I have an extra set of roller rocker arms that I could use in my coupe for a little while. They will, eventually, go in the motor for the Field Car. In the meantime, I'm thinking about putting them on the 327 to try them out. What considerations should I make to figure if this is a move in the right direction? Outside of valve clearance, pushrod geometry, spring bind etc., what are the performance advantages and disadvantages. Right now, I have a Comp Cams XE274H. It's a .490/.490 lift with 230*/236* duration at .050. With the 1.6 rockers I would be at .522/.522 lift. (.490/1.5=.326 lobe lift and .326* 1.6= .522 valve lift) I think the duration will increase about 20* to 250*/256*. I ran the Comp Cams CamQuest program and loaded in "Pro Street and Mild Racing" and it recommended a 294H grind that is a 519/523 lift and a 250*/256* duration. Almost identical to the numbers I'm coming up with by exchanging rocker ratios. The rpm range lists it as making power to 1000 rpm higher, right to where I'm shifting now. So, will putting these rocker arms on the motor have the some characteristics as changing to the 294 cam? It's temporary, so I'm not as concerned about the street manners. If it's a big issue, I'll just switch back sooner. I'm more concerned with the all out performance difference they may make.
from what i remember when i was playing with 5.0 mustangs changing to the bigger ratio rockers gave the effect of a cam change i.e. more lift and duration the only thing i heard was that it tended to put more wear on the guide. I have also seen people run two different ratio's from intake to exhuast to get the dual pattern effect. i say run it and see what happens. my .02
If you do decide to run 1.6 rockers make sure they have clearance in the pushrod slots. I always ran 1.6 on 383 and 400 engines. Just always made sense since most of the cam grinds were aimed at the 350. The lift rate is quicker so it puts more strain on the springs at high rpm. It also puts more pressure on the lifter at the same ratio.... Try it if you have guide plates and good springs that can handle the extra lift.
I don't think the duration will increase that much, but you are right on the money on the lift increase. I can't think of any real disadvantage to changing to the 1.6 rocker, other than making peak power at a little higher rpm. Just make sure you don't have coil bind, and that your valve to piston clearance is sufficient. Also make sure have enough clearance at the top of the valve guide for the extra lift, and that your pushrod slots are big enough. You may be able to put a little more fuel and timing into it to make some more power, but to be honest I don't think you'll see a whole lot of difference from the driver's seat. At any rate, Smokey Yunick said that the 1.6 ratio was an improvement for small blocks, and of course he was pretty sharp.
I remember in Smokey's book he even put 1.7 BBC rockers on a smallblock. Had blueprints and all. Looked cool with the dimpled valve covers....
The valve lift increases, the duration does not. This is because the grind on the cam remains the same, so the angle of rotation where the valve starts to open, close, etc.... is still the same. I've used them in a "stock" upgrade, making the cam seem larger, and have worked out well. They will help performance out some, but the lope at idle and rpm range remain about the same. As long as you have clearance, go for it.
Funny if the software actually specs this-- I just always thought that increased rocker arm ratio mostly affected the lift. I guess that .050" lift might happen a little quicker-- but I doubt that it's a full 10° quicker on the front side, and 10° longer on the back side of lift profile. My thinking about the 1.6 rockers was that it was similar to having a head with larger intake runner volume/valve diameter. But since it is a ratio that we're talking about, the bigger the cam profile, the more dramatic the change. Theoretically, anyway.
I just did the swap in a 350 and it felt like it had more bottom end right away, I figured it would show at top not on the bottom. I have yet to really try it out yet but like I said so far it seems like it has more out of the hole.
