roller rockers, how do they affect torque?

pjtoledo · Feb 8, 2017

2000 4x4 3.0 ATX.
I'm getting ideas about the 1.7 ratio roller rockers. anybody know how they affect the torque curve? since they basically emulate a bigger cam I would suspect they reduce the bottom end and move the peak torque to a higher RPM.
currently the gears are 3.73, its turning about 2500 RPM at 70 Mph. 4.10s are in the works, that would put the hi-way RPMs at about 2750.

torque improvement below 3000 is my goal, so do you think the 1.7s in combination with head porting would help or hurt?

also thinking of an intake spacer to make the runners longer.

55trucker · Feb 9, 2017

The 1.7's won't necessarily move the torque curve up, they may actually lower the rpm curve where it begins to peak (which would be good ..where this anemic engine is concerned).

The 1.7's will not increase the duration at all, the cam profile opens & closes the valves, so if the cam profile doesn't change neither will the duration. What Will change is the speed at which the valve moves, the 1.7 rocker will cause the valve to open & close faster thru the duration that the valve is off the seat as well as extend the valve deeper into the combustion chamber. That should allow more air into the cylinders increasing the torque (marginally), remembering that the PCM controls the A/F ratio.

I've considered this move as well, but have just never got around to actually doing it.

gw33gp · Feb 9, 2017

I don't know how much the 1.7 ratio rockers will effect torque. My guess is it will basically improve high rpm horse power. I don't know what the stock ratio is but am guessing 1.7 is a higher ratio than stock and will open the valves a little more to get more air flow. It will also open and close the valves quicker which provides a little more flow. All of this should provide more horse power at higher rpm and may shift the peek torque but I would not count on it.
Head porting can help or hurt engine performance depending on how it is ported. Porting is a real science. I have been building my race engines since 1986 but I leave the head porting to an expert. I have seen race engines ruined by bad porting.
Just remember engine performance is optimized by tuning everything. Just working in one area may provide some improvement but overall tuning makes a huge difference. Sometimes changing one thing can upset the balance and give you negative results.

stmitch · Feb 9, 2017

I'd expect the rockers to increase tq throughout the rev range.

This article ( http://www.hotrod.com/articles/roller-rocker-ratio-test/ )involves a small block chevy doing dyno runs with various rocker arms. They saw an increase in hp and tq at the 3 listed engine speeds just from switching from stamped rockers to roller tips of the same ratio. Increasing the ratio had a similar effect. Pay special attention to the chart that compares the various runs.

As for porting, you'll probably find that there is plenty of material in the heads, and especially the lower intake manifold that can be removed if you so choose. They're rough castings. Even if you don't open them up very much, I think that removing some of the roughness and mold marks will only help you out. Mine are gasket matched, and smoothed throughout the runner. I removed a decent amount of material, but they could be taken quite a bit further too. My focus was trying to eliminate obstructions to flow as much as I could, rather than massively reshaping the runners/bowls.

If you're taking the heads apart, I'd also suggest polishing the combustion chambers to reduce detonation that the 3.0 is so well known for. It might reduce your compression ratio a touch, but it will keep it from pinging and cutting spark when you try to accelerate. Milling the heads can always restore your compression ratio.
Here are mine before/after:

pjtoledo · Feb 9, 2017

my biggest concern would be scavenging and overlap. if the exhaust is held open too long it may kill the temporary vacuum in the exhaust runners that gets the intake moving sooner when both valves are open. true the timing events are the same, but the valves being open further late in the cycle may allow the exhaust gases back into the cylinder rather than seal them out.
I'm thinking some long narrow headers may increase the velocity so the exhaust can't do reversion as fast.

for the intake I'm going to take a cue from my Taurus SHO. they use a large surge tank as a pressure reservoir that feeds long intake runners. the tank gets the air moving sooner, the long runners cram more into the cylinder.

wanting better low rpm power, I'll try spacers to lengthen the intake runners.
the surge tank shouldn't be too hard to fab.

let the bench engineering begin!

55trucker · Feb 9, 2017

pjtoledo said:
if the exhaust is held open too long it may kill the temporary vacuum in the exhaust runners that gets the intake moving sooner when both valves are open. true the timing events are the same, but the valves being open further late in the cycle may allow the exhaust gases back into the cylinder rather than seal them out.

There is no change at all in the amount of time that either valve is off the seat, all the larger ratio rocker does is cause the valves to move faster & to open deeper into the combustion chamber. The valves open & close at the same position of either the upramp & downramp of each cam lobe regardless of what rocker is used.

Rustbucket350 · Feb 9, 2017

They shouldn't affect overlap at all. They'll just open the valves further for the same amount of time. The power gain would depend on how the head flows at the new lift vs the old lift. Adding lift might actually hurt performance if you open the valves farther than the lift at the peak head flow. To really see a difference you'd want a cam with more duration and a tighter lobe angle.

roller rockers, how do they affect torque?

pjtoledo

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