Thread: GT 40 P heads Biggest cam with stock rockers

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In the performance world I am unacknowledged . I bought these heads and at another great performance site they are to the book . Lets skip a page and say I want to run the stock rockers and replace the cam what would be the biggest lift I could go . I have one that is .456 lift with a 110" LS but I do want the Cammy sound but nothing that would hurt the heads or cam it's self . I know I need to change springs and then there is bind . So some straight up advice would be greatly appreciated . This has to be on a budget to many things in the fire right now .

Sorry Bobby, your question concerning max lift is unanswerable. There are just too many variables and each motor has to be measured on its own.

Stock rockers will not be the limiting factor. Coil bind, rocker slot interference at the stud, retainer to seal interference and valve to piston interference will be the limiting factors.

Get a set of checking springs and install them on one cylinder position with the retainers and locks you got with the heads. Like you are going to shoot an arrow from a bow, push the spring down out of the way and position the valve stem between your fore finger and middle finger and place your thumb on the valve stem tip. Pull and twist while pushing the head away from you with the other hand. What we are trying to do here is to seat the retainer and locks onto the valve stem so that you get good results from this operation. Bolt the head on with an old head gasket using just a couple of bolts snugged down. Roll the crank over so that you are about 30 degrees before TDC on that cylinder. With a dial indicator on the intake valve retainer, push the valve down until it touches the piston and read the travel on the indicator. Write it down. Now roll the crank clockwise until you are at 20 degrees before TDC and do it again and write it down, then again at 10 degrees and write it down, then at TDC and write it down, then at 10 degrees after TDC and write it down, then at 20 degrees after TDC and write it down, then at 30 degrees after TDC and write it down. Deduct 0.080" from the smallest figure you wrote down. That will be the max lift for the intake valve.

Now, move your dial indicator to the exhaust valve retainer and repeat the operation. Deduct 0.100" from the smallest figure you wrote down. That will be the max lift for the exhaust valve.

When mounting the dial indicator, bend down to the level of the indicator and eyeball it in both planes, from the front of the motor and from the side of the motor. You want the indicator stem to be parallel with the valve guide bore. Also, push down only on the valve stem. Pushing on the retainer will dis-lodge it.

Tech I do understand what your saying because I do this type of work a lot just not on engines . Looks like a day with math and indicators . Though I was hoping someone has done this and has the science already worked out easier on me >

Here are just a few variables that come to mind and the reason nobody can give you a definitive answer....
Piston deck height
Piston crown configuration
Piston valve pocket placement and size
Valve size
Gasket thickness
Valve seat location relative to the deck of the head

When you first think about my explanation, you might think that you should just check it with the piston at TDC. The thing is, at different piston locations in the bore, the valve might drop down into a pocket, whereas it missed the pocket in the position before or after. That's the reason I said to check at different piston locations in the bore.

Valve to piston clearance will normally be tightest on the exhaust valve during the overlap period when the piston is chasing the exhaust valve back onto its seat.

Ok some info I have stock pistons , 1979 block . valves on the P heads are 1.84 and 1.60 pistons are super cut for low compression .

Are going to convert it to a roller motor? I did a bunch of mods a few years back with a roller 302 block.. it had stock crank and rods, 306 probe pistons that were .030 over with 8.5:1 before the blower. The p heads were worked and had 1.6 ford motorsport roller rockers. I went with a custom ground cam that was equivelent to the lift of an "X" cam. ( about .540 lift) my pushrods were not stock length and cannot remember the little specifics of it.. I built an 83 nonroller motor and had nothing but problems with that one.. it wasn't worth the money.. and to tell you the truth... either was the GT40P heads.. the explorer heads were okay back in their day.. they are a far cry from the GT40 heads. today I would have went with a set of RPS cast irons for the same price. Don't take this as gospel.. I had a lot of heart ache with this combo.. I wanted to build a race motor for the street.. I broke more things than I care to write about.. I paid more cash than I would admit.. and have made friends with the local flatbed driver, all for the sake of the Need for Speed... The motor finally crapped out when I lost a valve keeper down into the motor and you can figure out what happened there..
The motor ran 12.90 in the 1/4 and just didn't have any more pull on the top end due to the heads.. Now I have a 331 with reinforced everything!! 210 cc flowing heads with a custom .600 lift cam, 8 lb supercharger and big injectors.. It has almost 600 horses and I am afraid to run the damn thing because everything is just so expensive if it breaks.. does it make sense? I have about $8K in this motor and now the car sits in the garage most of the time... Now my new project is an 86 Buick Regal with a BBC and an 8-71 supercharger.. Once that is done right.. I am guessing it will also sit in the garage.. go figure????

Bobby,
With the stock pistons you will be limited to not a lot over .500 lift even with the dished pistons and OEM fly cut - that is unless you do some more fly cutting of the valve reliefs. My last 351 I stayed right at .512 and it was fairly close even with the fly cut hypereutectic pistons. Of course if you choose to replace those OEM pistons with something else, then the sky CAN be the limit. I do remember that you are trying to build on a reasonable budget, so do understand that your options become somewhat limited as to where you want to go.

As far as stock rockers - an out of the box Performer Plus will sound nice, run decently http://www.edelbrock.com/automotive_..._ford_sb.shtml Of course, some roller rockers........

AS far as HP3445's comments, he is correct if were building a race engine, not a streeter with a nice idle.