we are talking max horsepower not along a curve, not accelerating down the road, not anything but what his max number is and what his max to the road number is.Thats all we car about is that magazine cover blurb. I've got a 800 hp motor in my Prius, its awesome!! do we care how much power it takes to accelerate from a stop light, how much power loss I experience as i melt those little 13 inch hard rubber high mileage tires as i put it in gear and take my foot of the brake...no its just the number that we are told when you get it back from the dyno. When we dont have the engine dyno numbers we want to try and figure it out. The only way to do it is to know what your drive train actually needs to spin at to get your max number at the wheel. IF it takes 100 hp to get your tires spinnning it will always take that same amount of horsepower to get there. 100hp from a 150 hp motor is not the same percentage as 100 hp from a 500 hp motor....it just cant be.

 

I believe we are in agreement in the "big" picture, but differing in the details.

At the same speed or acceleration rate, the drivetrain will eat up the same absolute power (as I said before, I don't say "percentage") regardless of the engine maximum capability. I believe we have been in agreement there. (Correct me if I am mistaken.)

Just so we can be on the same page, when you say "get your tires spinning", are you taking into account time or acceleration rate to that particular tire speed?

 

I have no idea how this got this complicated with acceleration down the road, air resistance, acceleration dependent stuff, or aero drag or anything else. No dyno can measure aero drag, air resistance, or any type of road resistance. So when talking dyno numbers all of these are irrelevant, LOL. Having actually owned and operated a chassis dyno at one time in my life, I saw nothing wrong with the 153 RWHP on the 165 HP rated engine if a couple of mods had been done. Just removing or emptying the pellet cat on those engines will make a huge improvement, and a manifold carb change could easily get him there... First no 2 dynos are equal, and there will be differences even on the same dyno on different days with different atmospheric conditions. An engine dyno will measure crank HP, a chassis dyno will measure rear wheel HP with all drivetrain loses.
It has been long accepted by the industry that a 15-20% HP loss is normal from an engine dyno to a chassis dyno. Most use a 15% figure as their baseline. This number has come from hundreds and hundreds of vehicles that have had their engines dynoed and then they were put on a chassis dyno to do final tuning. The tuning on the engine dyno will not be the same as when the engine is in the car with full exhaust, air cleaners, under hood hot air etc... The op is correct to assume about a 20% HP loss to what he would see on an engine dyno. We usually used the 15% figure, but it is impossible to know exactly what it is.
When an engine is on a dyno, there are no parasitic engine loses, there are no power steering pumps, AC compressors, alternators, engine fans, and even the water pump is normally run with an electric motor, There is no exhaust and no mufflers. You also do not run an air cleaner or filter, but a huge velocity stack that is open. The PS, AC, ALT, fan, and water pump take quite a bit of HP to run. When you are on a chassis dyno all that stuff is connected to the engine and is creating parasitic loses. Then you have your air cleaner, air filter and hot under hood air. On top of that you have the loses from the transmission, and if you have an auto transmission you have loses from the slippage of the converter. You have your mufflers and exhaust, your differential, and your bearings, brakes and everything else that slowly adds up. You could have a car that from an engine dyno number to a chassis dyno numbers could be less that 15% loss, but it is usually an outlier. If you have no PS, AC, an electric water pump, electric fans, an underdriven alternator, a manual transmission, a small differential, drum brakes etc... you will probably be under that 15% but normally 15-20% is about right. Auto trans and lots of engine driven accessories, full exhaust you will be closer to 20% losses, manual trans with little engine driven accessories closer to 15%.
At the end of the day dynos are tuning tools, they are not there to tell you exactly how much power your engine has, but to help you get the most out of your present combination.

 

I agree with what your saying and I myself am to blame when I use inappropriate word that misconstrue what I am actually try ing to say. I shouldnt say get your tires spinning because thats like comparing a refrigerator or air compressors operating amps to its surge amps. I was using get your tires spinning as a metaphor. anyways im not talking about what the car needs to do or the HP it takes to get to the max number. Im just talking about what the engine is producaing and consuming at the max number. And if we take a moment to do some research all the car magazines and video hosts are now coming around to the belief that you cant use a percentage anymore. The variables are far to great now a days with all the different combinations drivetrain types, dynos, the people who run them, etc. We are all victims of "back in the day" especially if you live long enough and have a bunch of hobbies or jobs. But times change and hypothesis change. And until a hypothesis becomes a law it can go back and forth. Now is the time that the 15% isn't applicable anymore. When I was a kid if you got 1 hp/cube you were doing great. Now if you arent past 600hp its considered tame. Lift below .650 is considered small and nitrous, turbos and blowers are just considered power adders, you can get them anywhere including China and people dont care. Hell, the big three are all close to selling 4 digit motors over the counter. With a range that big versus when we started the 15% "rule" there wasnt much hp difference in many of the performance motors. My 450 hp 327 wasnt a big deal. Im currently trying to build a 427 to stay UNDER 500hp. Wrap your noodle around that. Im trying to find a good deal on small oval port heads and most on the market are big rectangle ports.......

