valve lash
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Depending on the lobe, the last sets the opening and closing points of the valve.
Yes...
Ray
Yes is a short answer. As Steel has said, it depends on the lobe design. Some cam grinders use a very aggressive "ramp", a steep ramp, that gets the lifter to the point of zero lash, (and the start of the valve "event"), very quickly. The grinder knows when he wants the event to start, and the aggressiveness of the ramp, so he says "lash" the valves at .xxx to keep the designed duration of the "event" accurate... Other grinders use a less aggressive ramp design in an effort to make things easier on the valvetrain components.. And as such, recommend the lash be tighter to once again, keep the advertized duration within their specs.... As you can see, if you run a tighter than spec'd lash you will increase the duration by making the valve event happen a few degrees sooner because the lobe will reach zero lash sooner and start the valve event. Usually a tighter than recommended setting, (.001, .002, .003) will give an engine a little more up top, but you will sacrifice a little down low. By the same token, a looser setting will be a real bear down low, but you'll give up a little up top... Anytime you're messing with the timing of a valve event be sure to recheck piston to valve clearance carefully. Just a few thousandths in a tighter lash can create an exhaust closing later, or an intake opening sooner, piston to valve collision during valve overlap... Sure, we're only talking a minute difference in duration, but sometimes things were already super close. So check it closely if you try "playing" with the settings...
Ray
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I've always wondered this also.. The last 3 cams that I have ran, duration @ .050" have been around 290 on INT and 305 to 310 on EX with moderate lift, .840/.800.. The lash on them have been .028" to .30" on EX and .030" to .032" on INT.. My new cam is 285/300, .873"/.823" and the lash is quite a bit tighter, .020"/.022" per the cam card.. I thought that was pretty interesting.. Obviously the ramp is a lot faster on the Intake which I would have thought the lash would be around the same as the others.. Should be interesting!!!!
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Some lobes are considered "tight lash" lobes, usually with lash <.020".
One thing we can do with lash is change the relationship between the intake and ex lobe. I see all too often where a customer has a poor intake to ex flow ratio yet the split on their cam is only 4-5 degrees @ .05 in favor of the ex., when the cam really needs 10-12 or more to help the ex side. To validate this for the customer, I'll tell them to tighten the ex. lash a few thou and maybe even loosen the intake a few thou. Almost invariably they will come back and report a significant performance gain, either on the dyno or the track. Changing intake or ex lash independently will let us know what the engine needs.
On my last 565 I did one final tune up to the ex ports before the engine went together which (expectedly) killed some of the flow. Unfortunately, I had already given Chris Straub some "final" flow numbers and the cam was already ground based on the original intake-to-exhaust flow ratios. Knowing that they were now different, when dyno testing I made some lash adjustments to the ex., tightening it to add some duration and make up for some of the lost flow. The last pull we did was nothing more than a .002 adjustment, ex. only, and it gained 10hp.
"Lash is the best tuning tool you have in your tool box."
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Most lobes are not designed from scratch per a customer's combination. Instead they're chosen from a library of lobes that are "close" and lash can be used to a certain extent, to help "tailor" the lobe more to the customer's specific needs but again. that's all "calculated" and in the end the engine will tell you what it wants. Lash adjustments or "lash loops" should be a part of every engine builder's or racer's testing/tuning program.
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kinda loose for the cams I've run. lifter type might also affect lash...
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With the Lunati it had the intake and exhaust lash reversed.. The EX was tighter than the intake which I found unusual at the time.. I called them up and they could only tell that it was a cam design directly from WJ.. I said ok and ran it that way.. But I agree, it was a lot looser than I have ever ran.. But it's still together and in my oldman's Blue boat as a back up motor..
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The bottom portion where you loosen and tighten for the lower and upper end is what my engine build explained to me a long time ago. I guess a person would never play with the lash like that unless your at the track to help back it up with numbers unless its that noticeable. Don't know! Might have to play now just to see!
As for the top portion of the page. That is what I was looking for. all you guys have so much good info I just keep making copy's of all of this info. Thanks a lot guys....I'm sure I'll have more questions later. Troy
"Seat of the pants"
Ray
Well Troy, you might be able to "feel" a difference in the seat of your pants... If a motor seems to fall off, or the boat noses over before you think it should, you can try tightening the lash a couple of thou and see how that feels.. BUT, as I said, messing with the valve "event" timing can create piston to valve clearance issues, so unless you KNOW you have plenty of room to play, DON"T!!!
Ray
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You'd have to be running p/v clearances really tight in the first place to have a real problem by adjusting lash a few thou. You're looking at about 1* duration for every .002" lash.
Yes, but....
Ray
Yes Scott I agree, but since I have no idea what, or how, his deal is set up now, I erred on the caution side and suggested he be sure....
Ray
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602 Posts
valve lash is something that I have have always tried to learn more about , like when it comes time to setting it on an engine that you don't know what cam is in it . I have always thought it best to be a little tight rather than too loose . maybe the guys that work with cam design can help here but if your lash is more than whats in the clearance ramps doesn't the follower get slammed into the opening ramp and the valve seat get hammered without the slower set down of the closing side clearance ramp ? maybe loosen a little to see whats happens to the power or rpm , but if you need less duration get another cam .
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lash
With so many combinations out there, the best way to design a cam is come close, then put her on a dyno. Make changes to lash, timing fuel... till it's optimised. then take thos enumbers and make another cam and do this again, and agiain till you've eeked out every hp and tq you can find.b Few of us have this capability.
then reality would set, in. What if i went bigger cubes, heads... It's a never ending quest.
Jerry
With so many combinations out there, the best way to design a cam is come close, then put her on a dyno. Make changes to lash, timing fuel... till it's optimised. then take thos enumbers and make another cam and do this again, and agiain till you've eeked out every hp and tq you can find.b Few of us have this capability.
then reality would set, in. What if i went bigger cubes, heads... It's a never ending quest.
Jerry
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Definitely a limitation on loosening lash before damage can/will occur for the exact reason you mention. Usually no more than .004, or .006 for just a pass or two to prove a theory. You can tighten lash as about a far as you want until something hits, or the valve doesn't close.
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You do have to be a little careful with the ramps get real long and have a large lash, like the 30-30 Duntov flat tappet. There are some old school rollers that fall into this same category. If you lash them too tight in something like a boat, its hard on the exhaust valve. Not only will it open the valve very slowly while on the clearance ramp, and if tightened enough, much sooner than designed. The escaping gases thru the small slow lift, being opened a little early, can torch the valve and possibly the seat. Plus it adds to the exhaust valve heat because its not on the seat as long as it would normally. Just .001 is enough to stop heat transfer to the seat/head.
Not a real issue for a drag test, but if you try it in a boat, it can lead to premature failed exhaust valves. Since exhaust timing doesn't seem to have nearly impact on power that the intake timing does, I try not to reduce the exhaust much more than half the normal lash.
Like Steel said, on the intake, you can pretty much tightened to the point the valve is held off the seat.
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I used to lash the 30-30 @ .020 hot , we were afraid to be too tight but we weren't sure why lol , ran pretty good . the racer brown stx-21 with 12:1 and a scorpion intake was a big step up :|err... have got so I look at the ramps when I degree a cam now , you can get a pretty good idea how the ramps are and when things start to open or should be closed .
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