BHP of 4.2 engine.

[skodajag]

I know that Jaguar's quoted output for their engines was optimistic but I can't now recall what is the typical true output (at the wheels) for the standard UK spec 4.2 E type engine. Please advise.

[politeperson]

About 180 bhp at the wheels if it is fit.

Of course it is the twisting force at lower revs that is far more important for the XK unit (torque) which is why it is so quick, developing over 320n/m at 3,000 revs..

[Tbob]

About 180 bhp at the wheels if it is fit.

Source?

[politeperson]

The real world is my source!.

Emerald. They do quite a few Jaguars on the rolling road.

The whole 265 bhp thing comes from the difference between Brake/ SAE and DIN figure I think.

[chrisfell]

The late Dennis Jenkinson (Jenks) owned an E-Type or two. He estimated that a the 4.2 in his car produced “an honest 180bhp”. His seat of the pants dynamometer was honed by years of experience of testing cars, both roads cars and race cars. So I call that a good source.

Others may chime is with real world dyno readings. It would be interesting to see a before and after standard engine that had matched ports and perfect valve and ignition timing.

[Tbob]

Others may chime in with real world dyno readings.

This.

[Nickleback]

I had my 4.2 engine with triple SU's fully rebuilt by Classic & Modern Engine Services in July 16 to full Jaguar standard specification, then ran it in over 1000 miles, had it fully tuned up on rolling road dyno by Classic & Race. It then produced the following figures on the dyno with the std drum air cleaner on:
Peak torque: 196.05 @ 3300 RPM and Peak Horsepower @ 142.34 @ 4200 RPM, all actual at the rear wheels.

[politeperson]

Sounds like a pretty healthy engine Mike,

After I had spent quite alot of money on my FI engine, it ended up producing 210 bhp at the rear wheels whilst producing 347 n/m of torque from about 2200 rpm. I would think this woulds equate to about 250 bhp DIN. Not sure how that would have been published in the 1960's bu Jaguar. Probably 290bhp SAE! Someone on here might have a clue.

DIN measures the power with all standard accessories installed, the way the engine is used.(if power steering or AC is option, than this is not installed, if standard, it should be attached).

BHP (not too often used british standard IIRC) is the bare engine, with exhaust and the air filter taken off.

SAE meaures the power without any accessories (alternator, water pump [basically anything belt driven], air filter, exhaust etc) to get the max value engine can produce.

For lb-ft, HP = torque * RPM / 5252 so when the RPM = 5252, HP = torque

Of course, HP is a good indicator of top speed as the engine will be revving close to maximum on many engines.

Torque is a better indication of acceleration as it is the twisting force lower down the rev range. You never put your foot down when you engine is already at 5,000 rpm do you?

The best way of appreciating the difference between torque engines and high HP engines is to think of the difference between a steam engine from 1890 producing 1hp and a 20019 Honda CB1000 bike producing 190bhp.

The steam engine can pull up tree stumps at 100 rpm because it has so much torque, whereas the motorbike would definitely stall in that situation.

However the Honda will produce large quantities of torque at 10,000 rpm.

[Mark Gordon]

Excellent write-up, James. I've always had trouble understanding the relationship (or lack thereof) between horsepower and torque and how horsepower is measured by various standards. You make it very clear. Thanks!

I once saw a Honda motorcycle on display that had an engine which produced something in the neighborhood of 500 hp. I always wondered how it would work in my little Spitfire. I guess from what you have explained that the Honda engine would have been a bit of a dog in a Spitfire because of the lack of torque at low rpms in a vehicle that weighed considerably more than the car.

[politeperson]

Thanks Mark,

I am pretty sure most motoring journalists haven't a clue about BHP either. You just have to read what they write!

Generally, engines only make maximum bhp when you are about to change gear and have already done your accelerating.

L J K Setright of course, knew about everything.

James

[69E4.2FI]

This is my dyno run, at the wheels.

[Jeremy]

To expand a little on James's excellent explanation, this article explains the relationship between power, torque, and RPM very clearly:
http://www.epi-eng.com/piston_engine_te ... torque.htm
James, your picture of a slow, relentless stump-pulling steam engine Vs the whizzing Honda bike captures the distinction perfectly. I have read more pieces of rubbish than I could count on this topic by professional writers who would have benefitted from your post!

[christopher storey]

With all due respect, that article is ok ( leaving aside his assertion that 1 Ton is 2000 lbs ! ) until it comes to this passage

"Think of it another way: In cars of equal weight, a 2-liter twin-cam engine that makes 300 HP at 8000 RPM (197 lb-ft) and 400 HP at 10,000 RPM (210 lb-ft) will get you out of a corner just as well as a 5-liter engine that makes 300 HP at 4000 RPM (394 lb-ft) and 400 HP at 5000 RPM (420 lb-ft). In fact, in cars of equal weight, the smaller engine will probably race BETTER because it's much lighter, therefore puts less weight on the front end. AND, in reality, the car with the lighter 2-liter engine will likely weigh less than the big V8-powered car, so will be a better race car for several reasons."

Since he has already postulated that the cars are of equal weight , his observations about relative weights are plainly wrong , and his suggestion that the lower torque engine will pull you out of the corner better than that with high torque is , putting it kindly, debatable except just possibly ( and purely theoretically ) in the case of an engine with virtually limitless rpm capability and a flat torque curve over almost all of the capability. I know of no such internal combustion engine , either reciprocating or turbine, and those reciprocating engines which can reach extraordinary rpm - say 18,000 rpm , invariably have a very narrow power band , as do gas turbines

[politeperson]

I have completed 100,000 miles in a car with a virtually limitless rpm capability and a flat torque curve over almost all of the rev range. My Tesla Model S!

Admittedly no internal combustion takes place. Its effing quick though.

Not as much fun as an E type either. I gather Mr E Musk has an E type too.