OK, so with 1.7 rocker arms I have .510 lift, swap with 1.8 rocker arms and end up with .540 lift. How much HP is this swap worth in a 6,000 RPM, BBC- engine. I like the duration The cam has, but I think a little more lift will help. The cam in question is a Comp. Cams 270 HYD. in a 408 CI, BBC.

Depends on alot of things... mostly, how much difference in head flow there is, averaged over the entire valve cycle (i.e. not just the difference in .510" flow vs .540" flow). It also depends on whether the valve springs will be happy with that.

Probably won't hurt anything though.

On the other hand, as cheap as cams are in general, you could likely get the same results, or more, by changing that out; maybe for less money.

The valve springs in the Edelbrock oval port performer heads will support up to .700 lift, so no problem there. The intake is an Edelbrock daul quad with Edelbrock 600 CFM carbs, all good up to 6,500 RPM. I ran 9 different cams in this engine before I made my pick on the comp. cams 270 HYD. This cam is Awsum up to 5,700 RPM. My thinking with the 1.8 rockers is it will raise the power band up to at least 6,000 RPM. I was even thinking 1.8 on the intakes and 1.73 on the exhaust just to see what that might do.

It always helps when talking about cams, to tell what the rest of the motor is, up front. Nobody can pick a cam in a vacuum, and get it right. Knowing what heads the motor has, and its compression, are critical.

They won't really raise the power band; that's set by the duration mostly, not by lift, and changing the rocker ratio doesn't change the duration significantly enough to havemuch of an effect.

All it will do, is to maybe allow a little better cylinder fill in the mid-high RPM range. Meaning, the torque curve will raise up a little bit, in its upper part; but its overall shape, of torque vs RPM, will remain about the same. On the other hand, if the valve lift isn't what's limiting the engine's performance now, but rather something else is the limit (intake manifold, headers, exhaust, whatever) it won't make any difference at all.

Putting higher ratio rockers on one side or the other is only helpful to balance the 2 sides of the motor. I.e. if the exhaust flows better than the intake, then increasing the exhaust flow still further, won't improve the overall output; and vice-versa. In other words, there's no hard-and-fast rule about put higher ratio rockers here and lower ratio ones there and you'll make more power.

That said, it's ALMOST CERTAIN that if the rockers make any difference at all, the motor will make more power with higher ratio on BOTH sides. The reason people put them on one side only, is for when that side is weak on flow, but they would exceed the limit of reliability by putting them on the other side (break valve springs for example). It is VERY VERY VERY rare that reducing the ratio on either side, will result in a higher power output; that is, that a 1.8/1.72 combo would make MORE power than 1.8 on both. And with that little Magnum cam, I don't think you're in any danger of breaking anything, so odds are you'll get the most results (if any results at all) by using them on both sides.

I figure that you actually need a 272 to a 274 true "Magnum" lobed cam to up your rpm like you want.

I don't know if CC even makes a true 274 Magnum lobed cam?? Not the XE 274 jobber, imho IF you really like the older 270 Magnum that much.

You might contact Isky and see if they have a Magnum/Mega lobe for a 274 cam??

They make the older 270 and 280 Mega cams so just maybe??

pdq67

I doubt you'll even be able to tell you've changed the rockers with that mild a setup and it will put the puchrod very close to the inside edge of it's hole where it passes through the head. Not the best way to get more power. If you really want something you can feel i think your going to have to take the heads off and have them worked on.

I agree with Harold, I myself don't particularly like changing the rocker ratio because it throws off the geometry. I can barely clear the pushrods in the heads with 1.7 rockers. If it's more rpm, I think a more modern grind will get you what you want without a drastic change in cam specs.

Thanks for the input guys, just something I was toying around with. The engine is a 1970 402 L-34 bored out to 408 CI, compression was 10.25:1 but droped down to 9.1:1 with the Edelbrock heads, it is a very responsive engine, and I do plan on getting the heads milled to get the compression ratio back up to around 10:1 this winter.

I am running a combination of 1.8 on the intake and 1.7 on the exhaust. These are the Crower Stainless Steel "Enduro" rockers.

My cam is an Isky mechanical roller 248/252 @ .050" and .638" (1.8) on the intake and .646" (1.7) on the exhaust. I wanted abit more lift/aggressive profile on the intake side.

The 1.8's not only add some lift but makes the cam "seem" a little bigger and more aggressive at the valve. Make sure the springs will work with additional lift, there is no piston to valve clearence issues and the pushrods clear the "pushrod holes" in the head. They get kinda close.

IMO, if you are wanting to extend your rpm range a little, you might want to consider switching to a dual pattern cam. Maybe something with 6 or 8 degrees more duration on the exhaust side. Just a thought.

Thanks Darren, that is one Awsum looking Chevelle you have.

Thanks for the compliment. :waving: