Difference between LT1 & LT4
Le Mans Master
Joined: Jun 2009
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From: South Dakota
In practice, the duration does not change that much, but it may be as much as 2 to 4 degrees "effective duration".
Last edited by QCVette; Jun 29, 2016 at 05:13 PM.
Burning Brakes
Joined: Dec 2009
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From: Finger Lakes Region, New York
They do change the effective duration. Most cams are rated for duration at 0.050 lift. The cam is a smooth curve from 0 lift to full lift. So the valve lift that is seen at 0.050 with 1.5 rockers is seen earlier (on opening) and later (on closing) when a 1.6 ratio is used.
In practice, the duration does not change that much, but it may be as much as 2 to 4 degrees "effective duration".
In practice, the duration does not change that much, but it may be as much as 2 to 4 degrees "effective duration".
The valve opens faster so there is a gain in duration BUT the valve also closes faster so there is a loss in duration. Therefore canceling out the effect of the other. (The faster openeing and closing speed is super slight as well)
Am I missing something here?
Burning Brakes
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From: Finger Lakes Region, New York
Burning Brakes
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From: St. Petersburg FL
Burning Brakes
Joined: Dec 2009
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From: Finger Lakes Region, New York
While V8VETTE84's reply of "Both," is correct, the more urgent reason is, as you suggested, to prevent binding. There are cases of the LT1 springs binding with the 1.6 rocker ratio. Changing from the round spring of the LT1 to the oval (flattened) spring of the LT4 is a really, really good idea!
Burning Brakes
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From: Connecticut
Le Mans Master
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From: South Dakota
I don't see how this is possible. The lifter "lifts" the 1.6 rocker a certain amount, same amount that it lifts the 1.5 rocker. There is a mechanical connection and the only way to extend the time that the valve is open is to make the cam lobe bigger or the mechanical connection has to be lost. I would really like to know how it's possible.
The valve opens faster so there is a gain in duration BUT the valve also closes faster so there is a loss in duration. Therefore canceling out the effect of the other. (The faster openeing and closing speed is super slight as well)
Am I missing something here?
The valve opens faster so there is a gain in duration BUT the valve also closes faster so there is a loss in duration. Therefore canceling out the effect of the other. (The faster openeing and closing speed is super slight as well)
Am I missing something here?
kg4fku gave a link that I copied part of below.
"Makes Your Cam Bigger, Too!
An increase in rocker arm ratio nets more than additional lift. It will also change the cam's duration characteristics. Because the increased ratio effectively speeds up valve movement, that means the valve will reach any opening height sooner than it would with a lower-ratio rocker arm. Higher ratios open the valves quicker and close the valves a little later. Since the increase is symmetrical on either side of the cam lobe centerline, a higher ratio will lengthen the overall valve timing, making your cam act bigger. The higher ratio also causes valve timing to increase proportionally as the valve opens farther (see chart above)."
An example might help explain.
If the cam lift is 0.050", the valve lift with 1.5 ratio rockers will be 0.075"
However, with a 1.6 ratio rocker the cam lift only needs to be 0.046875" to get the same valve lift.
So to the duration in degrees from the cam between 0.046875 on the opening side to 0.46875 on the closing side is longer than the duration from 0.050 opening to 0.050 on the closing side.
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Burning Brakes
Joined: Jan 2007
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While V8VETTE84's reply of "Both," is correct, the more urgent reason is, as you suggested, to prevent binding. There are cases of the LT1 springs binding with the 1.6 rocker ratio. Changing from the round spring of the LT1 to the oval (flattened) spring of the LT4 is a really, really good idea!
Safety Car
Joined: May 2004
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From: Naples FL
Burning Brakes
Joined: Dec 2009
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From: Finger Lakes Region, New York
An example might help explain.
If the cam lift is 0.050", the valve lift with 1.5 ratio rockers will be 0.075"
However, with a 1.6 ratio rocker the cam lift only needs to be 0.046875" to get the same valve lift.
So to the duration in degrees from the cam between 0.046875 on the opening side to 0.46875 on the closing side is longer than the duration from 0.050 opening to 0.050 on the closing side.
