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 jonnyb
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| posted on October 16th, 2009 at 05:43 PM |
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ratio rockers
how much difference to the engine does the 1.25 and1.4 rockers make the motor is 1600
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68AutoBug
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| posted on October 17th, 2009 at 12:59 AM |
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Hi
Not sure in Horsepower...
but they will let your engine breath better by opening the valves further...
they are also better than a high lift cam as you can run standard springs...
1.4 to 1 are fairly expensive...
so unless you are going to hot the engine up with big carbies etc..
I'd go for 1.25 to 1 which i hope to get for my engine one day
LEE
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 vw54
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| posted on October 17th, 2009 at 07:42 AM |
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the 1.4 will push the valve open 2.75 futher than the 1.25 rockers
the time the valve is open longer allows more ari / fuel to be dumped into the chamber making a bigger bang
dont for get u will need different push rods as well
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waltermitty
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| posted on October 17th, 2009 at 08:22 AM |
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1.25s dont make a huge difference
1.4s will make more but to make use of potential the longer valve opening you need better flow , ie cam , heads(valve size) exhaust to make use of
the potential.
A lot depends on what you are aiming for drivability / usage wise. Mitchell
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clinker42
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| posted on October 17th, 2009 at 10:59 AM |
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I wouldnt bother with a rocker ratio change, it achieves nothing. This engine in the pic was dynoed with 1.5 rockers, after the first dyno runs we
changed to a big block chev 1.7 rocker and it gained 8hp. We then changed from the 292@50 cam to a 298@50 cam (exact same lift) and it netted 42 hp
but back with the 1.5 rockers.
Lift is no where near as important as duration, opening the valve a few degrees earlier and closing it a few degrees later adds much more than just a
lift change.
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 matberry
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| posted on October 17th, 2009 at 11:16 AM |
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The ratio rockers do add some effective duration but I would be careful using 1.4's with std springs and an unknown cam....be very careful, you may
damage your heads at minimum.
| Quote: | Originally
posted by 68AutoBug
you can run standard springs...
LEE
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reub
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| posted on October 17th, 2009 at 11:47 AM |
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I've some scat procomp 1.25:1 rockers up for grabs...
Reub
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clinker42
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| posted on October 17th, 2009 at 01:11 PM |
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Mat
How do rockers make duration changes. Duration changes are made on the opening and closing ramps of the cam. Doesnt matter what ratio you have on
a rocker, the lifter starts to open at exactly the same part of the cam ramp, changing the ratio justs changes the amount the valve opens not the
timing of the opening.
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matberry
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| posted on October 17th, 2009 at 03:03 PM |
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Yeah but as you may know you can hardly even measure when the valve actually starts opening, and so you compare cam spec @ .050" and it is there that
there is a change with the ratio rockers, and flow is all about the volume or available area between the valve and the seat which is increased due to
the different ratio. This I believe is why they say 'effective' duration is increased.
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clinker42
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| posted on October 17th, 2009 at 03:51 PM |
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Matt
Now I get where your coming form, if you measure standard lift @50 you may be supposed to have say .350 lift, but when using ration rockers you will
have .375 lift @50, simply due to the increase in ratio. so effectively, at a given degrees your lift will be greater so it appears to affect your
duration. But really, i physically cant, the lifter is still moved by the opening and closing ramps, nothing can be done to change these except for
ramp changes, the lifter will still start to open at exactly the same point no matter what rocker ration you have. When you degree a cam, one should
never set the dial indicator on the valve, should always be put on the lifter.
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BiX
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| posted on October 17th, 2009 at 04:38 PM |
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It will give an effective increase in duration. As matt said most durations are measured at 0.050" as this is when you can sctually start getting
flow through the port. By having ratio rockers, it means the valve reaches 0.050" sooner after starting to open, and closes later.
rather than looking at the lift at a certain rotation, think of it as a rotation to achieve a certain lift.
Ie std cam (made up numbers) takes 3 degrees to reach 0.050" and the same to go from 0.050" to clsoe.
With ratio rockers it will take 2 degress to reach 0.050" from open and the same amount of rotation to close. hence you have no gained 2 degress in
duration. I am not sure on the exact amount of rotation between opening and reaching 0.050", but it will be dependant on the came shape, in the early
and late stages of the camp opening.
For a small capacity engine, a few extra degrees of duration and some extra lift for a few hundred (or less if they are second hand) coulod be a
valuable upgrade compared to splitting the case for an new cam.
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clinker42
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| posted on October 18th, 2009 at 11:55 AM |
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Guys
I hate saying this and it will make me out as a prick.
But you guys are completely wrong. You guys are talking about a change in lift not duration. Sure the duration will be different due to you having
a higher lift ratio. But trust me, you guys cant even degree in a cam if you have ratio rockers. All you are seeing is a different figure. The
valve doesn't open or close sooner or later, it opens at exactly the same time as it did with standard rockers, the increase in ratio simply gives
you a larger lift, sure if you want to measure your lift at the rocker then you will see a duration change in when its gets to that 50 thou mark. But
I seriously suggest that you guys dont try and set a cam up by measuring at the valve.
The duration is is set by the opening and closing ramp of the lobe. Its fixed, nothing you can do can change that.
All you guys are seeing is a change in where your lift is at a given duration, exactly the same as if you advance or retard the cam timing.
