99Hawk120

Here's some pics of the rocker on the #1 exhaust. Clicking should give you a bigger pic if all is working well.

Is it just me or are both lengths too short? I wish I had another pushrod length checked but mine stops at 7.95", and another one is a Summit "fondling" fee away...

(edit

The picture are, from top to bottom:
Valve closed
Half lift
Full lift

<TABLE>
<TR><TH>7.8" Pushrod</TH><TH>7.95" Pushrod</TH></TR>
<TR VALIGN=TOP><TD><A HREF="http://www.camaro.dnsalias.com:8008/79Vette/rocker%20arm-short%20pushrod-open.jpg"> </A></TD><TD><A HREF="http://www.camaro.dnsalias.com:8008/79Vette/rocker%20arm-med%20pushrod-open.jpg"> </A></TD></TR>
<TR VALIGN=TOP><TD><A HREF="http://www.camaro.dnsalias.com:8008/79Vette/rocker%20arm-short%20pushrod-half.jpg"> </A></TD><TD><A HREF="http://www.camaro.dnsalias.com:8008/79Vette/rocker%20arm-med%20pushrod-half.jpg"> </A></TD></TR>
<TR VALIGN=TOP><TD><A HREF="http://www.camaro.dnsalias.com:8008/79Vette/rocker%20arm-short%20pushrod-closed.jpg"> </A></TD><TD><A HREF="http://www.camaro.dnsalias.com:8008/79Vette/rocker%20arm-med%20pushrod-closed.jpg"> </A></TD></TR>
</TABLE>

AJ_92RS

The rockers look like they're REALLY long. They're sitting pretty far out on the valve tip.

What type of cam/lifters are you using? And what brand and ratio are those rockers?

99Hawk120

The cam is a comp cams XE268 I believe, with their lifters. The rockers, I don't know offhand what brand they are, but they're stamped 1.55...

ede

as a rule you'll need to increase push rod length by .1 for every .1 increse in cam lift. trouble is you can get cams in all sorts of lifts but push rods only come in .1 increments, unles you get a set made. i made a few length checkers by cutting a push rod and tapping one part and and silver soldering a screw in the other part. use soem dykem or white greese on the valve tip, use it to show where the contact is. you'd want to keep the rocker contact area as near to or in the center of the valve tip as you can.

kevinc

Black sharpie marker works good for this also, just clean up the valve stem surface and color it black. Assemble the rocker, rotate the engine thru 2 full cycles of that rocker, and disassemble...you'll see the marker worn off where the rocker was in contact.

RB83L69

I can't tell looking at it.

Like ede said, you typically would increase push rod length by .050" for every .050" of lobe lift that the cam has above about .275" (i.e., @ .325" of lobe lift, go to a .050" longer, @ ,375" go to .100" long, etc.), all else being equal. But there are way too many variables on a modded motor to predict with any certainty; everything that affects optimum push rod length can be different besides just lobe lift, from valve length, to how deep the seat is cut in the head, to how the rocker is made, to head gasket thickness, to how much the block & heads have been decked, etc.

I usually use white lithium grease as the "marker" on the valve tip, and choose a push rod length that makes the smallest possible area of contact, in the direction that the tip rolls back and forth as it moves through a cycle. It doesn't matter whether it's centered or not. You can't control that since it's determined by the distance from the stud to the valve and the length of the rocker, and having it off center doesn't hurt anything. The narrowest contact patch indicates that as the rocker roller follows its arc, the rocker is horizontal at half lift, and tipped equally forwards and back at full open and closed.

http://www.compcams.com/catalog/278.html
http://www.compcams.com/catalog/279.html

Vader

Since we cannot see the indicator face in the photo, it's all a guesstimate, but I'd guess that you're still a little short. What I have been instructed is that at 1/3 of the lobe lift, the line created by the rocker push rod cup and roller surface should be perpendicular to the stud.




And yes, the roller over the valve stem appears to be over-center, but if we analyze the angle of the photo indicated by the rocker fulcrum, we have a parallax issue and can't definitely determine where the roller is in relation to the valve.

EDIT: Since you're apparently using checking springs, if you have an older flat tappet SBC push rod, hacksaw, and 12-24 or 1/4-28 tap/screw/nut, you can make a longer push rod length checker and not have to wait for Summit.

BTW - You're gonna need some fairly tall rocker covers for that setup.

99Hawk120

Yep, for some reason I thought it was at half lift, not 1/3 lift. If it's 90* angle at half lift, it's probably something less at 1/3 lift. I'm guessing I need about 8.1" pushrods. The cam is spec'd at 0.496 lift, but with those rockers it ends up being closer to 0.506

The rocker looks like it's way too long, but the center of the roller never seems to go more than 2/3 of the way across... in other words, the roller sometimes reached 1/3 of the way to the edge on the exhaust side. Not anything to worry about IMO.

