I know this forum is for El Camino’s but I have a 1972 GMC Custom 454 Sprint with cow induction, TH-400 transmission and a postraction rear and my Sprint is basically an El Camino with a GMC name plate. So I figure this web site should also help me, I hope.
I am the original owner of this vehicle and have put 96000 hard miles on it and had parked it in my garage for the last 15 years (it was giving me engine trouble and I did not have the money to rebuild it then). I am now ready to fix her up.
I hope the members of this forum can guide me in the right direction with this engine rebuild.
The 1st question I have is what is the difference between a 1971 vs 1972 454 that dropped the hp from 365 to 270. When I look at the bore & stroke they are the same. Was it the cam, heads or I have read GM de-rated it just on paper and there was no difference.
The 2nd question is the rebuild itself. I would like to get the HP up into the 400+ range and I am not sure what is the best way to do that. I have started to look at a 4.25” stoker kit and punching it out to 496ci but I am not sure what compression ratio to shoot for. I still want to run pumped gas in it.
The main thing is I do not want to reach the HP level to where I would have to increase everything in the drive train to handle it if that is possible. I would like to stay with the stock torque converter.
So anyone that has gone through this process that would like to share it with me please let me know what you did and what part vendors you used.

The horsepower drop was because GM took the horsepower readings off of the flywheel up until 1971, then switched to taking the reading off of the rear wheels. That and their quest for for gas milage (smog motors) the horsepower ratings dropped. I'm a smallblock guy myself, but I've had my share of big blocks and I know 400 HP is easily attainable, with heads,cam, intake, carb and headers. Not sure of part numbers, some of the big block guys could be of more help with this. I'd keep the compression below 10:1 for pump gas.

If you use aluminum heads don't go any higher than 10.0:1. Iron heads, keep the compression below 9.5:1. If the iron heads on your big block are the peanut port configuration, they are low performance and restrictive to flow. The horsepower numbers of 400-500 are easily attainable but on a street engine, torque is king. A 496 will get you well over 500lb/ft. and that moves the vehicle. Horsepower is attained at higher RPMs and you really don't need to wind up a street engine, especially a big block.

I would run the Edelbrock heads and Performer RPM package, a bolt on and go system, your pistons & combustion chamber will determine your C.R. like the other people have stated keep it to 10.to 1,, sounds like a nice ride,, smoke um if ya got um!! The tires that is,, Matty man

The rebuild also depends on how much ya wanna spend on it!! If iron heads with the good ports would do it for you, along with a suitable cam to keep your vacuum and stock converter, that is worth looking into also. Using the stock converter etc is pointing in the direction of better street manners, and gas mileage you can afford.
It's all about money in the end.:smileyb:

This forum is for El Caminos and their close cousins, the GMC brand Sprint!! You are in the right place, and welcome!!:smileyb:

If it was originally equipped with a big block then the rest of the drivetrain should handle some arbitrary h.p. number of 400, but in reality h.p. isn't all that crucial with a big block. Torque is where it's at. These days, with a few mouse clicks and a fat checking account, you can get a 400 horse small block delivered to your door, but it still won't pull like a big block. Assuming no ring/bore issues, a set of heads, intake, and cam will easily make 400. It's probably doable even with your stock heads. Even easier, just add a plate style nitrous kit and presto!, instant 400 h.p. I'm curious though as to why you mention keeping your stock converter. IMO you can achieve your 400 goal with the stock converter, it just seems odd to mention it. At any rate, torque is what accelerates the car, not h.p.

Welcome Doc, in answer to your first question, the majority of the drop of almost 100 HP is as stated a change in the rating system by the US manufactures that occurred in 1972. I pulled one of the better explanations from a Wikipedia article for you.

SAE gross power</SPAN>

Prior to the 1972 model year, American automakers rated and advertised their engines in brake horsepower (bhp), frequently referred to as SAE gross horsepower…... As with other brake horsepower test protocols, SAE gross hp was measured using a stock test engine, generally running with few belt-driven accessories and sometimes fitted with long tube (test headers) in lieu of the OEM exhaust manifolds. The atmospheric correction standards for barometric pressure, humidity and temperature for testing were relatively idealistic.
SAE net power</SPAN>

In the United States, the term bhp fell into disuse in 1971-72, as automakers began to quote power in terms of SAE net horsepower…... Like SAE gross and other brake horsepower protocols, SAE Net hp is measured at the engine's crankshaft, and so does not account for transmission losses. However, the SAE net power testing protocol calls for standard production-type belt-driven accessories, air cleaner, emission controls, exhaust system, and other power-consuming accessories. This produces ratings in closer alignment with the power produced by the engine as it is actually configured and sold.


