I read an article (High Performance Chevy Jan. Issue) that states that they found low end torque and horse power increases by putting 1.6 rockers on the intake and 1.5 on the exhaust. They tested this on a 350 and had dyno tests to back up their claims. I was just wondering if anyone has tried this, and if so what were the reults?

 

It is a tuning aid, and like any other tuning technique, results depend entirely on the rest of the combo (especially the heads in this case).

If you don't have access to a dyno, you will have no way of knowing what the effects are in your particular engine. It is rarely helpful to downgrade either of the rocker ratios in a motor with stock heads since you usually need all the flow you can get.

 

Do you happen to remember what heads they where using? I remember reading an article years ago that had the same sort of results but I don't remember if the heads where stock or not.

It seams counter intuitive considering most cams either have the same profile for int/ex. or dual pattern cams have higher duration and lift for the exhaust. And also the fact that everyone is looking for a better E/I ratio and it would seam increasing the ratio on the exhaust would help that.

 

I read that article also. I am pretty sure they were using aftermarket heads in this article. I read another book where they were doing the same thing, but on stock heads. TPI swappers guide or a lingenfelter book can't rember which one. They also were saying it yeilded better results when mixing rocker ratios but I thought they were using 1.5 on intake and on the exhaust 1.6's.

I agree with RB8 if you don't have all access to a dyno, how will you ever know? Was there really that big of a difference though?

 

I am going to mix rocker ratios on my engine...however i will be going 1.5 on intake and 1.6 on exhaust.

I wish I had a dyno to see the difference

 

so u are going to have a doublely large exhaust and a very restricted intake compared to the exhaust flow. i would rather have them balanced. more air/fuel in and the same out rather then less in and more out since there wont be more to send out

 

 

It might help a little for a head that doesn't have good flow on the exhaust port. It is the same idea behind dual pattern cams.

Usually it is worth like less than 5 hp or so

 

doublely large exhaust and a very restrictive intake? What is that supposed to mean, and how could rocker arms affect that? Changing ratios from 1.5 to 1.6 has barely any consequence.

I'd think that running 1.6 ratio rocker arms on the exhaust and 1.5 on the intake would be following the same logic as a dual pattern cam having more exhaust duration to make up for lower flow.

 

the ratios is what also the valve open bigger and longer. it helps with the cam in doing that where as the cam is the main part. i dont understand why annyone would want more exhaust flow when there isnt more to flow with a smaller anmount of intake.

 

I was thinking about this also. From what I have read on here about the vortec heads, the Exhaust flow is one of the only sore points. Wouldn't the extra lift of a 1.6 on the exhaust side help balance that slightly? I really don't think it would hurt anything unless it put you over the .450"-.480" lift limit of the heads. And most of the lifters cost the same or very close to the same in 1.5 and 1.6 ratio.

 

it would help but what i am getting at is that fact that u cannot flow whats not there. yeah the exhaust gases will get out faster but what gonna flow the rest of the time that valve is open? nothing which is why it would make more power to increase the intake as well

 

Usually when you do a mock-up of greater lift, as posed by a 1.6 rocker on the exhaust side, it will show a slight gain at very high rpms. Like 5 hp or so at 5500 rpm, probably due to the 2 or 3 degrees more duration forced by the increased ratio. So when you're really revving there is a slight benefit.

 

OK, the way I see it, the higher exhaust ratio makes some sense, if the cam profiles are the same (or close) for the intake and the exhaust.

Keep in mind that the Intake valve has a 2" diameter, while the exhaust valve has a 1.6" diameter. If the lift on both valves is the same, say .450", then the availabe area for flow would be equal to the circumference of the valve, times the lift.

(dia) x Pi x lift

Intake
2 x 3.14 x .450 = 2.87 sq in

Exhaust
1.6 x 3.14 x .450 = 2.26 sq in

To get the change in valve lift from 1.5 rockers, to 1.6 rockers, divide the stock lift (@ 1.5) by 1.5, and then multiply by the new ratio, 1.6.

.450 @ 1.5 becomes .48 @ 1.6

So, back to our flow equation:
(dia) x Pi x lift

Exhaust
1.6 x 3.14 x .480 = 2.41 sq in

So, if the cam provides the same (or similar) lift and duration for the intake and exhaust valves, then changing to 1.6 exhaust rockers does not make for " ...a doublely large exhaust and a very restricted intake"
As we've seen here, the intake valve still has more flow area than the exhaust valve

Even if you go to a dual pattern cam with an Int/Exh lift of .427/.454, and mix rocker ratios, you get:

Intake @1.5
2.0 x 3.14 x .427 = 2.68 sq in

Exhaust @ 1.5
1.6 x 3.14 x .454 = 2.28 sq in

Exhaust @ 1.6 (.484 lift)
1.6 x 3.14 x .484 = 2.43 sq in

The available flow area for the intake is still greater than the flow area for the exhaust. To flow more gasses through the exaust valve, we will have to increase the amount of time the valve is open, compared to the intake valve. most dual pattern cams have slightly higher duration on the exhaust side to do just that, so that the same amount of air, at a given speed, can move through both valves.

