1.8 Rockers and High Ramp Rates??
09-20-2003, 01:41 PM
#1
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1.8 Rockers and High Ramp Rates?? Most of the vendors warn against using 1.8 rockers with thier high ramp rate, high lift cams.
Is this just a PV clearance issue, that can be mitigated with flycutting or is there something else??
Is this just a PV clearance issue, that can be mitigated with flycutting or is there something else??
09-20-2003, 04:57 PM
#2
Re: 1.8 Rockers and High Ramp Rates?? Due to the 1.8 ratio rocker, the already fast ramp rate becomes even faster. Its not a problem if you have stiff enough springs but that usually requires a duel spring (which requires head modification).
Then, the added spring pressure and high ramp rate can collapse the factory hydraulic lifters (might even collapse Comp Rs depending on how big the cam and springs are).
If you have the spring pressure, lifters, and P-to-V clearance, then there is no problem (will gain performance) with 1.8s on high ramp rate cams.
Then, the added spring pressure and high ramp rate can collapse the factory hydraulic lifters (might even collapse Comp Rs depending on how big the cam and springs are).
If you have the spring pressure, lifters, and P-to-V clearance, then there is no problem (will gain performance) with 1.8s on high ramp rate cams.
09-20-2003, 05:10 PM
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Re: 1.8 Rockers and High Ramp Rates?? Funny you should mention this. We're just about to do a 1.6 to 1.7 swap on a buddies XE ramped custom (ford). It just so happens that the ramps on his cam are just a little more aggressive then the XE-R line for the LS1.
Couple of issues you tend to run into with faster ramps & high rocker ratios.
First is that the stock lifters can take only so much pressure. The rocker is a simple fulcrum. Spring pressure on the nose of a cam (peak lift) is multiplied by the rocker ratio...this is the pressure on the lifter. Going from 1.7's to 1.8's with springs that have 300lbs on the nose would add 30lbs of pressure on the lifter....& that's not accounting for the added lift of the 1.8's. That'll add another ~20-25 lbs at the valve (depending on spring rate possibly more)....which is another 35+lbs on the lifter.
Going from 1.7 to 1.8 rockers will add effective duration at any given valve lift. For instance:
1.7 vs 1.8 at a give VALVE lift:
.006 values will remain nearly identical
.085 valve lift (.050 lobe lift) duration will increase by roughly 2 degrees. This means overlap at that valvelift is increased by 2 degrees.
.340 valve lift (.200 lobe lift) duration will increase by roughly 6 degrees! This is a LOT more duration where it matters most & with ramps like the XE-R's 1.8's will make the lobe nearly as aggressive as some solid roller profiles.
As you can see higher rockers can drastically affect power output. You just need to be carefull with the valvetrain.
First what you need to do is find out how much spring pressure the stock LS1 lifters can handle with 1.7's. Multiply the spring pressure by 1.7 & this is the total pressure the lifters can handle. Then divide by 1.8 & you'll get the maximum spring pressure the lifters can handle with 1.8's.
For instance:
Ford lifters can handle 425lbs with 1.6's. That is 680lbs. Divided by 1.7 gives me 400lbs. This is the maximum pressure Ford roller lifters can handle with 1.7's. & so you setup the springhieght accordingly.
The entire compcams catalog is online. This includes charts on every spring compcam makes with lb ratings in .050 increments. With these charts & a little math the RR swap with fast-ramped cams is not so difficult.
One trick to get the lifters to handle more pressure is to use restricted pushrods. Basically they are pushrods with a reduced oil feed tip at one end. This increases oil pressure in the lifter. Which brings up another point to check. Oil pressure plays a BIG role in how much pressure a lifter can handle before collapsing. This should also be reasearched & accounted for.
Hope this helps.
Couple of issues you tend to run into with faster ramps & high rocker ratios.
First is that the stock lifters can take only so much pressure. The rocker is a simple fulcrum. Spring pressure on the nose of a cam (peak lift) is multiplied by the rocker ratio...this is the pressure on the lifter. Going from 1.7's to 1.8's with springs that have 300lbs on the nose would add 30lbs of pressure on the lifter....& that's not accounting for the added lift of the 1.8's. That'll add another ~20-25 lbs at the valve (depending on spring rate possibly more)....which is another 35+lbs on the lifter.
Going from 1.7 to 1.8 rockers will add effective duration at any given valve lift. For instance:
1.7 vs 1.8 at a give VALVE lift:
.006 values will remain nearly identical
.085 valve lift (.050 lobe lift) duration will increase by roughly 2 degrees. This means overlap at that valvelift is increased by 2 degrees.
.340 valve lift (.200 lobe lift) duration will increase by roughly 6 degrees! This is a LOT more duration where it matters most & with ramps like the XE-R's 1.8's will make the lobe nearly as aggressive as some solid roller profiles.
As you can see higher rockers can drastically affect power output. You just need to be carefull with the valvetrain.
First what you need to do is find out how much spring pressure the stock LS1 lifters can handle with 1.7's. Multiply the spring pressure by 1.7 & this is the total pressure the lifters can handle. Then divide by 1.8 & you'll get the maximum spring pressure the lifters can handle with 1.8's.
For instance:
Ford lifters can handle 425lbs with 1.6's. That is 680lbs. Divided by 1.7 gives me 400lbs. This is the maximum pressure Ford roller lifters can handle with 1.7's. & so you setup the springhieght accordingly.
The entire compcams catalog is online. This includes charts on every spring compcam makes with lb ratings in .050 increments. With these charts & a little math the RR swap with fast-ramped cams is not so difficult.
One trick to get the lifters to handle more pressure is to use restricted pushrods. Basically they are pushrods with a reduced oil feed tip at one end. This increases oil pressure in the lifter. Which brings up another point to check. Oil pressure plays a BIG role in how much pressure a lifter can handle before collapsing. This should also be reasearched & accounted for.
Hope this helps.
09-20-2003, 08:49 PM
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Re: 1.8 Rockers and High Ramp Rates?? Sean ... this will probably be a spring project for me. You and Dave have mentioned working together on a project this might be a good one.
I have an existing hot cam with 1.8 rockers on TEA S2 heads w/CC 987 springs. The heads have good flow through .600 and I would like to take advantage of that and make use of the faster ramp rates as well ... M
I have an existing hot cam with 1.8 rockers on TEA S2 heads w/CC 987 springs. The heads have good flow through .600 and I would like to take advantage of that and make use of the faster ramp rates as well ... M
09-20-2003, 09:54 PM
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Re: 1.8 Rockers and High Ramp Rates?? 987 huh... Max spring pressure on those is 328lbs at 1.200 inches. Coil bind at 1.150...this leaves .050 clearance from coil bind. Typically I go with the standard .060 clearance for coil bind.
Still....I think we can set you up. Worst case is you have to get valvesprings along with the cam. I'd hate to see you hit valvefloat & bind up a spring or snap one. Seen it happen before.
I like the idea of making the cam more aggressive using a higher ratio rocker. A lot of racers/engine builders don't. We'll see here shortly how the little Ford responds to the same thing.
Still....I think we can set you up. Worst case is you have to get valvesprings along with the cam. I'd hate to see you hit valvefloat & bind up a spring or snap one. Seen it happen before.
I like the idea of making the cam more aggressive using a higher ratio rocker. A lot of racers/engine builders don't. We'll see here shortly how the little Ford responds to the same thing.
