There is no limit to the amount of performance or horsepower an I6 can lay down!. If an OHV NASCAR V8 can lay almost 2 hp per cube, then an I6 can do 1.3 on the street if you follow wining combinations. It all comes down to cost! If you want to shame some Ricers and V8's you gotta do one of two things:-
1. Use nitrous, a turbo or supercharger
2. If you don't want these, you're gonna have to drag 40 years of ancient intake design kicking and screaming into the 21st century!
I follow the Keep It Simple Stupid ideal, but in order to keep it simple, you gotta have all the facts.There is a skilled and unskilled section to getting the best out of your I6.
Best option (going right to the jugular, and assuming your skilled )The best single mod you can do to any I6 is to remove the integral intake manifold that restricts the cylinder heads gas flow. Most really good intakes swallow just over 10% of total air flow from the head itself. On the I6, it's more like 35%. Most people Stateside are on to this. Clifford and others have tri-power set ups for triple carbs. This is a good start. Even the twin 2 barrel set ups you'll see on this site are a huge leap in the right direction.
Along the "let's remove the integral intake" lines. Before 1970, and the advent of the 2V 170 HP 250 engine, Australians used this approach extensively in speedway situations.They would grind off the intake and weld a plate at the gasket face, in line with the exhast manifold face. They made an independent runner manifold to fit three twin choke sidedraft carbs or throttle bodies on. (10 years ago, Redline, Cain, and others in Australia had patterns avalible to suit this modification). Then they spent mega hours making the head flow well.
Simple, effective, no 2V head or Alloy head.
If removing the intake isn't your bag, the other hot tip is to use mutiple downdraft carbs. My thinking is that with a 200, which is lots shallower in the block than a 250, the best approach is to grab two 3 barrel 46 IDF3 carbs off a 911T Porche and slap them on a modified, gasflowed integral intake. There should be just enough space to fit them under the hood. The other option is to get three Weber 44 to 48 IDA's or IDF's mounted. These are 2-barrel down draft carbs which work real well.
Non-skilled
The economic solution for US integral manifold items is to get a 2V Argentine or Australian head to improve air flow, and then go stark raving mad on carbs, cam and exhast. There is 1.3 horsepower per cube for this, and still maintaining streetability. To do this, you need an after market intake manifold to suit three Italian Weber DCOE 45 OR 48mm carbs, or alternatively, DHLA Dell'Orto 45/48 carbs. Australians use these on worked sixes ranging from little 179 cubers to 277 cube Valaint Hemi Sixes. Motech or Halltech throttle bodies are now able to exceed the performance of even these carbs.
Other Things
The cam and cylinder head is the start, and finish, to performance. The seven bearing post 1965 I6 block, cranks, and rods are quite good, although the cast rods and rod bolts are pretty low-rent for high RPM. Oil windage/spray from the 250 crank limits its maximum RPM to 5400 rpm, and it's cast so it won't like 7000 rpm for long even if you look after all the security items for high rpm use. Argentine/Australian 188 and 221 I6's have steel cranks, and these can cop an absolute hammering without snapping. The little 200's are bullet proof.
A 200 I6 with triple carbs or throttle bodies and a 2V head should hit 265 hp net with a 300 degree cam with 480 thou lift and fully gas flowed head.The limit to performance is the rod:stroke ratio, similar to all I6's
250 sixes are not able to produce proportional increases without work on the cam and porting. As engines get bigger, the ratio between torque and power changes. The air flow of bigger engines needs to be more specialized to cope with this. In Australia, even basic 250 Cross flow sixes with 4 barrel carbs hit 290 horsepower on dynos with cast cranks. With 2V heads well worked and triple throttle bodies, 330 hp at 6000 rpm is possible. The piston speeds are huge on this engine for any given RPM. What was 1.3 hp per cube on a 200 will end up being less than 330 hp for a 250. The con rod ratio to crank stroke ratio is again a week point.
