Aluminum head vs steel valve adjustment dyno/track comparisons

Bbb,494 stage2 aluminum head,solid flat tapped cam,roller rockers. I've been reading this for years and never done either comparison. Read that if aluminum head go .006 tighter cold. It's been years since I've checked my lash hot. I've been setting at .020i,.022e.cold. So I tightened them up and did a compression check before and after. Lost 20# on each cylinder. Now I'm not sure yet if I've lost that hot but will be checking tomorrow night at the track after a pass. Has anyone seen performance gains or losses?

 

I used to forego any consideration for hot vs. cold and my car would run best on the first pass then slow down.

I tightened the lash .006 to account for the warm up and the car was more consistent and didn't slow down like before.

That was a factory block with aluminum heads. Tomahawk is down around .006 - .008 cold lash.

 

So a update....went to the track tonight. Really crap air-3300feet da.really humid. My predictor said 10.16 and it went 10.24,10.20,10.18. After the 2nd pass I removed a valve cover to see what the lash was and suprize!! It was still .014 just as I settled cold. Engine temp was 180.oil temp 180. Set them to .020 hot and thats when it went the 10.18. Compression was lower too. So I guess it isn't worth the effort to set them tight. Not for this cam anyways.

 

Awesome indie what’s the combo?

 

494 stage2 track elimator heads, solid cam .680 lift,

 

With a solid flat tappet cam or a solid roller once you reduce the lash so much that the acceleration ramp and the decelerating ramp of the cam is already guiding the lifter you will have the result of cylinder pressure loss since the valves are now opening sooner ( intake ) and ( exh ) closing later, but the cam is acting bigger.
It would now seem that even with not so great air you could use a tad larger Cam.

it’s always been said that in terms of race motor performance that once your into the cams power band you should never trade off breathing for compression, of course providing your 60 ft times do not suffer.

 

So you saying your cold and hot lash was the same Dan?..Interesting, I have a solid cam tomahawk in the shop now, will check cold vs hot lash..

I have always set cold lash at .012 less than spec on the stand.. on the dyno, the lash is in the ballpark usually once it goes thru warmup/cam break in.

JW

 

What's the .050 duration and static compresson?

 

Yes,after my run as long as it took me to remove my hood and valve cover bolts,covers. Was still at .014/.016

 

Be interesting to know what the head temp was, when you were checking the hot lash. As you said, you have to drive it back, and then get the hood off and valve covers off before you check.. If you like most racers, the fan and water pump are on while your taking the valve covers off, unless you specifically left them off to keep the heat in the motor.

I wonder if the heads are shedding enough heat in that time to tighten the lash back up.

Does any of this really matter?.. Probably more trivia than anything, and it's no where near the factor in ET change that other things are.

Like everything else in bracket racing the key is consistency.

JW

 

My 308s cam called for .022/.028 COLD but didn't specify iron or alum heads. When I set them like that with my alum heads it was a clattering mess hot.
Went with .014/.016 cold which is probably .020/.022 hot.

What are others setting 308s at with alum heads?

 

When I ran a 308s years ago I had good luck at .020,.024 cold with aluminum head.

 

And it wasn't noisy as hell hot?

 

Here’s some thought for some of you.A few of you know that I work on a Pro-mod team.760-960 ci.On our big inch engines( billit block and heads)I have seen when it was cold,in the 40”s at night.In the morning,I would have to heat the coolant and of course the oil in the dry stump tank to get some temp in the engine as it would have no compression from negative valve lash.After a warm up lash would be .024-.026. Generally we leave the starting line as close to 100* as possible to help curb some detonation,so I keep my optimum lash at that temp if possible

 

.020 .022 cold. Very loud. Cut it back to .016. .018. Quieted up a lot. 308s. Never checked it hot.

 

I have always ran my solid rollers on the tight side without any issues. .018”-.020” cold. Set it and forget it. I check everything in the winter/spring,and they are exactly how I left them.

 

Mine would be embarrassingly loud hot if I set to .018/.020 cold. I do have mufflers....I guess if I had open headers I would know if they were clattering or not.....
Maybe my roller rockers are worn?

 

Dan are you setting lash every 90 degrees on the balancer?

 

I have a tomahawk and stage 2 heads (AL) and did run 0.012" cold around 60F. This results in 0.028" lash on a hot 180F engine.
Cam card calls for 0.022 / 0.024 so I'm now I running 0.008 cold at 60F to get the 0.024" hot.

 

Wow! That's a BIG difference cold/hot. Aluminum block have something to do with it?