1.6 roller rocker question.

[Da Hawaiian Punch]

Was just wondering, I planned on purchasing a set from a fellow member, and was stunned to hear that they came stock in the LT-4's, is this true?
With a stock cam, Putting them on will be like stepping up one in cam spec's, true or false? What will the cam lift and all the spec. numbers be after install from L-98 1.52's to the LT-4's 1.6's?
and, if I'm at 13.9's at 98 mph, how much should I improve in the 1/4 mile?
I know, lot's of question's here.
Mahalo guy's

 

[Dave C. '04 Z06]

rockers>>

The 1.6 rockers will help slightly...the reduced friction being the biggest help.

Unless you have nice cylinder heads (IEorted and polished, or larger runners) you won't see the benefits like you would installing a high performance cam. The reason is that most of the benefits from an aftermarket cam is in the increased duration and valve overlap.

On stock heads, at .500" lift with a 1.94" valve, the heads are usually flowing as much as they can. Opening the valve further doesn't really increase the air flow because it's not the valve that's the choke point anymore. But open it earlier and close it later in the cycle like a performance cam does and the longer duration will allow more cylinder filling at higher RPM for more power. This coupled with the valve overlap (not to be confused with Lobe Separation Angle, although they're related) which also increases high RPM breathing is why a performance cam helps power.

The 1.6 rockers will only increase your lift. They do have a minor affect on opening and closing times of the valves since the valves will open and close more quickly, but this is fairly insignificant.

But you do lose alot of potential valve train friction with the full roller rockers (especially important on the closing), which will allow you to increase your engine RPM by about 500.

So the whole thing I'm trying to say here is don't expect a SOTP improvement after you install them, but rest assured that your engine is sliding free as a bird. You might even experience a cooler running engine because the oil won't heat up as much in the rocker pivots.

-Dave C. '97 Z28

PS: "roller tipped" rockers, IE rockers with a roller tip but a pivot ball fulcrum, are pretty useless. The friction in a rocker is at the fulcrum. The LT4 uses full rollers, and I believe the LS-1 uses roller fulcrum, slider tipped rockers which are good.

 

[Edmond]

Dave,

I've read about this before and some members have said that you gain upper end HP but sacrifice low end torque. Any thoughts on that?

So what you mean is that the choking point in the engine will be the heads? Do you think the exhuast manifolds are adequate for high performance?

 

[Vettelt193]

I like the mixed set (1.6 intake, 1.5 exhaust)... as a matter of fact, Dave C. actually installed the mixed set in my '93.

When comparing 1.6 rollers to standard 1.5 rockers, it is almost apples to oranges. Since you pick up power in both the added lift and the reduced friction, even with a loss of low end torque, the reduced friction usually compensates. I have seen dyno charts with a few comparisons... the mixed set added power accross the board, the 1.6's were close to stock on the bottom, but much better on top... the full set of 1.5 rollers gave an increase accross the board, but a bit less than the mixed set.

 

[CKA_Racing]

Yup, everyone is pretty much dead on. 1.6 intake and 1.52 exhaust for a stock engine.

Go here for more education on the LT4 http://www.lt4.com

www.corvettemagazine actually had a feature article on this very same subject and some other upgrades. You''ll have to got here and search for it.

Edit: here i found it for you. http://www.corvettemagazine.com/2000/march/l98buildup/l98p1.asp

 

[Dave C. '04 Z06]

Choking point

What I mean is that on the intake side the heads start to be a factor above 0.500" lift. Some will flow better than others, because they are factory castings so some are just slightly different due to the casting process. But with the 170cc intake runners that the stock LT-1 has (and maybe more so the smaller 1.94" valve), you get to a point where the restriction to flow is in the valve seat to valve stem clearance and the runner. Opening the valve more at this point makes no difference in flow because the runner and seat-to-stem clearance is the restriction. On a stock LT-1 head if you look at average flow numbers, they start to fall off around 0.5"to 0.55". Here is an average head flow measurement:

0.100 96cfm
0.200 120cfm
0.300 170cfm
0.400 199cfm
0.500 210cfm
0.550 207cfm
0.600 204cfm

Notice as you get near 0.500", more lift makes less difference, and actually starts hurting flow if you open the valve too much (because the air begins to swirl before it gets past the valve).

So unless you port and polish the heads, especially around the bowl area, your heads can only flow so much into the cylinder even if there was no valve there at all.

So I would expect you would only gain on the order of between 5HP and 15HP (+/-) with a 1.6 rocker swap. Not really enough to feel, but it'll be there. They certainly won't hurt anything, so there is no performance to lose really.

About the exhaust manifolds:

I think the Corvette exhaust manifolds are probably pretty good for regular performance stuff... of course, headers are usually better if they are of the tuned type not because they necessarily flow more, but because they are configured so the exiting exhaust gas pulls the exhaust out of the next cylinder, so you get like a mini supercharger effect, only it's on the exhaust side. That's why the long tubes are usually better than shorties... shorties only give you the benefits of better runner flow, whereas long tubes can help scavenge the next cylinder. You need usually on the order of 30" length on the tube to get into the low-to midrange area where our engines are designed to make power. There's a whole formula for calculating the primary length, but I won't go into that. Shorties would scavenge if your engine spun at 10K RPM.

-Dave C. '97 Z28