I've been searching the web for a site that really brings together the relationships between engine power, rear end ratio, and tire selection to help with my mild performance 72 malibu. Currently it has a 3/4 cammed, performance carbed 355 with (best guess) 280 horses. I hope to tune it to an even 300 or just over for next summer. The tranny is TH350 with a B&M street/strip kit, 2000 B&M stall convertor. Rear end is 2.73 open, tires are 26 inches. I cruise 65 mph around 2600 RPM and would like to keep it under 3000, 2800 would be great. Being in college I don't exactly have money to throw at it, but I'm always looking to improve an already great car. At this point it seems that the most effective performance choice would be posi. To keep RPMs manageable I was looking around 3.31 but I'm unsure of what my best combo would be. Any suggestions or predictions on what improvement posi could give and what would be involved in switching my 2.73 open to a 3-series posi? What do I look for in a used rear end and how easy is the swap? Finally how does tire selection fit into the whole deal? Would bigger tires give lower RPM but compromise the acceleration I'd be getting from the posi? Thanks guys,

Brent Christensen
72 malibu 350

Ahh Grasshopper,much to wern.
3/4 cam...yes much to wern...

You have lucked out and found a place that will give you very good information.

To get all the correct answers tell us as much as you can about your car.Be as accurate and truthful as you can about the spec's,parts, and your intent of the vehicle.

Lets start with the cam.
Who makes it?
What is the part number?
What are the spec's on the cam card?

What is the compression ratio?

Both of these factors are the key to performance.

For behold you will be rewarded with knowledge.

3/4 cam, I haven't heard that term since high school 20 some years ago. I never found out what the hell it was supposed to be 3/4's of a "full race cam" I guess, LOL. Go to this site and download the calculator it will help you with what you need to know young one.

http://www.virtualengine2000.com/index.htm

I will post the link to the 3/4 cam discussion from back in 2001.

Brent, This is in no way meant to poke fun at you. It is just an interesting term from back in the 60's a lot of people would be interested in reading.

Please list everything as Thrasher said an we will be more than happy to help you out on achieving your goals

Sorry about the ancient vocab. My dad has been my help with my car and I use the same words he did when he was my age. You say you need more specs to help. The engine work was partially done when I came along so I don’t know all the details, but here goes: The 350 is a 4-bolt out of a 68 Chevrolet. It was bored .030” over and stock cylinder heads were milled an unspecified amount, so combustion chamber size is probably between 72cc and 76cc, compression ratio is probably 8.5 to 9:1? The cam was also put in before my involvement. All I have is a receipt from Red Rooster Auto that says ASW ERSE11002 Cam & Lifter Kit, list $149.99. The idle on the car is still fairly smooth, bringing RPMs to 4000 really opens up which is probably more the exhaust system than anything. A hand-written memo shows that at one point a Crane cam was considered with 216/216 duration @ .050, .454/.454 lift, 110 degrees lobe separation. I’m not sure if this was decided on or not, but I’d guess that what's installed is probably similar. The TH350 was rebuilt with a B&M Shift Improver Kit. A Tork Master 2000 converter was installed. Proform HEI Distributor (50000V through 7500rpm) and 8mm wires replaced points ignition. We put on an Edelbrock Performer intake manifold and carb (600CFM manual choke), Hooker Competition Headers, 1.625” OD, dual exhaust and high-flow mufflers. I brought the column shifter to a B&M Megashifter on the floor, got an SS dash, and fitted a tach in the radial clock location. Last year I installed high lift springs from JC Whitney, which caused some bad vibration problems between 25 and 50 that I believe are due to the new pinion angle. I posted this message first, though, because if I end up changing out the rear end or tire size, I don’t want to have to correct the pinion angle twice. That’s the best I can do. Assume that all unmentioned components are stock or equivalent. The car has not been dyno tested so I can’t give an exact hp/torque. Based off the parts I’ve listed you can make a best guess of potential power. Any suggestions on my topic now? Thanks.