I'm interested in the idea of using 1.7 ratio rocker arms on a 302 (stock 1.6 ratio). I would want to avoid any piston to valve interference of course. I'm sure the camshaft grind and lifter rod length make a difference if there is an aftermarket cam used. However, for this question let's assume stock: Part Description : Camshaft Opens : -22 BTC/12 BBC Closes : 26 ABC/-8 ATC Duration : 184/189 Opens : 22 BTC/45 BBC Closes : 70 ABC/40 ATC Duration : 272/265 CamLift : .230/.238 Gross Valve Lift : .368/.380 Center Line C/L : 114/104 Rocker Arm Ratio : 1.6/1.6 Valve Lash Clearance : HYD/HYD Has anyone tried this? I was thinking it would be a different way to achieve more lift and duration using a smaller cam.
Valve lift is cam lift multiplied by rocker arm ratio. The 1.7 ratio will obviously provide more valve lift, but there is no way of predicting piston/valve interference without more information.
If we take the .238 x 1.7 = 0.4046 valve lift. I guess I need the maximum allowable lift, but trying to find out without pulling the head and testing with clay, etc.
It's all really simple. Plus...are we to assume that you are talking about a...Chevy 302 ? For all the hassle, the cost of some ill fitting rocker arms, just buy the cam that you really want..! 1.7 rocker arms on a small Chevy...na..! Mike
I've read where people have recommended anywhere from 0.480 (tappet) to a high of 0.520 (roller). Assuming the 0.480 - 0.4046 = 0.0754, is that enough clearance to avoid any problems? Ford 302 circa 1974
I agree with buying the cam that you need/want and passing on the rockers. I've seen problems with strange harmonics in the valvtrain using 1.7 rockers on standard cams. Be aware that you might need push rods and valve springs to run the 1.7 rockers.
Valve spring bind is what I was thinking about as well. This was more of a theoretical exercise than anything that I would start dumping money into. I'd rather get a crate engine on a roller block than invest the same money or more trying to massage stock stuff into waking up. Right now I'm satisfied with the dual exhaust and aftermarket intake and carb. Just put a Summit four barrel on there and I'll update how it does after I have driven it for a while.
The point of contact will be at tdc in overlap. If you knew how much clearance you had before changing rockers, you could make a calculated guess as to what you would end up with. People have been swapping ratios for decades. The correct geometry and push rod length is what's important. As an example, the 93 mustang cobra has 1.7 rockers where all the standard ho's have 1.6. I have the 1.7's on my 32. Better than 20,000 miles without issue.
If you are checking the intake is the one most likey to hit ,10 degrees or so after the valve will be closer, the piston doesn't move much in that 10 degrees less than the valve.