But that 223 HP (not 323) loss is roughly a 150 HP loss, which translates to about 40%.
Transmissions will suck up the horsepower (about 30 or so) with the Ford AOD being the worst of the lot. I think the spec on that one is around 40-50 HP.
Here’s how I arrived at the 223 figure. You take the 375HP that the 396 is rated at. Since 375HP is the gross rating we can deduct about 30% of that 375HP to covert to it’s net HP rating (I’m sure it’s not totally accurate but it’s in the ballpark). That would bring the HP down to about 263 HP. Then we take away another 15% from that for typical RWD driveline loss. That’s about 39 HP. We end up with 224 HP at the wheels. Which is pretty close what the Chevelle put down on the chassis dyno.
You might do a net search to verify it but I don’t think a 40% loss is normal or acceptable. It’s more in the 15-20% range and includes both accessory and driveline loss to arrive at the SAE net HP rating.
I’m pretty sure that driveline loss is not taken into account on net HP ratings. No manufacturer advertises RWHP, the HP rating advertised is always the power at the crank. The difference in gross HP as I understand it is that the engine is dyno’d without accessories and without an exhaust system, and with carb jetting and timing adjusted for maximum output. Whereas the net rating is taking with all accessories hooked up, full exhaust, and default tuning.
Here’s what I found,
SAE gross horsepower
Prior to the 1972 model year, American automakers rated and advertised their engines in brake horsepower (bhp), frequently referred to as SAE gross horsepower, because it was measured in accord with the protocols defined in SAE standards J245 and J1995. 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 horsepower
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 in accord with SAE standard J1349. 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 hp 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. The change to net hp effectively deflated power ratings to assuage the auto insurance industry and environmental and safety lobbies.
Edit: After further reseach I found this regarding the Ford 302.
Emission regulations saw a progressive reduction in compression ratio for the 302 two-barrel, to 9.0:1 in 1972, reducing SAE gross horsepower to 210 hp (157 kW). In that year U.S. automakers began to quote horsepower in SAE net ratings; the 302 two-barrel carried a net rating of 140 hp (104 kW).
That’s a 30% reductiion in rated power going from Gross to Net HP. When you factor in driveline loss. a 40% reduction from rated Gross HP to RWHP is a quite reasonable expectation.
Here is the trick to going fast. You need to increase the amount of air the engine displaces per foot of linear travel of the vehicle.
Lots of ways of going about this. Simplest is changing to a numericly higher gear ratio, or installing shorter tires. Then the engine speed is higher and it is displacing more air so it is able to also burn more fuel.
Then there are volumetric efficiency changes to the motor, Remember the motor is a pump. These are things like Heads and manifolds, exhaust.
Then there are camshafts. Cams are a compromise. You are pretty much just moving the peak torque (aka best volumetric efficiency) to a different spot in the RPM range.
Then there is super charging and turbo charging. This is basicly turning your engine into a two stage pump. The one drawback there is the first stage heats and thus expands the air. This leads to a larger volume of air being pumped but not nesasarily a greater mass (weight) of air.
No mater what you do. the sticiometeric A/F ratio for gasoline should be kept close to 14/1 by mass (weight) But that is the job of the tuner/ chip burner, or whatever you want to call him.
I left out a lot to try and keep this simple. I also left out chemical means of increasing effective displacement, such as nitromethane and nitrous oxide.
So what I think happened here is your mechanical changes to the engine moved your peak volumetric efficency up higher in the RPM range, but you haven’t changed the gear ratio in order to take advantage of it.
Either that or your mechanic or your chip guy made mistakes. All the right parts without proper tuning will get you nothing, or less than nothing.
You’ve got a lot of good advice in this thread, but I think the thing that you absolutely have to do -and this is a tenant of EFI performance- is get it on a dyno and get a baseline. You can play bench racer until the cows come home, trying to guess at the horsepower, but you’ll never know where you are if you don’t get it on a dyno.
Go to a shop that will tune on the fly. Once you put down a couple of runs and get a baseline, they’ll look at the overall engine performance vis a vis the spark and fuel curves and change them as necessary for optimal performance. It’s really, really hard to get a completely optimal tune from a chip that just assumes things about your motor.
I’m begging you: dyno tune it, it’s the ONLY way to truly know that your motor is performing as well as it can.
No mater what you do. the sticiometeric A/F ratio for gasoline should be kept close to 14/1 by mass (weight) But that is the job of the tuner/ chip burner, or whatever you want to call him.
