The Stovebolt Site Forums The Shop Area Engine Shop adjusting valves for more compression

I read an Isky article once about increasing the intake valve gap which changes the valve closing time which raises the compression for better low end torque, better economy and more overall power. Is this something that will help my 261 with stock cam? Right now the valves are set at .007I .018E Dave V

Nope- - - -all it will do is cause the valve to batter the seat, and cause the rocker arm to hammer the tip of the valve stem into a mushroom shape. You can get away with a wide gap on a solid lifter race cam because the engine isn't going to last long enough get that kind of wear. The race engines we built for the round track were antiques if they managed to make it 1,000 miles. Most got pulled out for rebuilding or hand grenaded long before that.

Doing race engine stuff on a street machine is stupid.
Jerry

Thanks for the reply Jerry. Just trying to get the most out of the engine the way it is built. Dave V

DaveV, Isky was correct. We alter valve gaps for different tracks which shifts torque range. It's a good testing way to see if another cam would be better suited. I will advise that you only adjust valves when oil is thoroughly hot. Get out there and drive it. Don't think running it in the driveway for a while is adequate. The oil must be hot. Chev measures oil feed temp at rockers. The valves are adjusted while it's running. It can be done with feelers if rocker and valve tip faces are not worn. Or a dial indicator type tool called a Gapper is used.Clearance varies with load or work asked of the engine. A truck working hard needs prox .010 Int, .018 exh . These valve stems expand which closes gap and the valves will close later stealing a bit of power. In essence, lowering Dynamic compression, not the Static compression most people know or talk about. Never set the valves too close. You'll have low manifold vacuum and erratic idling. Myself, I wouldn't run the Int at .007. I've found it's too close. .008-.010 is better, even if you hear them. David

Great explanation David. I will increase the gap some like you suggest. Thanks for the reply. Dave V

The 48-51 Truck Shop Manual has two valve clearance settings.
Normal and Heavy Duty.

Normal: Intake .006 to.008
Exhaust .013 to .015

Heavy Duty: Intake .010
Exhaust .020

There is no mention of why.

Ignoring the slow-rate acceleration and deceleration ramps that are built into the cam by altering the lash specification causes the opening and closing of the valves to become a violent, destructive action. If the clearance is specified as .007", for instance, there is an area of the cam lobe .007" tall where the transition is made from the base circle to the beginning of lifting the valve that is very slow until the clearance is taken up and all the parts are solidly in contact with each other- - - -then the rate of lift increases dramatically. Ditto for the lobe shape just before the valve face touches the seat on closing- - - -the rate of speed slows down.

Increasing the lash adjustment results in the rocker arm hammering the valve stem at a high rate of speed on opening, and the valve slamming down on the seat at closing. Anyone who has studied even rudimentary camshaft design should be able to understand those principles. Every 9th. grade high school kid who ever attended one of my engine principles classes left with an understanding of basic camshaft design!

Carl, the increased lash for heavy duty use is to account for valve stem growth during high speed, high load full-throttle operation due to increased valve temperature.
Jerry

Very interesting, Jerry

I think I can understand the resulting extra hammering/slamming.

Carl asked "why" the higher specified gap for heavy duty use of engine?

Check out my edit. Hydraulic cams also have acceleration/deceleration ramps, but they're very short, only about .003", and they only last a few degrees of cam rotation.
Jerry

Thanks for the explanation Jerry. Would you have a recommendation for the settings on my 261? I do pull a 2000 lb camper with it quite often. Is that considered working hard enough for the wider gap? I'm a little confused also about the settings on my son's 235. The cam is a 254RV from Patrick's which has specs on the box of .012 gap for both intake and exhaust. I thought that was too much for the intake so I have it set at .08I and .016E. Should I change to the .012 on both? Dave V

Use the cam grinder's specification, as they know what clearance ramp they put on the cam when it was made. Always do a valve adjustment on a fully warmed-up engine, and if necessary stop halfway along and run the engine a few minutes to keep the temp up to normal. I absolutely do not like trying to adjust valves on a running engine. With a little practice you can do a companion cylinder valve adjustment on a stovebolt engine in 15 minutes or so with perfect accuracy.

For an engine that pulls hard a lot of the time, just increase the lash adjustment by .001" or .002"- - - -a little rattle is better than burning a valve from not enough clearance. Just don't get radical on increasing the gap for some imaginary difference in performance- - - -it just ain't gonna happen!

I remember reading the same info on altering clearance specs from Isky in the mid-1950's. They were using that method to compensate for cam grinding errors back when all camshaft grinding machines used a mechanical "master cam" to set the lobe profile. Today's CNC grinders are much more accurate, so the old "alter the clearance" compensation while degreeing the cam is as obsolete as caveman Og's stone axe!
Jerry

Thanks again for explaining this. I'll do some adjusting today. Dave V

For more compression, short of replacing pistons, one can have the head milled .062" or so, the next time it is removed, if it is not already removed, make sure to inspect it carefully first to see if there is enough 'meat' left. Reasonably priced operation. Good idea to mark head for future operations.

