The V8 Owners Forum -

With an automatic transmission, ratios other than 1:1, would they absorb more engine power?

I would imagine that the 1:1 ratio would be the most efficient, that is to say delivering the highest percentage of the applied engine power through to the rear wheels.

Ron.

Ron,

Old style automatics such as the BW 35 and BW 65 and probably the ZF4 HP22 are said to absorb more power than manual and new type auto boxes.

Even in top gear you are driving the hydraulic pump so power will absorbed.

Kevin.

I would agree that driving any transmission at 1:1 is going to be more efficient than driving it at a ratio.

In addition the torque converter slips a few %. However, newer boxes such as the ZF4HP22 will lock up the torque converter when in 4th gear to eliminate that loss.

I would have thought the best solution would be to have a manual transmission with 1:1 for top gear, and select a diff ratio accordingly, rather than this idea of 4th is always 1:1.

Chris.

Hello Kevin and Chris,

Many thanks for your replies.

I wonder just how many horsepower my BW 35 does absorb? Would the loss be essentially fixed, independent of engine power?

Or would the loss be directly related to engine power?

I have seen arguments from both points of view.

Ron.

I would think that the loss has two components:
One related to engine speed, caused by bearing friction and pumping oil.
One related to torque, caused by geartooth friction.

Exactly how / who measures this kind of thing though.... I don't know.

Chris.

You need to know the stall speed of yout torque converter first. For road use you should be stalling at around 1800rpm for max efficiency. I race my car and use a 3000rpm stall converter.
If you put the car on a dyno you will get a good idea of transmission losses.

Hello Perry,

The standard stall speed for the torque converter as fitted to the BW35 behind the 3.5 Rover V8 in the P6B is 1950 to 2250 rpm.

My 1974 P6B was run on a dyno prior to having the engine removed.
The run was undertaken in second gear, which is a ratio of 1.45 : 1. This yields a top road speed of 128kph @ 5200 rpm.

The differential ratio is 3.08 : 1.

Maximum power as measured at the rear wheels was 69kW (93 horsepower) @ 120kph, which is near enough to 5000rpm.

The engine had an original CR of 10.5 : 1, so initial dynamic timing had to be set to TDC @ 600rpm to avoid pinking with the local fuel, but even so it would still pink when pushed. The quoted maximum power output of the engine for the Australian market was in the order of 106kW (143 horsepower), and 273Nm (202lbs/ft) of torque.

I have seen printouts of other P6B dyno runs, and the outputs were very similar.

After my 4.6 (running a CR of 8.37 : 1) was installed, at some 3500km (2000miles) the engine was tuned up on the same dyno. All other areas of the car remained the same, so the power difference would be soley due to the engine.

The run was done in second gear, with a maximum power of 105kW (141 horsepower) @ 4750 rpm being recorded at the rear wheels. The average power delivered by the 4.6 is considerably greater, as an example at 50kph (30mph) close to 100% more power is being delivered to the rear wheels, compared to the 3.5.

My 4.6 runs a custom ground high torque camshaft, designed to improve the low speed torque and performance of a HSE Range Rover. To that end, it works extremely well in my P6B with its tall gearing.

I am assuming that my 4.6 is producing somewhere near 150kW (200 horsepower) and probably close to 400Nm (300lbs/ft) of torque.

What do you think?

Ron.

Ron,

If you have a look at the figures produced at the forum rolling road day last year and compare perry's automatic to the manual cars rear wheel BHP it will give you an idea of the additional power loss of the auto box.

Perry's car runs a TH350 which is probably fairly similar being an old style 3 speed auto box, I think the difference was as much as 20 BHP so your figures look comparable.

Not sure how rolling road operators calculate the frictional losses to arrive at an estimated flywheel BHP figure, I feel some could be best guesses!

Kevin.

CastleMGBV8 wrote: Not sure how rolling road operators calculate the frictional losses to arrive at an estimated flywheel BHP figure, I feel some could be best guesses!

Kevin.

It's a bit of a finger in the air job really. After the power part of the run, the rolling road drives the wheels. It measures how hard they are to turn, and then adds this to the measured RWHP to get BHP. Estimate at best!

Chris.

Hello Kevin,

Thank you indeed for the pointer! The dyno results are most interesting indeed. My 4.6 with its twin SU carbs holds its own quite nicely too...!

Maybe it is time for me to buy some new glasses, for until you gave me that pointer, I had missed those posts altogether.

Thanks indeed!
Ron.

ChrisJC wrote:

CastleMGBV8 wrote: Not sure how rolling road operators calculate the frictional losses to arrive at an estimated flywheel BHP figure, I feel some could be best guesses!

Kevin.

It's a bit of a finger in the air job really. After the power part of the run, the rolling road drives the wheels. It measures how hard they are to turn, and then adds this to the measured RWHP to get BHP. Estimate at best!

Chris.

I thought it timed how long it took for the rollers to stop spinning after the power up- like it it takes 3 seconds for everything to come to a halt you know its pretty damned inefficient and if it rolls on for 10 mins it is bloody good! Obviously those times are exaggerated but I thought that was the principal...

TC

I just looked at my Dyno day print and it says I lost 51hp (38kw) on the resistance run

Now I can push my car fairly easily and don't think I an 51 ponies strong!

Also the resistance on the back of the gears will be different to the resistance on the front face of the gear.

I'd go along with it being an oily finger black art thumb suck!

Ian

What drive train do you have Ian?

Renault R21 Turbo Transaxle - UN1 (The Turbo box gives longer gearing or the V8 ends up revving it's nuts off!)

Ian

Here's some useful information, although related to Subaru's it also would apply to proper cars.

http://www.andyforrestperformance.co.uk/26063.html

Kevin.