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Define cheap as I don't think there is such a thing when trying to get more power out of an engine.
Power flow and port the head first. You could change to a B Head which is the straight ported head found on the 240.
Larger HD8 carbs from the 420 for more fuel or change to throttle bodies and injection.
Change the cams for ones with more lift.
Increase the compression ratio by fitting 9:1 compression pistons.
Enlarge the valves.
Stronger valve springs.
Tubular exhaust manifolds for better exhaust gas flow and a tuned exhaust.
All these things need to be done together to get the best from the engine as doing one just makes all the other areas the weak spots for losing power.
Lastly you could fit a turbo or a super charger with all the electronics to tune it.
You might consider fitting the Megajolt/ EDIS ignition system, that produced a noticeable torque bump in my car. There has been quite a bit of discussion about it on E Types, some who did before and after dymo runs were getting nearly an extra 30 ft-lbs under 2500 rpm vs a distributor.
I'll second the comment on properly mapped ignition timing. Conventional mechanical, vac capsule and centrifugal springs and weights, are severely compromised by what's possible and the need for a safe margin against knock. Beyond that, if we're staying economic, I'd try increasing the diameter of the exhaust system by about 50% from just after the flexible section going back. And remember that these cars are over half a century old: some simple servicing, restoring and renewing can be a transformation.
Flow bench the head
Increase carb size & tune inlet length (e.g. 3 twin side draught Webers & tuned length tumpets from a filtered plenum.)
High performance cams with suitable valve springs
Fit largest valves possible ~ cut 3 angles on the seats.
Balance crank & flywheel. Lighten flywheel. Forged lighter, weight matched, rods. Fit short skirt pistons if you don't mind the noise. Increase rev limit.
Maximise compression ratio without knock or up octane of fuel used.
Tuned exhaust manifold & freeflow tuned exhaust system.
If money is no object increase capacity as Eagle does.They have build some greater output models for clients but standard ~ Capacity is enlarged to 4.7-liters, enough for an output of 380 horsepower at 5,750 rpm and 375 pound-feet of torque at 4,000 rpm. 0 to 62 mph under 5 secs. Top speed over 175mph.
Fit a 123 Ignition distributor that allows accurate ignition curves. There are other high energy ignition solutions. Curves must be repeatable & accurate.
Ensure adequate fuel line sizes,
Forced induction.
Friction modified synthetic oils.
WBO ~ we know your engine is worn. Just taking it back to full original spec will help a lot.
Last edited by Glyn M Ruck; Dec 11, 2021 at 04:34 PM.
Since we've abandoned cheap and easy (it's the price of enthusiasm), I'd definitely add a Rotrex supercharger and either asymmetric pistons or a twin plug head.
Since we've abandoned cheap and easy (it's the price of enthusiasm), I'd definitely add a Rotrex supercharger and either asymmetric pistons or a twin plug head.
In which case a full dry sump ali engine from Crossthwaite and Gardiner.
I'd use the Rob Beere pistons that effectively redesign the combustion chamber. The current hemi combustion chamber isn't very efficient, and a twin plug system is simply a bodge to try and make up for it's shortcomings. The different pistons remove the need for twin plugs.
In the 50's it was though that swirl around a vertical axis was ideal, now we know that swirl around a horizontal axis gives much better air-fuel mixing, especially at part throttle. The Rob Beere pistons work to change the swirl axis. Although they are forged pistons, and RB has said they are designed as racing pistons and not really suitable for a road car.
Rob Beere seems to spend his life telling people his pistons are not suitable for road use. ~ I wonder why?
Mercedes twin plug 3 valve V6, V8 & V12 engines were a huge success & did massive mileage. Blew some of the cleanest exhausts on the market. Alfa's twin spark engines were great.
Last edited by Glyn M Ruck; Dec 11, 2021 at 06:46 PM.
Rob Beere seems to spend his life telling people his pistons are not suitable for road use. ~ I wonder why?
My guess, based on the description saying "there isn't another gram we can take out of them" suggest they are not durable enough for the average road user who wants a long life. In racing, if it fails after a single race, that's life. A rebuild every 200 miles isn't in the average owners expectations. Maybe Rob Beere doesn't want to deal with combacks? Only my guess.
If I was good enough with CAD I'd be tempted draw up the profile and send it to one of the custom piston makers and get a cast set made.
There's nothing wrong with forged pistons, they're miles ahead of old fashioned cast pistons, I'm running forged pistons in my 51 Bentley and I don't drive that car carefully, it cruises at 70 mph all day and between 80 and 90 mph when passing.