The duration as such does not change when you change ratios. (It does very, very slightly, if you go though some fairly complex calculations, but in the order of fractions of a degree.) What happens is that you add area under the curve. The easiest way to understand this is to actually plot the lift on a x/y axis in relation to the cam rotation in degrees...then do the same thing with the new ratio. What you will see is something like this: ...^... & then .../\... in other words, the opening & closing points are in the exact same position (therefore the duration hasn't changed), but the valve gains lift more quickly with the larger ratio rocker, holds it at a higher peak/near peak longer, and then shuts it more quickly. The average healthy 355 often likes 1.6 on the intakes & 1.5s on the exhaust, with most of the OE Chevy iron heads. Without knowing much about your heads, & their flow numbers, it's difficult to say what will happen. Since you have a 327 the effects will be a bit more apparent. If you are running exhaust manifolds you may find that you gain more by running the 1.6 on the exhaust side only. As others have mentioned, you need to check piston-to-valve clearance, coil bind, & retainer-to-guide clearance, along with all the other stuff...but it sounds as if you're aware of this already. I need to add that if possible, you need to check the SIDE clearance of the piston valve relief, not just the face...you are significantly changing this relationship and if the clearance is marginal you might see a problem now. If you have TRWs, the reliefs are usually very large, as they were designed knowing full well that the avg. hotrodder doesn't generally check that kind of thing.
Homespun-- I'm sure that it's not that black-n-white, particularly when you are considering "effective" valve lift @ .050". It's definitely more than a "fraction" of a degree- but most likely not a full 20°. For one thing, the net effect (on duration) of increased rocker arm ratio takes place twice every 360°-- on the ramp up, and then on the ramp down to valve closing. You could look at the exact bell curve that you mentioned and see that ".050 valve lift occurs sooner with a higher rocker arm ratio- It's just simple math. the subltey lies in exactly HOW MUCH sooner-- that's what buzzard is asking about.
Then I guess I'm not explaining myself well enough, partly because camshaft design is too damn complicated to discuss effectively on the Internet- too much typing. I think we are saying much the same thing, but let me try this again. Advertised duration does not change at all (actually, as I said, it usually does by a fraction, as a rule, because of geometry changes). .050 is used by most cam grinders, as a standard, because it represents the point when any clearance ramps are taken up for a hydraulic cam, or lash for a mechanical tappet, and the lifter starts to "rise"; also accelerate. Some makers used .020, or .006, but the general accepted standard now is .050. If you measure that duration at .050, there is, almost always, very little difference, between one ratio & another. Nothing has really happened as yet...you are merely getting slack out of the valvetrain, or preparing the lifter to change its angle in relation to the lobe. As you say, after that point, as you plot the lift curve, you see a difference, more so as lift increases. Some of this depends on the particular lobe profile. The XE BBC lobes are very aggressive on the entrance ramps and as a result, if you did a 1.5/1.6 comparison with an XE, and then did the same thing with, say, an older GM lobe with the same advertised duration, you would see that the XE shows an exaggerated effect with the 1.6 as compared to the GM lobes, which have extremely lazy profiles (as do many older cams, generally because spring technology wasn't up to today's standard). BTW, one other thing to consider is that many lobes are asymmetrical, i.e. the entrance ramp & lobe profile is different than the exit side....so you will have a different ratio profile on the graph as well. If you REALLY wanna get complicated, a larger lifter diameter, when used with a lobe profile that is specifically designed to take advantage of it, will show more effect with a larger ratio than a smaller diameter...even if you use the same advertised & .050 specs when designing the two respective cams. As to your point about it not being fractions of a degree as I stated...well, it depends. Sometimes, as hopefully I've shown, there is a difference @ .050 with the increased ratio, depending on the lobe...it can be a couple of degrees, meaning one or two, and sometimes it's on both entrance & exit. Frequently- in my experience the majority of the time- there is no real difference at all. It all depends on the lobe profile, and whatever the cam designer had for lunch that day. As I mentioned, Comp has very aggressive profiles in the XE series...which isn't always a good thing. There are other lobe designs out there which seem to make as much power but aren't nearly as noisy; the XEs slap the hell out of the lifter and you can hear the clatter as a result. I won't pretend to be a cam expert by any means but I have worked with a fair amount of SBC cams (as well as BBC, BBF, & BB Mopar), and this has been my experience. We did a lot of work with the SBC for circle track, drag, & street applications, and profiled a lot of cams with the Cam Doctor software, as well as by hand...again, just my experience. Your mileage, as they say, may vary.
