1.5 vs 1.6 ROCKERS

[mikesduster]

if you had a choice to either run 1.5 or 1.6 rockers,what would your choice be and WHY??

another question is,you have NOT picked a cam yet,now which rocker would you choose,1.5 or 1.6??

 

[rumblefish360]

Before you even think about a different rocker ratio, try and find the cam in the lift & duration you want. It really should not be that hard to do. A custom ground cam will fit the bill. You can also spec/ask about one from a manuafacturer like this;

Say to them; "I would like a cam to take full advantage of my heads. There 2.02-1.60 S/B moPar heads that have been on a flow bench with these numbers (Start fax) and I would like the cam to perform well in the 2,000 - 6,000 rpm range. What can you do for me?"
They'll also want to know more about the car. Lots actually.

I think like this, for a max performance in mind. Intake manifold and exhaust not considered in this. This is just an example.

The head flows "300" cfm @ 600 lift and stalls there after. So a cam of 600 lift is the limit since theres no real sense of exceeding it.

Cam lifts the valve to 500. To small in lift with the 1.5 to take full advantage of the heads flow.
500 lift / 1.5 then X 1.6 equals 533. Not enuff to take full advantage of all the head flow. Wrong cam.

Next cam has a lift of 560 @ 1.5 rocker ratio. At 1.6 rocker ratio, it's 597. Thats close to max flow. A 1.6 rocker it is.

Your better off getting the cam ground with the lift (If possible of course) than jumping to the next ratio. It makes it easier on yourself with less to fuss over. In general.

 

[mikesduster]

the reason i ask is because i have both 1.5 and 1.6 rockers but have not chose a cam....my J-heads will flow around 260-270@.550-.575 lift and im running a AIRGAP intake.

ive talked to scott brown about a cam with 1.6's but not the 1.5's.

 

[C130 Chief]

Also consider the increased valvetrain loads when going to a higher ratio. Force at the lifter goes way up with RPM. Reducing upper valvetrain mass is always a good idea.

 

[Superelbert]

This is interresting!

I have chosen to go for 1.6 ratio lifters my self, and here is my reasoning:

Currently, i have a summit 6900 cam installed which works rather OK.

The only issue I have is that fuel economy is not too good, and a little bit more "umph" in the bottom range would have been a good thing.

At the same time, I came across some roller lifter kits on e-bay one with 1.5 ratio and a similar with 1.6 ratio.

So then I figured, shorter duration and less overlap will give better economy and better low end torque.
But this gives less lift and restricts engine breathing capacity on higher RPM's, i.e. less area under the (lift) curve.

But what then if I went for a shorter duration cam, but increased the resulting lift with higher ratio rockers?

Well, that's what I'm gonna find out this spring, but some desktop dyno simulations I've done points towards interresting results. My plan is to use the 410/425 lift second mildest mopar cam with 1.6 rockers, and compared to the Summit cam, calculated max power is down by 10-20 hp, yet the torque curve is virtually flat up to 3500-4000 RPM.

All in all, it should be like running a low duration cam with fairly aggressive lift profile.

Since the cam I'm going to use is fairly moderate in the first place, I don't have too many concerns about the increase in loads and accelleration forces on the valves.

I think that unless you are planing to run a cam that allready have high lift and agressive profile, higher ratio rockers and a mild cam is like both having your cake and eating it.

I'm a bit surprised this topic doesn't surface morte often, be it on forums or in the performance literature, I'm sure you agree it's a very interresting topic!

 

[moper]

The small block valve train in Mopars is bad to begin with in terms of angles. With the .904 lifter, you can get a very fast rate of lift as compared to GMs and Fords. So my choice would be get a cam for the 1.5s, with the rate of lift that works with the heads. Like C130 said, stress is higher, and parasitic loss is greater from having to push against thoase bad angles and a 1.6 rocker. Also , my feeling is most cams companies are spec'ing springs for thier cams with 1.5s in mind. If you run 1.6s, you may run into issues from not enough spring pressure even when running the "right" spring per the manufacturer. For flat tappets, the lower the ratio the more stable things are. So a street or street/strip car gets 1.5s and a cam to match.

 

[388dart]

This is interresting!

I have chosen to go for 1.6 ratio lifters my self, and here is my reasoning:

Currently, i have a summit 6900 cam installed which works rather OK.

