L-76 Engine Performance w/H & W Valve Adj.

[Stuart F.]

This past week end I performed the Hinkley/Williams (or W/H) valve adjustment on my original 63 L-76. I set them at a cold .008" (I) and .016" (E). my first impression, as I went through the procedure was that my intakes were about right from my previous adjustment (about 6 mos/ago), but all the exhausts had been about .002" loose. I went through them twice. After having applied a timing tape to the balancer, I barred the engine over using a torque wrench with a ratchet head and 5/8" socket on the center bolt of the balancer. From the onset, I found the center bolt to be loose (untorqued), so it took no more than 40 ft lbs. to turn the engine in the normal direction of rotation (CW from the engine front, CCW from the driver seat). Conditions were that the plugs were still in place, battery disconnected and car up on ramps - none of which probably mattered.

Upon my first road test, I noticed that my low end performance had suffered some, where as, the overall road performance showed noticable improvement. It pulls stronger in the mid to upper RPM range in all gears. It had always had good torque in 4th gear at normal cruising speeds, but the sense now is that it is unnecessary to downshift.

All in all, I'd say the change has awakened the beast. The difference is quite noticable and I strongly recommend it to any one with an SHP equipped Vette.

Special thanks to both John Hinkley and Duke Williams for their efforts to develope and document this procedure. I only wish it had been done 55 years ago. It would have saved me a lot of oil burned hands and severe head aches, to say nothing about the improvement my ET slips would have shown.

Stu Fox

[Tim S.]

Re: L-76 Engine Performance w/H & W Valve Adj.

The same procedure in my 65 L84 stabilized the idle, valve clatter, and improved the overall performance as well.

Tim

[Duke W.]

Re: L-76 Engine Performance w/H & W Valve Adj.

The recommended clearances are based on the observed constant velocity clearance ramp heights from the engineering drawings times my measured 1.37:1 lash point rocker arm ratio.

The .008/.012" tall clearance ramps heights times the "design" 1.5:1 rocker arm ratio yields .012/.018", which are the specified Duntov cam clearance settings prior to 1963, but they are .010/.016" using my measured lash point rocker arm ratio.

The Duntov cam was somewhat milder is the longer stroke 327 compared to the 283, so apparently the engineers felt it was worth specifying tighter inlet clearance to get a little more effective duration. The .008" spec was established in 1963, and back in the fifties an article in Corvette News recommended this setting for Corvettes used in "weekend sporting events".

Tighter than OE clearances will increase effective duration and overlap, which can hurt low end torque, but will also reduce valve train loading, which will help minimize valve seat wear/recession. The OE clearances result in the valve being jerked off the seat and then slammed back into the seat at higher than the .0002"/cam deg. clearance ramp velocity.

If you detect a loss of low end torque, you can set the inlet valves at .010" and still keep valve train loading and noise to a minimum. It's a tiny change, and I'm not sure if your SOTP could feel it or not, but it's worth a try.

Since you're pulling a 3.36 axle through a CR tranmission, a little bit of extra low end torque can mean a lot, but the difference might not even be noticeable with a 4.11.

Duke

[Stuart F.]

Re: L-76 Engine Performance w/H & W Valve Adj.

Duke;

Thanks for responding. Can't say I mind the low end torque loss too much because this car certainly is not a stop light performer. I did go so far yesterday to check out my carb settings, i.e. where it was idling at 14" to 15"Hg before it is now at 12"Hg., and it seems more sensitive to minor adjustments (1/4 turn either way on either screw will cause a 1/2" Hg. change right away). All my typical IFR temperature measurement points seem to be the same, including the exhaust manifolds.

Perhaps I may revisit my carb jetting at some point in the future, but it seemed very good before. I don't know if you noticed or not on another thread, we were discussing the amount of vacuum signal one should see at a typical automatic choke. I related how I had tested mine at the clean air hose and saw no more than 2"Hg, and others said that I should be seeing close to full idle vacuum. I did that test before I went to the electric choke to determine what affect that eliminating that vacuum draw might have on my jetting considerations. As I saw so little signal there I disregarded that as a factor. If, as others say, I should see full manifold vacuum there, to me that would be a big vacuum leak and is not likely that it was intended to be that way. I recall changing heat tubes in manifolds back in the late 50's and they appeared to be pinched in an "X" shape to restrict flow through them. Others report recent replacements are straight through with no apparent restriction. The discussion left me baffled.

Would be interested in hearing of your experience in this area.

Stu Fox

[Duke W.]

Re: L-76 Engine Performance w/H & W Valve Adj.

The choke hot air system is driven by manifold vacuum, but at some point in the flow path there has to be a restriction that limits the flow rate. I've never looked into it in any detail, but I know the vacuum signal port between the AFB choke housing and the carb body that is sealed by the little O-ring is pretty small.

Whatever the smallest diameter in the flow path is the restriction that limits flow rate.

Duke

[Stuart F.]

Re: L-76 Engine Performance w/H & W Valve Adj.

Duke;

If you get a chance, I'd be interested what numbers you might come up with by connecting your vacuum gauge to the clean air hose at the carb and doing a normal start and warm up - assuming you still have the hot air loop in place.

Thanks;

Stu Fox

[Duke W.]

Re: L-76 Engine Performance w/H & W Valve Adj.

One would need a very sensitive vacuum gage calibrated in "H2O.

The manifold vacuum port to the choke housing acts on the choke vacuum break piston, so leakage past the piston is all that enters the choke clean air tube at the air horn, as long as the choke housing cover gasket is in reasonable shape and doesn't leak.

So there is very little flow through this circuit.

You could get an idea by plugging the inlet at the carb air horn and measuring the RPM drop at idle - hot or cold.

My guess is it won't even be noticeable.

Duke

[Stuart F.]

Re: L-76 Engine Performance w/H & W Valve Adj.

Duke;

Your theory corresponds with my actual tests. I did that before I changed to an electric choke in order to evaluate how much of a vacuum signal was present in this circuit and, therefore, whether it would be a factor to consider in jetting. I found it to be, as you say, very little to the point of being insignificant.

When connecting a good quality gauge to the hose at the carburetor, I was able to read practically in 1/10ths Hg. because the signal is well dampened.

Several others in another thread disputed my findings saying that they were getting almost full manifold vacuum there and felt that only the piston was imparting any restriction in the circuit.

It seems to me that if you were to get almost full vacuum at the hose to the carb air horn, you would see a change in idle speed by plugging it, as you noted. After all, to me, that would be a major vacuum leak!

Again, I would be interested in you or anyone else conducting a little verification test on this. I may be old and lost some of my marbles, but I think I have this figured right.

Have a great week end.

Stu Fox

[Larry B.]

Re: L-76 Engine Performance w/H & W Valve Adj.

Does anyone have a link to a tech paper for a adjusting a '61 with the Duntov cam? Thanks Larry

[Stuart F.]

Re: L-76 Engine Performance w/H & W Valve Adj.

Larry;

Check the archieve. It has been discussed many times. The paper was developed by Duke Williams and John Hinckley and may be referred to as either the Hinckley/Williams or Williams/Hinckley method, depending on which one of the Authors is deferring to the other for credit. The last title I saw was: "30-30" AND OTHER OEM SB SOLID LIFTER CAM VALVE ADJUSTMENT (Revised 09-23-2008), By John Hinckley and Duke Williams.

The title says it all as far as your 61 w/ Duntov equipped Vette is concerned. I highly recommend it. I could list out the specs from it here for you, but I feel you should really print out a complete copy for yourself for future reference - as once you adjust your valves with this method, you won't need to know any other way.

Stu Fox

[Joe R.]

Re: L-76 Engine Performance w/H & W Valve Adj.

Does anyone have a link to a tech paper for a adjusting a '61 with the Duntov cam? Thanks Larry

Larry,

Here you go:

http://www.lbfun.com/Corvette/Tech/vettetech.html

Click valvetrain, there are a couple of articles.

Joe

[Larry B.]

Re: L-76 Engine Performance w/H & W Valve Adj.

Thanks Joe!

[George J.]

Re: L-76 Engine Performance w/H & W Valve Adj.

I thought it wasn't recommended to turn the engine using the balancer bolt for fear of stripping the threads. Others have stated you need to use a torque strap, which seems difficult, and that you need to take the plugs out.

George

[Stuart F.]

Re: L-76 Engine Performance w/H & W Valve Adj.

George;

As I noted earlier in this rhread; My plugs were still in, I used the balancer bolt which I found to be loose. I put my torque wrench on it and I could turn the engine at less than 40 ft lb. I posted a thread about this and was advised that I needed to torque the bolt to 80 ft lbs., twice what it took to turn the engine. now what does that tell you?

After I was done with the valve adjustment, I put the tranny in 4th gear and set the E brake so that I could torque the bolt w/o turning the engine. My 63 has the L-76 with advertised compression ratio of 11.25 to 1. I turned it slow, no less than 8 times CW from the front of the engine, and even backed it up a short amount a couple of times.

A month or two ago, I did a compression test and all cylinders were up to snuff. My car has a little over 45k miles on it, it burns very little oil (normal) and still revs to redline freely as well.

So now then, can any one tell me what I have done wrong? Is my engine about to fall apart? It recently survived a complete short block overheat and purge without any lasting ill effects when a new thermostat stuck closed.

Stu fox

[Duke W.]

Re: L-76 Engine Performance w/H & W Valve Adj.

I thought it wasn't recommended to turn the engine using the balancer bolt for fear of stripping the threads. Others have stated you need to use a torque strap, which seems difficult, and that you need to take the plugs out.

George

I've always used the balancer center bolt, when necessary, to turn the my 327/340 and Cosworth Vega engines, even backwards. Both are speced at 80 lb-ft, and if it takes that much static torque to turn the engine, even with the spark plugs installed, there's something wrong inside.

Duke