Duntov cam

[David Bodily]

Can someone tell me the lift @ .50 and duration of the duntov cam for a 283 sbc Thanks David

[Duke W.]

Re: Duntov cam

Timing method.....IO-IC/EO-EC, duration, ICL/ECL/LCA, gross lobe lift

Lash point timing 35-72/76-31, 287/287, 108/112/110, .2625"/.2665"

SAE J604d timing 26.5-63.5/67.5-22.5 270/270, same, same

.050" lobe lift 5.5-42.4/47.7-2.5, 228/230, same, same

The SAE J604d timing is at .006" valve lift assuming clearances of ..012"/.018" and a 1.5:1 rocker ratio. The actual rocker ratio is NOT 1.5:1. The lash can be set to achieve these timing numbers, but valve lift will not be 1.5 times lobe lift minus clearance.

E-mail me if you want the valve adjustment procedure, which includes an explanation of actual rocker geometry and how it affects proper clearance, timing, and lift.

I highly recommend the LT-1 cam for all mechanical lifter engine rebuilds. The Duntov is a great cam, especially considering it was a purely intuitive design by Duntov, but the LT-1 cam was the culmination of over 15 years of research by Chevrolet and it shows with broader torque bandwidth and more top end power.

Duke

[David Bodily]

Re: Duntov cam

Thanks Duke I think I'll take your advise on the LT-1

[Duke W.]

Compare to the LT-1 and 30-30

The Duntov data is from specs and third party analyses, so I can't personally vouch for their accuracy, but they appear to cross check satisfactorily and are probably correct.

Here's comparison data for the LT-1 and 30-30 cams. The data for the LT-1 is based on an actual lift-crank angle diagram I measured. The data for the 30-30 is based on a lift-crank angle diagram taken by Bill Clupper. These data will not look like ANY published specs you have ever seen for these cams except the total lobe lift!

LT-1

Lash pt. 34-80/89-40, 294/309, 113/114.5/113.75, .306"/.323" (note 1)

J604d.... 26-71.5/83.5-29.5, 277.5/293, 112.75/117/114.9, same/same (note 1)

006/010.. 26-68/83.5-26, 274/289.5, 111/118.75/114.9, same/same (note 1,2)

050 ......08-49/64-04, 237/248, 110.5/120/115.25, same/same (note 3)

30-30

Lash pt. 40-90/90-50, 310/320, 115/110/112.5, .323".323" (note 1)

J604d.....31-79/80-38, 290/298, 114/111/112.5, same/same (note 1)

006/010.. 31-75.5/80-34, 286.5/294, 112.25/113/112.6, same/same (note 1,2)

050...... 12-56/58-10, 248/248, 112/114/112, same/same (note 3)

Note 1: The LT-1 and 30-30 cams have asymmetrical lobes so the lobe "centerlines" do not correspond to the points of maximum lift. The LT-1 cam, in particular, treats the exhaust vavle like and egg on closing, which the valve seats will really appreciate! Points of maximum lift for the LT-1 cam are 110/122 and 112/116 for the 30-30. Angle between points of max lift are, therefore, 116 and 114 respectively. Both these cams have early phased exhaust lobes, which help compensate for the relatively restrictive SB exhaust port. Typical aftermarket cams have exhaust lobe centers of about 110, which are okay with headers, but will kill torque with manifolds. The LT-1 exhaust lobe is very close to the 30-30 exhaust lobe, but is phased even earlier. Inlet lobe phasing is typical for a high performance cam. The slightly later phased inlet lobe on the 30-30 accounts for the 30-30s slighly higher peak power, but this kills low end torque. The late closing 30-30 exhaust lobe is also a torque killer. The LT-1 cam creates a more street useable torque bandwidth at a very slight loss of power in the upper ten percent of the rev band, which is a good tradeoff for a street high performance engine.

Note 2:Because of the asymmetrical lobe I developed a "modified" J604d file for DD2000, which applies a little "Kentucky windage" to adapt these asymmetrical lobes to the symmetrical lobe that DD2000 assumes from the timing data. Picking the closing point at .010" valve lift shifts the torque curve down the rev scale a bit and may yield more accurate simulation results.

Note 3: Timing, etc. at .050" above the base circle, which includes the clearance ramps.

Other notes: The top of the conatant velocity clearance ramps on the LT-1 and 30-30 cams are 016"/.020" and .020"/.020" respectively above the base circles. Since actual rocker ratio at the lash points is 1.37:1, lash should be set at .021"/.026" and .026/.026" to take up lash just short of the top of the ramp and set the valve down on the seat at ramp velocity.

Rocker ratio increases with lift and peaks at 1.44:1 at max lift, so actual approximate peak valve lift can be computed as lobe lift times 1.44 minus clearance. All OEM and aftermarket cam data ASSUMES a constant 1.5:1 rocker ratio, which is not the case in the real hardware.

