409/409

No matter what results are produced on an engine dyno, the rear wheel dyno will show a lower result. Not in percentages, but how much actual horsepower loss is attributed to engine accessories, clutch, transmission, rear end, side shafts etc.? It seems that loss on a C2 would be the same number regardless of engine horsepower.
Thanks

DansYellow66

That was sort of my thinking also initially - if accessory and driveline losses are 75 HP on a 327/300 they should be 75 HP on a 427/435 with the same accessories, transmission and rear end.

It was pointed out that as more torque is applied to a gear set the frictional, lubricant drag and inertia losses increase and absorbed horsepower increase. So driveline losses are not fixed at an absolute number. The question is do they increase linearly as power increases, or exponentially increase or exponentially decrease? I don't know - probably need one of the Mechanical Engineers out there to weight in on that.

Edit: The above applies to using that HP from a bigger engine. At a steady-state cruise down the highway, if it takes 125 HP to maintain that steady cruise speed on level ground, the driveline doesn't really care if it's being powered by a 327/300 or a 427/435 - 125 HP is 125 HP. Driveline losses to friction, drag and inertia are pretty constant. The only purpose of a bigger, more powerful engine is to accelerate quicker or go faster. Either one will result in trying to spool up or rev the drivetrain quicker (acceleration) or travel at a higher speed (driveline turning faster). Both are when the friction, drag and inertial forces increase due to the more powerful motor.

409/409

I am hoping someone will know a range of numbers like 5 to 7 or 10 to 15 or 30 to 40. Some motorhead has to have tried to figure that out!

SWCDuke

Not true. As I said in a another thread earlier this week exhaust pumping power increases with the cube of indicated horsepower, which is the actual power produced in the cylinders minus internal engine friction.

I offered empirical factors, derived from actual test data to estimate SAE net HP at the flywheel based on SAE corrected RWHP and another factor to compute honest SAE gross from SAE net at the flywheel for a SHP small block producing about 275 SAE corrected RWHP.

For a SHP big block producing something on the order of 325 RWHP these factors will be smaller. I don't have any data to compute these numbers, but you can use the cubic relationship from the small block data I provided to estimate big block exhaust pumping power.

From a given baseline everything has to be computed proportionally, but the relationships are not necessarily linear.

Duke

409/409

SWCDuke,
While I appreciate your reply, I just can't wrap my head around that technical talk. I just want numbers or a formula and instructions on how to calculate. Even though I still think somebody knows a number range. Or maybe I'm just a blockhead.

SWCDuke

I doubt if you're a blockhead, but unfortunately most car guys don't understand the physics going on in an IC engine and the various relationships to do the calculations, and it's not something I can explain here in great detail in a reasonable amount of time.

Duke

65tripleblack

Since nobody answered your question yet, then I'll give it a shot:

First, it's important to know that dynamometer output is a function of engine torque, which is converted to power, which is a function of engine torque at a given RPM. The dyno measures torque and then converts it to power.

That said, the reason that drivetrain power losses are a percentage of engine power is because power losses are a function of friction between meshing gears and u-joint trunnions/caps/needle bearings. This friction is in direct proportion to the (normal, or perpendicular) force applied between the involved components, and this force is directly proportional to engine torque.

Poorhousenext

Just do a search on "manual transmission drivetrain lost". Then read the numerous write ups on subject. Like the man said lot of things play a part in HP/TQ lost between Engine Dyno numbers and Chassis dyno numbers.

They don't try Shuck & Jive talk to impress in below write up, just straight talk with some examples of things that can cause loss.

http://www.hotrod.com/articles/ccrp-...in-power-loss/

Also, depending on Manufacture of Chassis Dyno you dyno car on, it will produce higher or lower numbers based on the way their software calculates numbers.

Weather/Intake air condition play a part in numbers also. Most Dyno Manufacturers do use the same SAE formula to provide corrected HP/TQ numbers based on weather numbers.

