LT-1 cam timing ..
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Safety Car
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From: northern CA
LT-1 cam timing ..
I just checked the LT-1 cam specs that are printed on the exterior of the Federal Mogul box against the figures I got using a dial indicator setup and degree wheel.
The FM specs are as follows: @ .050 cam lift
Intake Exhaust
BTC ABC BBC ATC
10 52 68 6
(9) (51) (measured) (67) (5)
The method I used in setting up the dial indicator was to install the spring loaded point of the dial indicator on top of the rocker arm in line with and parallel to the push rod. The Intake and Exhaust rocker arms were adjusted to zero lash. Is this an ok method to use in checking cam specs . .? .. Is there a better way .. ? . The measured results (one degree difference across the board) would seem to suggest consistency.
I also measured Intake and Exhaust valve lift with the dial indicator set up to read from the tops of the valve spring retainers. The measured result was Intake, .438 -Exhaust, .470 .. The published specs are (I), .459 - (E), .485 ..
It would seem one degree of advanced cam timing wouldn't make much difference in performance with respect to the published specs. Should I leave it as is . .? . .
John
The FM specs are as follows: @ .050 cam lift
Intake Exhaust
BTC ABC BBC ATC
10 52 68 6
(9) (51) (measured) (67) (5)
The method I used in setting up the dial indicator was to install the spring loaded point of the dial indicator on top of the rocker arm in line with and parallel to the push rod. The Intake and Exhaust rocker arms were adjusted to zero lash. Is this an ok method to use in checking cam specs . .? .. Is there a better way .. ? . The measured results (one degree difference across the board) would seem to suggest consistency.
I also measured Intake and Exhaust valve lift with the dial indicator set up to read from the tops of the valve spring retainers. The measured result was Intake, .438 -Exhaust, .470 .. The published specs are (I), .459 - (E), .485 ..
It would seem one degree of advanced cam timing wouldn't make much difference in performance with respect to the published specs. Should I leave it as is . .? . .
John
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Joined: Dec 2002
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From: Cincinnati Ohio
Re: LT-1 cam timing .. (mrg)
I got similar results when I measured mine but concluded the difference was within the tolerance of my ability to measure. The cam is changing by about .004 per crank degree at this point and there is rounding.
The lift numbers are the same as what I got and, given the valve train geometry seem reasonable.
Bottom line, I was happy and left things alone.
Dave
The lift numbers are the same as what I got and, given the valve train geometry seem reasonable.
Bottom line, I was happy and left things alone.
Dave
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Re: LT-1 cam timing .. (mrg)
Your measurements are within the combination of manufacturing tolerance and measurement error.
The valve lift spec is based on 1.5:1 rocker ratio at zero lash times lobe height of .306/.323. I've discussed rockers before, and the OE rockers are not 1.5 nor are they constant.
Based on your zero lash measurements the rocker ratio at peak lift is:
438/306 = 1.43
470/323 = 1.45
The specified zero lash valve lift with the "theroetical" 1.5 rocker is 1.5 times lobe heights of .306/.323 = .459/.485. If you back out the GM specified lash you get .435/.455, but you will never measure these numbers on an engine because or rocker ratio geometry variation.
I measured 1.44 rocker ratio on my engine. If you had used a second dial indicator on the valve retainer you would have found that rocker ratio starts at about 1.37 and increases to the max of about 1.44 at peak lift.
The actual rocker ratio at the lash point is the basis of my recommended clearance spec of .021/.026. I believe the Speed Pro data with the cam recommends .020/.025. The ramps are .016/.020 high and Chevrolet just multiplied this by 1.5, which is wrong. The valves will be too loose at the Chevrolet spec of .024/.030
E-mail me if you want the Word document on adjusting valves.
Duke
[Modified by SWCDuke, 10:40 AM 6/22/2004]
[Modified by SWCDuke, 9:58 PM 6/22/2004]
The valve lift spec is based on 1.5:1 rocker ratio at zero lash times lobe height of .306/.323. I've discussed rockers before, and the OE rockers are not 1.5 nor are they constant.
Based on your zero lash measurements the rocker ratio at peak lift is:
438/306 = 1.43
470/323 = 1.45
The specified zero lash valve lift with the "theroetical" 1.5 rocker is 1.5 times lobe heights of .306/.323 = .459/.485. If you back out the GM specified lash you get .435/.455, but you will never measure these numbers on an engine because or rocker ratio geometry variation.
