Caprice Power Difference in Different years

Hi all, and happy new years! I was reading all the 4bbl 350 C.I.D. engine specs for different years in my Caprice Hanes manual. The specks from 1969 - 1973 went like this:
1969 - 300 HP - 300 Ft/Lbs - 10.25:1 Compression
1970 - 300 HP - 380 Ft/Lbs - 10.25:1 Compression
1971 - 270 HP - 360 Ft/Lbs - 8.5:1 Compression
(No 4bbl 350 in '72) 2 bbl: 1972 - 165 HP - 280 Ft/Lbs - 8.5:1 Compression
1973 - 175 HP - 260 Ft/Lbs - 8.5:1 Compression

My question is why did the amount of power put out from the 350 drop so massively in those years? Also why from 1971-1973, while the compression stayed the same did the power drop so much?

Camshafts changed... and somewhere in there the manufacturers agreed upon a standard was of providing HP numbers. I think the earlier numbers were given with just the motor on a dyno. When it changed, they gave the numbers with all the accessories, alternators, PS pumps, AC etc, mounted to the engine with an exhaust system possibly in place as well.

Also, there was a 4bbl option in 1972 - it was 185hp.

The HP difference was the switch from brake HP to SAE net HP, as CD above states, this is the difference between wide-open crank HP on the dyno to at-the-wheel HP with everything on.

So if they changed the way they dyno the engines, why didn't the re-dyno the pre-'72engines to get a more accurate comparason?

Also, there was a 4bbl option in 1972 - it was 185hp.

The HP difference was the switch from brake HP to SAE net HP, as CD above states, this is the difference between wide-open crank HP on the dyno to at-the-wheel HP with everything on.
Not at the wheels but it was a change from bare motor to as it would be in the car but still an engine dyno test not a rearwheel dyno test.

In there the emmisions era began too which at first took a BIG bite out of available HP but these days emmisions equipment is not as big a hinderance to power as it was then.

So if they changed the way they dyno the engines, why didn't the re-dyno the pre-'72engines to get a more accurate comparason?

$$$ Why spend the money on recertifying something they have already sold as having X HP? Plus the potential liability if the engines didn't make the HP. Did someone mention Ford and Cobra Mustangs???? Ask Ford how that worked out...

So instead of 300 HP and 380 Ft/Lbs torque from the 1970 350, what would you estimate it to be in the vehicle tested like the others?

Keep in mind, too, that compression ratios dropped from 1970 onwards, not across-the-board-in-one-year, but older engines were phased out and replaced. One GOOD example of this is the 1970, 1971 and 1972 flavors of the LT-1 engine - the 1970 version was 380 lb ft and 370 HP (this was even considered a derate by some who feel the 1970 LT-1 was the most powerful carbureted small block ever to come from the factory), in 1971 it was down to 360 lb ft and 330 HP, and in 1972 it was 280 lb. ft and 255HP.

Compression on this engine was 11:1 in 1970, 9:1 in 1971 and 8.5:1 in 1972.

So to get that kind of power out of lets say a 1980 350 during an engine rebuild, I should get raised top pistons to raise the compression ratio, along with the bigger cam I've already gotten (.29" lift at the lobe) and possibly larger volume, non-EGR heads?

There are no "EGR" or "non-EGR" heads, only heads that have heat-stove plumbing and those that do not. The former were/are used on all carbureted cars from the factory - the exhaust for the EGR was simply drawn from these heat stove passages.

A larger chamber will yield LOWER compression, not higher. There are a few reasons why compression dropped - one was emission related, another was the impending switch to unleaded fuel for catalytic converters - Chevrolet began making their engines unleaded-friendly starting in 1970. Unleaded gasoline back then was only available in 87-octane, therefore necessitating a lower compression ratio. Today, you can increase compression as unleaded gasoline comes in all grades, and in fact is all you can buy in the US. I'd recommend flat top pistons over domes - first, a dome piston doesn't transfer power as efficiently as a flat-top does, likewise a dish piston is better at power transfer than a flat-top (due to pressure dynamics - the dish lends a more spherical shape for expanding gas to act upon). If I was building the engine, I'd go with (on a 350) 64cc chamber heads with ~20cc dish pistons. Specifically, KB hypereutectic 22cc reverse-domes.

The engine in my car is a mixture of parts, including a 1985 two-piece RMS block (which is actually set up for factory hydraulic roller lifters), 1.94/1.5 valved 3991492 heads (these are similar to the mid 60's '462' heads in flow and chamber shape, but have accessory bolt holes, and are the castings used on the 1970 LT-1), Silvolite KB Hypereutectic 22cc reverse-domes, Fel-Pro head gaskets, Manley Race-Flo one-piece SS valves, Crane PowerMax 2030 cam, Sealed Power lifters, Edelbrock true-rolling timing set, Edelbrock 3701 Performer EGR intake, Clevite "Michigan 77" bearings, TRW high-volume oil pump with 35psi relief spring, TRW chrome-moly oil pump driveshaft, COMP pushrods, COMP springs, COMP titanium retainers with wide-angle locks, posi-lock intake valve guide seals, o-ring seals on all 16 valves, COMP Magnum roller-tip rockers (as an aside, these things lend a diesel-like quality to the engine's sound), and Sealed Power chrome moly piston rings. In addition, I had the block align-honed (not really needed but done to get the block closer to 'blueprint' spec), the heads blueprinted and the block decked, bored .020" over and then honed with torque-plates.