On two different Mustang dynos on total different times of year, my Transgender'ed C2 made same corrected numbers. On a Dynojet Dyno, same day as pulls on one of the Mustang Dyno's car made 49 HP more than on the Mustang. So on one I have a 25% power lost due to everything from Intake System to rear wheels. On the DynoJet 409HP, only a 14.5% lost...LOL

Use a dyno to tune car with. With a Good Tune, on either one, an car should perform at it's best regardless of Max numbers. Don't worry about numbers unless you are willing to spend, $$$$$.$$. If I wanted to up my HP/TQ the first place I need to start is with it's air intake system, then intake manifold & throttle body size, then headers from 1 3/4" tubes to 1 7/8", ect.

If you want to brag about how much HP you car's engine has when parked, dyno on a Dynojet...LOL

63 340HP

The answer is torque and rpm related.

Torque is what squeezes together the gears and components suffering from the friction causing the drivetrain loss. More torque at a given rpm will result in more drivetrain losses.

In the same chassis a heavy equipment truck BBC 454 with 400lbft at 2000 rpm will exhibit more driveline loss at 2000 rpm than a SHP BBC 454 that is only producing 300lbft at the same 2000 rpm. The truck BBC will have less driveline loss than the SHP BBC at 5500 rpm, because it is delivering less torque at 5500 rpm. Same displacement and engine series in the same chassis, with two different driveline loss curves because the torque curves are different.

The friction loss per unit rpm at the friction surfaces in the driveline increases exponentially by the cube factor (SHP Duke is right). Unfortunately the friction at each surface and fluid loss is not the same, so the driveline loss hp/rpm you experience on the dyno does not look like a clean cube equation line. The loss in percent of total power increases as rpm increases so the loss curve "smiles" with a turned up end as rpm increases.

Apply this to the OP question, and the C2/C3 chassis:

A 300hp 327 will see ~7% loss at 2500 rpm growing to ~20% at 5500 rpm in 4th gear (no trans gear reduction, just fluid drag).

A 450hp 427 will see ~10 loss at 2500 rpm growing to ~30% at 5500 rpm in 4th gear.

Runs made in a lower gear will see more driveline loss, and a more dynamic curve with more loss as the rpm increases.

A C2 that is tied down to where the rear axle driveshafts are straight will see less driveline loss than a lowered car with more power that has to be tied down with more force, a set up that results in angles at the axle shaft u-joints and more friction at contact surfaces

If the driveline components are new and lubed properly, and low loss lubricants are installed the loss can be reduced. Transmission loss can change 1 or 2 percent less with light ATF in the transmission as opposed to traditional 90wt. The same driveline after a neglected 20,000 miles over ten years will have considerably more loss as rpm increases. Lubricants and materials have changed a lot over 50 years, so power loss reduction methods used in a 2015 car, when applied to a 1965 car can provide loss reduction benefits, but with the risk of durability concerns (yellow brass synchro rings in a Muncie are not as forgiving for wear with light lubricants as the coated components in a Tremec T56).

Use the rules of thumb, but watch your thumbs so they don't get caught sticking out to make you look foolish. Everyone has a different bench racing strategy, so boasting and sandbagging numbers is your choice.

W Guy

That reminds me of a story one of Bill Jenkins' team members told me from 1962. At one race, the car was down a tenth of a second and they couldn't figure out why. After some thought, Bill told them to change the oil in the 3rd member. The next run they picked up a tenth. True story

Verne

63 340HP

Nice personal numbers and article reference. Thanks.

Notice the article loss is highest at the torque peak of both cars tested.

The loss dips after the peak torque & peak friction, because the gears and bearings are designed to sustain low friction at a much higher rpm than the runs.

The torque converter loss of the Comet is not surprising. In College I tested the slip and losses of an Old's switch-pitch modification to my '70 GTO's T400. The rpm to road speed slip changed from ~4% set tight cruising steady at 50 mph to ~11% loose under power at 100 mph at 1/4 mile lights. The power loss at the lights on 1/4 mile runs was ~15% more set loose, based on trap speed loss on back to back runs. Instrumentation on a chassis dyno today is so much better than Saturday afternoon runs at a a track with changing weather and air quality.

