Can anyone give me the specs & GM or Crane part number for the 1970 LT-1 cam? Also, is the '70 LT-1 cam different from the '71-'72 cam, as the hp ratings are different. I know the '71-'72's have slightly lower compression, but I'm trying to figure out if I have the right cam or not. My car has the Crane Musclecar Blueprint LT-1 350/330 hp cam in it now, and I'm just wondering if the '70 cam was indeed different. Thanks in advance!!
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Re: LT-1 Cam Spex
Jeremy-----
The LT-1 cam was the same for 1970 through 1972; no changes were made to the cam or the specs for it as a result of compression ratio changes in the engine over the period. The GM part number for the piece was GM #3972178 (camshaft ID 3972182). The Crane part number is 969551. The Crane-manufactured piece is also available as a kit complete with lifters under GM #12364054.
The specs for this (and other) cam will vary according to the source and convention used. The GM-published spec is as follows:
duration at lash point:
I= 300 degrees
E= 312 degrees
duration @ .050"
I= 242 degrees
E= 254 degrees
lift with 1.50:1 rockers
I= 0.435"
E= 0.455"
lobe centerline
116 degreesIn Appreciation of John Hinckley -
if you have a low compression engine
one of the after market cams that helps build cylinder pressure could give better performance over the GM camComment
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Re: LT-1 Cam Spex
Like Joe said, the specs vary with convention used, and the LT-1 cam has some features that make it very unique. First, it has asymmetrical lobes so the lobe centerlines do no correspond to the points of max lift. The 116 degree "lobe center angle" quoted by GM is really the angular distance between the points of max lift. The actual lobe centerline angle is narrower. The following are actual measurements off the lobes.
Lash point timing;dur;CLs/LCA 34-80/89-40;294/309;113/114.6/113.8
@.006" vlv lift..............26-71.5/83.5-29.5; 277.5/293; 112.8/117/114.9
Specified duration at .050" lifter rise is not meaningful, especially when compared to hydraulic cams because a lot of this duration is clearance ramp. Subtracting 10-20 degrees can get you in the ball park to compare to a hydraulic cam. Use the higher number for cams with large clearances. The LT-1 cam would equate approximately to a hydraulic cam with 226/234 duration at .050" lifter rise. This is only a very gross approximation because the LT-1 cam has a very unique acceleration profile compared to a typical hydraulic cam, which has a significant favorable effect on the torque curve.
The LT-1 inlet lobe is similar to the Duntov cam inlet lobe with a few degrees more duration and phased a few degrees later. The exhaust lobe is very similar to the 30-30 cam, but phased earlier with more aggressive initial opening acceleration. This helps mitigate the SBs relatively restricted exhaust port.
The LT-1 cam has very unique characteristics and works EXCEPTIONALLY well on a street high performance engine with the OE manifolds and a low restriction exhaust system.
DukeComment
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Re: LT-1 Cam Spex
Thank you very much, guys....I've been looking all over in my books and I couldn't find the darn specs....!Comment
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Re: if you have a low compression engine
Clem....Do you have any specific cams in mind? Would the compression rate affect your choice? Say from the 70 (11:1) to the 72 (9.5:1) GM used the same cam but could we( because of the technology now) build more HP with the cams presently available? (all things comparable, except for the cam)
Thanks Bruce
PS I can run the comparisons on the Dyno program.........Comment
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Re: if you have a low compression engine
if you have the 9.5:1 engine there are aftermarket cams out there that will help to build cylinder pressure but i have been out of the business for about 10 years so you will have to check with the cam company.Comment
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Re: if you have a low compression engine
How does the cam "build compression"? Are you saying that the deign of the lobes will allow the engine to reach peak compression for a longer duration than a vintage stock cam design?Comment
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Re: if you have a low compression engine
it is done by the lobe timing,less overlap of intake and exhaust, that prevents "bleed off". you can slso use a cam that "allows bleedoff" if you have too high a static C/R and have "pinging". this is called dynamic C/R. go to http://cochise.uia.net/pkelley2/DynamicCR.html for a full explainationComment
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Re: if you have a low compression engine
Chris, compression ratio is often thought of as a "ratio of pressures", but by definition, i.e. the engineering defintion, compression ratio for an engine is the ratio of the swept cylinder volume at BDC to the cylinder volume at TDC. In other words, if the cylinder has 9 times more total volume at BDC than it does at TDC, then the compression ratio is 9:1.
An engine with a low compression ratio will still have a low compression ratio using a cam like Clem's describing, since the ratio of cylinder volumes doesn't change; the valves simply close sooner in the compression stroke than a typical cam, leading to more pressure building in the cylinder than with conventional cams.
The other extreme would be to have NO camshaft to open/close the valves: then pressures in the cylinder could reach an extremely high value before you ran out of sufficient power to turn the crank, or something broke, or over time, the cylinder pressure simply leaked past the rings.Comment
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Re: if you have a low compression engine
For a given static CR the dynamic CR is determined by the point that the inlet valve closes. The earlier the valve closes, the higher the dynamic compression ratio and the better the low end volumetric efficiency both of which enhance low ene torque. The tradeoff for this is less top end power, so it really comes down to cam choice.
The LT-1 cam is biased toward the top end, and works best with high compression, The CR of the LT-1 engine was dropped to 9:1 in '71 to comply with GM managements "91 RON unleaded fuel edict", and this had a big effect on output.
The LT-1 cam does not have excessive overlap, but the inlet valve does close fairly late. For a true static CR of 10.5:1 the dynamic CR is only 7.42:1 using lash point timing or 8.07 at .006" valve lift timing. This should allow operation with pump premium without having to excessively retard initial timing or slow the centrifugal curve.
When building an engine with the LT-1 cam I recommend establishing the true static CR in the range of 10:1 to 10.5:1. With OE machined block and heads and a .040" composition head gasket, it should be in this range.
DukeComment
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