The result is an engine that has run strong and well for over a quarter million miles, and produces roughly 275 HP and 350 lb. ft. torque.

Thanks for the recommendations! I was thinking a larger chamber for more volume, and raised top pistons to bring the compression up past 8.5:1 But your idea sounds much more reasonable. I'm looking at some hyperutectic pistons from summit racing right now. Back in 1970, how did they achieve such high compression? Small chambers, or raised pistons, or both?

Smaller chambers with flat-top pistons.

Would 58cc Heads be too small? Heres the description of a set I found on eBay:
"SMALL BLOCK CHEVY CYLINDER HEADS. COMPLETELY REBUILT. 58CC,ALL NEW 2.02 AND 1.6 STAINLES VALVES. 500 LIFT SPRINGS, RETAINERS AND KEEPERS. HEADS WERE PRESURE WASHED, BEAD BLASTED, MAGNIFLUXED FOR CRACKS, RESURFACED ON HEAD AND INTAKE SIDES. VALVE POCKETS OPENED UP BEHIND SEATS.NEW 3/8 SCREW IN STUDS AND GUIDES. ALL WORK WAS DONE BY ASE MACHINE SHOP, THEY HAVE ALL THE ACC. HOLES. READY TO BOLT ON AND GO."
I'm not gonna buy them b/c they end in about a half an hour, but I just want to know what'd be good to get. I also read in other descriptions about intake port and exhaust volumes... how much of an impact would this have on my purchase?

58cc is the standard 305 series. If you're wanting to run them on a 350, figure on getting pistons with at least a 22cc dish/reverse-dome just to keep the compression ratio in the pump gas range - even still you're looking at 91 octane with them and the 22cc dish pistons. You can go a full point higher on compression if you run aluminum heads instead of iron ones.

Roughly guestimating, what kind of compression would I be looking at with that combination?

Here is an article (http://www.chevyhiperformance.com/techarticles/46778_engine_compression_guide/) which spells it out. You have to know things like deck height (that's piston installed height, the distance between the top of the piston crown and the deck surface at TDC), cylinder head volume, head gasket thickness (these have volume, too), etc.

Wow, now that article was loaded with information. Thanks! I even downloaded the free compression ratio calculator software they offered.

If looking to make power you do not want to look at anything the "musclecar era" had to offer. A set of Vortec heads off a 96 half ton truck with 1.94/1.50 valves are substantially better than the best heads to come out of the "good old days" even the big valve stuff.

It is not just the injection and compression that allow the LT1s of the 90s to make the power they do it is the heads and the roller cams both of which you can easily upgrade in your build. Look for a one piece rear main 350 probably best to look at the b-bodies just because the trucks came with flat tappets in roller blocks, get a good b-body motor and it will have roller lifters you can reuse after inspection. A set of L98 heads might be OK or Vortecs but those offer some extra challenges, they are a great bang for the buck in power making though. Then something like the GMPP 12370845 cam and you should have a nice little torquer.
With the carburator you are going to have to stay conservative on the compression probably around 9.5 would be decent.


One of these days I will put together a motor like this, I still know a few people who like carburators and one day I will talk one of them into letting me blend a little technology into their motor. At this point they still think it scary and exotic though but I think my car's current performance has one or to ready to listen too me.

If looking to make power you do not want to look at anything the "musclecar era" had to offer. A set of Vortec heads off a 96 half ton truck with 1.94/1.50 valves are substantially better than the best heads to come out of the "good old days" even the big valve stuff.

Very true, but when I built this engine in 1993, the LT1 was a new beast and the Vortecs didn't exist. Short of buying more exotic heads (of which I looked at available aluminum heads at the time), I decided to run a pair of '492s, which were already mine, having been given to me 4 years prior. Basically, I ran what was good then. Now, having said that, my next motor is likely going to have better aftermarket heads with centerbolt valve covers, if only because they seem to not ever leak.

I did not say it to criticize your build but to point him in what I feel is the right direction on his build.
Trust me I realize how things change and understand what you are saying far as options you had in 93.


I remeber just a few years ago when it was impressive for a heads/cam/nitrous LT1 car to run just a few tenths faster than what my car runs now and I am not done searching for it's limits yet. The community continues to learn, weed out the good parts and vendors from bad and grow.

On another subject, what would you think of an L99 complete in a Vega?

It's an idea that's been bouncing through my head lately. I think I want another Vega, and I want one that's fun.

But, but... the L99 is the same size as any other SBC. Why restrict the displacement? You can get a tall deck and crank to make 454 cubes in a small block these days. At least consider a 377 or 383.

But, but... the L99 is the same size as any other SBC. Why restrict the displacement? You can get a tall deck and crank to make 454 cubes in a small block these days. At least consider a 377 or 383.

I don't want to have to convert it to a rollcage frame to keep it from twisting like a soda can the first time I pounce on it!