Printable View
well, I started a post the other day and the whole post seems to have disappeared so I'll ask it again.
I have a big block 402 going in a pickup in front of a 5 speed with a 3.89 rear. The compression is slightly less than 10:1, stock iron heads.
This will be for the street only, I am looking for a cam to replace the 255 @ .050 duration cam that somebody stuck in there.
Can anyone offer suggestions on a good grind?
Can I use old pushrods with new lifters?
How about re-use the bearings?
I don't know how many miles the motor has on it, I bought it from somebody, it is not new though. I don't want to change the pistons. It won't be a daily driver, a weekend driver though. I'm looking for power, not economy.
I don't want to change the bearings if I don't have to, I just didn't know if you could use old bearings on a new cam.
I think the valve springs are capable of some pretty descent lift, as the cam that is in it is pretty aggressive. I recently moved though and can't find the cam card so I don't know for sure what the lift is.
Call Comp Cams Help Line 1-800 999-0853. Tell them what you have and what you want.MY guess would be either , A --Magnum part#CB270H-10 B--XTREME ENERGY part#XE262H10, or a little milder steet cam would be C-- High ENERGY part # CB268H10 . Make sure you use new lifters.Roll Your pushrods across a flat piece of glass or a very smooth table top to make sure they are not bent. If they wobble replace any bent ones,also check the ends to look for any excessive wear.If the valve springs would handle the bigger cam you have ,they should more than handle these cams but ask anyway because the springs might too strong. It is best if yoy order the complete cam kit.
thanks alot
I'll give them a call
archives?
I checked the "shop talk" section, which was where I initially posted it and nothing was there. I looked well past the date when I posted it, couldn't find it.
no matter.
when I got the motor, the guy told me about the pop ups, which I later saw for myself when I had the heads off. He didn't know exactly what the pop up was, but he said it was small, around 10 cc or so.
When I used this info. to calculate comression I came up with less than 10 to 1.
I think I even used 15cc dome to account for any error in what he told me.
I'll post a picture of the engine when I had the heads off so you guys can see the dome.
Maybe somebody can estimate the dome volume, as I cannot because I have no prior experience to draw from.
I'll post that pic first thing tomorrow morning.
Tcodi, dont forget you need the combustion chamber volume also to somewhat accurately check the c/r of your engine. Also, the deck height, and thickness and bore diameter of the head gasket...a few cc's can make quite a difference. Also, and I think this was said in an earlier reply, just because you're going to go with a smaller cam, the springs on there now may still not work...they might be ok for the lift, but may have too much pressure, and that will kill your new cam within a few minutes on the break-in. Check the things mentioned here, and go from there....its more work, but will be worth it in the long run. Yes, you can use the cam bearings if they're not scored deeply..check them when you pull out your cam (carefully) Also, use a new timing chain and gears if you have any doubt at all of the condition of the ones in there. Keep in touch here, we'll help you. Oh, nothing against Comp Cams, but keep in mind, they're mostly reading from a computer screen, not from experience, so you might not get the best recommendation. I would check with a smaller cam company for the info you need, Isky for example, Howard's cams ( I use them exclusively), there are others. You get much more personal service from the smaller ones.
ok, I checked my book with the casting number and my heads have 105cc chambers. I calculated c.r. using:
deck height: 0 (to be safe)
gasket bore: same as cyl. bore (to be safe)
dome: 15cc (I think it's less, but check the pic)
gasket thick: .04
bore: 4.128
stroke: 3.76
I get 9.34:1, and I think I was very conservative with items that will end up decreasing the comp. a tiny bit in reality.
Here is a pic of the pistons when I had the heads off.
I have a set of pistons very similar to those, and the dome volume is more like 28cc...but again, it's hard to be accurate looking at a picture. It's always hard to figure what you're trying to do if you're not pulling a head to measure a chamber volume. I have seen the published cc's of stock heads to be way off, 10cc's or more in some cases. This will change your C/R quite a bit. But I know what you're trying to do, and I guess being conservative is a better way to do it. Does your friend have the spec sheet or number on those pistons?? That would tell you the exact volume. I have a fair knowledge of big blocks, not an expert by any means, but based on what I have seen in the past, if your chamber volume is about 105cc, I'll bet your c/r is all of 9.3/1, probably closer to 10/1....The deck height will also change that quite a bit, as will the gasket bore...they both add lots of cc's. But it sounds like a great combo so far, have fun with it. Be careful with your cam choice, remember, smaller duration = low end torque, at the expense of high end horsepower....but unless your're racing it, you want your power at lower RPM's...if you want a really lumpy idle, look for a cam ground on a 108 LSA vs. 110 or 112....but if you do that, you'll lose a little vacuum...it's all a trade off, so shop carefully. Talk to a cam grinder if you can, like I suggested above. John