I will respectfully disagree. The government mandates a 14.7.1 A/F ratio for minimal pollution. However, peak fuel mileage occurs at a slightly richer (fuel) ratio than 14.7.1, and peak power occurs at even a richer ratio.
Where those peak ratios for mpg and power are depend on all the variables that affect whether every precious oxygen molecule has an adjacent fuel molecule next to it during the brief period that the flame front is moving across the piston.
Up through the early to mid 70s, auto manufacturers used a 110% richer for best mpg and 116% richer for peak power.
With the improvements since the 70s in combustion chamber design, I would expect those 110% and 116% numbers to be lower for today’s engines, but they are still above the 100% (14.7:1) sticiometeric chemically-correct ratio.
If you are going by feel, and driving conservatively, I’d expect the changes you made all moved the peak torque from the very low RPM range to mid-way up the RPM range in exchange for HP from Mid- to High-up the RPM range.
Driving it all nice and nice might actually feel slightly weaker.
Just based on the changes you made, I suspect more power, not enough to overwhelm you and the kind of power that it hard to feel, but should rear it’s head by how the numbers shake out and how the tach needle moves from 3300 RPM on up.
I think you got it right. Make a lot of changes related to top end power, and the bottom end could well drop. Those ‘shorty’ headers are just such a mod, if I remember right.
I respectfully disagree and I think the way you’re looking at the figures is wrong.
Consider the 302 Ford as an example.
You seem to be comparing pre-72 302s with post-72 versions during the same calculation and this is not correct.
If I read this right you’re saying that the post 72 engines have 210 HP gross, are rated at 140, and therefore there’s a loss of 70 HP right there before even figuring in drivetrain power loss.
This is not correct because the post 72 engines had nowhere near 210 HP to begin with. They were more like 175 HP gross and that kind of thing. They were detuned through several factors with cam timing being the most important one.
Well it seems I got a lot of really great information here and I can’t thank those who helped me enough. I think after all the advice I received, I found the culprit that keeps the car from achieving it’s peak power. Whomever owned the car before me put some Hypertech chip in the car. When the chip was burned for the new mods, the chip was burned for the stock PROM, not a hypertech. According to what I think I know now, the Hypertech, which piggybacks the new chip, is hurting the entire motor. I was told to do one of two things. Either get a brand new chip that will run without the stock PROM or replace the Hypertech with the stock PROM since that is what the chip builder was basing his burn around anyway. Once I get the computer chip correct, whenever that happens, the car should be a serious little street machine. Right now it has great low end power but it does not feel like it has that same power throughout.
You are probably right. I just threw a balpark figure out there. It was more to emphasize that there is no magic. If you want more power, you got to move more air and burn more fuel.
That is provided everything is tuned right to start with.
I kind of forget gasoline tuning a bit. Been working with other fuels for a long time.
I know that this is an old thread, but I discovered it today and someone else may need the information in the future. I have built a lot of high performance engines over the years and one thing that happens from time to time is that, even though you’ve done everything exactly as you’ve done it many times before, the results are not the same. The engine is stronger–Great–or weaker–Damn! Either way the thing is, sometimes, as others have told you it may get better with time and miles after break-in or even much later. I have seen dramatic power increases after the rings finally seal against the cylinders. The valves are a major factor in power production. If your valves can’t seal all that extra cylinder pressure you’ve created with a bigger cam, 1.6:1 rockers, intake and larger injectors; you’re not going to see the performance you were expecting. New heads are not necessarily “ready-to-install” there’s no substitute for lapping-in the valves. Some ignorant people have stated–to me–that lapping valves into their seats is simply wearing-out new parts. Untrue, it is actually the matching of the two wear surfaces to make a better seal that holds the cylinder pressure when it’s needed most–the first 90 degrees after TDC. The same principle applies to piston ring sealing. There is lapping compound available for rings too. Moly and low tension compression rings require a perfectly round, super smooth and taper-free cylinder to work in. Piston ring end gap and piston-to-wall clearance are critical factors too.
True, the computer has some “fuzzy-logic” capability on these old cars–more so in the Vette PCM(computer)–but it only takes a couple days at most to “learn” the new adaptives. Your car is most likely sealing-up and that can take thousands of miles sometimes, especially when the items above are not dead-on spec.
Hope this helps the next guy.
Rob Nissen