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Hello everyone, I just had my 435 engine rebuilt. the engine is 30 over. it was built by a reputable engine shop locally. the engine was built back to stock specifications. the engine dynoed at only 426 horsepower (using 114 octane and using the stock dist and carbs). the engine builder had said that Chevy was notorious for overstating the horsepower ratings. I've had others tell me that Chevrolet was notorious for underrating horsepower. What am I to believe. Any shared experiences, knowledge is appreciated. TIA Tony
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Other Considerations
Hi Tony...
I know these things may be obvious, but I have to ask:1) What induction was used for the Dyno? If you used your original 3-2V setup- what shape are the carbs in? New? Rebuilt? Freshened up? Or just slapped back on? This could make a difference. Even "professionally" rebuilt carbs will sometimes fall short of a full deck. 2) What ignition was used? Did you freshen up the distributor? A sloppy distributor shaft, considering both shaft/bushing clearance and end-play will make a major difference in performance. 3) Advance Curve- did you "curve" the distributor? This is one of the most ignored exercises. The factory springs/weights were not "blue printed" items. When I put these distributors on distributor machines, no two were alike. They all needed some work done to bring them up to PAR. 4) Break In- What type of rings did you choose? Cast? Chromoly? Did you observe the correct break in for the type of rings selected AND the final hone of the bores? 5) Did you "degree" the cam? I'm sure the 435 HP rating was achieved with a "selective" set of components where the valve train was right on the money.
One other possibility (conjecture on my part): did the factory "port match" the engines (heads/intake) that they used for determining HP? -
Re: Other Considerations
Tom, Carbs were rebuilt by Kunz, distributor rebuilt by Fiedler, etc. I assume the correct break in was followed by the builder. I was told by another engine builder that once I had a few miles onthe car, the horsepower would increase. Again, I don't know what to believe. TonyComment
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I'm impressed.
Tony -
In 1967 Chevrolet claimed to be giving "gross" horsepower for their engine ratings. As you know, this means that most normal engine parasitic power losses were eliminated (water pump, fan, alternator, belt system, fuel pump, etc.). I believe that they also used tubing headers with no mufflers, pressurized air intakes, and optimized incremental tuning. In other words, they changed the timing and carb jetting as necessary to maximize the ratings at several different rpm points on the dynometer curve. Then the marketing team would "contour" the curve into a smooth hump that constantly rose to an interesting peak number.
You will note some incredible horsepower drops during the early seventies when Chevrolet went to the "net", or as-installed, ratings. If you just looked at the numbers, you'd think many engines suddenly went down 20% in horsepower. What really happened was an injection of reality into the rating system.
You did good to get more than 400 horsepower. I'm very impressed. Uh.....with all due respect to your engine builder, keep in mind that some shops have a slightly generous dyno for testing their own engine work.Comment
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Re: Straight talk on horsepower ratings
Tony, what type of dyno, flywheel or rear-wheel. If rear wheel that is a GREAT hp number, 426 at the rear wheels. If flywheel, agree with Tom's post about all the varables.Comment
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Re: Straight talk on horsepower ratings
There are lot of variables here, not the least of which is the self-serving aspect of an engine builder/tuner being able to publish a "favorable" number to his clients. What was used for (the dyno's) induction? Was it atmo or pressurized? What was the temperature of the intake charge? Are your numbers corrected to STP or just arbitrarily "indicated"? How about ignition? Even the cooling method employed affects the numbers,etc,etc.
Folks in the engine business will tell you that the only thing a shop's dyno does reliably is provide a comparison or relative standard. Meaning that if you test two engines on the *same* dyno and correct HP and T readings to STP, you can then use their respective readings to compare those two engines to each other or to compare modifications and different configurations of the same engine to each other. The actual numbers don't necessarily have ironclad meaning but the difference between them is absloute.
In my experience Chevy didn't necessarily inflate HP figures across the board but they did monkey with them and, of course, always gave themselves the benefit of the doubt. Think the 57 FI really came out to 283HP? Ever wonder why the L88s were 5 HP less than the 3X2s?
Maximum HP is a somewhat meaningless number, anyway. What you really care about is how soon and over how wide a range whatever HP the engine develops is available. 400HP in a slow revving engine may be less useful than 300 HP delivered, right this second, by an engine which winds up quickly. Add to that mecahnical losses in transmissions and right angle drives and the engine HP number, however it's arrived at is just one factor which affects overall performance. And there are tricks to enhance not only actual performance but even the "perception" of performance (like the 50s and 60s cars with the non-linearly weak throttle return spring so you couldn't possibly help but squeal the tires).
Regards,
JPComment
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Re: Other Considerations
Guys, It's been a long time since I reviewed the old test procedures, but the manufacturers used a procedure known as "Test 20". This test was run at room temp (70 degree) air temp at the carb, no accessories, (water pump. fuel pump, etc) and open exhaust. The tests were optimistic, but not unobtainable. A few years ago, I dyno'd a 427/425 horse engine (fresh) stock cam, correct carb etc and it ran right at 427 hp on our local shop dyno, just me and the owner playing, so the numbers were real. I believe I remember the tests were available through SAE or somewhere, I rember looking some of the up when I was in College, but the was too long ago.Comment
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