Please trust me, I know what Im talking about, been doing this stuff for 20 yrs and on average, I spend 10 hrs a week in a dyno room working on heads
and cams.
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1303Steve
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| posted on October 18th, 2009 at 12:10 PM |
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Hi clinker42
We are all here to learn and hopefully with what you know about motors will help us all learn.
I had 1.4s on my old 2.1 WBX, 110 hydraulic cam, 10.5 comp and dual 46 IDAs. I bent a pushrod once and went back to stock 1.1 ration rockers for a
week, the difference was very noticalble between stock and 1.4 rockers.
Steve
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clinker42
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| posted on October 18th, 2009 at 12:19 PM |
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Steve
Sure you could see a difference, but it is lift and a timing change caused by the rocker ratio change, not a duration change
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clinker42
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| posted on October 18th, 2009 at 12:36 PM |
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Guys
Maybe this will help
This is what duration is
Duration is the time (in degrees) the valve is of its seat during the opening period of the valve event. This means that as soon as the lifter
starts to rise of the opening ramp until it cease to rise.
Thats duration. You guys are talking about a timing event change at the valve due to the rocker change.
If you guys ever look at cam figures then everyone of them is at the lobe, not the valve. When you degree in a cam you do it of the lobe not the
valve. Have a look at the pic, then you may see that you cant change duration unless you change the lobe. But what you guys are thinking about is a
timing event change not a duration change.
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BiX
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| posted on October 18th, 2009 at 01:58 PM |
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Clinker, as long as no body gets nasty (which no body has) a good robust disscussion on a topic is a great way to learn.
I agree with nearly all of your comments and yes by going to ratio rockers there is no duration change at the camshaft.
What i suppose a few of use were trying to say, was the EFFECTIVE duration at the valve is increased by using ratio rockers. (maybe this is what you
define as a timing event change, though i define this as the timing of the cam compared to the rotation of the crank).
As you rightly say the actual duration at both the cam and valve are the same no matter wihch rocker you use.
Though I define effictive duration as the duration of the cam that allows usable air flow to occur into the combustion chamber. In most instances this
is 0.050".
As stated above the closing/opening ramp shape will not change, but with a higher ratio rocker, the time (in degrees) taken to reach the point at the
valve at which air flow will occur is quciker. Therefore the valve will be open at a point able to EFFECTIVELY allow air flow (defined as 0.050")for
a longer period, given the same cam is used. Mu understanding this is the duration ( though measured at the valve when open more than 0.050")
Cheers
Drew
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clinker42
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| posted on October 18th, 2009 at 04:34 PM |
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Drew
But that's what I'm trying to get across Drew, duration is a fixed thing no matter where you measure it, if you measure the duration as it is
supposed to be, the start of the opening and the completion of lift, then even if you measure it at the valve with a ratio change, the duration has
not changed, it can never change. The thing that changes is the amount of lift you have at that duration. You guys cannot measure any of these
things the way you do, If you try to measure the way you guys are, then for a start you would have to multiply 50 thou by the rocker ratio to get
anywhere near close to the timing. This whole timing and degrees and duration has actually nothing to do with valve lift, its only position the cam
in the correct realtionship with the crank and we just use the 50 thou thing as a way of doing that.
You have some good points, especially on the usable air flow. Thats the one major draw back with a ratio change, you can get an increase in airflow
on the piston down stroke, but the really bad offset is that when the cam is in the overlap period, that's when the piston is on the compression
stroke and the valve is still open. At that point we are using the air speed and the high rpm's to create very high velocity and still allow the cyl
to fill when the piston is coming up. If you use a ratio change you are holding the valve open further at that period. Unless you have the heads
and inlet and rpm to use that then you will loose hp. When you change the duration on the lobe you can bring in the opening earlier and have the
closing later but you can also reduce valve lift in the overlap period to reduce the loss.
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Craig Torrens
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| posted on October 18th, 2009 at 08:17 PM |
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well while everybody is getting bog down with duration BS maybe its worthwhile actually answering the original Q.......My Answer is: 1.25 and 1.4's
make very little difference to a stock 1600........1.25's would be the max i would use if you really wanted to go down that route.
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matberry
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| posted on October 18th, 2009 at 10:38 PM |
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Dont think it's bs Craig and I have to say here that I agree totally with what clinker is saying, but I think bix agreeing with the concept is also
right. Effective is the key word here as the ratio rocker is in fact for the same duration cam (because you do measure duration at the cam/lifter) is
increasing the lift at any given point of crank angle. If we look at the area under the curve of a lift vs duration graph, the area relates to volume
of inlet charge, and this curve gets fatter with the rocker ratio increase, including increasing the area between the overlapping inlet and exhaust
increasing, thus giving the effect of increasing duration and lift.
I realise the actual camshaft duration cannot change, but buy opening the valve quicker, we are getting more lift but also starting the flowrate
sooner during opening (and later during closing)will give improoved flow over the otherwise same cam duration with more lift with say the og 1.1:1
ratio rockers.
Maybe it is all bs and Craig is right. I know by the time I write it via the keyboard it all comes out wierd anyway......Bix made it sound good but
:-)
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jonnyb
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| posted on October 20th, 2009 at 05:40 PM |
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thankyou
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