440 compression raising?

[dtpolk]

I have here a standard 1971 non-HP engine that I want to improve compression on within a low/non-budget. It has the stock pistons about .160" in the hole, and the 346 heads. I guesstimate the compression at 8.5:1 or probably less. I realize that different pistons would be the best idea, but simply can't afford them.

I have a set of 915 heads for it with steel shim gaskets, which should raise the compression a tad, but I'm wondering if camshaft choice would help with this as well? What cam choice would folks here suggest? Thanks!

[DartGT66]

With the pisons .16" in the hole and assuming 84cc chambers in the 915 heads, you should have 8.0 CR with the steel shim gaskets. A small cam would give you more compression pressure, but if you want performance, there really isn't any other reasonable way than to change the pistons to get the CR up.

[cudabob496]

Just increasing CR will not buy you much. Maybe 20 hp. Probably get same increase and more with headers and free flowing exhaust, cold air induction ram air box. Also, every loss of 100 lbs is like 10 rwhp as far as performance goes. A good torque converter is worth 30+ hp to the rear wheels. Then of course 3.55 or 3.91 rear gears will get you the quickness increase you would see from more hp. Even a good tune up with a quality ignition system is worth 20hp! Also over time, due to carbon buildup, will see at least a .5 increase in CR. So all the trouble of going to new pistons may not be worth it.

[ehostler]

A proper cam will do a better job than just raising compression. The lower compression will allow you to run the low octane pump gas the the right cam added to that will give you more usable power.

If you are doing anything with the heads, make sure that you have hardened exhaust seats installed.

As cuda said, there is a lot of power to be found in exhaust, induction, torque converter, etc.

[cudabob496]

In the 60's, they made power with CR. Now, with cam technology, cams are the way to go, among other things. Make sure the cam is designed for your CR. Too much overlap and you'll make less power!

[Cudadrag]

CR is the ratio of Cylinder volume to combustion chamber volume (minus head gasket and +/- piston height and/or dome/relief) thus the only way to increase compression ratio is to increase cylinder volume (Bore/stroke) or decrease combustion chamber volume (new or head work or higher CR pistons)

What you are really asking is how to increase cylinder pressures. Compression Ratio influences cylinder pressure the most, but it certain is not the only consideration. A supercharger for example can and will also increase cylinder pressure for the desired power levels with far more flexibility. (Different pulley sizes) I believe Edelbrock has a new under the hood supercharger with a built-in intercooler that would be fun to play with. This maybe a very interesting option especially if you are a little over carbed to begin with. (And also assuming your motor is strong especially piston rings) They advertise a 140 horse increase at max boost. You would feel that increase.

In my opinion these lower CR 440 would be ideal for these small superchargers. The valves and port config is the same as the older and tighter heads (Hell, the big open chamber design may and I believe do flow better.) The problem is the poor low CR motor has nothing to flow. They also used the same carb size thus were really over carbed for the fuel and air needed. And if I'm not mistaken even the cam profiles were similar thus underutilized.

If this was my motor and car I would research this option. I believe the cost for this unit is about $1500. (It seems everything is $1500)