Quote:

BTW - You're gonna need some fairly tall rocker covers for that setup.

Actually, the ironic part is the stock centerbolt covers almost fit. they clear the rockers fine, however they do not clear the stupid polylocks (which, in my opinion, are stupid tall to begin with).

Damon

Using my "calibrated eye" I'd say niether one is very far off. I like the longer pushrod angles maybe jsut a smidge more. Impossible to say for sure without being there myself.

BTW- most people go too long with pushrod length. The rocker looks like it's leaned WAY back towards the pushrod and gets barely past 90* at max lift. They get all nervous about this, run out and buy longer sticks. WRONG-O! This is actually as it should be.

Valvetrain geometry is one of those "voodoo" subjects like you need to be a rocket scientist to understand it or something. It isn't, but there's TONS of incorrect information floating around out there. I used to think I understood it until I drilled into REAL experts about how it's supposed to work.

Here's the real poop:

The rocker tip is supposed to start out towards the intake side of the valve tip and move towards the exhaust side of the head as the valve lifts until it reaches 2/3 of the cams lift and then stop, and start coming back towards the intake side again. Not at 1/2 lift, not at 1/3 lift (I used to use 1/2 max lift point until I knew better).

Reason: The rocker tip will always apply lateral force to the tip of the valve as it arcs through it's travel and slides across the valve tip. However, you want most of the movement to happen at LOW lifts. Why? Because that's when valve spring pressure is lowest. You want as little lateral movement above 1/2 lift as possible since that's when valve spring pressures are highest and that makes for more firction and more force applied laterally on the valve tip. That's why you want the movement to stop and start back at 2/3 lift (that's where the rocker should be at 90* to the pusrod and valve tip)- very little movement will happen laterally as you go from 1/2 to 2/3 lift and then on up to full lift since the rocker is very close to being "flat" across from the pushrod tip to the valve tip in the higher lifts. So simple you probably wonder why you didin't think about it youself. Factory rocker geometry is just about perfect for near-stock cams. This does NOT mean that the contact pattern will be centered perfectly on the top of the valve tip!! A simple thing like the location fo the rocker stud, the valve guide location or the brand of rocker arm you use can move the pattern a little. In a race motor they will get everything dead-on, but in a backyard-built street engine it'll never be perfect. It's not the absolute LOCATION of the contact pattern- it's MOVEMENT of the rocker arm tip across the tip of the valve that matters!

There is absolutely NO reason to automatically use longer pushrods with a higher lift cam, by definition. If you think about what I wrote in the previous paragraph you'll probably come to the conclusion that in some cases SHORTER pushrods may be required. If you're coming to this conclusion then you get the gold star for the day. There ARE NO RULES OF THUMB!! You measure and check for every particular combination- PERIOD! I have found that near-stock lenght pushrods work almost every time, even with higher-lift cams.

However, MANY things affect proper pushrod length. Cam base circle diameter is a big one but also consider that a simple thing like decking the block can affect you ideal pushrod length. ALWAYS remember that the goal is proper rocker arm travel geometry across the tip of the valve. That is the one and only thing to go by. Everying else is trivia.

On a street engine if you're within .050 of the ideal pushrod length you're plenty close, in my experinece. Rarely is there a need for custom length pushrods. That's why they sell them off-the-shelf in .100" increcrements. If in doubt- stay on the shorter side rather than too long!

99Hawk120

Damn it, I had a whole long reply typed up, and then my net connection went down. And of course IE thinks it is far more important to tell me this than to let me copy the post to the clipboard!!!

Anyhow, paraphrasing from memory, I ended up getting the longer checking pushrod anyway. I followed all the (non-conflicting) advice above with different lengths, and it looks as though 8.150" is the magic number. Why so long, I don't know.

I tried doing the "parallel to the head surface" thing, but the top of the orcker isn't flat and eyeballing the axis is just too prone to human error. I gave up and positioned the dial indicator so that the vertical pointer was in exactly the same plane as the valve stem and then used that to watch the movement of the rocker tip across the valve.

The best results were at 8.150"; with the rocker tip beginning slightly on the intake side of the valve, proceeding approximately 1/3 of the way across the valve as the valve opened, and just barely coming back across as the valve was almost at full lift. Beyond this length, the rocker arm began to have too much movement at the far end of its travel; I could maybe use 8.2" pushrods and no longer.

Fortunately 8.144" happens to be the stock length on a F*rd 351W, which helps out on both the cost and availability of the pushrods (should I ever bend or break one). For some reason my Chevies keep getting contaminated with F*rd parts... first the 9" rear in my Camaro and now F*rd pushrods in my Corvette.