250 hop up

[maverickhead]

Thinking of building a 250/5spd for my '70 maverick to be a daily driver.The 250 is a '74.I have a '70 and 71 200,should I use one of those heads?Which head is best for performance and economy?Was thinking of running a holley 2bbl,3.55 gears,T5,What kind of header should I use,dual outlet or single?Want to keep it quiet,may be a 2.5" single?Like it to run at least a 10.50 or better in the 1/8 mile,get 22-25 mpg.Any suggestions?Thanks

 

[CZLN6]

Howdy Maverickhead:

And welcome to the Forum.

Q- "Which head is best for performance and economy?"
A- While none of these heads are "the best", they are better then some earlier. The "Best" factory head for performance is likely the '78 and later with 1.75" intake valves (the heads you have will have 1.69" intakes), hardened seats and a slightly larger intake tract volume. While 1.75" intake valves and hardened seats can be added to any of the heads you have it is hard to make up for the extra volume needed at higher rpms.

The "Best" head for economy is likely the late '60s head from a 170 with the smaller kidney shaped combustion chamber. It will have a higher quench to bore ratio. This would only be beneficial if you deal with the excessive deck clearance on the 250. This head will also have a 52 cc chamber so CR would be way high and have to be dealt with too. This head would not breath much past 4,000 rpms due to smaller carb, intake and shrouding.

Q- "What kind of header should I use"
A- I'd recommend a single outlet for less weight an fewer fitting problems. I'd also recommend a 2 1/4" system. A dual system and/or larger diameter pipes may have an advantage ar higher rpms, but only slight one.

Q- "Any suggestions?"
A- The hardest hurdle with building a performance 250 is dealing with the huge deck clearance. The goal is to achieve a total deck clearance of .040" or slightly less. 250s were designed to have the piston down in the cylinder about .125", mine was down .150". The purpose for this was to reduce CR without having to redesign the 200/250 head. The downside of this move was the loss of quench. A proper quench of .040" improves combustion by adding turbulence as the piston and head deck come together and squeeze the fresh mixture into the chamber and piston dish to be burned.

To deal with this issue you have several options. Do a search, cuz I recall that our web host, "El presidente", Jack posted options recently.

I made the mistake of building my head before I got to the deck clearance issue and fell into a too high CR trap. I ended up decking the block .070", using small dish pistons and settling for a better, but less than ideal deck clearance.

Jack's solutions post listed his path with his 250 Mustang of using 255 V8 pistons which are .085" taller than 200/250 OEM pistons and decking the block to goal, and-

Using a longer rod from a HSC Tempo 4 cylinder for an improvement in rod to stroke ratio.

Again, the pit fall is if you build your head first, specifically milling. If you do you will end up with a Compression Ratio that is too high. The CR can be brought back to an acceptible range by unshroading the intake valve in the combustion chamber. This will also increase flow potential. A CR goal of 9 to 9.7:1 is reasonable depending on your cam and other details.

I's suggest that you read the sticky at the top of this forum for the pitfalls of replacement head gaskets.

Other than that, I like the sound of your build. 250 projects have been quite popular lately on this forum. Be sure to read and study the threads to glean the best info for your build.

Enjoy the journey.

Adios, David

 

[CZLN6]

Howdy back:

Oops! The search should be for "MustangSix", in the author box. The thread is dated 12/19. It give a good scenerio of Jack's 250 build and the HSC rod build.

I have not yet begun to look for a source for the 2.5 HSC rods, but it is on my list, which is getting longer.

Also note that when doing a mix-&-match build like this it is important to add an engine balancing to the build.

And. It will be essential to use hardened washer under the head bolts on assembly, or head studs, better yet. The problem is the head bolts can bottom themselves out in the bolt holes and never allow the head to be properly torqued and sealed.

Adios, David

 

[maverickhead]

Thanks for the info.I think I'll skip on the expensive bottom end,maybe just mill the block & head to get some comp..I've had v8s all along,will this run at least as well as a mid '70's 302?If so I'll probably go along just to be different.

 

[CZLN6]

Howdy Back Maverickhead:

If you're using what you've got then use the '74 250 head. It will have 1.69" intake valves that can be replace with 1.75" when rebuilding. Be sure to specify a three angle valve job and back cut the intakes.

CAUTION- do not mill the head until you've decked to top of the block and assessed you CR. You will likely need to make a truing cut on the head to assure flatness. Measure carefully, before milling the head to attain your goal CR. AND-

Be sure to use flat washers under the head bolts on reassembly.

I'd recommend the Holley 500 2V and mount it directly to the log by modifying it rather than the adaptor route.

The other down side with mid 70s 250s is that FoMoCo retarded cam timing to gain low end and improved emissions. When you get around to replacing the cam timing gears and chain, specify parts for a '69 to '71 250 to avoid the problem

This should be a great daily driver with more than a little "Fun" factor thrown in.

Adios, David