Gernby

I think you (or BC) has these numbers backwards. I've never seen any valve lash spec where the exhaust lash was smaller than the intake. Exhaust valves run much hotter, so they expand more.

I completely agree with everyone above about WHY valve lash shouldn't be too large or too small. The ONLY thing that I'm disagreeing about is what determines valve lash requirements. I believe that it has everything to do with the expansion of the materials used for the valves, rockers, head, seats, etc. when they reach operating temperature. As everything expands, the valve lash decreases. Of course, the cam will expand as well, but I don't think that is a signifcant percentage of the expansion.

chris_barry

FYI, valve clearances increase as the engine heats up as alloy heads expand more than the valves. The exhaust side of the heads run hotter than the inlets so they get taller on that side.

A few race engines I've worked on (eg Spiess Opel Formula 3 engines) have almost no clearance when cold. Once the engine heats up the clearances come up to spec. Any valve seat wear or really cold weather and you cannot start the car. My Dallara had dry breaks on the cooling system so you could preheat the motor.

Long duration camshafts achieve their long duration by starting the opening/closing events much earlier than stock. Running tight clearances helps achieve this with the downside that more regular adjustment is required to avoid lash closing up with valve seat wear.

Gernby

The expansion of the head being larger than the expanstion of the valve is a very good point that I hadn't considered. However, it still seems to support my point that valve clearance is not dictated by the cam profile. I would agree that running large clearances for the purpose of long maintenance intervals would be safer on a stock cam than an agressive cam, but I still don't see any reason why a safe clearance on an agressive cam wouldn't also be safe on a stock cam.

Your point about alloy heads causing valve clearance to increase as the engine heats up has me thinking. The first car I ever adjusted valve clearance on was a Datsun 240Z, and the valve lash specs were larger for a cold adjustment than they were for a hot adjustment. Did it not have an alloy head?

Final thought ... if valve clearance increases as the engine heats up, is there any reason why we shouldn't all be running .005" clearances for intake and exhaust to get that little bit of extra lift and duration? Of course, I know that doing this would require more frequent valve adjustments ...

RedLineS2k10

Honda manual calls for the motor to be less then a 100 degrees when doing a valve clearence. This is all pretty new to me, but from what im understanding from chris and BC, is that a cold lash at .008 is what is spec, as the motor heats, and the expanision begins to occur the tight valve lash compinsates for the higher cam profile i;e expansion rate is the same, but cam lift has changed......If that makes sense.

Stock lift is .498"/.459"..BC stage 3 Cam is .525"/.518.

Again, i am new to this, but the higher lift on expanision, would call for a tighter tolerance on a cold valve. If im going in wrong direction with this please tell me...

Gernby

Damnit ... my memory sucks. I just looked up the valve lash specs for a Datsun 240Z, and I had it backeards. The cold clearance specs are smaller than the hot clearances.

Now I'm really wondering why we wouldn't want to run significantly smaller clearances than OEM spec. As long as the clearance stays bigger than 0, it should be fine, right?

wadzii

the clearance has to do with the ramp speeds and profile of the cam.

a hair less than zero might work, it might not.. i dont design cams so i cant say.

also, a thou here or there isnt going to matter much

Gernby

Okay, so I'll quit arguing.

However, I'd still like to know what could possibly cause the BC cams to need larger valve clearance than stock on the intake side, but need smaller valve clearance than stock on the exhaust.

The OEM specs are:
Intake: .008" - .010"
Exhaust .010" - .012"

The BC spec from above is:
Intake .010"
Exhaust .008"

wadzii

call em and ask.. haha..

there are a few articles out there the detail the why's but i cant find them for the life of me.

Rocket made a post on his old forums and possibly honda-tech discussing it

Gernby

Something that I forgot to mention is that the first time I adjusted the valves on my '02 AP1, I set the clearance to the wrong spec, and lost about 10 ft-lbs below VTEC. I measured the clearances between the cam lobes and roller rockers instead of between the valve stem and adjuster screw. This meant that my clearances were all about .005" too large.

We had an S2000 club dyno day a few weeks later, and my car put down really weak numbers on the low cam while making good numbers on the high cam. Basically, the VTEC kick was much bigger than normal compared to the other S2000's at the dyno day.

When I discovered my error, I corrected it, and went back to the dyno. My low cam power jumped up 10 ft-lbs, but my high cam power was virtually the same. This experience was why I consider every .001" to be significant. It may not make much difference on the high cam, but it sure seems to matteron the low cam.

I'll try to find my dyno files when I get home tonight ...

wadzii

valve lash has a pretty big effect on duration

1 thou one way or the other wont matter much, 3 matters a little, 5 matters alot... its not exactly a linear relationship from my experience.