Ed

Jerry -

I don't visit the forum as frequently as I used to. However, every time I read one of your posts, I learn valuable info that I can use on my 3104 and my MGTF ... both have old-school running gear. Shame on me for not visiting more frequently.

Thanks for continuing to educate me.

- Lonnie

Glad to help. Lonnie- - - -one of my favorite cars back in the dark ages was a 1957 MGA 1500 that I bought in almost-running condition (rod knock) in 1965 for $50.00. Wish I still had it!
Jerry

Pardon me for barging into a topic I know precious little about.... But ... going back to the OP and the article referenced -- Are we trying to apply advice given for high performance/racing engines to work-a-day/occasional use truck engines?

Is Isky's advice good ... for race engines? If so, we may want to clear that up.

Just trying to keep the peace here.


Thanks,
John

John, Isky's advice was good 50 years ago, when cam-grinding technology was in the stone age. Now, with ultra-precision computerized machine tools, it's as obsolete as kerosene headlights and hand-cranked engines, sort of like manual transmissions.

I remember the little paperback book written by Ed Iskenderian in the early 1950's "Top Tuner's Tips", where the concept of degreeing a cam by altering clearance for individual valves to assure each valve got the exact same duration was suggested. I also watched my father use that method on flathead Ford race engines, using Johnson adjustable tappets, and then brazing the adjusting screws in place so they wouldn't change position.

Increasing clearance effectively shortens valve duration, which, in theory, allows for a longer compression stroke. The same result can be achieved by installing a slightly shorter-duration cam , such as using a Comp Cams 260-degree item instead of a slightly more radical 268. The idea of influencing compression pressure with a minor adjustment of valve lash looks good on paper, but I challenge anyone here to show me a measurable difference on a dyno. I'll even provide the dyno, if anyone is up for the challenge!
Jerry

You can also change the static compression by using offset timing keys. This will shift the whole cam timing forward or backwards a few degrees, thus changing the valve opening and closing time in relation to crank rotation. This does the same basic thing valve clearance does to timing without loosing the total duration time.

Joe

Good point, Joe- - - -we used to advance the cam 3-4 degrees for better torque off the turns, or retard it for better horsepower at the end of the straightaway. In the late 1970's I installed a "Vara-Cam" timing gear on the race engine with centrifugal weights built in that retarded the cam at high RPM so a gain could be achieved at both ends of the power band. Now variable-advance cam sprockets on dual overhead cam engines are computer controlled.
Jerry

I never see lash settings for engine assembly in any manual and know one seams to agree?

Every "Motor" manual I've ever used has valve lash settings, and so do all the factory service manuals, unless the valves are non-adjustable like Ford and Pontiac V8's etc. Then the length of the pushrod sets the depth of the hydraulic lifters and the top of the valve stem must be ground to the proper height during a valve job.

It's easy to reverse-engineer the proper lash setting by using a degree wheel and a dial indicator- - - -just look for the height of the area of slow lift just off the base circle of the cam. That's "race engine building 101" stuff.
Jerry

I'm sorry if left out the word 'COLD' for setting the lash during assembly. Anyone?

You set the lash a little wider:

+ .002" to the intake lash setting
+ .003" to the exhaust lash setting

Start the engine, bring it to operating temperature, then turn off the engine and adjust the valve lash using a static adjustment technique, such as this technique.

Here's a rule of thumb that has worked for me for 50+ years- - - -turn the lash adjuster down with your fingers or very gently with a screwdriver until it gets snug, back off 1/4 turn, and lock the nut down. It might be a little noisy during warmup, but it won't be too tight.

I just noticed your location- - - -I spent about 5 years living in Merced and running a 24-hour diesel truck road service up and down Highway 99, then I was a maintenance man and pipe welder for E&J Gallo, all in the late 1970's.
Jerry

Ok that's what I meant, thanks guys.

Yes Hotrod Lincoln the Gallo boys put Modesto on the map. I also worked in several of their plants just a few years back, we were outside contractor doing calibration work. I bet TN looks mighty nice compared to this area, nothing but traffic jams, you got out at the right time. I enjoy driving my old stuff on the back roads towards the mountains.

I used to spend a lot of time roaming around the gold country on Hwy. 49, and in Yosemite, Tehachapi Pass, Mono Lake, etc. Ever been to the Bodie ghost town?
Jerry

Yes I'm on hwy 49 quite a bit, lots of old car buddies up there. Stopped once in Bodie a looooong time back.

It doesn't alter anything in this discussion but what you are referring to Joe is dynamic compression. Static compression does not change in relation to valve timing.