I would have to agree with the opinion that mixing rocker ratios is a tuning aid, dependent on the specific cam profile, and head flow characteristics of a specific engine.

Besides, why would you not want your exhaust to flow as high as your intake? I would think you would want to clear as much of the post-combustion gasses out of the cylinder as possible, I mean, you can't burn gas twice, can you? I'd think you'd want to get all of the exhaust out, so you can get a nice, clean cool charge in to the intake.

Just my $.02

 

i am not saying it would hurt or anything i am just saying i think u would see more of a gain if u increased the intake as well. the exhaust will flow out faster so whats gonna come out the rest of the time th evalve is open? which is why i say increase the intake as well to increase the performance. utilize that empty space is all i am saying. i reread some of my posts and it kinda sounds a little confusing but that is all i am getting at is utilizing the space provided. i feel that a wide open exhaust with not enough intake to utilize it is a waste

 

Gosh I hate to see a guy go through all that work in math just to be told that he used the wrong formula

Dia. X Pi (3.14) = the circumference of a circle, not the area.

Redo what you've done, then use

Pì (r²)

to find the area, then multiply by lift to find the area of a cylinder (the one formed from the lift).

2.02 ÷ 2 = 1.01" (radius)

1.01 x 1.01 = 1.02" (radius squared)

1.02 x Pì (3.14) = 3.20"² (area of circle)

3.20 x .450 = 1.44"³ total area

No flame intended..... I get those formulas mixed up sometimes to.

 

doh!

 

wouldn't improving the exhaust side, if it is a known problem, help pull more air through the intake side? kinda the same principle behind headers improving the exhaust "scavenging" effect?

 

The biggest thing about split ratio rockers is that you can make cam timing changes on an already installed cam. If you were building a motor and needed to buy a new cam, there would be no need for split ratio rockers. You would just order a cam that had the timing you wanted.

 

Actually, I used the right geometry. If you imagine the valve face and the valve seat as the 'ends' of an imaginary cylinder, with a height equal to the valve lift, my calculations are right. For a cylinder of the proportions we're looking at, the area of the 'ends' will be greater than the area of the 'sides'. I purposely did'nt use the are of the valve face in my example, because air cannot flow through the valve head, being as the valve is a solid object.

The formula AJ_92 used is correct, but it is the formula for the volume contiained in that same cylinder.

We're both right, but in different ways. I think i'm conceptualizing too much.

 

Your math is accurate, but oversimplifies the issue at hand. Other factors are at work, so no matter how correct it is, it doesn't accurately model the real world. There's more to it than just flow area.

The exhaust has compression pressure available to push the gas out; the intake side has a limit of atmospheric pressure (14.7 psi more or less).

As I said, it's a tuning technique. The results are almost impossible to predict. The usual reason people do it isn't to get more power by using less valve lift (which doesn't happen); it's to get the same power without stressing the rest of the valve train, particularly the valve springs. If higher lift on one valve or the other produces no benefit, usually because the other one is the limit to the engine's total flow, then it makes no sense to add the unnecessary lift which just makes the springs work harder and fail more and not last as long even before they start to outright break in half.

The only way you'll get more power by lowering the lift is if the valves float with the higher lift, but don't float at the lower lift. And if that's the case, then the correct answer isn't to downgrade the rockers, it's to install the correct springs in the first place.

Note also, you can make a FAR bigger change in the flow characteristics of the port, by installing undercut valves; and back-cutting the valve face; and so on. Higher rocker ratio is just another little "edge" you can make for yourself.

So sure, you can de-rate one side of your engine or the other; but do you know if it's the side that isn't the overall flow limit? If it is, the engine's power will go down by the same amount as it would have if you downgraded both valves. And if you don't have a dyno or a track time and 2 full sets to try all 4 possible combinations then you won't have any idea whether you did the right thing or not. You are on a par with the monkey: 50% chance of getting it right at best.

 

How can you make cam timing changes with split ratio rocker arms? The only difference I can see them making is the obvious one of lift and a slight change in effective duration.

 

Having a dyno would truly be good, some say that it's a good way to see if your cams big enough. However a larger cam will create more duration and that may not be a good answer for everyone yet. Tall lift without more duration in a roller cam will push torque closer to horsepower in the rpm band which gives fantastic efficiency. However a comon mistake is to add a 1.6 rocker ratio on an already running high performance engine and have contact with the pistons. just a note..

Also G.M. did a test on one of their 358 Nascar spec engines and ran ratio's 1.65 - 1.70 - 1.65 - 1.60 - 1.60 - 1.65 - 1.70 - 1.65
on each bank to achieve about a 40 horse increase.

I run 1.6 rockers with a set of heavily ported pre 1975 big valve 993 cylinder heads and a ramjet cam. Hauls A$$.

 

Switching to a bigger rocker starts the valve opening sooner and holds it open longer. That is the timing I was refering to.

 

Measured duration is what opens sooner, not the valve. The valve just accelerates faster to the openeing point that's measured.

Since the common measure (at least the most consistant) is .050" for duration numbers, the duration time from .050" O/C on a 1.6 ratio would be a little longer. Enough to add more RPM (that's what more duration is supposed to do)? Probably not.