I suppose the weight can be reduced so much that it would weaken the piston, but with today's alloys, how can it be a problem under normal use.
Normal use means, not red lining the engine and keeping it there like in racing.
That part doesn't make sense, road use means less abuse, if they stand up to racing, they will stand up to road use ?
I suppose that if one were to make the piston so thin just to shave off a bit more weight, but wouldn't that defeat the purpose in the end ?
Glyn, The article has some interesting thoughts. Increasing the valve area would allow more power from a 4.2 so long as it can cope with the consequent higher engine speed. And what happens to driveability at part load?
I'm reminded of a catchphrase from the 1970s and 80s: the most critical aspect of engine performance is how the flame goes across the chamber. The asymmetric piston (first suggested for the XK by Klat and Watson in the 1960s), twin plugs, and the central plug in the 4-valve head all serve to minimise the distance from the initial flame kernel to the furthest corner of the combustion chamber. This reduces the time that the last bit of unburnt mixture sits around at high temperature and so it's tendency to knock. In consequence, it allows a higher compression ratio. It also reduces the sensitivity of peak pressure to small fluctuations in the initial burn rate, the cycle-by-cycle combustion variations that reduce driveability, especially with weaker mixtures. The story goes that the D-types gained little from the second spark plug, but did Jaguar try to increase the compression ratio at the same time?
Swirl is something that Ricardo and Weslake favoured. It is important in Diesels. I suspect that in spark-ignition engines the performance improvement is more due to an increase in the turbulence intensity that arises from the same inlet modifications. To some extent, the same goes for tumble, though it will generally carry the flame towards the hot exhaust valves, helping to avoid knock. It would take some effort to design a 4-valve in-line engine without tumble.
Apart from cubes, another advantage of the 4.2 XK is the small squish area that generated some turbulence to accelerate flame propagation. I wonder if the more roof shaped pistons of the 2.4 and high compression 3.4 could be arranged to produce some squish?
To come back to the cheap upgrade to the XK to gain power. For a few hundred spondulix, convert to a 4-valve head and aluminium block. It gives an extra 40 or 50 hp and is
commonly known as AJ6/16. The only downsides are the cam cover aesthetics and arranging all the fittings.
Glyn, The article has some interesting thoughts. Increasing the valve area would allow more power from a 4.2 so long as it can cope with the consequent higher engine speed. And what happens to driveability at part load?
I'm reminded of a catchphrase from the 1970s and 80s: the most critical aspect of engine performance is how the flame goes across the chamber. The asymmetric piston (first suggested for the XK by Klat and Watson in the 1960s), twin plugs, and the central plug in the 4-valve head all serve to minimise the distance from the initial flame kernel to the furthest corner of the combustion chamber. This reduces the time that the last bit of unburnt mixture sits around at high temperature and so it's tendency to knock. In consequence, it allows a higher compression ratio. It also reduces the sensitivity of peak pressure to small fluctuations in the initial burn rate, the cycle-by-cycle combustion variations that reduce driveability, especially with weaker mixtures. The story goes that the D-types gained little from the second spark plug, but did Jaguar try to increase the compression ratio at the same time?
Swirl is something that Ricardo and Weslake favoured. It is important in Diesels. I suspect that in spark-ignition engines the performance improvement is more due to an increase in the turbulence intensity that arises from the same inlet modifications. To some extent, the same goes for tumble, though it will generally carry the flame towards the hot exhaust valves, helping to avoid knock. It would take some effort to design a 4-valve in-line engine without tumble.
Apart from cubes, another advantage of the 4.2 XK is the small squish area that generated some turbulence to accelerate flame propagation. I wonder if the more roof shaped pistons of the 2.4 and high compression 3.4 could be arranged to produce some squish?
To come back to the cheap upgrade to the XK to gain power. For a few hundred spondulix, convert to a 4-valve head and aluminium block. It gives an extra 40 or 50 hp and is
commonly known as AJ6/16. The only downsides are the cam cover aesthetics and arranging all the fittings.
Exactly ~ I'm a swirl & squish believer (gasoline engines ~ if some tumble comes with it all good). I have no idea what the twin plug did in the D Type or what they did with compression ratio. (The heads have become collectors items). Equally I don't know what larger valves & higher RPM did to driveability at part load. All good questions.
Aston Martin DB4 GT had some success with 12 plug heads. Twin distributors were a pig to time.
Note pistons:
Last edited by Glyn M Ruck; Dec 12, 2021 at 06:02 PM.
