Power output of 4.2 series 3 with su carbs?

[RobXjcoupe]

A question of curiosity really, does anybody know what output power can be achieved on the series 3 xj6 4.2 engine fitted with a set of triple su hd6 carburettors?

[a8hex]

RobXjcoupe said:

A question of curiosity really, does anybody know what output power can be achieved on the series 3 xj6 4.2 engine fitted with a set of triple su hd6 carburettors?

Depends on how much you want to fiddle with it. The one in my XK150 was showing about 290 on a dyno (can't vouch for the over all accuracy of the dyno, but it showed a stead increase as CKL worked on making it go in my car, it wasn't one of their engines) when it wasn't working too well. It had been fettled and was running rather extreme cams, keeping the lairy exhaust one and putting a standard series III inlet cam made it much easier to drive and it didn't seem any slower around Goodwood.
When I was researching engines for the XK I found a few places offering a reliable 250BHP, the XJ is quite heavy so you probably want to concentrate of torque more than outright power. That's probably why my XK which was also pretty weighty felt just as quick over all with the normal inlet cam.

[RobXjcoupe]

a8hex said:

RobXjcoupe said:

A question of curiosity really, does anybody know what output power can be achieved on the series 3 xj6 4.2 engine fitted with a set of triple su hd6 carburettors?

Depends on how much you want to fiddle with it. The one in my XK150 was showing about 290 on a dyno (can't vouch for the over all accuracy of the dyno, but it showed a stead increase as CKL worked on making it go in my car, it wasn't one of their engines) when it wasn't working too well. It had been fettled and was running rather extreme cams, keeping the lairy exhaust one and putting a standard series III inlet cam made it much easier to drive and it didn't seem any slower around Goodwood.
When I was researching engines for the XK I found a few places offering a reliable 250BHP, the XJ is quite heavy so you probably want to concentrate of torque more than outright power. That's probably why my XK which was also pretty weighty felt just as quick over all with the normal inlet cam.

I was hoping for the 250bhp mark. My engine was built as standard series 3 injection with a nos block, crank, pistons and rods. Head is standard series 3 injection which had the bigger inlet valves as standard. That had a skim as it was a used item. I’m sure it was .3mm skim, bronze guides fitted and re-lapped valves. Cams inlet and exhaust are both standard series 3 injection. The carbs have been re-built but never set up properly but both fuel pumps are now kerput. Oh and exhaust is a nice Hayward and Scott if that helps?
I was curious to what I’ve created so when it goes for a tune I have a rough idea of what it should be making.
Do you know what amount of torque it should produce? Max bhp on these standard engines seems low as they don’t rev much but not bad for 5.5k rev limit

[a8hex]

RobXjcoupe said:

Do you know what amount of torque it should produce? Max bhp on these standard engines seems low as they don’t rev much but not bad for 5.5k rev limit

Hunt back true posting here or in the classic forum by "lowdrag" he's often posted the printouts from his Lynx, I can't remember the figures but at the top it hit the limit of the dyno they were using.
When you say "Low" remember this is basically a 1948 engine :-)
It was also always about torque not power, or area under the torque curve since that's really what matters.
If you want more poke I've seen an XJ Coupe running an AJ6, so I presume you could get the SC AJ16 running in the car relatively easily. But a well setup XK is always a wonderful thing.

[RobXjcoupe]

a8hex said:

Hunt back true posting here or in the classic forum by "lowdrag" he's often posted the printouts from his Lynx, I can't remember the figures but at the top it hit the limit of the dyno they were using.
When you say "Low" remember this is basically a 1948 engine :-)
It was also always about torque not power, or area under the torque curve since that's really what matters.
If you want more poke I've seen an XJ Coupe running an AJ6, so I presume you could get the SC AJ16 running in the car relatively easily. But a well setup XK is always a wonderful thing.

I like my xk unit, it won’t be changed It looks fab under the bonnet and more so with its triple su setup. Just wondering what it’s capable of that’s all

[mph]

Any reason you've gone for HD6's ? HD 8's are more usually fitted when going for performance.

I wouldn't worry too much about the HP, it's torque you want for a heavier car such as yours. I would suggest that 250hp may be slightly optimistic.

[RobXjcoupe]

mph said:

Any reason you've gone for HD6's ? HD 8's are more usually fitted when going for performance.

I wouldn't worry too much about the HP, it's torque you want for a heavier car such as yours. I would suggest that 250hp may be slightly optimistic.

The above is my engine bay. To be honest it’s been 10 years since I have worked on the jag properly. So hd6 hd8? My hands are up as I don’t know lol
Any advice is welcome though

[mph]

That looks very impressive.

