Which way to go?

[63comet]

Heh everyone, I will be rebuilding a 250 this winter to drop in the old comet, my questions are, which pistons are better? The 255 V-8 or the tempo flattop, if I use the flattops, how much can I deck the block and what CR should I be looking for? If I use the 255's, what should I deck the block to, I know that the 255's are .085" higher pin height. Which cam would be a good one for all around driving? I am already working on the head for it, it is a D7XX, and it will be modified pretty good, opening the ports up, eliminating guide bosses, port matching and unshrouding to the head gasket as well as polishing exhaust and chamber,(as best I can) and glass beading the intake side. It will also be fitted with a 2V plate to the log, in place of the single. : : :

 

[MustangSix]

If you use the Tempo flattops the piston will still be down in the hole by as much as .140" just like the stockers. You can't deck the block enough to get a good quench distance.

That's the reason for using the 255 pistons. You can get the piston higher in the bore, deck the block just .045 or so (please measure your assembly) to zero deck and get decent quench. CR will be higher without having to mill the heck out of the head and in fact you will probably be able to relieve the chambers considerably around the valves without losing significant compression.

The quench will help prevent detonation even at higher CR. High CR without the quench will only give you an engine that likes to ping....

 

[Hot 6t Falcon]

Jack gave you good advice. I too think the V/8 pistons are the better choice, BUT DON'T MILL THE HEAD any more than to get a flat surface.

Go to my web site by clicking on the link in my signature. Then click on the compression calculator. Change the stroke to 3.91. The combustion chamber to 62cc. The head gasket thickness to .045. The deck clearance to .035, and clink on "Calculate" It will show a compression of 9.9 to 1.

Now just change the deck height to .010 and click "re-calculate" to see what happens to the compression. Too high?

Try adding 4cc to the combustion chamber to allow for unshrouding, and click "recalculate"

Fun to play with, isn't it?

This "playing" will help you see how important it is to do as Jack says and measure everything. And I'll add to Jack's advice - don't just measure it, WRITE IT DOWN.

Good Luck

 

[63comet]

Thanks for the help guys, I ordered the 255 pistons this afternoon. Dennis, that's pretty amazing how little changes make that much difference in the CR. I also changed the dish reliefs from 7cc to1.5cc, which it says they have and that raised it up .6. Guess I'll work a little more on the unshrouding of the valves. By the way, I had planned on going with the 1.45 intakes from the early 144 as the exhaust valves. What are your opinions on that? I also have teflon seals and Clifford double valve springs to go with adjustible rockers. Any ideas on which cam to use? And yes, remember measure, measure, measure, and record.

 

[Hot 6t Falcon]

I notice in your signature that you have a C/5 auto but aren't happy with it. Why?

The cam should not be more than the automatic can stand. Your first choice of 264 is about right. Even though the 250 will give you lots more seat of the pants torque, don't go crazy with the cam. Don't get sucked into the DPE syndrome - That's the Dolly Parton Effect Syndrome - everybody always thinks bigger is better when really balanced is better.

Both my brother, David, and I have the 144 intake valves being used as exhaust valves. Neither of us has had a problem with them. They help solve a malady of this engine - poor exhaust flow.

Are you boring the block for the new pistons? If so, add that to the to the bore dimension just to see one more change in the compression calculator 3.68 plus .030 = 3.71

Good Luck

 

[63comet]

Dennis, thanks again, it's not that I'm not satisfied with the automatic, so much as it the fact that I am having difficulty getting it to perform as it should. From the beginning, I ordered a master rebuild kit and also a shift kit for the tranny this spring, I rebuilt it according to specs and several books I purchased including Ford's shop manual for C5's. When I installed it and tried it, I had no reverse, pulled it apart and checked everything, couldn't find anything wrong. re-installed with new seals and plates, still no reverse, checked pressures at the port and everything checked out OK, pulled it again and tore down, everything looked OK, put it back together and sent tranny down to transmission shop, 100 miles away, very respected shop, lot's of recommendations. He tore it apart and said everything looked OK, except the torque converter had a problem, he cut the torque converter in half to take the weights out for overdrive, because he said they were a problem, but someone had beat him to it, they were already gone. So I realistically had a C4 with C5 guts. He put it on the dyno and said it worked fine. Installed it and lo and behold I had reverse, but, when I first take off with it in drive, it will not shift into second until I reach about 35-40 mph, this is not floored but just slowly accelerating, then it will not shift into third until I am going about 55-60 mph. After it warms up, it shifts into second at 10-15 mph whether I'm going easy or floored, and shifts into third at 35-40 floored. I have adjusted the modulator several times and it still does the same except when it gets warm, I can increase or decrease the shift rpms with the adjustments. Also, if I take off in drive and let off on the gas before it shifts to second, it will go into neutral and I can rev it up, if I floor it it will go to about 3500 rpms before it comes into low again. So, I have a C4 also to rebuild this winter. I'm outta here