How much more can I expect from a 250


Well-known member
Heh everyone, How much more oomph can I expect to see when I install the 250 vs. 200 ? The 250 will have 255 pistons, and a ported and polished loghead, that has been modified to accept a 2 barrel directly without adapter. It will also have headers and 2" dual exhaust. I'm not sure which cam I should use ? Would the 264* clifford from my 200 be the one ?(rpm range 2000 to 4500), or should I look at another manufacturer that makes power at lower rpms ? Am I correct in assuming that the 250 will not turn the rpm's of the 200 ? I will have a C4 tranny and I have 3.20 gears. I'm also assuming I will need to get a different torque converter with a higher stall speed. (what would be a good tc)? The 250 is out of 75 comet.I will try to put a picture of my ride up. It's
Howdy '63 Comet:

With the right combo of CR, Carb and cam you can expect 200 hp, possibley a little more. My 260 Comp cam is fairly conservative in a built 250. I'm using a stic tranny. With a C4 it might be about right. The Clifford 264 has about the same specs at .050".

A built 250 is an easy rev-er. Maybe not a spritely as a 200, but it will wind. How much did you deck the block in your rebuild?

I'm not aware of anyone marketing a performance torque converter for a 250 C4. Let us know what you end up with. SBF may bolt up, but I don't know how the stall ratings would compare.

Adios, David
Heh David, I haven't decked the block at all yet, I don't plan on doing anything there except maybe just true it up as well as just true up the head. I haven't measured the chambers yet but I know they were not touched until I worked on it. I also unshrouded the valves and plan on installing the 1.44" 144 intake valves for the exhaust and undercut the valves. I also am installing double springs and teflon valve seals with adjustible rockers. I know everyone is going the Aussie 2v or waiting for Argie heads, but I like Doug, would like to see what I can get out of the LOG. Thanks for the info.
The stock C4 converter is the same from 302, I was told when I bought my rebuilt converter. By the way, what's the diff in Tempo pistons? I noticed someone else wants to know too.
Duane. I believe that has been answered in the tempo piston question, but I will answer it anyway to bump this back to the top. The ford tempo HSC pistons are a flattop design with no dish or valve reliefs in the top, they are direct replacements for the 200, and because they are flattops, they raise the compression higher than the dished pistons so you don't have to mill so much off the head to achieve this. There are some who feel that the dished pistons are better for avoiding detonation problemsThis may be true as I have had this problem since I fired it up and have to burn 93 octane to avoid this. Hope that helps
Howdy Back 63Comet:

The reason I asked about decking the block is because the 250 engines typically have about a mile of deck height built in. That plus a composite type gasket and aftermarket, rebuild pistons and you'll still be way down in the bore. The advantage of using the 255 V8 pistons is that they have approximately .085" taller pin height then the stock 200/250 piston. My 250 block started with .150" deck height. What does yours have? Deck height is from the top of the piston to the deck of the block at TDC.

Ideally, in a performance engine you'd like enough deck clearance (deck clearance includes deck height and the compressed thickness of the gasket) to avoid valve to piston interference. Any more than .045" of quench and you begin to lose combustion efficiency. An aftermarket composite gasket will measure .045" to .055" thick.

If I were in your shoes, I'd use the 255 V8 piston, deck the block to as close to zero as posible, find the thinest head gasket available and open up the chambers to achieve your desired CR. Check with MarkP in the classified for a gasket.

Adios, David
Heh David, If I remember correctly, Jack said that it was not a good idea to deck the block to .000", I can't remember what the reason was. Also your brother wrote in his reply to me that the compression ratio would be too high if the deck milled too much. Here is Dennis's quote,
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?

David, even with .125 deck clearance, the CR is 8.9:1. I guess I need to wait until the block comes back and assemble everything and measure,measure, measure.
Decking to zero is preferable to get decent quench, but you'll have problems keeping the CR in check. 9.5:1 is about the max you should shoot for to use pump gas.
Howdy All:

You're right on 63Comet! I'm glad you've got a memory.

A couple of details- the 255 V8 pistons have valve relief eyebrows cast (maybe machined) into them. They may have a cubic centimeter volume of 1 1/2 to 2 ccs. When using the calculator, specify a dished piston and add what ever that volume is. Before installing the pistons champher, or round, the ledges of the eyebrows, and polish the top of the piston while you're at it.

Still, you'll need .015" of deck height, open the chambers to 62ccs and a head gasket with .055" compressed thickness to get to 9.6:1.

Zero deck height is a worthy goal, but you must maintain a reasonable CR, right Geezer?

My plan, for my next 250 block build is to deck to zero using 255 V8 pistons and mill a "D" shaped dish into the top of the pistons that mirrors the shape of the chamber in the head. That dish would be 4 to 7 ccs depending on the depth.

The CR is still quite high, but I'm at 5,000 ft elevation, so it's less of a problem.

Sorry to mislead you. I'm glad you caught the goof!

Adios, David
Zero deck height is a worthy goal, but you must maintain a reasonable CR, right Geezer?

Yep....Unless you're a "six packer performance freak" like me ;)

or if youre slightly looney tunes....whichever applies :LOL: :LOL: :LOL:


Thanks for the info David, I believe the valve reliefs for these pistons were 1.5cc which I used in figuring the CR. When I figured the CR I used flattop with reliefs and not dished. I didn't know about chamfering or rounding off the edges though. Make sense sharp angles not needed for hot spots.