Thanks for that Chuck, I knew what I meant, just didn't use the right word. I looked it up and found this article, it's pretty interesting.
http://www.enginebasics.com/Advanced%20Engine%20Tuning/Static%20vs%20Dynamic.html

Here is another from Comp cams, look at question #10.
http://www.compcams.com/v002/Pages/404/camshaft-faq.aspx

In my 250 I have a set of Rhoads variable lifters. They bleed off at low rpm and pump up at higher rpm, thus changing the duration and cam events. I get a much smoother idle yet still have the full benefits of a slightly larger cam.

Joe

Very interesting answer to #10. There must be an acceptable safe limit to changing the gap. Wonder how much? Dave

Based on several years of dyno testing, half the clearance ramp height is about the maximum variation I'd recommend. Beyond that there's a danger of holding the valve open when hot if the clearance is reduced further, and a real risk of battering parts if it's increased too far. It's still a "race only" modification, or something a really stupid street engine builder would try. Doing race engine stuff on the street is a surefire indicator that somebody shouldn't be tinkering with anything more complex than a hand-cranked can opener!
Jerry

How often did you have to replace those, engines, Jerry?
(I know the answer, because you have posted that clarification/warning in the past).

Thanks,

A race engine that cost somewhere in the neighborhood of $12K to build was usually good for about half a racing season on a 1/2 mile track- - - -that's way less than 1,000 miles of repeated full throttle operation followed by a quick chop of the throttle to get around the turns. Accelerating from a minimum of 3K RPM in the turns to 7500-8K at the end of a straightaway maybe 800 feet long, repeatedly, has a tendency to wear stuff out, even if the engine doesn't hand grenade itself. Dirt track cars on a 1/4 mile course were even shorter-lived because of all the dust and dirt they ate. Those engines had wider clearances when brand new than a 100K mile street engine! It has to be that way to keep them from tightening up and seizing from piston expansion and too-close bearing clearances! A race engine with mufflers on it sounds like there are a bunch of little guys with sledgehammers inside, trying to get out!
Jerry

The good thing about trying different adjustments is that you get to see what the engine likes or doesn't like. Then you can order up a custom camshaft with those changes added or subtracted from it.

You may find it makes zero change, then it was all for nothing, or it really woke it up and now you get to spend lots more money!

I was able to do a LOT more to influence real-world performance on the track (and the dyno) by using an offset Woodruff key or an offset dowel pin bushing on the timing sprocket than I could by tinkering with the lash adjustment. The Vara-Cam timing sprocket was the best of all, as it was able to accomplish the best of both worlds- - - -good holeshot off the turns, and better HP at the end of the straightaway. The biggest problem then was wheelspin off the turns, a good way to spin out on an asphalt track. On dirt, there's a little bit of wheelspin all the way around the turn. One of our drivers claimed he was turning 200 RPM faster in the turns than he was down the straight!
Jerry

10-12 thousand was about what I spent on drag race engines at the level I was racing at. You got a lot more out of your engines than I did. 200 runs was it, about 50 miles of full throttle. Add in the burnouts and maybe 100 miles. The engines were still quite strong at that point, the issue was the manufactures recommendation of tossing the aluminum rods at 200 runs.

Drag engines can be built a lot hotter than round trackers. When the load is only on an engine for seconds, not minutes, and there's no on/off/on power application, it's simpler to push the envelope toward a complete blowup. Aluminum rods absolutely cannot survive round track racing, as a few of my competitors learned the hard way. A set of Carillo tool steel rods are so hard they're impossible to recondition- - -they wear out the honing stones! A King's lightweight billet crankshaft (38 lbs.) used to cost $5500.00, and the rods, pistons, and rings to complete the rotating assembly would add another $2,000.00 or so. A standard OEM crankshaft weighed 55 lbs. Those prices are probably out of date now.

BTW, a real quarter mile race track has four turns and is paved with chert gravel! LOL!
Jerry

My dad used a Vari-cam on his Pontiac for a while, it work really good. James Rhoads invented them way back when, then switched to the variable leak down lifters still produced to today. The lifters in my 250 work pretty good considering the weak valve springs. Stronger springs and steeper cam lobes make the lifters work better, as mine don't have the pressure needed to really bleed down. They change the cam profile by not opening the valve as fast, not lifting as much and closing the valve sooner. Once the oil pressure comes up and over powers the springs, the lifters go almost solid so you get the most out the camshaft.
A simple way to describe them is having very loose valve lash, till about 2500-3000 rpm, then it goes back to normal.

Joe

Rhoades lifters still have the same flaw as increasing clearance on a solid lifter cam. At low speed, the valve action is too violent, particularly on valve closing where the valve slams down on the seat instead of making use of the deceleration ramp. It's a hokey attempt to compensate for making the wrong choice about cam duration and lift during the planning stages of building the engine. The two most egregious errors an inexperienced engine builder can make is installing too much cam and using too much CFM on the carburetor, ending up with an engine that only works well in a very narrow RPM band near full throttle.
Jerry