An engine question reguarding cam lift, head clearance, and rockers

[Stomis]

Ok I wont drag this out I've got the 442 I just bought and since the truck is coming apart I want to throw the 355 in it.

The truck has a mild "rv" cam in it. I have to find the card, its in the paperwork I have back at my parents. Now I just bought a summit carb and an edelbrock intake from a guy on a gbody forum for a $100 in preparation of getting the 355 in the olds.

I really dont want to tear into the 355 because I know the thing runs damn strong and is low miles. Therefor I really dont want to pull the heads to swap the cam and change the timing gears etc.

What I thought was a good idea was to get some 1.6 or 1.7 roller rockers to bump the duration and lift of the cam that already in the motor. Basically since I dont know if the heads on the 355 are stock, worked, clearanced for springs, and all the other jazz that a high lift cam requires I want to know if pulling more lift/dur out of a cam using bigger rockers has the same affect as just putting a larger cam in the motor.

Man I dragged it out anyway lol.

 

[Wyomingboy]

Hey, You can always set your #1 cylinder at TDC on the compression, get a air pressure kit and fill your cylinder with air feed via a compressor. then you can take the valve spring retainer off and carefully, take off your springs and install a "check" spring( the lightweight ones) and put your retainer back on. Then roll the engine through a cycle by hand and check your clearance when the valve is fully lifted on the cam of both intake & exhaust. You didn't say, but most stock or near stock heads are probably good for .6" on the valves. You'll probably run into problems with the guide seals, or whatever that particular head has long before you hit the pistons. Measure how much you have to go before you touch the valve guide seals and then you'll know how much you have to work with. You can calculate your new lift by using the ratio of 1.6/1.5 times your present lift.
Just my $.02.
Good luck,

 

[shovelbill80]

i'm new here, but i'm pretty good with Harley engines, so in my opinion the bump in duration would be negligible at best. assuming they're stock heads you'll need to do the math and see what total valve lift you'll end up with and you need to know when the stock head's springs and guides are maxed out at too. if the heads flow won't support the extra lift it wont matter what you put in there aside from different cam with different timing events. on a good flowing head short duration and high lift usually equals bigger torque on the bottom, least for my other motors. we did this exact same experiment on a 74" Harley motor leaving all else stock but the carb. it made quite a difference in my friend's ass dyno. just took some math. i don't see why it wouldn't work here with your new intake.....also just my $.02 and someone will correct me if i'm wrong..........Bill

 

[shovelbill80]

hell Wyoming i was typing that whilst you posted.........i like the way you think

 

[4X4HIGH]

Chaning to a higher ratio rocker arm WILL NOT change duration of the cam but it will add a very small amount of lift. The larger rocker ratio will add some more dwell time to the valves though at TDC but like I said, will not change duration (duration is built into the cam).

 

[shovelbill80]

maybe i'll just stick to harley engines...swapping from hydro to solid lifters will minutely change the duration.....i'm here to learn about Chevy engines and what works for them......i'll be asking plenty of questions soon enough.

 

[76zimmer]

anytime you make a valvetrain change, you should recheck valve to piston clearance, and also rocker arm geometry between the valve tip and the rocker arm. Roller tip rockers are more precise for geometry than stock type rockers. Checking the clearance as suggested above will work fine, don't worry about the duration as Scott says, but check that clearance every 10 degrees or so of crankshaft rotation around TDC when the valves are moving. You should have a minimum of .080 on the intake and .100 on the exhaust with Steel (stock type) rods.

 

[Stomis]

Reading an article on one of the chevy mags they claimed that the response times of the valves opening quicker/slower depending on the rocker ratio will minutely change the duration.

That being said I think my easiest route would b to get my cam card, do the 1.6 and 1.7 math for the lift and research the casting numbers on my heads to see if the lift is within the stock spec.

Thanks for the help guys.

 

[folkenheath]

Reading an article on one of the chevy mags they claimed that the response times of the valves opening quicker/slower depending on the rocker ratio will minutely change the duration.

That is correct, when the valve begins to open and close will happen at the same spot, so the advertised duration (@ .006" lift usually) will remain pretty much the same, but the duration at .050" will change because you are opening the valve further in the same amount of time, so it will get to .050" lift quicker. With that said, these guys are correct, the duration change will be pretty insignificant.

On a side note, if you want to check piston to valve clearance without removing the head, you can put a dial indicator on a valve retainer, and then adjust the rocker arm too tight, this "pre-opens" the valve, keep going until the valve is open .100" (some companies say .090, , some .100, and some companies say .100 intake, .120 exhaust, I would use the safer side). If it's a hydraulic lifter, keep adjusting it until it remains at .100" (the lifter is bled all the way down). Once you do this to both valves for one cylinder, then slowly turn the engine over at least two revolutions. If you feel resistance increase, STOP. But if you can do that without any interference, you should be fine. Return the lash to normal and then assemble the rest of the cylinders.

If you have larger than stock valves, I would not trust this method, only the clay method.