4.2 engine power output

[Alty Ian]

May I please ask you experts out there to enlighten me,.....I have read that the US series 1 4.2 engines delivered less power than the UK cars due to emission control regs etc and had a different differential.

What are the actual technical differences between UK and US cars in terms of bhp, acceleration, top speed and economy?

And presumably the US 4.2 engines could be uprated/improved with some simple tweaks then?

Ian

[christopher storey]

Virtually all US 4.2 cars were supplied with a 3.54 final drive which gives only about 21.5 mph/1000 rpm in top and with corresponding reductions in the intermediate ratios. For traffic light drags up to the then US 55 mph limit - stunning, but for anything else pretty useless

From late 1967 on, all North American cars had twin Stromberg CD carburetters, with a dual plenum crossover device which heated the mixture from the exhaust manifolds. Quite what this did to power is hard to know, but the probable effect was to lower it from what might be guessed in modern terms to be 210 - 220 DIN hp ( as a guide the S3 XJ6 with EFI but with lots of emission equipment had 205 ) to perhaps 170 - 180 DIN hp

[Moeregaard]

With the 3.54 final-drive ratio, I suspect there is very little difference between the cars with emission controls versus those without, since first gear is used up so quickly right after launching and second maxes out pretty quickly as well. I lived with the 3.54 (and the damned 55-mph limit) for many years, and fuel economy on road trips was typically around 23 miles per U.S. gallon (3.8 litres) in my '65 4.2-litre coupe. I suspect that fitting the 3.07 would have added two or three more miles per gallon.

The 4.2 in stock form has always been regarded as a slightly "lazier" engine than the 3.8, which for years was favored by tuners. It was designed to produce greater mid-range torque, making it more suitable for automatic gearboxes. The downside to this is that the cylinder-bore repositioning resulted in some harmonic issues with the crankshaft and a lower redline. Much of the 4.2's laziness can be eliminated by fitting a lighter flywheel. With the bores closer together than those on the 3.8, the head gasket has a pretty big job, so I'm not a fan of high compression ratios on the 4.2 either.

[PeterCrespin]

I lived with the 3.54 (and the damned 55-mph limit) for many years, and fuel economy on road trips was typically around 23 miles per U.S. gallon (3.8 litres) in my '65 4.2-litre coupe. I suspect that fitting the 3.07 would have added two or three more miles per gallon.

Wow. Ethanol fuel must be worse than I thought. I don't think I've ever topped 16-17 miles per US gallon on my Strangleberg 3.54 FHC. It runs well and is not over-rich.

Remember too when comparing emissions poser outputs around this period that BHP testing protocols changed in the industry. Lots of published outputs fell dramatically for basically unchanged engines, simply because the test methods were closer to as-sold trim.

Pete

[Moeregaard]

Peter, my figures were taken from my annual runs from the Los Angeles area to the Monterey Historics every year. Even during the bad old days of the double nickel, the CHP would give us some leeway that weekend and a steady 65--70 MPH would yield some pretty good figures. Mileage around town was more like 14--15 MPG, although I could get 12 if I worked at it

[Alty Ian]

Thanks for the info, so what top speed would the car achieve with a 3.54 diff then, mine has SU's BTW. Do the early 4.2's have any emission control devices compared with the later ones?

[Moeregaard]

An E-Type with 3.54 gears will have a maximum speed of about 120 MPH, at which point you will be approaching 5,500 RPM. With 3.31 gears you can bump this up to 130 MPH, while the 3.07 gears will give you a theoretical maximum of 150 MPH. What's annoying about the 3.54 is that it has the engine turning at around 3,200 RPM at 70 MPH, which is a little busy for cruising.

The only emission controls fitted to the earlier cars consisted of running the crankcase breather line over to the air filter. An oil separator was incorporated into the breather housing during 4.2 production, but it wasn't until the advent of the 1968 model year and the Stromberg carbs that emission controls became serious--in the U.S. and Canada anyway. The rest of the world got to keep their SUs.

Incidentally, my very early '65 FHC had the oil separator fitted, but the breather outlet was only about 5/8" in diameter--far too small for effective crankcase ventilation. I replaced it with a breather housing from a 3.8 and this worked well. My much later '66 FHC had a different oil separator with a much larger outlet, so Jaguar apparently recognized the problem. So, to answer your question, the early 4.2-litre cars did not really have what we would recognize today as emission-control devices.

[38E]

An E-Type with 3.54 gears will have a maximum speed of about 120 MPH, at which point you will be approaching 5,500 RPM. With 3.31 gears you can bump this up to 130 MPH, while the 3.07 gears will give you a theoretical maximum of 150 MPH.

That seems a bit low. So what axle ratio did the initial Motor and Autocar 150 mph road test cars use?

[38E]

And what happened to the rest of my post??

It was all there in the preview.

[christopher storey]

As far as I recall, the initial road test cars were on 3.31 axles, but used Dunlop R5 tyres so that 150 mph equated to about 6200 rpm allowing for the expansion of the diameter as a result of centrifugal effects. This is consistent with the account of the max speed test given by , I think, Charles Bulmer

[Moeregaard]

It's worth keeping in mind that the early 3.8-litre cars given to the press had received some "special attention" from the factory before being turned over to the journalists. An accepted figure for a bone-stock 3.8 is probably closer to 140--145 MPH.

[38E]

As far as I recall, the initial road test cars were on 3.31 axles, but used Dunlop R5 tyres so that 150 mph equated to about 6200 rpm allowing for the expansion of the diameter as a result of centrifugal effects. This is consistent with the account of the max speed test given by , I think, Charles Bulmer

Thanks Christopher, that's what I had thought all along, 3.31:1 and R5s. As for the testers exceeding the rev limit to get these high speeds, personally, I never worried about that in those days. With most of the sports cars I had at that time, I would generally just keep my foot hard down until the speed stopped increasing if I wanted to check out the top speed. About the only engine I remember that needed the revs to be watched in those days was the twin-cam 1600cc Lotus/Ford in the Elan. That engine would blow up if over-revved, but then it normally had a cut-out in the distributor.

[malcolm]

That's interesting Christopher. I thought both the 3.8 and 4.2 had the standard Uk axle (3.07 for 4.2 manual) but had to over rev by 500 revs to hit the 150 mph. This meant 6000 for the 4.2 and 6500 for the 3.8.
Certainly, 6000 revs on the 4.2 (end of the red band, but 500 revs more than the handbook max permitted revs) would give 150.
As we know, the engines used to achieve these revs weren't standard!
Have I been mis informed?

[christopher storey]

Yes, your information is not correct. The original UK 3.8 cars were turned out with 3.31 final drives. In about early 1964, this was changed to 3.07, ( about the same time as some of the late 3.8 cars were fitted with the all-synchro gearbox ) and for all European spec short wheel base cars thereafter 3.07 was the standard . For 2+2s , 3.07 was used for manuals, and 2.88 for autos in Europe

[pmansson]

This combination is, in my limited view, the best combination, providing 5th is OD and not 1:1.
120 Km at 2.500 and very good acceleration.
Just spent an hour in the Swiss mountains in a 3.8 with that setup. The car is very, very well sorted and an absolute killer esp going uphill where I am on 3rd between 3,000 and 3,500 except for 2nd in the hairpin bends (preceeded by a good revving with the left foot on the clutch..) In that RPM range I am doing between 70 and 110 on the fairly straight sections, but still with nice curvs along the way. Brilliant 10 km stretch plain uphill !!!