If the cam lift is 0.050", the valve lift with 1.5 ratio rockers will be 0.075"
However, with a 1.6 ratio rocker the cam lift only needs to be 0.046875" to get the same valve lift.
So to the duration in degrees from the cam between 0.046875 on the opening side to 0.46875 on the closing side is longer than the duration from 0.050 opening to 0.050 on the closing side.
Ok, so a lifter change increases duration by a nanosecond.
I understand the idea but I don't think it's enough of an increase to really do that much. I feel like 1.6rr's gain is in the reduced friction mostly.
Race Director
Joined: Dec 2002
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From: SCMR Rat Pack'r Charter Member..Great Bend KS
The LT4 is an LT1 with more aggressive cam, better heads with better porting, more injector, more rocker, and a bunch of "other" stuff. It only got the LT1 to 330-340hp. How can doing the same thing the factory did get an LT1 to 425hp??
Last edited by rocco16; Jul 1, 2016 at 06:17 PM.
Burning Brakes
Joined: Dec 2009
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From: Finger Lakes Region, New York
We're talking a "head and cam" package here.
The LT4 is an LT1 with more aggressive cam, better heads with better porting, more injector, more rocker, and a bunch of "other" stuff. It only got the LT1 to 330-340hp. How can doing the same thing the factory did get an LT1 to 425hp??
The LT4 is an LT1 with more aggressive cam, better heads with better porting, more injector, more rocker, and a bunch of "other" stuff. It only got the LT1 to 330-340hp. How can doing the same thing the factory did get an LT1 to 425hp??
Race Director
Joined: Oct 2011
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From: Dallas Texas
2025 C4 of the Year Finalist - Modified
We're talking a "head and cam" package here.
The LT4 is an LT1 with more aggressive cam, better heads with better porting, more injector, more rocker, and a bunch of "other" stuff. It only got the LT1 to 330-340hp. How can doing the same thing the factory did get an LT1 to 425hp??
The LT4 is an LT1 with more aggressive cam, better heads with better porting, more injector, more rocker, and a bunch of "other" stuff. It only got the LT1 to 330-340hp. How can doing the same thing the factory did get an LT1 to 425hp??
Team Owner
Joined: Dec 2007
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We're talking a "head and cam" package here.
The LT4 is an LT1 with more aggressive cam, better heads with better porting, more injector, more rocker, and a bunch of "other" stuff. It only got the LT1 to 330-340hp. How can doing the same thing the factory did get an LT1 to 425hp??
The LT4 is an LT1 with more aggressive cam, better heads with better porting, more injector, more rocker, and a bunch of "other" stuff. It only got the LT1 to 330-340hp. How can doing the same thing the factory did get an LT1 to 425hp??
They did help, but you are far better off buying aftermarket parts doing a combo than following the gm recipe.
Burning Brakes
Joined: Jan 2012
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From: Gilbert AZ
2018 C1 of Year Finalis4
I never understood why GM invested that much in the LT4 for a 1 year run and it only netted them 30 hp more. They would have been better off to introduce the LS1 as the motor for the Grand Sport and 6 speed cars. At least that was a bump to 350 hp.
Race Director
Joined: Dec 2002
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From: SCMR Rat Pack'r Charter Member..Great Bend KS
BTW, a little research will reveal that the LT4 heads are a BIG improvement over LT1 heads. GM went milder on the LT4 cam than I would have, but GM had to meet gov't regulations and I don't.
Le Mans Master
Joined: Feb 2000
Posts: 9,061
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From: Epping NH
And thankfully glad they didn't...the 97 LS1 was woefully overrated by GM in 97 and most LT4's had more HP then 1997 LS1's. Later years of the C5 the LS1 got better with HP.
Drifting
Joined: Sep 2014
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From: Trieste Italy
Bigger valves
Different cam profile
Different lifter
Different roller rocker
Lighter pistons
Crankshaft differen too, as far as i remember
Last edited by Christi@n; Jul 3, 2016 at 05:37 PM.