Im working on a 71 Cutlass Supreme. The 1971 350 is rated at 260HP and the 1970 350 is rated at 310HP. Id like to keep the original motor in the car and build it like the 1970 motor. Does anyone know what I would have to change on the 71 motor?

 

Pistons (8.5:1 vs 10.25:1), carb (potentially), timing.

Heads are different numbers, but have similar design characteristics.

I believe the cams are the same between the 4bbls in those years.

Note that between '71 and '72 HP ratings changed from SAE Gross to SAE Net, so the 260 HP engine from 1971 is essentially the same as the 180 HP engine from 1972, with the latter number (SAE Net) being the same system in use today.

Welcome to Classic Olds.

- Eric

 

Speed Pro makes a set of Flat Tops that will get you close to the 1970 compression numbers.....

 

That's pretty close - the data I have shows the 180 hp rating was for the single exhaust 350-4bbl engine and the dual exhaust 350-4bbl was rated 200 hp.


The data in my 1971 Owners Manual that shows both Gross and Net ratings:

Gross: 260 hp @ 4600 RPM and 360 ft-lbs @ 3200 RPM
Net: 200 hp @ 4600 RPM and 300 ft-lbs @ 3200 RPM

So a 60 hp / 60 ft-lb loss just due to the rating methods for the '71 engine.

I wouldn't worry too much about those ratings, though. Back in high school I raced my bone stock '70 Supreme 350-4bbl against a friend's '71 Supreme 350-4bbl, both with 2.56 gears. The cars were neck-and-neck off the line and up until the top of first gear, then I started edging ahead of him. I was kinda bummed since my "high compression 310 hp" car didn't waste his "low compression 200 hp" car.

 

As has been stated, the biggest difference is in the pistons. In 1970, low comp 350s had a 14cc dish, high comp a 6 cc dish. In 71-72, all were the big dish 23 cc pistons. Hard to get decent Cr. If you are rebuilding, you can replace with flat tops, but Cr will probably be over 10 to 1, a bit dicey on today's fuel. Speed Pro also makes a 6 cc dish, maybe a better choice. You will also need a modern cam, not the old-school W-31 (or was it W-30??) copy.

 

Thanks for all the info (and the welcome). I might be thinking ahead a little because I am just starting to replace a rusty trunk floor, then Im gonna change interior to bucket seats, floor shift with console, and rally pak gauges. Then I will rehaul the motor while someone else paints the car. Doesnt hurt to plan ahead does it?
Anyway I will go with either flat top or the 6cc dish piston.
Is there anyplace I can look at specs for the original stock cam for the 310HP motor and compare to new modern cams?
I would like to build this as a street motor with a little rougher idle than the stock motors.

 

Here's data I archived from old posts:

350 auto cam #400084:

.400/.400 lift
186/202 duration @ .050" lift
250/264 advertised duration
108 intake centerline
109 LSA



350 Manual trans cam:

.440"/.440" lift
206°/218° duration @ .050"
262°/274° advertised duration
108° intake centerline
109° LSA

 

^^^^^^ Good info! I think both would be too "small" for a 355 that is close to 10 to 1. Also, might as well get more lift. Something around .500 and 215 @ .050 on the intake would work and sound good.

 

FWIW, if you're rebuilding the engine, go ahead and make a budget and plan. The stock cams aren't anything to get excited about - there's much better stuff available now. If absolutely nothing else, get the basic "RV" cam. It's also not great, but it's a few steps above stock. There are cam dealers on this board that can help, and there's a wealth of information out there. The simple path is to find someone with a build that you like and duplicate it.
Bumping the compression up needs some serious thought to make sure you won't have detonation problems. Been there, done that, broke parts. The drive to lower CR was due to gas changes (no lead) and emissions controls.
Once you change one area, you also have to make sure your entire driveline is still matched. There's tight relationships between the cam, compression ratio, exhaust system, transmission, torque converter and rear end.

 

Also good advice. I had a 9 to 1 355 with a 210/218 cam and 3.42 gears than ran high 13s in a wagon, was fun to drive with ghreat manners and reliability. No "lope" to the cam, but so what??

 

Thanks for all the suggestions. I was looking at Edelbrock website and looks like they make 2 cams for the olds 350. The Performer RPM has good numbers for duration and lift, but the cam card (footnote 1) says "Adjustable pushrods or rocker arms required." How hard is it to put adjustable rocker arms on stock olds heads?

 

The Comp Magnum roller tip rockers are popular. Here's how to install them; http://www.compcams.com/Technical/In.../Files/250.pdf

There are much better cams out there than those generic Edelbrock cams. Get something better.

 

It'll get interesting really quick. Do some reading on the 442.com FAQ site (good place to get a basis of understanding) and more digging around here.

The faults with the Magnum roller tip set is the pushrods are horrible, and you are betting off using poly locks instead of the pinch nuts. They are nice rockers and I use them on my 350. The adapter studs and guideplates are also good.

 

I agree that you should look for something other than those Edelbrock cams.

Decide what type of driving you plan on doing with the car (weekend cruiser, daily driver, street/strip.etc) and then ask for cam recommendations based on that, and you will get some really good information. Alternatively, look at other folks engine builds, pick one that matches what you want, and use their recipe.

 

just adding more head ache for you guys lol!!n wouldn't low compression cause a lower horse power?