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My engine builder, who also works with local NASCAR teams,and I were having a disscussion around solid lifter lash settings. He suggested, if I chose, opening the lash from my current settings of .08/.18 to .12/.20 if I wanted a bit more torque at the lower rpm range. I didn't think this would really feel any different given my 3.08 gears. Can anyone comment? Thanks
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Re: 097 cam lash settings
Setting the lash loose will cause the valves to be jerked open and slammed shut at greater than clearnace ramp velocity, which will increase valve train shock loading and increase the rate of seat recession. And since the Duntov cam has symmetrical lobes rather than the soft closing asymmertrical lobes of later OE cam designs, you don't want to slam the valve shut unless you are willing to refresh the heads every 50K miles or less.
I definitely DO NOT recommend it!
My recommended Duntov cam settings are .010/.016" based on actual rocker ratio measurements and inspection of the lobe design data that clearly shows that the tops of the clearance ramps on the lobes are .008/.012", inlet/exhaust. If you don't have the lash adjustment paper co-authored by John Hinckley and me, email me and I will send it to you.
Also go down a few threads to the one titled "Vacuum" and read my "Additonal thoughts" response for more background info on this issue. The Duntov cam was designed for SCCA racing and has more overlap that is effective for a good road engine. Back in the fifites most guys thought the lumpy idle, lack of low end torque, and need for short gears was "neat", but as we aged and drive less aggressively these operating characteristics have become a nuissance for many. It can be "fixed" to some degree, but requires a custom cam to be ground, which is not a big deal.
Loosening the lash reduces effective overlap and closes the inlet valve earlier, which can enhance low end torque, though the effect with constant velocity clearance ramps is not huge. Lash is a much more effective "tuning device" on racing cams with constant acceleration clearance ramps, but either way it's tough on the valvetrain.
Keep in mind that race engine folks are not too concerned about longevity as long as the engine doesn't break until the next teardown and inspection, which can be as little as one race. I have constant problems dealing with "engine builders" because they are nearly all oriented toward building race engines - not road engines with broad torque bandwidth and OE or better longevity.
Duke -
Re: 097 cam lash settings
Jim,
That is true. Added benefits include smoother idle and more idle vacuum. All of this is due to the decreased duration and resultant decreased overlap because of the wider lash.
Now for the downside:
Lower lift, causing a corresponding drop in peak power output. Increased valve seat wear due to higher acceleration forces of the valves. The net effect of this is, to use a colloquialism: "a smaller cam".
JoeComment
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.10/.16
I'll go to .10/.16 this weekend. Thanks DukeComment
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Re: 097 cam lash settings
Duke, Did you ever wonder why our Corvette shop manuals have conflicting info on setting the valves on the 097? The Corvette Servicing Guide and 63 Shop manual have two different settings. One is 8-18 and one is 12-18. Forget which manual says what. Now you and others are saying 10-16 which is the middle of the road of the two service manuals. So you must have done your research and experimented to come with this setting. Interesting. JOhnComment
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More...
To illustrate the point here is the Duntov cam lobe lift and dynamic data as the exhaust valve approaches the seat on closing.
The first column is total lobe lift above the base circle. The second column is lobe velocity, which is negative since the valve is closing. The third column is acceleration, and I began the table at the end of the peak acceleration period, which is "3200". Don't worry about the zeros, just look at the relative integer values without the zeros. The last column is jerk - rate of change of acceleration, which is associated with shock loading if positive, which it is on the opening side.
The units of measure are inches and CAM degrees, so velocity units are inches/per cam degree, but can be converted to inches per second (or MPH or any other common velocity dimensions) at any engine speed, and the the actual acceleration and jerk is one and two orders of magnitude, respectively, less than listed on the chart, which allowed me to reduce the number of leading zeros. Don't get hung up on units or orders of magnitude. The important thing is to look at the relative values.