Chassis dyno runs in lower gears with an auto also produce questionable results. At Cal Poly in the 80's the ME Technician at first would not let me run my GTO in 3rd because of flywheel speed concerns with the Jennings equipment we had in the IC Engines lab. I also could not make anything close to a full power pull with the pumps and pump head we used for our class testing. Later a few of us were allowed to configure the dyno for more rpm and power for 3rd gear pulls (to settle a bet with the Tech that I could get tires to stick on the drum in a full power pull). The 3rd gear curves were a bit different than the prior 2nd gear runs (and I won the bet with soft Caldwell recaps driving the drum, and probably the only time the school's Jennings dyno recorded more than 300lbft of torque).

These article tests are also straight rear axle cars, without axle u-joints, so C2/C3 curves are different. YMMV.

kolsen911

409/409
I still don't think anyone answered your question. In the Porsche world we use 15% driveline loses. When I built my 3.0L we broke it in and tuned it on an engine dyno for two days, 38 pulls, 245 HP at the flywheel. My engine builder, Mike Bruns, said to use 15% to get rear wheel horse power.

That's probably pretty close in the Corvette. The main difference is the driveshaft and separate differential. The stub axles are close to the same size.

It's like pushing your car up or downhill, more resistance expends more effort.

GOSFAST

It is about a 16%/18% loss according to a supporting vendor up here (on Long Island), and one that does Vettes for a living! Had a conversation about this specific issue with them some years ago.

It is still a "nominal" number but will put you in the ballpark.

Thanks, Gary in N.Y.

P.S. This percentage is also confirmed from our own (engine) dyno testing and many of the rides we've had customers test after the fact!

DansYellow66

That's what the rest of my response said Duke.

SWCDuke

The drivetrain/tire power transmission efficiency is ALWAYS going to be a percentage of transmitted power, and 0.85 is a generally accepted figure for front engine rear drive cars with a manual transmission in direct drive and a hypoid axle, and figure 0.80 for an automatic with a non-lockup converter.

Look at any machine design book, and it will tell you that loss through a gear train is approximately a constant percentage of transmitted power as long as the teeth are robust enough to have very little deflection. If overloaded to the point of excess tooth deflection, the percent loss increases, and the teeth will have short life due to being stressed well over the fatigue limit.

So a small block with 300 SAE net HP at the flywheel is going to loose about 45 HP, and a 400 HP big block will loose about 60 HP between SAE net at the flywheel and SAE corrected RWHP.

This does not include vehicle exhaust system loss, which is accounted for in the SAE net/SAE gross conversion and is often the biggest loss in the whole conversion from SAE gross to SAE corrected at the rear wheels. It has to be calculated empirically for each situation.

In the thread on the difference between under-the-car and side exhaust on a '65 FI engine along with data I have on similarly configured engine lab dyno tests with manifolds and no mufflers I was able to compute a reasonable comparison. Under-the car lost a little over 5 percent and side exhaust was over ten percent, which is double in absolute terms.

On a big block that makes 30 percent more gross HP (which means about 30 percent more exhaust mass flow) than that '65 FI engine exhaust system loss can be estimated using the '65 FI conclusions and the knowledge that vehicle exhaust system pumping power loss increases with the cube of exhaust mass flow, so .05 (1.3)**3 = 11%, and with sidepipes .10 (1.3)**3 = 22 percent.

That's why you hear about big blocks that make well over 500 gross HP on a lab dyno, but may not even achieve 350 SAE corrected RWHP.

The cubic power relationship applies to any fluid flow system, whether you're computing the power required to increase a vehicle's top speed, fixed radiator fan power absorption with increasing speed or any other fluid pumping situation such as computing the additional required power to pump 30 percent more exhaust mass flow through specific exhaust system design.

The trouble with the referenced Hot Rod article is that it is full of engine specs and dyno data, but it fails to reveal the basic relationships that I doubt the authors or editors understand, so it's basically worthless IMO.

Duke