Seems I've been researching SB mechanical lifter cams most of my life, and I may be at the point where I have enough data and analysis to write an article for the Corvette Restorer that will make my tire article look like something for written for children!

Duke

[Dennis C.]

LT-1 P/N

Duke, Maybe I missed it, but do you gots a GM P/N for the favored bumpstick? Dennis

[Duke W.]

Re: LT-1 P/N

For reference the service part number is 3972178, but is discontinued.

I understand that GMPP sets a kit with cam and lifters that is manufactured by Crane, if I recall correctly. Crane and/or some other aftermarket cam manufacturers may also offer clones.

Also, the Speedpro CS1145R is supposed to be a clone and use with the AT840R lifter, which are supposed to be clones of the old edge orifice lifters. Though GM used the piddle valve lifters on SB mechanical lifter cams from 64 to 69, they reverted to the old edge orfice type (as used on the Duntov cam) for the LT-1 cam in '70.

Duke

[Joe L.]

Re: LT-1 P/N

Duke-----

I think that the 67-69 Z-28 302's were the only small blocks that used the piddle valve lifter, GM #5231585, in PRODUCTION. In fact, I think that the later 69 302's may have reverted to the edge-orifice lifter, GM #5232695, too. I believe all other Chevrolet small blocks with mechanical lifters used the edge-orifice lifter in PRODUCTION.

All big blocks with mechanical lifters used the piddle valve lifter, though.

[Duke W.]

Re: LT-1 P/N

For some reason I always thought the 64-65 SHP/FI engines with the 30-30 cam used the piddle valve in lieu of the edge orifice type used in previous mechanical lifter SBs with the Duntov cam, but a review of my available information indicates that I and wrong and your untilization synopsis is correct.

However I believe you have reversed the part number-type data. The 5231585 is the edge orifice and the 5232695 is the piddle valve type. I prefer the edge orifice type for SBs due to the reduced overhead oiling, but the piddle valve should be used for all BB mechanical lifter cams because they can use the extra rocker box oiling.

Duke

[Joe L.]

Re: LT-1 P/N

Duke-----

Yes, I did reverse the part numbers for the 2 types of mechanical lifters. The GM #5231585 is the edge orifice lifter and the GM #5232695 is the piddle valve lifter.

[Peter M.]

Re: LT-1 P/N

Joe,

So what part number would I need for a 71 LT1? Is it also discontiued? If so, who makes a reproduction, if any? What type of lifter should I use and where can I get them?

Thanks as usual for educating dolts like me.

Long live the Vette! After owning 6 muslcecar4s(all of which I like) I am surprised at how much I love driving my LT1-it just feels special!!!!

You and guys like Duke are the best!!!!!

Peter

[Joe L.]

Re: LT-1 P/N

Peter-----

The 71 LT-1 used the edge orifice lifter, GM #5231585. This lifter is GM-discontinued. The Federal-Mogul/Speed Pro lifter AT-992 may be a direct replacement for this type lifter, but I am not absolutely certain. In any event, the AT-992 is a functional replacement for the original.

[Duke W.]

Re: LT-1 P/N

Clem previously posted AT840R as the replacement for the GM 585 edge orifice lifter. Guess we need to get this resolved.

The Speed Pro (Federal Mogul) number for an LT-1 cam clone is CS-1145R, but I have not personally verified that this is the correct number. GMPP also sells the cam and lifter kit. I don't have the part number.

I understand some of the aftermarket cam vendors also sell clones, but I don't have part numbers.

Dukue

[Joe L.]

Re: LT-1 P/N

Duke----

The AT-840R is an edge orifice lifter. However, it's designed for racing applications. I don't know if it's ok for normal street use, or not. The AT-992 is specified by Federal-Mogul as the OEM replacement for Chevrolet small blocks with mechanical lifters, but it's not specified as an OEM replacement for big blocks. That implies that it's the edge orifice style used for most small blocks. However, it's also possible that it's some alternate design and neither edge orifice or piddle valve.

The Federal-Mogul/Speed-Pro CS-1145R is the GM #3972178 copy.

The GM part number for the Crane-manufactured 3972178 kit is GM #12364054. This kit includes camshaft and lifters and GM lists for $179.00.

[Clem Z.]

AT- 992 are piddler valve and AT- 840R are

edge orifice. F/M lists AT-992 as the solid lifter for all SBC and BBC and list the AT-840R for roller rockers only because of the reduced oil flow. the chevy power manual even says the edge orifice should only be used with roller rockers but does say they were used on some SBC. i think that the use of the stock chevy low tension valve springs will allow the use of the edge orifice lifters with stock type rocker arms but heavier springs and the stock stamped steel rocker may have a ball pivot friction problem.. LT-1 cam kit from GM is # 12364054 CS-1145R is the F/M for the LT-1 cam