Lift............Velocity....Accel x 10...Jerk x 100
0.02525 -0.002630 0.003200 -0.000125
0.02278 -0.002310 0.003175 -0.000625
0.02063 -0.001995 0.003075 -0.001375
0.01879 -0.001695 0.002900 -0.002000
0.01724 -0.001415 0.002675 -0.002625
0.01596 -0.001160 0.002375 -0.003375
0.01492 -0.000940 0.002000 -0.003875
0.01408 -0.000760 0.001600 -0.004000
0.01340 -0.000620 0.001200 -0.004000
0.01284 -0.000520 0.000800 -0.004000
0.01236 -0.000460 0.000400 -0.003500
0.01192 -0.000440 0.000100 -0.001875
0.01148 -0.000440 0.000025 -0.000250
0.01104 -0.000435 0.000050 0.000000
0.01061 -0.000430 0.000025 -0.000250
0.01018 -0.000430 0.000000 -0.000125
0.00975 -0.000430 0.000000 0.000000
0.00932 -0.000430 0.000000 0.000000
Picking the top of the clearance ramp is somewhat subjective, but in the case of the Duntov exhaust lobe .012" is a decent call (.1192" on the chart). Note that velocity is just barely above final clearance ramp velocity of -430 and acceleration of 100 is insignficant compared to the max acceleration of 3200. As a point of reference, peak velocity on the opening and closing flanks is plus or minus "6080".
Since the lash point rocker ratio is about 1.37:1, the highest operating lash should be 1.37 x .012 = .01644, which is the basis for my .016" cold lash recommendation. Cold and hot (idling) lash are essentially the same (based on both materials engineering principles and my own measurements) because the steel pushrod and cast iron block and head have about the same thermal expansion rates, and there is little more valve stem expansion because the exhaust valve runs cool at idle and low load.
If run hard one can expect the exhaust valve stem to heat up more, expand another couple of thou, and reduce running lash an equal amount, and this is why the Duntov cam has a higher clearance ramp on the exhaust side. It allows plenty of room for exhaust stem expansion during hard running - more than the cooler running inlet valve. The clearance ramps are there to allow for more stem expansion than the bulk engine, and if lash closes up a few thou during hard running that's okay because they are being seated as gently as possible.
Now let's increase the exhaust lash to .020". At the 1.37 lash point rocker ratio the valve will hit the seat at .020/1.37 = .0146 lobe rise above the base circle.
Looking at this lobe rise value on the chart - between .01492 and .01408 - you can see that velocity is around -900, which is about double clearance ramp velocity, so the valve is hitting the seat much faster, which will not have a good effect on valve seat longevity. And at high revs, valve bounce is likely.
Setting the lash at my recommended specs will result in a very quiet valve train - so quiet that you may be able to fool some into thinking it has hydraulic lifters. Valve train clatter is merely the manifestation of high shock loads from slamming the valves into the seats at high velocity - like slamming a door closed rather than closing it gently.
DukeComment
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Re: .10/.16
Okay, but keep in mind that the slightly tighter lash settings may slightly amplify the unfavorable characteristics you referred to. On the plus side your valvetrain will be quiet, it should last forever, and the engine will love to rev!
With a Duntov cam it's okay to set both inlet and exhaust lash of each cylinder at TDC of the compression stroke since the lobes are NOT "on the clearance ramps" at this point. But this is NOT the case with the 30-30, LT-1, or L-72 cams, which is why I admonish all to set lash of these cams at 90 degrees ATC on the inlets and 90 BTC on the exhaust side. This indexing scheme allows you to adjust two valves on the engine at each TDC point (but they are on different cylinders). If you set both valves at TDC with these cams the actual lash will be as much as .003" more than you measure, and the OE specs are already "too loose" due to Chevrolet not considering actual lash point rocker ratio.
The Duntov cam lash can be set using this indexing too, but TDC is also okay - but only for the Duntov cam.
DukeComment
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Re: 097 cam lash settings
Chevrolet based the recommended early .012/.018" lash setting by multiplying the .008/.012" clearance ramp heights by the "design" rocker ratio of 1.5 and did not take into account actual rocker ratio behavior over the entire lift cycle. I measured 1.37:1 at the lash point, which is the basis for my recommended settings. The peak rocker ratio I measured at maximum lift is 1.44 so the rocker ratio never actually achieves 1.5.