The only issue I have is that fuel economy is not too good, and a little bit more "umph" in the bottom range would have been a good thing.

At the same time, I came across some roller lifter kits on e-bay one with 1.5 ratio and a similar with 1.6 ratio.

So then I figured, shorter duration and less overlap will give better economy and better low end torque.
But this gives less lift and restricts engine breathing capacity on higher RPM's, i.e. less area under the (lift) curve.

But what then if I went for a shorter duration cam, but increased the resulting lift with higher ratio rockers?

Well, that's what I'm gonna find out this spring, but some desktop dyno simulations I've done points towards interresting results. My plan is to use the 410/425 lift second mildest mopar cam with 1.6 rockers, and compared to the Summit cam, calculated max power is down by 10-20 hp, yet the torque curve is virtually flat up to 3500-4000 RPM.

All in all, it should be like running a low duration cam with fairly aggressive lift profile.

Since the cam I'm going to use is fairly moderate in the first place, I don't have too many concerns about the increase in loads and accelleration forces on the valves.

I think that unless you are planing to run a cam that allready have high lift and agressive profile, higher ratio rockers and a mild cam is like both having your cake and eating it.

I'm a bit surprised this topic doesn't surface morte often, be it on forums or in the performance literature, I'm sure you agree it's a very interresting topic!

Food for thought - run the 1.6s on the intake and 1.5s on the exhaust. I think you will be pleasantly surprised :thumbup:

 

[rumblefish360]

Since you have both ratio, still try for the cam with the lift gound into it with the 1.5 in mind.
However, lets say a off the shelf cam has everything your looking for, but not in the lift area and the math work in your favor to be able to use the 1.6 rocker, you could do that.

Whatch out for rocker tip alignment and proper travel over the full range from just before the rocker lift to after the rocker allows the valve to close. Make sure it stays on the tip right...correctly.

 

[mikesduster]

Food for thought - run the 1.6s on the intake and 1.5s on the exhaust. I think you will be pleasantly surprised :thumbup:

WHY????

 

[C130 Chief]

Depending on exhaust flow numbers, I might be inclined to do the opposite, especially if i was running something like early A-Body iron manifolds. A compliment perhaps to the asymetrical cam profiles which perform so well in a street car with full exhaust.

 

[mikesduster]

Food for thought - run the 1.6s on the intake and 1.5s on the exhaust. I think you will be pleasantly surprised :thumbup:

you just cant do that for ANY cam...can you???

 

[C130 Chief]

Oh you could do it for any CAM, but certainly not for any VALVESPRING/RETAINER combo.

This brings up another point, regarless of port efficiency, at some point valve spring compressed height will limit total lift.

I'm still not convinced that the increase in lift would make enough more power to offset the horsepower required to open the valves farther, not to mention the hassle of correcting possible geometery messes. Remember, while the force to compress the spring is linear, the force to open the valves in a running engine increases at the square of RPM.

 

[mikesduster]

im STILL lost about running 1.5 and 1.6 at the same time. :sad1:

 

[Coyote Jack]

Mike,
I'm making a bit of a guess here, but I think the theory behind it is, the exhaust ports don't flow as well as the intake ports. With this in mind, if you use a 1.6 on the exhaust side, then you will be able to scavange more of the exhaust gasses out due to the increased lift and marginally longer duration. This should increase low end torque and make overall effiency better.
As I said, just a guess. People like BJR Racing and moper would have better knowledge than me about this.

Jack

 

[moper]

Close, yes. Many performance cams are single pattern. They have the same lobe shapes for both intake and exhaust. Most performance engines like a ratio of 70% exh flow to intake flow in the port designs. So a good head will be close to that, and the single pattern cams work well. If you have smallish ports on one side or the other, you can help tailor the valve timing events by running a staggered ratio on the arms. There can be power found by running 1.6s vs 1.5s on any valve. But, things like good flowing ports, pushrod clearance, good scavenging headers, and free flowing intakes can complicate things. IMO, the lower the ratio, the easier it is on parts. So I'll size the cam properly taking all that crud into account..then stick with 1.5s. Higher ratios can also have retainer to rocker clearance issues. If you're starting from scratch, no need for 1.6s really. Eventually, you get to the limit on rate of lift using just the lifter alone...That's where the 1.6 can come in handy. If you're looking for more beyond what's available.