For the best mileage a used offy dual port intake is one good option along with the annular booster carb i mentioned especially with those huge ports in those heads . . I would also run a 192 thermostat because you can run leaner jetting with a warmer engine and therefore get better mileage . the engine will also last longer due to reduced wear . . If you want your power in the lower rpm range then yes a smaller cam than the 268 would be better . . The 256 would be the smallest I would go and will probably give better mileage than the 262 but the 262 would be a better match for those heads due to their huge ports imo . . FYI . . If I remember right the 73 4v heads have 2v valves .
The intake is one of the things to figure out...once I've got the piston/head combination figured out. I've wondered about the dual port Offenhauser and if it lives up to it's claims.
I've heard different things about the 73 4v valve sizes. Including some reports that they've never found the small valve 4v heads in a Mustang or that it was a late year thing. I'm wondering if that might have something to do with the two different hp ratings, 248 and 266 for the 73 4v. I'll measure the valves this week, but since the heads didn't come from a Mustang, not sure how much that will me. The date code on both heads I think is 2H8.
Remember, the 351M/400's turned into pinging monsters on low octane due to excessive deck clearance - in no small part thanks to those dished pistons.
You've got a good idea to use closed chamber heads though; the quench helps fight pinging issues.
Just don't duplicate the engineering issues of the 351M/400, or you'll wind up running 93 just to combat the pinging. Funny thing: My stock 400 is so prone to pinging (with or without EGR, with or without timing recurve, etc.) and has such a low compression ratio that I've come to believe that sticking a turbocharger on it may be the second-easiest solution (!). First solution would be to get rid of it.
-Kurt
The books say 78 cc for the 351m/400 and 75.4 for the 73 4v/CJ. That is one of the questions that prompted this thread, is the pinging issue more related to the open chamber or the dished pistons or just combustion chamber volume regardless of head/piston. Since the complaints are typically with stock type OC heads, then I was wondering if switching to the Trick Flow 72cc head with dished pistons (factory or aftermarket) to bring compression back down would counter the pinging. The heads being aluminum would also help reduce heat and therefore pinging.
Build what you want. But if you run CC heads with dished pistons, you are really wasting money. Increased compression results in increased efficiency.
Quench height is the free space between the piston at the top of its stroke and the head and has to be held to a fairly tight range to allow a quench head to . . . well quench!
So here is a different formula-build a 302 Cleveland
You can use quench 2V (Aussie) heads and a small 600 CFM Vacuum Secondary Carb on an appropriate dual plane intake. (If you run 4V heads, you'll probably want to try out some port fillers to improve low speed airflow) Run it in a car with a 3.25 or 3.00 rear end and you'll be able to milk decent mileage out of it and get some fairly impressive performance. A custom Cam builder can help you acheive your goals. I would not be afraid of 10:1 compression ratio-I get by regularly with 93 octane in a 11.5-1 393 Cleveland and while it requires a good tuning, it isn't rocket science.
Better mileage will never come from reduced compression ratios. A smaller displacement engine can be more efficient however.
Personally for your goals, a 5.0 EFI transplant from a late fox body would probably work and be much cheaper.
Which leads to Jeff's post about the Aussie heads. The Trick Flow heads have the 2v ports like the Aussie heads (and raised ports at that) to keep up the intake velocity. Since the Trick Flow heads are offered with 62 and 72 cc options, I went looking for pics of the 72 cc head to see exactly what the combustion chambers look like, but have only found various pics that appear to be the same type head with no clear indication of which one it is (although I suspect all the pics I see show the 62cc head for promotional materials). I still do not know how different the 62 and 72 cc chambers look on the TF heads. It all might be a pointless mental exercise.
I'm assuming you mean the actual 302 heads with the 59cc chamber. I contemplated the 302 crank and pistons/rods for a destroked motor (similar to Paul's 330 suggestion). Not sure what the compression ratio is on that motor.
I was hoping for MPG thru conservative tuning (small carb/cam overdrive and the right final drive) without high compression and fussy fuel considerations. That's why all the postulating on head/piston combinations. One of the key ingredients is the small intake runner/port velocity. My desire to figure out a way to run OEM 4v heads without falling flat was my own monkey wrench. The TF heads looked like a way around that, not OEM, but a certain amount of cool factor with advantages of their own.
The 5.0 EFI (Ford not aftermarket) was an option I'd considered originally for a 65-66 coupe idea I had with the same high MPG freeway commuter concept. MUCH lighter car so it would have been even more effective. I think that car could have hit 25-28mpg on the highway at 60mph-ish with 205 tires. Not much less than my Protégé gets now.
I think my plan is changing slightly. Since I am diving into a new learning curve with the Cleveland and I've acquired the 4v heads for too good a price, I am seriously considering building the components I have into a more or less stock OC 4v motor with either dished or flattop pistons. This gets my feet wet building a motor myself with some idea of what it should be and as I learn what I'm doing I can plan for the other motor. This will keep costs down for now and get a livable motor in the car sooner. Trying to hotrod while learning is usually a good way to burn money and be wrong, so I will spend a bunch less and learn more for now.
