71 340 Compression Test
Can anyone recommend a maximum CR to run on say 91 to 93 octane pump gas? Could 10:1 work.Thanks
Everyone who has commented. Good information.
It has been said many times that 9.5 is the highest Scr to run with iron heads, at sealevel. The problem with that is it doesn't leave much room for a performance cam, before the Dcr is a runaway train to being a slug off the line. The usual work-around is a hi-stall TC and higher, numerically, gears; like 3.91s or better. So again it comes down to your intended useage.
A racecar is gonna run race gears (like 4.30s), to trap at an optimum rpm. It will also run whatever TC it takes to blast off the line. And a race car has better fuel available to it. So, in this case compression can be increased to optimize the cam, which are optimized to the head flow.
A DD will run a much lower gear like 3.23s, and a very low stall TC like 1800(or less). In this case a cam will be chosen to suit the DD needs, like the teener 240*. So now the Static compression(Scr) can be much lower, so the Dcr doesn't get into detonation.
A streeter is an unusual thing in that every combo has to be optimized for the user. That means it has to do reasonably well under a multitude of different uses.Like straight line, and hiway, and bombing around, and maybe hauling a trailer now and then. So....... sometimes the combo has to be biased one way or another, in other words sacrifices have to be made.
So you have to sit down and make some hard decisions, as to what you are willing to give up.
And one of the first ones will be,where do you want the power to be? If you want big power, you have to have big airflow.That leads to big heads, big revs,big cubes, or supercharging.Those are your choices.
If you are focused on using the 340, then big cubes are out. If your budget is limited then supercharging is out.,and probably a stroker as well. So that leaves just big heads and big revs. Big heads can be worth one or two, or possibly three cam sizes. That makes headflow very desireable on a streeter. Cuz running a really big cam on the street, with all the attendant junk that goes with it, a super annoying experience. Sure for a weekend warrior a 300plus cam with 4.30s and a 3500TC will be big fun. For a while.But if you can make the same power on a 285* cam and now use 3.73s and a 2800TC, that is much more fun. Hours and hours atta time,fun and day after day,fun.
The point is; choosing the Scr is not the first thing on the to-do list. Choosing how much power you really need/want, and where in the rpm band to put it, is. And then choosing the support works, and then Scr to make it all work.
For a street-340,your power choice is from about .8 to 1.2 horsepower per cubic inch. Less than .8 will likely not satisfy and more than 1.2 will get expensive in a hurry. The stock 1971 340s at about 300 hp, were .88 hp/cid, and 1.2 is 408hp.
FWIW
Having enough power to break 295s loose at 35mph(on kickdown), and a small enough gear to cruise the hiway for a couple of hours atta time, is IMO, a delightful combo. And it doesn't take 408hp,or 4.30s, to break 'em loose. And 3.55s are a nice all-round street gear. They will put you right at 2200 or so rpm at 35mph in second gear. Very comfortable. At kickdown, the tach will jump to 3800(plus slip), and the car will explode. And 3.55s will cruise at 65mph =2870 with 27s.
If you build the engine to work with the combo (your combo;work it out with whatever parameters you choose), it is hard to go wrong.
I run a stick car with a GV overdrive. I have run just about every rear gear you can imagine, and with a clutch, I have an adjustable "stall". But my favorite has always been the 3.55s. They do no-one-thing GREAT!,but they do every-thing just right.
If you put the power in the heads, you can run a streetable cam and have loads of fun, and buy your gas the same place you buy your lawnmower gas. How sweet is that?!