Most low buck street performance valve springs don't accept more than 490-510 lift. That'd be my biggest concern, spring bind and depending on the valve seal/spring retainer combo you could run into a secondary clearance issue. Good luck
Ok, correct me if Im wrong, and I offten am .. the whole reason guys started doing this with the different ratios, was because there just was not alot of choices in cam grinds avalible. with the avalibility of cams out there now and resonable cost of them . isn't this just a waste...or is it still a fine tunning thing? Im Just asking (info hunting ).
In this case they're an item on the shelf, and thus sorta free. Otherwise 1.6 rockers tend to target extreme ends of the spectrum. They're good for people wanting a little tiny bit more, but don't wanna go into changing the cam. They're also good for people with really non-stock geometry, because when you start jacking around with pushrod and valve lengths, or aftermarket heads with subtly moved valves, or just plain huge lift, a 1.6 rocker can improve the rocker/valve interface. Sometimes.
Geez, I love this place. With all the bitching about drama, rat rods, magazines etc. etc., if you ask a decent question you'll get a bunch of well informed, intelligent responses. Thanks for taking the time for the explanations. The heads are Iron Eagles and the pistons are KB hypers with about a 10:1 compression. The pistons are flat tops with 5.4cc valve reliefs. I also run open headers, so exhaust breathing should be decent. I didn't do math, or have a formula, to come up with the duration increase. I read that in an article online from one of the magazines. Maybe I misread it and I can't find it now. It made sense to me that at .050, the 1.6 rocker was lifting a bit higher/sooner and closing a bit later, but I have know way to quantify any actual duration change. I'll assume that it's not enough to matter. Since we are talking about relatively small cams, I didn't think I'll have valve to piston clearance with the flat tops. I'll check, or course My main concerns are coil bind and valve float at 7000 rpm with the sharper "net" lift profile. So, are ya'll generally agreeing that it should make a little more power at a little higher rpm range? Making the most power between 4500-7000 is what I'm looking for with this change. Since the rockers are sitting there, and I may be racing in a week, I thought I'd try them out. The 1.5's will most likely go back in after that.
A lot of guessing going on here. Duration at 050 will change because 050 will happen at a different spot. 1.6s dont autmatically give more power. Sometimes they do sometimes they dont. If the cam design is already oprtimized than often they cost power and et. I have seen losses as high as 3/10s and just to be sure this fellow who races out of my shop dorve to town and got his 1.5s and returned to the track and reinstalled them . His times came back to what they shold have been. There is a nasty tendancy to think more is always better when in fact it isnt. Right is better and too much or too little is worse. Doesnt matter what Hot Rod Mag says or anything else, That s just the way it is. With some engines like for instance BB mopars with iron heads (even ported ones) there is a point at which flow revesres as lift rises above a certain point. On our flow bench we have seen heads flow wonderful at 540 and flow only 60% of that at 600 lift. So if the rockers push the lift above the heads sweet spot power goes down. Did a lot of testing both on the bench and in the racecar to see if the was actually born out in practice. It is. I think it is wonderful people do this though especially when we are running against them. A famous cam grinder once told me the best way is to try and see what happens. Sometmes it works and sometimes it doesnt. I think that was the real truth. The real hard trick for many to learn is when it doesnt work (and your time slip and MPH will tell you without question) then be man enough to remove them and return to what does work. I just returned right now from removing a set from a friends race car and installing and adjusting the old 1.5s. He would really like to run the anodized red 1.6 rockers but the old iron iskys just go so much faster what can he do? As for roller rockers make a car go faster. BALONEY. That we have proved is false many time over. Don
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