F0x Slaughter

Rick Abare's setup is a little strange to me.

He is running 1.6RR on the intake and 1.5RR on the exhaust side.
Why would you want to do this?

2nd Car down

He was using the stock fuel pump also.

Ed Wright is running 1.7RR intake and 1.6RR exhaust.

joelster

Just taking a guess here, but it's quite possible that they've both tested with different rocker ratios and settled onto the ratios you see right there. They may run stock pumps but I doubt they run stock fuel pressure. It only need to last sub-10 seconds in their cases.

F0x Slaughter

Well on a N/A car doesn't lift matter?

Well I guess in the case of the exhaust isn't it being "sucked" out? Since air naturally moves from high pressure to low pressure. Also having a lower lift would create a higher velocity, because the air is trying to squeeze by the smaller opening creating more pressure correct?

So by creating a fast exit velocity of the gases it would suck in more air on the intake side correct?

Melkor

It's one of the easiest things you can change at the track mechanically, so a lot of guys do. Sometimes a 1.6 or 1.7 rocker can cause problem with the valvetrain, where as one step in ratio down, works fine. Sometimes it just works out that way, unless you can afford to spin-tron every engine you build, it's probably better to just try different rockers at the track.

joelster

There's probably a lot of theories as to why a car would pick up if you install a lower ratio rocker, just as there are theories of going to a higher ratio. Those 2 guys are dedicated racecar drivers. I'm sure they'd rather test a theory out at the track and see results right in front of them. Lowering the ratio also reduces the amount of effective overlap a teensy bit, and retards the exhaust opening a teensy bit.

bowtienut

iTrader: (1)

Nothing at all strange about that.
I'm running 1.6I and 1.52E. It's what it likes.

nitrous2fast

iTrader: (2)

Less ratio on the exhaust side causes many things.
1 less exhaust to intake ratio creates more torque in area under the curve.
2 exhaust valves get beat to death compared to intake valves. Lower ratio can make the lobe seem less aggressive.
3 less ratio can help the larger cams keep the exhaust in the chamber at low rpms keeping low speed torque higher.

Melkor

I've seen plenty of guys run down a rocker ratio simply because the lobe is so aggressive that a move up even 0.5:1 in ratio, and they couldn't keep a set of valvespring alive for long.

There are a LOT of variables at work there, but like in everything... one test is worth a thousand theories.

F0x Slaughter

Why are you running this setup?

Is there theory behind it?

Or does your engine just like it better for some reason?

Puck

iTrader: (17)

It could also be something as simple as a PTV issue. I had to flycut for my 1.7 shaftmounts, but a 1.6 would have made it acceptable clearance.

They can also be used for other reasons like taming aggressive lobes and adjusting for a skewed intake/exhaust ratio after porting.

F0x Slaughter

Flycutting is cutting valve reliefs into the piston correct?

Why wouldn't you just run a higher lift cam with the 1.6 rockers? Why 1.7?

It wouldn't make sense to run 1.7 on the intake and 1.6 on the exhaust because of PTV clearance?

I could see in your case where you are running the same RR where you need to flycut, but with a 1.7 and a 1.6 you would have to cut only the intake relief?

guppymech

iTrader: (7)

It is easier on the valvetrain to add more lift with a rocker ratio change than increased lift at the cam as you don't have to accelerate the lifter and pushrod the additional distance. Changing to a higher ratio rocker arm can be used as a tuning tool or to indicate if the engine would respond positively to a larger or smaller cam.

Puck

iTrader: (17)

Higher ratio doesn't just add lift, it essentially makes the lobe more aggressive as well. Plus, you can only run so much lobe lift on a cam without causing clearance problems with connecting rods, so to get big lift numbers you have to run higher ratio rockers . This is also why small base circle cams are beneficial as well. Instead of raising the lobe higher, you drop the base lower to get the same effect.

I was just using it as an example of one reason why someone may run split rocker arms, like say if a certain cam spec causes one valve to come too close to the piston - either intake or exhaust. Changing the rocker ratio of just that valve will free up enough space to put both in spec, without sacrificing the higher ratio on the other valve.

On an aggressive cam with higher lobe lifts rocker arm ratio changes make an even bigger impact on lift at the valve, so while it may not make a big difference on a stock cam, jumping from 1.6 to 1.52 can free up quite a bit of clearance on a high lobe lift grind.

gjohnsonws6

iTrader: (12)

Has nothing to do with aggressive lobes or ptv clearance. Think along the lines of cams with a reverse split. There is nothing special about it. Been done for many years.

96lt1m6

iTrader: (31)

Our motors don't have exhaust flow issues, so using 1.6 rockers on the exhaust does nothing.....

I am at work but I will post up info gathered from a Top Engine Builder on this topic.

bowtienut

iTrader: (1)

It shifts my powerband slightly lower, which results in better 60' and better ET. If I were running an aftermarket ECM and weren't limited to 7100 rpm, I may have found the 1.6's on the exhaust to work better.
My power peak is at 6400. RPM drops to 5400 on the shifts. It's all about keeping it in the highest part of the hp curve as you go down track. I guess you could call that "theory".

96capricemgr

iTrader: (1)

If you want to pick apart theory look at the top three cars, a huge LSx barely any faster than a couple of LT1s??????????????
Then the faster of the LT1s is heavier, with less compression, smaller heads, less cam lift(but more duration) and less rpm to get the jo0b done and both of them are using "little" 2.00/1.56 valves the benchracers would have you believe are too small for any stroker.

There is a lot of "theory" that should be questioned after comparing those three cars.

Melkor

I had a Comp TK series roller in a race engine that was spintron tested with 1.8:1 and 1.9:1 rockers, running the assembly to 8100 rpm. Found controllable bounce in the 1.8 combination at less than .015", where as the 1.9:1 was somewhat erratic, often bouncing the valve .020" and more; enough to break valvetrain parts. Shimmed the valvespring, checked coil clearance and retested with some 20lbs more on the seat; still erratic. Spintron testing cost roughly the same as engine dyno time fwiw and well worth the money imo.

It's not really a matter of theory vs reality. If you can test and don't then, this probably isn't the hobby for you. Even in advanced engineering fields, we'll test any time it's feasible and we have plenty of simulation software...

Either way, there could be lots of reasons why the engine did better with one rocker or the other. Maybe the blowdown on the exhaust port was a bit much? Who knows... who cares... test and see what yours likes.

Puck

iTrader: (17)

Thank you both for two real world experiences.

PS: TK lobes are badass .