I was just wondering if anybody knows what ratio rockers come in the x303 aluminam heads in the GT-40 crate engine. Also how much would I benifit from going to higher ones.
If you ran 1.7's you would add .03 lift to the camshafts specs right now. For example if you had a cam that had .480 lift (b cam) it would be .510 with 1.7s
The spec sheet for this motor (I've got one in my '85) states "standard rocker arms" are utilized. I believe the standard ratio is 1.6. I don't know if there would be clearance issues if you move to the 1.7 rockers. I'm curious about the potential improvement myself, so if you go through with the upgrade, would you please post your results? Thanks.
Most of the 302-based Ford Racing crate engines come with regular old stamped steel, stock, 1.6 ratio rockers. If you're worried about clearance issues and want to run a higher ratio, go with the 1.7 Cobra (-A50)rockers they have the best valve cover clearance and usually sell pretty cheap by comparison to other roller rockers (around $190). If you're worried about piston to valve clearance, an E- or B- cam and 1.7 ratio rockers will leave you plenty of p-to-v clearance.
To calculate the change in peak lift, you simply use the ratio of the rockers. 1.7/1.6=1.0625. Multiply 1.0625 times the peak lift with the 1.6 rockers and you have your new peak lift. The .03" mentioned above is an approximation that works with cams that have 1.6 valve lift in the .48-.500 range. The B cam (.480" peak) goes to .510". The E cam (.498" peak) goes to .529".
Ford Racing's tech line suggested to me that with either the B or E cam, their Y or X heads, 1.7 rockers and stock pistons that p to v should be measured. With Y heads, stock pistons, E cam and 1.7's I had .110" intake clearance (min - .080") and .210" exhaust clearance (min - .100"). Plenty in my case - but each engine/component is dimensionally slightly different. If you already have the motor running without problems, and you want to make the swap, you shouldn't have any clearance issues. The rockers only reduce p to v about .005" with the E cam.
So am I right to presume that my B cam in X heads would yield an even greater p to v clearance than your E cam in Y heads (presuming all manufacturing tolerances are in spec.)? That's how it reads to me.
You are incorrect in your presumption - p to v clearance has nothing to do with peak lift. Duration/overlap/LSA are the issue. The earlier the cam opens the intake valve, and/or the later the cam closes the exhaust valve - with all else equal, the less p to v clearance. At peak lift, the pistons are WAY down the cylinder - never any p to v issues at peak lift. Clearance is tight when the intake is just opening, exhaust is just closing (overlap period) and the piston is passing through TDC at the end of the exhaust stroke and the beginning of the intake stroke. So what you'd need to do is look at the valve timing events of the B compared with the E and see how it compares. If it's opening the intake earlier than the E, then the clearance will be reduced there. If it's closing the intake later than the E, then the clearance will be reduced there. There's only one way to determine whether your engine has enough clearance - and that's to measure it; there's simply no way around it. There are so many variables that even two engines with the same components can have significantly different clearances due to tolerance stacking.
More peak lift = less p to v clearance is one of the most common misconceptions there is out there. It just goes to show how little a lot of folks really know about how cam/valve timing relates to the movement of the pistons. Even this month's Hot Rod magazine and MM&FF had articles or ads that referenced not having to worry about p to v clearance with a cam up to such and such lift - what a blunder!