I measured 1.44 rocker ratio on my engine. If you had used a second dial indicator on the valve retainer you would have found that rocker ratio starts at about 1.37 and increases to the max of about 1.44 at peak lift.
The actual rocker ratio at the lash point is the basis of my recommended clearance spec of .021/.026. I believe the Speed Pro data with the cam recommends .020/.025. The ramps are .016/.020 high and Chevrolet just multiplied this by 1.5, which is wrong. The valves will be too loose at the Chevrolet spec of .024/.030
E-mail me if you want the Word document on adjusting valves.
Duke
[Modified by SWCDuke, 10:40 AM 6/22/2004]
[Modified by SWCDuke, 9:58 PM 6/22/2004]
Thread Starter
Safety Car
Joined: Nov 2001
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From: northern CA
Re: LT-1 cam timing .. (SWCDuke)
Dave ..
I'll take your cue and leave well enough 'alone' the timing specs I measured as they are. Looks like it worked out just fine in your recent engine build up! .. I read your dyno test results with interest. My own build up is along the same line as yours, but based more on the 300 hp engine ('backyard' pocket ported heads with 1.94/1.6 valves, cast iron intake, AFB, flat top pistons - .040 over, Scat con rods, Rams Horns, etc.). I'm hoping the combination will yield an honest 300 hp, perhaps slightly more.
Duke:
I just finished up setting the valve lash on my engine. Thanks for sending the Word document on adjusting valves. Piece of cake using the method you and JohnZ came up with. I used the .021/.026 lash setting as per the Word document. But I see now in your reply to this topic that you recommend an exhaust lash setting of .024 .. Should I use the .024 setting instead of .026 .. ? . .
John
I'll take your cue and leave well enough 'alone' the timing specs I measured as they are. Looks like it worked out just fine in your recent engine build up! .. I read your dyno test results with interest. My own build up is along the same line as yours, but based more on the 300 hp engine ('backyard' pocket ported heads with 1.94/1.6 valves, cast iron intake, AFB, flat top pistons - .040 over, Scat con rods, Rams Horns, etc.). I'm hoping the combination will yield an honest 300 hp, perhaps slightly more.
Duke:
I just finished up setting the valve lash on my engine. Thanks for sending the Word document on adjusting valves. Piece of cake using the method you and JohnZ came up with. I used the .021/.026 lash setting as per the Word document. But I see now in your reply to this topic that you recommend an exhaust lash setting of .024 .. Should I use the .024 setting instead of .026 .. ? . .
John
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Re: LT-1 cam timing .. (mrg)
My mistake. I meant .026". I went back and corrected the post. Your power curve should be similar to Dave's. The difference between the CI and SHP aluminum manifolds and carbs
is relatively small. Also the valve sizes don't make a huge difference.
Dave's head guy knew his stuff, from both the flow numbers and the top end power! The heads are the key to power above 5000. Dave also sweated the assembly details like achieving a high uniform compression ratio within the bounds that were established to allow operation on premium unleaded. Such attention to detail pays off in the power curve!
Duke
is relatively small. Also the valve sizes don't make a huge difference.
Dave's head guy knew his stuff, from both the flow numbers and the top end power! The heads are the key to power above 5000. Dave also sweated the assembly details like achieving a high uniform compression ratio within the bounds that were established to allow operation on premium unleaded. Such attention to detail pays off in the power curve!
Duke
Thread Starter
Safety Car
Joined: Nov 2001
Posts: 4,424
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From: northern CA
Re: LT-1 cam timing .. (SWCDuke)
Duke ..
.026 is the number .. ? . . Ok, thanks for clarifying. Yes, Dave's dyno numbers were impressive. I especially liked the low end torque readings. Sounds too like it revved easily to 6000+ like nobodys business .. ! .. I had to figure he took great care in all the engine build-up details - special internal engine parts, professionally ported heads, engine dyno testing, etc. While I won't have my engine on a dyno stand, when the time comes I would like to put my car on a chassis dyno to see what the RWHP numbers turn out to be.
John
.026 is the number .. ? . . Ok, thanks for clarifying. Yes, Dave's dyno numbers were impressive. I especially liked the low end torque readings. Sounds too like it revved easily to 6000+ like nobodys business .. ! .. I had to figure he took great care in all the engine build-up details - special internal engine parts, professionally ported heads, engine dyno testing, etc. While I won't have my engine on a dyno stand, when the time comes I would like to put my car on a chassis dyno to see what the RWHP numbers turn out to be.
John