I think the carbs are probably HD8's (2 inch) rather than HD6's (1 3/4 inch)

An interesting project - let us know how it develops.

[RobXjcoupe]

mph said:

That looks very impressive.

I think the carbs are probably HD8's (2 inch) rather than HD6's (1 3/4 inch)

An interesting project - let us know how it develops.
I was going to say 2inch but honestly couldn’t remember one way or the other. Nothing runs at the moment, the engine will be out again soon to get the bodyshell up to scratch so whilst in bits again I’ll do a proper job and make sure all is perfect

[a8hex]

I'd thought they looked more like 2" SU's than the smaller ones, I spend ages yesterday searching back through old photos trying to find some of HD6's and 8's to compare.
I don't think Jaguar ever fitted triple HD6's to a production car. The first triple carb models were the XK150S models and even the 3.4L version used triple HD8s.

[RobXjcoupe]

a8hex said:

I'd thought they looked more like 2" SU's than the smaller ones, I spend ages yesterday searching back through old photos trying to find some of HD6's and 8's to compare.
I don't think Jaguar ever fitted triple HD6's to a production car. The first triple carb models were the XK150S models and even the 3.4L version used triple HD8s.

The inlet manifold and carbs were from a 420G. I found them on an auto jumble stall, all seized up but for £60 I couldn’t really argue that price.

[mph]

From a 420G they'll definitely be HD8's then. £60 - what a bargain !

You might want to fit a freer flowing air filter to get the best out of them, especially if your exhaust system is uprated.

When I uprated the XK engine on my own car, changing the air filter made a huge difference to the power output.

I currently have this one from Rob Beere Racing, scroll down the page for ITG filter. A bit expensive but it does work and also includes revised needles to suit.

http://www.rob-beere-racing.co.uk/induction.html

[RobXjcoupe]

mph said:

From a 420G they'll definitely be HD8's then. £60 - what a bargain !

You might want to fit a freer flowing air filter to get the best out of them, especially if your exhaust system is uprated.

When I uprated the XK engine on my own car, changing the air filter made a huge difference to the power output.

I currently have this one from Rob Beere Racing, scroll down the page for ITG filter. A bit expensive but it does work and also includes revised needles to suit.

http://www.rob-beere-racing.co.uk/induction.html

The filter currently is a hotch potch home made affair. The itg filter looks free flowing but wouldn’t it draw in hot under bonnet air without some kind of casing around it? Again it’s a detail that needs looking at but without a rolling road session I don’t know what exactly is needed to get the most from the engine with the bonnet shut. It would be nice to get v12 power from the modded 4.2.

[RobXjcoupe]

That’s the exhaust side of the engine. Needs a clean up, probably get it ceramic coated rather than try to keep polished

[a8hex]

mph said:

From a 420G they'll definitely be HD8's then. £60 - what a bargain !

You might want to fit a freer flowing air filter to get the best out of them, especially if your exhaust system is uprated.

When I uprated the XK engine on my own car, changing the air filter made a huge difference to the power output.

I currently have this one from Rob Beere Racing, scroll down the page for ITG filter. A bit expensive but it does work and also includes revised needles to suit.

http://www.rob-beere-racing.co.uk/induction.html

Or just go for trumpets and socks. There's a lot more room under the XJ bonnet than there was under the XK150's.
Rolling roads don't always give the same characteristics for under bonnet air flows as actually trying it out on the road or track.

[mph]

a8hex said:

Or just go for trumpets and socks. There's a lot more room under the XJ bonnet than there was under the XK150's.
Rolling roads don't always give the same characteristics for under bonnet air flows as actually trying it out on the road or track.

The inlet manifold in the OP's picture doesn't look like the Mk10 version but I may be mistaken.

Socks over ram pipes actually give a rather limited area of filtration. You also have the issue of what length of ram pipe is best.

As the OP has a reasonable amount of under-bonnet room perhaps a custom made manifold with a large readily available filter stuck on the end would be something to look into.

Also to to be considered is that many of these aftermarket set ups will greatly increase your intake noise. Not sure how noticeable that will be on the coupe or whether it's a concern.

I'm far from being an expert in this subject but I did read an awful lot when trying to liberate more power from my own engine. The more I read the more confused I became until I now know no almost nothing..

Unfortunately my preferred option, stub stacks and ITG filters, won't fit under my bonnet. The Rob Beere arrangement is relatively compact.

Edited by mph on Saturday 27th January 21:28

[RobXjcoupe]

Although there is room on the exhaust side, intake side is limited due to the brake servo. The intake filter is basically made from the original two carb item screwed to a plate with 3 holes to suit the triple carbs with a cone filter attached to the front of the intake plenum so to speak. It’s too small an internal capacity unless I design something that is a little taller.