Speaking of accuracy, can someone explain to me how you can set valve lash with a feeler gauge while the engine is running??? This would be impossible.

 

.021/.026" for the LT-1?

The paper (revised in 12/15/05) specifies .016/.023...

 

What does the "30-30" mean?

Will that work for my BB?

 

Not really, several people do that method when they dont know of any other ways. Fairly simple by tighting the rocker to have a slight drag on the feeler. Horribly messy, oil everywhere including on the person acting this proceedure out. They even sell some rocker clips for this, but not worth the effort. Some even suggest cutting the top out of a old valve cover. Same result, oil everywhere. My method works best and with the engine off. Clean and easy.

 

lash is .030 on both intake and exhuasts. 69Z's had this cam...I loved it. This is sb cam. Larry

 

Might be what he is recomending now for LT-1's. I have not been hanging around over there in a few years and I have not seen Duke here in the C3 area for a long time. Thanks for catching that. I need to update my files.

I do like to use this procedure though for my "30-30" cammed 302 Camaro. You can barely hear the tappets when they are set right. If they are making a lot of noise, something is wrong.

-Mark.

 

Can this rotation/adjustment be used on a BB?

 

JUST FOR the record, I have owned either a LT-1 Vette or 302 Z/28 Camaro since 1978. I have a few years of solid lifter Chevy motors under my belt. I only adjust valves cold now, hot is BS! because you cant work well on a hot engine and temps vary as you set the the valves unless you continually try to keep the engine warm (how can you do this when you need the valve covers off to get at what you need to adjust?) I adjust valves by refrencing the harmonic balancer and you can easily mark your factory balancer by using a paint pen or silver color "Sharpie" felt pen and a little math (8" diameter balancer times PI equals about 6 1/4" of diamter per 90 degree interval on you balancer) Steal your wife's measuring tape she uses for sewing and mark your balancer at the 90 degree intervals and make a note of which cylinders you should be on according to the firing order TDC 1+6, 8+5, 4+7 and 3+2 at the 90 degree intervals(you may also want to make a mark at about 36 degrees for ignition timing if you don't have a advancible timing light (at about 2.5" from BTDC is close enough to 36 degrees.) The people who say the exhaust cam is on the clearance ramp when the cylinder is a TDC on the firing stroke are full of BS, while the cam duration of the 30-30 cam is BIG the ramps are not THAT long. Get the balancer marked and adjust the cam to 0.24"int/0.26"exhaust while cold. Your motor will idle rough and you will lose low end HP when the cam gets out of adjustment (yes the motor will idle rough when the Holley carb gets out of adjustment too!) My "stock" LT-1s have run low 14/high 13 second 1/4 miles times and my "stock looking" 72 LT-1 has run low 12's using this method of valve/ignition timing adjustment. Adjust the valves, replace the VC gaskets, change the oil and go drive it!

 

Kinda the same process I use with BBC....Get out and drive,works good lasts long time!

 

Any one of the above proceedures will work, BB or SB as they have the same firing order. Just some are easier and quicker than others.

 

BS, huh? Okay, Einstein, the following table is the beginning of the opening clearance ramp of the 30-30/LT-1 exhaust lobe directly from the GM drawings - lift off the base circle, in inches, measured prior to the point of maximum lift in cam degrees. The LT-1 exhaust lobe POML is indexed at 122 BTDC and the 30-30 POMLs are 110 ATDC/118 BTDC inlet/exhaust. This same lobe is used on both sides of the 30-30 cam and the exhaust side of the LT-1 cam, and the POML is not the same as "centerline" because the lobes are asymmetrical.

Anyone who knows that a circle has 360 degrees and the cam rotates half crank speed ought to be able to figure out if the LT-1 exhaust lobe is on the opening clearance ramp or not at TDC of the firing stroke. The answer can be easily calculated from the table and POML. Care to tell us the angle and corresponding lift? Anyone else? How about on the 30-30 cam?

110 0.00782
111 0.00746
112 0.00710
113 0.00674
114 0.00638
115 0.00602
116 0.00566
117 0.00530
118 0.00494
119 0.00458
120 0.00422
121 0.00386
122 0.00350
123 0.00314
124 0.00278
125 0.00242
126 0.00206
127 0.00170
128 0.00134
129 0.00098
130 0.00062
131 0.00031
132 0.00013
133 0.00006
134 0.00002
135 0.00000
136 0.00000
137 0.00000
138 0.00000

From my dynamic analysis of the lobe design data I determined that the first hint of jerk to bring the clearance ramp up to constant velocity occurs 137 cam degrees (hint: 274 crank degrees) before POML.