As noted both measurements are still taken from the crankshaft. Both the 71 365HP and 72 270HP engines ran an 8.5:1 compression ratio (which is really ideal for todays crap gas with iron heads).

Without actually looking at your engine there may also be a couple of other factors involved in the HP drop. A few things that come to mind added emissions equipment that the factory was starting to install at about this time (EGR, catalytic convertors, smog pump), cam profile and ignition timing

Chances are just simple changes made during the rebuild can make a very noticeable improvement over stock. Personally I would look at simple things to enhance what’s already there. A better intake and exhaust and a more modern cam design.

John pretty much nailed nailed it, don’t worry about some arbitrary HP number…..unless you actually dyno the engine you will never know anyway. The vast majority of HP figures that people throw around are nothing more than wishfull thinking and a BS number they are pulling out of their a$$ anyway.

The first step you should take is to realistically look at what you want the car do once you get it back on the road, then look for a combination of parts that work well together to make it happen.

If you can define what your goals are a bit better I’m sure people can come up with some specific recommendations for you.

Thanks for setting the record straight Mike, I was mislead and in turn I mislead, sorry.

I'm not a engine builder... yet. But I read alot about chevy motors and talk Chevy power to guys who know there stuff. You dont need heads to make power out of a big block, or even a forged crank and rods to hold up that power... It's a big block have you seen the size of the internals? There huge.
Really all you need flat top pistons, a good cam to match your setup, intake and carb. And you'll wake that beast up. Check this combo out.




Street combination:
Hiperf 454 4bolt block (std boring)
"Peanut" oval port heads (mild port work)
2.19/1.725 Manley raceflow valves + multiangle valve seats (30,45,60 deg.)
CompCams magnum rocker arms
stock 8.75 to 1 cast pistons
stock rods
stock cast crank
Edelbrock torker intake
552/555 230/236 @.050 (CompCams XE274H)
Holley 850DB carb
FlowTech 1-7/8 headers
Dyno result with 3-inch exhaust & pumpgas: 526ft/lbs@3500rpm, 468hp@5100rpm (over 500ft/lbs torque from 2800rpm to 4800 rpm!)

A few things that come to mind added emissions equipment that the factory was starting to install at about this time (EGR, catalytic convertors, smog pump), cam profile and ignition timing
EGR has no impact on peak HP. It isn't even an emissions device. It is a fuel economy device. During high-vacuum operation, the EGR valve dilutes the intake air charge with an "inert" gas (exhaust). When you punch the throttle, the EGR valve closes, and you're running air/fuel, with no exhaust mixed in.

The first step you should take is to realistically look at what you want the car do once you get it back on the road, then look for a combination of parts that work well together to make it happen.
This is the key point that many people ignore. A car is a system. Everything has to work together. You need to consider what you'll be using the vehicle for, what level of performance you want from it, and how much you're willing to pay for gas. This will to make informed decisions that will allow you to achieve your goals.

Here's another overlooked question....do you have a big block budget? It ain't cheap.

Like I said, it's all about money:smileyb:

“……EGR has no impact on peak HP. It isn't even an emissions device. It is a fuel economy device……”

Whisper I’m afraid I’m going to have to at least partially disagree with you. The primary reason for the addition of the early EGR systems was to reduce Nitrogen Oxide (NOx) emissions. Rather than basically re-type a complete article here are a couple of quotes from another Wiki article



“……..exhaust gas recirculation (EGR) is a nitrogen oxide (NOx (http://en.wikipedia.org/wiki/NOx)) emissions reduction technique used in most petrol/gasoline (http://en.wikipedia.org/wiki/Petrol_engine) and dieselengines (http://en.wikipedia.org/wiki/Diesel_engine)….”

“……A properly operating EGR can theoretically increase the efficiency of gasoline engines via several mechanisms…..”

“…..Early (1970s) EGR systems were unsophisticated, utilizing manifold vacuum (http://en.wikipedia.org/wiki/Manifold_vacuum) as the only input to an on/off EGR valve; reduced performance and/or drivability were common side effects…..”

the EGR systems have evolved over time and with todays compter control capability the EGR system can in fact increase economy, however the 72 era system would definately fall into the unsophisticated category.

The whole article can be found here:


http://en.wikipedia.org/wiki/Exhaust_gas_recirculation