70 LT-1 cam specs.

[robert leazenby]

I just checked the lift of my cam at the lobe, not at the valve with the following results.

intake .292 to .294
exhaust .309 to .311

Factoring in the supposed 1.5:1 rocker I calculate .438/.467.
Duke, if you are listening, does this seem as it should.

[Joe L.]

Re: 70 LT-1 cam specs.

robert----

Lift specs for the GM #3972178 70-72 LT-1 cam are as follows:

at the valve with 1.52: 1 rocker arm

intake= 0.435"

exhaust= 0.455"

[Duke W.]

Re: 70 LT-1 cam specs.

I don't have the OEM specs for the LT-1 cam lobes, but they should be in the '70 to '72 Chassis Service Manual. Maybe someone with one of those manuals can post them.

Are you measuring a new or used cam?

Duke

[Duke W.]

Addendum

From my notes: Published in Corvette News Feb/Mar 1970. Gross lift at a 1.5 rocker ratio: .4586/.4850, divided by 1.5 yields .3057/.3233, but you should still check the service manual spec.

Duke

[Jeff Burkey]

Re: 70 LT-1 cam specs.

Just checked the ' 72 factory service manual and it reads: cam lobe lift +/- .002"(X) intake .3057 / exhaust .3234 at 1.50:1 Hope this is what you need.

[Duke W.]

Thanks, Jeff

Those are the actual lob lifts, which jibe with what I calculated from the Corvette News article. Actual valve lift will depend on running clearance and rocker geometry.

Duke

[John H.]

Re: Thanks, Jeff

The Chevrolet "Power Book" lists gross lift for the LT-1 cam with 1.50:1 rocker arms as .435"/.455", which equates to .290"/.303" lobe lift; this ignores the fact that factory rockers are NOT 1.5:1, but that's the design number they use in all the published data.

[Duke W.]

Actually...

John - those are the net valve lifts with a 1.5 rocker and .024"/.030" clearance with would be .314"/.323" at the lobe with a 1.5 rocker - pretty close to the shop manual lobe spec, except the inlet. The actual inlet lift figure should be .432" which would yield .304" at the lobe. The Power Manual is in error. The fifth edition says .452". I don't know if they ever got it right!

The actual lift at the valve is less because the rocker ratio at max valve lift is about 1.44:1 not 1.5:1. And, of course, since the rocker ratio is only 1.37:1 at the top of the ramp, the cold clearance should be set at .021"/.026" to insure that the valve is set down at ramp velocity. Looser running clearance than .022/.027 will set the valve down on the seat at greater than ramp velocity and contribute to valve seat recession and valvetrain noise.

Duke

[Stan F.]

Re: 70 LT-1 cam specs.

I've always been curious if the production cams were identical to what Chevrolet was selling over the parts counter so I sent three cams (two production and one service) over to Jere Stahl's to have him plot the acceleration curves, lobe lift, etc.

I'll be happy to post the results when Jere is finished if anyone is interested.

Regards,

Stan Falenski NCRS# 24272

[robert leazenby]

Re: 70 LT-1 cam specs.

I would be most interested in the reuslts. I am toying with the idea of finding a correct vintage block, correct vintage 186 heads, and building a spare motor for my car to LT-1 specs. That way I can save my original LT-1 for the day when I don't drive the car so much.

[John H.]

Re: 70 LT-1 cam specs.

Stan -

Please post the results when you get them - will be very useful.

[Duke W.]

Let's get Jere to do 'em all

Jere has my original Duntov cam, but hasn't provided the data yet. It would be nice if someone has a 30-30 to offer up. A little wear on the top of the lobe is okay, because the critical data is the first 50 thou of lift above the ramps.

Since Stan is near Jere's shop he can stay on top of Jere to make sure we get the numbers and the cams back.

My understanding is that Jere's test fixture provides the complete lift, velocity, acceleration, and jerk profile - a complete engineering characterization, and "camshaft nirvana" for engineering geeks who want the lowest level of detail.

Those lobs will tell the story of Chevrolet's high performance mechanical lifter camshaft development from the mid-fifties to late sixties. After that, it was all emissions development.

Duke