintence.

[bill jones]

-I really don't like to compress new valve springs into a solid coil bound situation because I've seen the pressure drop about 30# or more when I did.
-Some people and some cam companies say to do it but I don't because I don't want to waste that first 30 to 50#.
-So I'd say that you need to be paying attention to what happens if you clamp the spring solid like in a Rimac tester.
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-If you have plenty of leeway in the springs vs valve lift I suppose that you could do like a lot of people do and just do the math to see where you are---but if the engine is for racing and I am doing it I check---"at the valve, race ready, every valve, every cylinder".
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-I have a pair of transport stands that I use to sit engines down on the floor or into truck for delivery and I have a heavy steel table on wheels that is suitable to support a complete engine so I can set the engine on this table to do the valve train checking.
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-Deflection in a valve train is just about as radical to the metals as the wrinkles are in your tires to the rubber when you stress the tires.
-So if you test the valve train with a light spring and then with the valve train completely assembled and race ready you get two totally different pictures---and the one that really counts is the loaded-raceready situation.
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-AND---if you are using a rubber band for a timing belt you really need to know how all that load and deflection affects valve to piston clearance and on the stroker engines how it affects rod to cam lobe clearance---AND what happens if you ever need rotate the engine counterclockwise.
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-To test the loaded valve to piston clearances means that you need a tool like I described so that you can rock the rocker and valve down to touch the piston near TDC.
-This is where your tool design becomes a problem---because the valve cover rail and the upper valve cover bolt bosses interfere with the hook of ther tool that lifts the pushrod side of the rocker---you have a real problem getting it thin enough to get in between there and still maintain adequate strength.
-And you have to check EVERY rocker to make sure the tool fits'm all.

[Wolfplace]

I check coil bind in a spring tester. Check open & closed pressures here too.
It's like a Rimac but electronic & gives you the actual rate at the height you are installing the spring, not what the catalog tells you.
Also,once you test & save a spring you can change heights on the computer & it will give you all your new specs.
Pretty neat deal.
I have seen springs drop in pressure after being cycled but nothing as radical as Bill has come across.
My feeling is I would rather cycle it in the tester if it is going to lose a few pounds & set it up at this point.
I use light springs for initial testing of clearances & if it gets to a point I feel is close then I check with the real springs to see what I gain.
If I have enough with the light springs I know I will have more in runnin trim.

I have run some pretty close clearances using these methods & so far not had an issue.
Not saying it is right or wrong just the way I do it,,

And, as Bill mentioned, when you are running close to the piston belt drives can get real interesting when you turn the engine backwards