4.2 engine tune and dyno

Got my Mk2 back from the dyno today.

As you may recall I built a 4.2, with some head porting, D type cams and 9.5:1 compression ratio.

I’m running a pair of HS8’s with 100 thou jets from an early XJ6.

To get it to fuel properly he ended up using BCA needles. Any more would have required changing to 125 thou jets.

Peak power at the wheels was 136.3 HP at 4900 rpm.

Peak torque was 294.7 lbft at 3400 rpm.

To put the power in perspective a 5l V8 fuel injected Holden Commodore produces 140 HP on his dyno. The quoted engine power for the Commodore is 165kW, or about 221 HP and 284 ftlb . So I’m pretty happy with that.

He says a 3.4l engine makes about 90 HP, and 3.8 around 110 HP on his dyno.


That torque figure is awesome!
But there is something incorrect here. That torque figure equates to a HP of 191 or 140kw at 3400. And reading off your torque curve you would have 219hp at 4800rpm.
But just a note on dynos, yes some dynos read very low. Many years ago I had my 4.2 E type on the dyno at a well known race shop in Sydney. The engine had been rebuilt and was fitted with Webers. It rated 165hp and the dyno shop said this was quite good as XJS V12 were only making 180hp on the same dyno.

How do you calculate that John?

There are plenty of calculators on the net if you google “torque to hp”
The formula is HP = Torque (ft/lbs) x rpm / 5252
In your case 295x3400/5252 = 190.97hp
No idea why your dyno sheet shows figures that are nowhere near this.

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Hi Andrew,

here is the Dyno test of a rebuilt 4.2 engine I got for $300 (the seller did not know it was rebuilt)

It is 9:1 with twin HS8, standard needles, do not know what cams, its is a S2 XJ motor

fitted to my 420G as the original motor burned too much oil (fouling the plugs…and had very low idle oil pressure)

I have more graphs on this dyno test somewhere, cant find them atm

I also have rigged up a manual choke setup for triple HS8 set mounted to a 420G inlet manifold, but I havent fitted it yet

The correction figure of 0.64 would account for the difference in HP figures.

Question is where do they get that from?

So looks like I’ve bought myself about 25 HP for around $10k :slight_smile:

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I got hold of the rebuild parts sheets as well as the dyno tests and the cost of the rebuild was around $6k ( about 10yrs ago)

Be good to have the extra HP and Torque you have, and a full rebuild would probably cost 10k today (or more)

The seller sure was not in a good mood when he found out, lucky I had the engine loaded and strapped down on my trailer

I was asking him about a few features on the engine, such as the manual chokes, and it had “heat pads” attached at various points on the engine.

So he rang the PPO, I saw his face fall

His Ebay ad said “XJ engine for sale”

He was a mechanic, and wrecking the car for the front & rear end for an older non-Jag project

I vaguely recall reading somewhere that the blueprinted engines used in the original press E Types produced around 220 real bhp. The 150mph target was only achieved by backing off the bearing torques to their minimum to reduce friction, removing the front overriders and motif bar etc. Apparently, the very last few mph were hard to coax, but, if the story is correct, the road testers were told not to come back until the magic 150 had been achieved. Don’t forget, 9600HP, on its original road test returned 650 mpg OF OIL, so a very loose engine.

I presume that would be engine dyno HP though, not rear wheel HP, which is reduced by the gearbox and diff (and whatever ancilliaries the engine is running)

Yes, I believe so. + 20

Turns out I have bonkers rich needles.

BAW is standard XJ6 S1.
BAM is Des Hammill’s recommendation.
BCA is what I’m using.

Cam spec is 0.380" lift, inlet 32/68, exhaust 71/28 with 237 duration at 0.050". Clive Cams 328A grind.

Took it for my first proper drive and the torque is amazing. Fuel economy will be interesting…


I think we all have read that, but in 1962 German Auto, Motor & Sport tested “6162RW” in Germany and Italy and it ran an average of 256km/h = 159mph measured over 1 km both ways. With overriders and a glass rear window etc. The legends regarding the early cars are much exaggerated IMHO, I think they had used perspex simply because no safety glass had been made yet. YMMV.


I have a copy of that magazine cover somewhere.

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Me too. I scanned and sent copies to a few fellow listers years ago. Copyright over here expires 50 years after the photos were taken and/or they were published.


Having given this dyno guy a try I’m tempted to take as few of my other cars to him to get some numbers.

Those sound like quite realistic numbers: congratulations!

I drove about 100 miles in the car today. The motor is fantastic.

With the front end rebuilt, uprated front springs and the new Koni shocks set hard it handles amazingly well. I was keeping up with the E types. The freshly rebuilt brakes are better than any of my E Types.

I’m beginning to really like this car.

Now to fix all the rattles…

After filling up I calculated 13.5mpg for yesterday’s run of spirited driving.

Today I did another 50 miles with the car in 5th gear pretty much the whole time. 17.92mpg.

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