Back in the fifties an issue of Corvette News recommended .008" intake lash as an optional setting for "weekend events". This was essentially a "lash tuning" recommendation for a little more effective inlet duration, which should make a little more top end power.
GM realized that the Duntov cam was not ideal for the 461 heads and longer stroke 327, so they tightened the lash spec on the inlet side in 1963 to the same .008" spec for a little more effective duration, but the lobe clearance ramp height did not change as evidenced by the same part number for the cam going back to 1957.
My other post "More" offers additional details into the nitty gritty world of lobe design and analysis.
DukeComment
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Re: More... addendum
Since the Duntov lobe is symmetrical the opening data is the same as the closing data except the velocity and jerk signs change - both velocity and jerk are positive on the opening side.
With the lash setting set at .020", not only does the valve train experience a high shock load from the valve being jerked off the seat at double clearance ramp velocity, but this same lift happens to be the point of maximum lobe jerk, so the shock load is increased even more.
This would be a formula for bending pushrods at a much lower rev level than if the cold lash was set at .016".
DukeComment
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PS
The rows are data every CAM degree, which is two crankshaft degrees. Sorry I did not specify this context in the earlier post.
The Duntov cam achieves its peak opening jerk of "4000" just TWO cam degrees beyond the tops of the clearance ramps. The initial opening dynamics are VERY aggressive. The 30-30 lobe achieves peak opening jerk of only "3000" a full 14 cam degrees beyond the top of the .017" clearance ramp, so the 30-30 lobe initially opens the valve much more gently and will be more forgiving of excess lash. And since the 30-30 lobe is asymmetrical is closes the valve even more gently than the opening.
The L-72/LT-1 inlet lobe is somewhat in between - peak jerk is greatests at "4625" but it occurs 7 cam degrees beyond the top of the .012" clearance ramp, so the jerk buildup is a little milder than the Duntov cam even though the peak is higher, and it's definitely more aggressive than the 30-30 lobe. Its closing dynamics are also a little more aggressive than the 30-30, but milder than the Duntov.
The details of each lobe design reveal very different personalities, which is likely a reflection of what Chevrolet learned about valvetrain dynamics as the years passed. The Duntov cam could bounce the valves at high revs. The 30-30 solved this, but was probably softer than really necessary. The LT-1 cam inlet lobe was from the L-72 cam, which is a bit more aggressive on both ends than the 30-30,but by retaining the 30-30 lobe on the exhaust side, the exhaust valve is assured of a gentle landing, which is good because hot exhaust vavles/seats will erode faster the more aggressively they are laid back on the seat.
DukeComment
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Re: PS
Thanks for the cam dynamics article.
The 192 cam was good for 7200 as I ran it in my 57 270 bored to 301, ported heads and Old's carbs (biger ventruries). With a wide ratio 4 speed and 4:88 gears it was a blast.
Every Friday night we raced on a local highway after dark - no tail lights or brake lights. The valves were set close for the fun and games. At the end of the evening they were a little looser even with set screw locking adjusting nuts.
7200 was good for 107. First gear was good for 50 ft @ 7200 and if you took off with your hand on the wheel instead of the gear shifter - you over reved it a bit. The only thing that gave me a hard time was a big Buick 401 in a 55 chevolet two door post sedan.
The 57 was sold for tution money - best investment I ever make.
These cars were built to be run and not put on a mantle.Comment
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Re: PS
What's a 192 cam? Doesn't ring a bell; 097, 346, and 178 are the camshaft and dowel pin "assembly" part number last three digits of the Duntov, 30-30, and LT-1 mechanical lifter cams for '57-up Gen I SBs; 140 and 754 were the OTC Chevrolet racing cams. T
The finished camshaft part number last three digits (not casting number) of those three production cams is 098, 347, and 182 respectively.
DukeComment
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Re: PS
Hi old friend, Well you got me again. WTH is a 172 camshaft. Maybe 6 cylinder??? Seriously I know a little about cams. But I know nothing compared to old Duke. Maybe someone will chirp in here and tell us what a 172 is. Maybe some after market cam. But I never heard of a GM cam with a suffix or nickname like that. See you in Carlisle in 3 weeks. JohnComment
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