After reading the recent magazines I measured the cranking pressure in my good ole 327. The articles recommend cranking pressures in the range of 160-180 PSI for decent street engines. Well, it seems that I could use some more PSI for my engine. I would like to hear if anybody has experience on swapping 1.6 rocker arms to 1.5 AND how this swap might affect on cranking pressures. If I understand correctly, 1.5 rocker arms decrease the cam duration a few degrees and thus intake valve closes earlier, resulting higher cranking pressure. Is this worth the trouble, or just the usual magazine BS?

My setup is: .030 over 327, 64 cc camel hump heads, CR around 9.0:1, Comp Cams XE262 installed straight up, 1.6 rocker arms. Cranking pressures 145 PSI in all cylinders.

Thanks in advance.

Tapio

 

Changing the rocker ratio does not change the duration, or make a valve close
earlier or later. It changes the "area under the curve". Going from a 1.5 to 1.6
ratio will increase lift throughout the entire duration.

If you have a realtively short duration cam, then increasing the rocker ratio will likely have the affect of increasing static cranking PSI because you can potentially get more air into the cylinder. A cam with more overlap will have less cranking pressure, even though the static compression ratio may be higher. I've seen 12 to 1 c.r. drag motors with really big cams not have much cranking PSI.

Lots of things affect cranking PSI. Your shorter stroke 327 may have a little less than a longer stroke motor (at cranking speeds), all else being equal. Cranking speed can also affect the PSI. Are you pulling all the plugs, or just one at a time? The engine will crank faster with them all out. Also, prop the throttle blades open.

Advancing your cam 4 degrees will increase you cranking PSI and result in more lower end torque.

In short, I wouldn't worry about the number too much as long as they're pretty consistent among all 8 cylinders, and it you're looking to increase low to mid-range torque, then look for increases from mods relative to what you started with.

Dan

 

Yep, I did pull all the plugs, and the carb throttle blade was fully open. My cam is quite conservative, 218 / 224 @ .050.

 

what head gasket? i am using a .015 shim to raise c.r.

.
i suggest you answer your own question by changing one intake rocker and re-test. might work well since the xe256 is the cam i would pick for a 327


[Modified by Matt Gruber, 10:28 AM 9/13/2002]

 

I measured the rocker ratio on my production L-76 - stock except for an LT-1 cam. It starts out at the top of the ramp at 1.37:1 and increases to 1.44 at max lift.

Rocker arm ratio will affect actual duration, depending on the clearnances you run, but the effect is very minor, and I doubt if rocker ratio would have any measureable effect on crankiing compression pressure. The static compression ratio and cam and the big determining factors, and you should look for consistency across all eight cylinders rather than getting hung up on the actual measured value relative to some theoretical benchmark.

The important point that fell out of my measurements is that specified clearances are incorrect for both OEM and aftermarket cams. They are based on the height of the ramp times 1.5. In the case of the LT-1 cam, the clearance ramps are .016" and .020" (actual measurements I made) and multiplying these by 1.5 yields the specfied clearnaces of .024" and .030".

If you run these clearances on the engine, the valves will be lifted and seated at greater than ramp velocity because of the short rocker ratio, so I "factor" the clearances by 1.37/1.5 and truncate to three decimal places, which yields clearances of .021" and .027". (I actually recommend .026" on the exhaust because the clearances tend to loosen with time.)

I've run simulations with different rocker ratios (the simulation programs assume it's constant, so they are not absolutely accurate, but the relative results are valid) and there is not that much difference - a percent or two, at the top end, with little or no gain at the bottom and mid-range - sometimes even a bit of loss at the low end because of the slightly greater effective duration with higher ratio rockers.

So OEM rockers are not 1.5 and the advertised 1.6 aftermarket rockers are probably not as advertised either except perhaps at max lift with a very high lift cam. No one seems to understand what is really going on with rocker ratios - the OEMs and aftermarket never discuss this, so I ended up making some careful measurements and figuring out the geometry myself along with the implications.

Duke

 

I also use Fel PRo 0.015 shim gasket.

Funny thing is that measured the pressures with my earlier, supposedly bigger cam (Crane 284, 222 @ .050) and got slightly higher pressures (around 150 PSI). Also much thicker blue Fel Pro gasket was used then.

 

That being the case, looks like getting more air into the cylinders (because of bigger cam) helps, so, going down on rocker ratio (which would lesson the lift) would hurt cylinder PSI.

How's the driveability performance differed between the two cams?

 

Well, the Comp Cams XE262 is definately better for my driving needs. It pulls strongly from 1000 RPM or so in any gear, the earlier cam was just a bit too aggressive for my liking. However, when the engine got into the power band, it pulled very strongly. But, as I mentioned, XE262 is much more civilized.

Tapio