From the beginning of the opening clearance ramp jerk to the end of the closing clearance ramp jerk this lobe occupies about 550 degees of crankshaft rotation, leaving a mere 170 degrees out of 720 when it is on the base circle.

An astute observer over on the C1/C2 board opined that "too many stupid people drove him away" when it was noticed that I was no longer posting, but I may come back once in a while to save the innocents from the clueless morons who should be doing some research while they eat humble pie!

Duke

 

Hey Duke! Thanks for dropping in.

I know from your lift/crank measurements that the "30-30" cam exhaust lobe will lift the tappet .004" at TDC firing position. That equals .006" at the rocker arm/valve stem assuming a true 1.5:1 rocker ratio. If you set the exhaust valve lash to spec at TDC you will be .006" too loose on the exhaust setting. This will cause the valve to slam down on the seat at high velocity. This causes a lot more noise and valvetrain/seat damage.

If you degree a "30-30" cam you can see on the dial indicator how long those ramps are. They lift very slow for the first .020" of lobe lift(around 100 degrees of crank rotation or around 50 degrees of cam rotation !!!) You can rotate the assembly and easily watch the dial to see where the cam starts to ramp up quickly. It takes a LONG time.

I think the POML for the exhaust lobe is 250 crank degrees ATDC-FP. The intake lobe POML is 470 Crank degrees ATDC-FP

Do I win?

-Mark.

 

I think you have the logic in hand, but your numbers are slightly off.

For the LT-1 cam, the exhaust POML is 122 BTDC, which is 360-122 = 238 degrees ATDC of the firing stroke, which is 238/2 = 119 cam degrees before POML. Now look at the chart and go "backwards" from POML 119 cam degrees to see whether or not the lobe is on the base circle (the abbreviated chart in my previous post shows the lobe lift data from 110 to 138 cam degrees before POML).

Obviously it is not because the nominal lift off the base circle at 119 cam degree before POML is .00458", which is about .006" at the valve. In the case of the 30-30 cam: 360-118 = 242/2 = 121, and read the chart. The same analysis can be used for the inlet valve using the last 10-20 degrees of the closing ramp data.

If you set 30-30 cam valve clearance at TDC of the firing stroke and then get the lobes back to the base circles, you will find that both clearances have loosened a few thou because both lobes are on the clearance ramps at TDC of the firing stroke - the early part of the opening clearance ramp on the exhaust side and the late part of the closing clearance ramp on the inlet side. The LT-1 exhaust lobe is also on the ramp at TDC, as we have shown, and as I said in the valve clearance adjustment paper, the LT-1 inlet lobe is not, but just barely!

Will you settle for a silver star? You at least earned that!

I first learned about this when I took a LT-1 cam lift-crank angle diagram many, many years ago, which is when I developed my lash adjustment procedure. I also measured rocker ratio behavior at that time, which allowed me to refine the clearance numbers. A few years ago a fellow NCRS member, engineer, and sharp car guy gave me the lift-crank angle diagram he took off a 30-30 cam many, many years ago. And within the last couple of years I have come by the GM design data from the engineering drawings, so all my previous analysis and conclusions were confirmed with the source engineering data - and then some!

The engineering drawings also confirmed that the LT-1 exhaust lobe is identical to the 30-30 lobe, which is used on both sides of the 30-30 camshaft. The paper also discusses where the LT-1 inlet lobe came from - another interesting historical tidbit. In fact, all the Chevrolet OE mechanical lifter cams, both SB and BB, are derived from only three basic lobe designs. There's nothing better than original engineering data to help you figure out what's going on and distill out some real history!

Analyzing the design lobe data also allowed me to pick - to much greater accuracy than from a dial indicator/degree wheel lift-crank angle diagram - the top of the constant velocity clearance ramps, and this along with my SB rocker ratio measurements allowed me to determine "optimum" clearance settings, which all who have used report excellent results including a very quiet valvetrain, good idle quality, and a rev forever top end. Looser clearance might trade a little top end power for a little more low end torque, but loose clearance will also be noisy, which is the sound of the valves being jerked off and slammed back down on the seats at greater than clearance ramp velocity, which will not benefit valve and seat longevity.

A properly lashed mechanical lifter cam can almost be passed off as a hydraulic cam to all, but the most astute observers!

Duke