 

To be clear, you are saying regardless of the vehicle's speed, using your example, the drivetrain loss is a constant 30 hp?

 

Welp, this gives me a little budget hope with my wore out 350. I've been thinking a big block was my only road to glory.

Actually, I still am...but I'll bet you can feel the difference!

 

If the drivetrain behind the engine does not change, the losses due to that drivetrain will be the same amount, regardless of the engine. You could put an electric motor on the driveshaft and it will absorb the same amount of work (horsepower, watts, whatever measure you choose to use) regardless of the power SOURCE.

 

From the Corvette Black Book listed engine power numbers, it looks like they changed reporting from gross to net in 1972.

 

yes the difference is huge. The price was alright. It seemed expensive. I did all the work myself(except the machine shop portion) spent $3350 for the motor, that’s everything, including new crank and the machine work to bore it out. By far the most expensive part was the exhaust. $1900 for the headers, custom true duals with cutouts, all done by a shop. So $5250 for an extra 123rwhp and 124rwtq.

 

Just ordinary info... I had a couple of late 80s k5 blazers/Jimmy’s. Tbi 350 factory ratings were something like 210 hp and 300 tq. To me that should be the basic standard of what an effortless, not trying, not caring run of the mill 350 should make as a flywheel power baseline. Gm must have tried very hard to bring those numbers down to 165.

 

I think its just showing how hard they tried to meet new emissions standards at the time.. from what Ive read power was down quite a bit on pretty much every make american car and it was just getting worse from there for the next few years for performance until it forced engineering to catch up.

 

yes I understand all of that... it’s merely an observation that goes hand in hand with another comment about the 165 hp seeming underrated based on sheer logic.

 

I was buying my first new car at the time, 1975, and 1975-1980 were some dark years for performance cars. The Vette and the Trans Am were it. 75 was the first year for the new "save the air" catalytic convertors, and they were really bad in the beginning. The O2 sensors would reportedly only last 20-30k miles, the convertors could easily glow red-hot when run behind a carb, that typically did not have good enough fuel control, The cars kind of regularly caught leaves and grass on fire. Even the owners manuals warned you not to park over that stuff. And the first convertors were literally pellets of ?? something that was poured into the container and while they worked for air quality, it absolutely killed performance. You may have as well had a 3/4" opening for a tailpipe!. They would clog up solid kind of frequently, stopping the engine from running, and requiring replacement. If you poured those pellets out of the convertor it was like waking up a caged up tiger, that literally had a cork up his butt. Yeah we did that.

By the 1980's all of that changed. The convertors got better, the cars got fuel injection, the computers got better, and a second O2 sensor appeared. The convertors even went to low restriction honeycomb at some point. The convertors were no longer a big liability, and performance came back!

If you put a 1985 convertor on a 75 engine I would bet it would gain an easy 30-40+HP.

 

 

How about this Cold War gem.....?



Jebby

 

All of this ^^^^^^^^^^^^^stupidity is why states that have emissions testing/emissions inspections on any car prior to the mid 90's is absolutely absurd!!!!!!!!

 

In a forum dedicated to 40+ year old cars there isn't much left to talk about that hasn't already been discussed many times over... What does change is the owners/members here and their knowledge. So what might be a dead horse to you, some young vette owner might be learning for the first time.
At least this is a bit more enticing than reviving 15 year old threads on the topic which is not encouraged here and thus would not promote this knowledge to be "rehashed" at all unless a new discussion comes up.

 

The reason acceleration was mentioned is because it matters in the real world. The flywheel, transmission gears, driveshaft, ring gear, and wheels/tires are, from a mechanical engineering viewpoint, a bunch of flywheels that are resisting the horsepower that the engine is trying to use to accelerate the car down the road (or, in the case of a chassis dyno, the acceleration of the inertia drum). Flywheels, by definition, are energy storage devices. They soak up horsepower during any acceleration, reducing the net horsepower available to accelerate the car/drum. (They then release that stored energy at throttle off, increasing the coast-down time.)
At high vehicle speeds, the aero drag is sufficiently high to where the acceleration rate is small, reducing the amount of horsepower/energy being added to the "flywheels". At top/terminal speed, the drivetrain losses are purely frictional, as there's no additional horsepower/energy being stored into the "flywheels".

I do agree with your concluding sentence.

 

That's one of the funniest engine pieces I have seen in a while!
Reminds me of an 8000 rpm NASCAR engine trying to breath thru a "restrictor plate"