afgunn

If this has already been answered just point me there.

LT1 with 1.6 Roller Rockers
Valve Lift:
Intake: .298 x 1.6 = .4768 (same as LT4)
Exhaust: .306 x 1.6 = .4896 (.010 greater than LT4)
I assume (we all know what that does) since so many have done this and advocated doing it that there are no piston to valve clearance problems with putting in 1.6 rockers?!

Duration @ .050:
I know the duration will change some with 1.6 rockers (longer duration), but I know of no way of calculating this, so has anyone actually measured the duration with 1.6 rockers?

I do not see how Lobe Separation would change with 1.6 rocker, but, hey... I have been mistaken before, so if anyone knows better enlighten me!

Many thanks in advance!

DanielRicany

Should have no problems with PtV.

Some say going to 1.6 rockers could be the equivalent of about 2 more degrees of duration... not sure how true that is.

Wouldn't change LSA I believe

afgunn

Thanks on the piston-to-valve clearance.

I think it would be about 2 to 4 degrees total increase in duration, but unless someone has actually measured it... I am just SWAGing it!

I agree on the LSA as well. Heck, I did not re-grind the camshaft!

Thanks much!

THE 383 admiral

Your calculation. Is spot on. 1.6RR adds +0.30 to your lift. OE LT1 .447I / .459E

you will not have any PTV issues. I will warn you if you have a manual and miss a shift. and over rev. you will EASILY break a spring or 2. the OE springs are pretty weak.
you will Honestly feel a slight improvement in response & HP.

cardo0

Well if u do the math for the LT1 spring #10206040 u take closed height 1.780" minus coil bind 1.260" gives u0.520". Minus 0.060" margin for safety = 0.460" max lift. Not enough for 1.6 rocker arms. U may get some coil bind at high RPM.

IMHO the LT1 head has good enough exh flow. I would try some 1.6 rockers on the intk vlvs only.

Good luck.

afgunn

I have the auto, 4l60E, but thanks just the same. I am not sure I could trust myself speed shifting with a manual! Lots of things could break!

afgunn

Coil bind... had not thought of that one. So you suggest 1.6 on the intakes only and not on the exhaust? What if I used new springs to avoid the possible coil bind? I do not do WOT often (mainly a street machine)... but once could be 1 too many. I have an air compressor, so I might should replace the stem seals anyway while there... springs would not be much more work... maybe.

I agree, the LT1 heads are fine for what I will do. I have not thought about the exhaust flow being good enough as is. I will do some checking on that. The new L99 in the 2010+ Camaros have a much smaller exhaust than intake, so you may really have something there. I have seen the specs on a 355 build that used the original LT1 heads and he had no trouble with about 450hp with the ZZ9 camshaft... IIRC.

THE 383 admiral

without doing a lil research. are you sure. The out of the box exhaust LT1 lift is .459 leaving .001 coil bind clearance from GM. that's a tight tolerance for error. your math Clearly proves this.

bjankuski

The advertised duration of the cam does not change with the rocker ratio. The camshaft specs are measured with a dial indicator off of the pushrod or lifter and they are usually measured at .006 lift for overall duration and .050 lift for the .050 duration. The lobe separation will also not change. What you are after is the effective change in duration at the valve when comparing crankshaft duration changes at the same valve lift with 1.5 vs 1.6 rockers. This cannot be calculated without knowing the ramp angles of the cam or measuring the cam in question and noting it. At .050 lift of the cam the valve with a 1.5 rocker will be .075 lift, with a 1.6 it would be at .080 lift. You would have to note how many more degrees it would take to get the dial indicator to read .0469 (.05 x 1.5/1.6) that would be .075 at the valve and would also be extra degrees of duration at the valve that the engine sees.

cardo0

Expected values dont seem to apply well to GM cams and springs. I u run the numbers for the LT4 Hot Cam and LT4 springs u get way outa spec using 1.6 rocker arms. But thats the way GM marketed the entire Hot Cam Kit so u have to see other details come to play. Like the LT4 heads had light vlvs that weighed 20% less than the smaller LT1 vlvs.

The only real answer to correct vlv springs is to use what the cam mfr recommends/states. Other wise we are kinda experimenting. But the 0.060" safety margin is from crane cams and u will read the same from comp cams too.

Hope this helps more than confuses.

afgunn

Thanks, I understand. I should have mentioned, I am using the stock cam that came in the LT1. If I do this, I will measure it and post. I was wondering/hoping someone had done this and measured the duration @ .050.

afgunn

Your info definitely helps. What if I used the LT4 springs? I have not done the math, but will do it later as I have the specs on the springs, but do you know off hand? Are there any problems using the LT4 valve springs on the LT1 cam?

afgunn

Really appreciate your post! So many posts here talk about and do switch to the 1.6 rockers that I thought there would be no problem. Surprising no one has reported trouble. I am seriously thinking of going with the TPIS springs for the reasons below.

Here is the way the rebuild manual I have says to calculate it:
LT1 Valve Spring 10206040, 16 = $155.84
1.780 installed height
-.4896 valve lift
1.2904
-1.260 solid height
.0304 should be +.100 for optimum reliability (but still well below the .060 others recommend)

LT4 Valve Spring 12551483, 16 = $97.12
1.780 installed height
-.4896 valve lift
1.2904
-1.220 solid height
.0704 should be +.100 for optimum reliability (but above the .060 others recommend)

TPIS Valve Spring 6.VSP510, 16 = $134.95
1.780 installed height
-.4896 valve lift
1.2904
-1.150 solid height
.1404 well above both recommendations

bjankuski

Your analysis is correct, either of the lower springs from a coil bind standpoint will work fine. The last piece of info you need is installed and open valve spring pressures. As long as the springs you are looking at have at least the same installed and open force as the original LT1 springs, and no more then 450 lb open force they will work fine.

afgunn

Thanks! I am leaning toward the TPIS springs even though they cost a little more (added prices) because it has plenty of clearance, .1404 and a damper as well. TPIS advertises 115lbs seat pressure @ 1.800" installed height and 325lbs @ 1.200". The LT1 spring is 85lbs @ 1.780" installed height and I do not have the open lbs and height, but I am sure it is less than 450lbs. Think this will be OK?

bjankuski

It will be fine.

cardo0

Well the problem w/LT4 vlv springs is i cant get reliable spring rate numbers. Ive been told the published spring rate for the LT4 spring is not the 332lbs/in as published by GM in there GMPP catalog and most everwhere else. 332lb/in x 0.4896" lift = 162.5lbs + 101lbs closed press is only = 263.5# open press. While the stock LT1 spring would give u 267.6#. Then again ive been told the 332#/" for the LT4 spring is wrong.

Myself i would have to measure the LT4 spring itself before using. Well we should do that anyways and a cheap spring tester is now 'bout $80 or less.

Now to really confuse u ill post the LS6 beehive numbers here. Be aware i copied these off the internet from an individual i have never met:
Seat pressure at 1.75" installed height: 108-111 lbs
"Over-the-nose" pressure at .500" lift (1.25" spring height): 293-297#
Coil bind: 1.185-1.190"

The LS6 spring has a spring rate of about 375 lbs/in, so the spring pressure at a height of 1.25" is just under 300 lbs, which is quite a bit more than the LT4 spring.

Overall, the LS6 spring will give you better valve control than the LT4 spring, plus they're a beehive spring. You will need to buy the appropriate retainers (Comp 787, or equivalent) and spring seats in the case of the LT1/LT4 heads.

Hope this can help more than confuse ya.