LT-1 cam idle

[Steve D.]

[Joe L.]

Re: LT-1 cam idle


Steve-----

The FM CS-1145R is supposed to be equivalent to the LT-1 cam, GM #3972178. However, the duration specs you mentioned as well as the complete specs which FM publishes for the CS-1145R are not equivalent to the 3972178. They are much more like the GM #3927140 which was a SERVICE-only, off-road camshaft. I do not recommend the 3927140 or any other camshaft spec'd like it for street use.

[Timothy B.]

Re: LT-1 cam idle

Steve,

If you are keeping the cam what you need is to get more air into the engine at idle. You are correct with the setting for the throttle blades, little squares or even a little less. This will allow the engine to idle with fuel from the curb idle ports and keep the transfer slot primed so when you open the throttle blades there will be no hesitation.

You can investigate small holes in the primary throttle blades or a larger PCV to get more air for idle.

[Duke W.]

Re: LT-1 cam idle

Steve-----

The FM CS-1145R is supposed to be equivalent to the LT-1 cam, GM #3972178. However, the duration specs you mentioned as well as the complete specs which FM publishes for the CS-1145R are not equivalent to the 3972178. They are much more like the GM #3927140 which was a SERVICE-only, off-road camshaft. I do not recommend the 3927140 or any other camshaft spec'd like it for street use.

Joe - I can assure you that the CS-1145R is an exact duplicate of the LT-1 cam. The problem with "cam specs" is that there are several ways to specify them, and often the measuring method is not described or explained.

I've inspected most of the GM cam drawings and the LT-1 cam turned out to be somewhat of a revelation. The inlet lobe is exactly the same as the L-78/72/71/LS6 lobe except for a smaller base circle, and the exhaust lobe is the 30-30 cam lobe. When looking at the cam lobe data, which is lobe height every CAM degree, in inches, to five decimal places, the CS-1145R specs published by F-M for .050"gross lifter rise are the same as I measured from both the drawing and on a real GM manufactured ...178 as are the POMLs and LSA, and regardless of the duration measuring method, both GM (in the service manuals) and FM specify gross lobe lift (which includes the clearance ramps) as .306/.323," which are identical to the big block SHP cam and 30-30, respectively.

The .050" lifter rise durations include the clearance ramps, so they can't be compared to other mechanical lifter cams that have different clearance ramp heights or any hydraulic lifter cam. The NET .050" lifter rise duration, which is .050" above the tops of the .012/.017" clearance ramp heights is about 231/239.

No doubt this is a long duration high performance cam, but by indexing the lobe POMLs at 110/122 for a 116 deg. lobe separation angle, effective overlap is reduced to about the same as the Duntov cam, which is much less than the 30-30.

BTW, the .050" "tappet lift" specs for the ...140 cam from the Chevrolet Power Manual are 243/254/108/116/112 compared to 229/237/108/122/116 for the LT-1 cam, which are close to the 231/239 net of clearance ramp .050" lobe lift that I measured off the engineering drawing, so apparently these "specs" are duration net of the clearance ramps, not gross lift that includes the clearance ramps. Lift is specified as max VALVE lift net of some checking clearance, which is not specified at a 1.5:1 rocker ratio (which is not correct. It starts out at about 1.37:1 at the lash point and peaks at 1.44:1 at max lift according to my measurements). The 140 is specified as .455/.481" compared to the LT-1's .432/.455". If you subtract clearance ramp height as specified above from gross lobe lift and multiply by 1.5 you get .441/.459". Using my measured 1.44:1 rocker ratio you get .423/.441", and that would be at .016/.023" clearance derived by multiplying the above clearance ramp heights that were derived from my drawing data lobe analysis times the 1.37:1 lash point rocker ratio that I measured on a real engine.

If some of you are thoroughly confused by all the "specsmanship" and rocker arm monkey motion, don't despair. Most guys are (especially on the Corvette Forum) and it's a lesson that comparing "specs" of different camshafts can often be an apples and oranges comparison, especially when comparing mechanical lifter cams or comparing any mechanical lifter cam to a hydraulic cam.

Steve - I notice that my Edelbrock catalog says the 1406 is calibrated for "fuel economy" and the 1405 is calibrated for "performance". It's noteworthy that in '63 the 300 and 340 HP engines used the same carb calibration despite very different vacuum characteristics. The AFB main and power systems were leaned out in subsequent years when it was only used on the 300 HP engine, however the idle jet was actually increased from .035" to .037"!

Similarly the '65-up 300 and 350 HP engines used the same basic Holley with identical calibration, despite the difference in vacuum characteristics.

I don't know if that carb offers full time or ported vacuum advance or whether it offers both. Full time is best and if you have anything other than a B28 VAC expect to have idle stability problems including stalling. The LT-1 cam should pull about 12" at 900 with something near 30 degrees total idle timing derived from the sum of initial timing and full vacuum advance. The B28 (OE 236 16) is the only VAC that passes the Two-Inch Rule with any OE small block mechanical lifter cam.

Have you considered obtaining a OE AFB? Since the 3721SB, which was the final OE model for '65 was available in the aftermarket for some years as a replacement for all prior AFBs, they are somewhat more common and less expensive than the earlier OE models.

Duke

[Steve D.]

Re: LT-1 cam idle

Spurred by Tim's comment about a larger PCV valve, I removed the valve and connected a hose from the carburetor to the vent. In this configuration it will idle at 900 with the primary plates set as described and timing (B-28 can attached) at 30 deg. Eureka! That is the good news (the really good news!). The old news is that it still dies after idling for 15 to 20 mins. But, after it died, I removed the hose from the carburetor and restarted the engine. It ran for 10 mins with out dying; previously, it would die in 1-2 minutes after restart. As a further experiment, while it was still running I reconnected the hose, and as soon as I got the tip of the hose on the fitting, the engine immediately died. I repeated this a couple of times (start with no hose, connect hose, die) with the same result each time.

The previous hypothesis was that the engine is dying at idle from a too-lean condition caused by a vacuum leak that begins after warm up. But now it appears that it is dying from lack-of-air after warm up. Any theories about what could cause that? While you ponder that, I'm going to restart with the hose disconnected and see how long it will run.

[Steve D.]

Re: LT-1 cam idle

I started it with the hose disconnected and it ran for 25 minutes without dying. I killed it and then it restarted on about half a crank.

[Timothy B.]

Re: LT-1 cam idle

Steve, one thing to also keep in mind is that any time you give an engine more air either from the throttle blades or PCV etc. the idle A/F ratio gets changed so be certain to re-adjust the emulsion screws again after any changes. Always do this when the engine is hot and the intake plenum is up to driving temperature.