Camshaft Itch. Scratch it or Leave it?

[politeperson]

My car is running really well. Fast, smooth, economical, dry (leaks) and reliable.

"If it aint broke dont fix it" (my aeronautical engineer grandfather used to say)

So, I think the best thing to do is ignore his advice and fiddle with it.

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If you have followed my progress, I rebuilt a standard short stud thick deck 1967 engine and fitted it in January.

It has a free flowing exhaust and Jenvey throttle bodies with an Emerald ECU.

However, although it sounds great and goes well I think my engine has much more to offer.

On the rolling road, it put out 200 bhp at the flywheel and 341 nm max. I want more (please).

The compression test shows that with new Mahle 8:1 pistons the cylinders show 180,195,195,195,195,195 psi warm with the throttle open. I think this is good.

The oil pressure is off the scale at 70 psi running, 40 psi idling hot with a mechanical gauge. This is also good.

Oil consumption is small. Oil leaks are zero.

The head has no modifications. It is just all clean and new apart from the cams.

Valve clearances are spot on.

I have cams from a 420 saloon. They looked OK but I never measured them. Everything else is new having covered 1,500 miles or thereabouts.

I checked the cam timing at the weekend with the Jaguar tool and a dial gauge. It is spot on TDC.

I put some electronic verniers across a couple of lobes. The indicated about 9.1mm of lift maybe (they are not that accurate).

So I think I should spice it up with a nice new pair of camshafts now it is run in.

These ones look interesting.

http://www.pipercams.co.uk/pipercams/ww ... =JAG6BP270

Means I don't have to take the head off, just need to dial them in.

Looks like fun.

What do you think? Waste of time and money? or the road to bigger thrills.

Opinions please.

[christopher storey]

I do not wish to be a killjoy but FWIW, for a road car I think you will be wasting your money and robbing the car of much of its attraction. It is significant that throughout the whole 44 year life of the XK engine Jaguar never altered the basic cam timing. They did in fact retard the whole thing by about 4 degrees in, I think, about the mid 80s but that was for emission purposes, and the overlap stayed unaltered at 30 degrees. What you are suggesting is to almost double the overlap to 56 degrees, and there is only one effect which will result from that : the low to mid speed torque which is the greatest attraction of these engines will be sacrificed for a small gain beyond about 4500 rpm which is an area hardly ever visited with a road car, particularly a 4.2. If your free flow exhaust is a fabricated manifold type, you will already have lost a not insignificant amount of torque in the 2250 - 2750 rpm range ( right where you need it ) and these cams will only worsen the position. We did try these cams some years ago in a 2+2 belonging to a friend of mine . Where did they end up ? In the dustbin

[MarekH]

Dear James,

You need to do the exact opposite to get more "oomph" where it is needed. More power high in the rev range is of no use to a road user. You want more torque in the low to mid range and to achieve that you need to do two things:-

1/ Generally speaking, you'll want to increase the length of the inlet runners- especially as you have throttle bodies with near to zero inlet runner length. This will increase torque low down.

2/ You want a camshaft with less overlap and lift. This is going in the opposite direction to what you suggest and it will mean less power, i.e. a lower headline number to boast about. The flipside is that you will have more torque low down which is where it is wanted "on the road", i.e. in the mid range when you go for an overtaking manoeuvre or at every gear change . The net result is that the peak torque is moved further down the rev range - to where you can use it and to where it has the most benefit.

Failing that, do nothing and spendf the money on a beautiful set of covered headlights.

kind regards
Marek

[Hugo]

I absolutely concur. You have to be VERY careful with camshafts or you will end up with the opposite of what you want. These claim a 15bhp increase - but at what speed? 6,000 rpm? 5,000 rpm? Useless unless you're going racing.
The maximum BHP is in any case a meaningless statistic. What counts is the bhp the engine is producing at, say 2,000 - 3,000 rpm. In other words, the torque is a far more valuable indication of what response you will get when you put your foot down.
Valve timing really is a dark art - you change one thing and everything else goes out of kilter. The one single thing that will increase torque is to close the inlet valve early. But that will affect, and be affected by, the inlet tract and exhaust back pressure.
It's surprising how much you tell about the characteristics of an engine just by looking at the cams. I like littel pointy cams, not the big bulbous things you get on racing engines. I have a 1917 LaFrance engine, 4 cyl, 9.5 litre, 60 bhp @ 1,200 rpm. The cams on that are like pimples - and that is all torque.
There is one thing I have never understood - I don't know if anybody on here can help me - how do you get 195 psi compression out of an 8:1 compression ratio? According to my calculations, the maximum you should get is about 115 psi - and that is not allowing for the fact that the valves do not open and close at the top and bottom of the stroke, so you're never getting the full swept volume.
Also, I would suggest that with those compression pressures, you are getting dangerously close to pinking/pinging territory. If you fit a high-torque cam (if there is one) it will push the compression pressures up even more.

[kingzetts]

Hugo, 8:1 mechanical compression ratio in a cold engine will give 116 psi or so only if compression takes place isothermally (without an increase in temperature) which requires the energy added by compression to be removed into the engine block/head. There isn't time for this to occur even at cranking speed and compression is closer to adiabatic meaning the air heats up while being compressed, and the increase in temperature results in the "additional" pressure.

James, I have a standard 8:1 3.8 with standard intake and exhaust systems running on EDIS/Megajolt ignition. It was rolling roaded at 265 bhp (flywheel) and about 260 lb.ft and has masses of midrange torque, and I virtually never rev it over 4000-4500 rpm even during "spirited" road use. Your figures compared to those seem low for a "tuned" 4.2 so sounds like there is potential to unlock - but like the others, I'd be looking at intake and exhaust systems rather than cams.

But it is fun to tinker, isn't it?

[politeperson]

John,

Have you got standard cams then?

[Hugo]

Of course! Thank you.

[kingzetts]

James, yes, standard cams.

[Heuer]

From the Forum 'Knowledge Base':
VSE Camshaft specs (standard, fast road, race etc): https://www.dropbox.com/s/og5m55kzkh9zoll/cams.jpg?dl=1
Piper Camshaft specs: https://www.dropbox.com/s/digbxgvwgp101 ... s.jpg?dl=1

I have VSE non standard cams, 9.5:1 compression, EDIS/Megajolt, straight through 1.75" silencers, ITG air filter. My car has been dyno'd at 280bhp and, more importantly, 340lb/ft of torque. Frankly it is too quick for UK roads but great on the empty roads of France!

[chrisfell]

I'd speak to the technical experts at Rob Beere Racing about their standard timing, high lift cam. If the rumours are correct this gives increase in wide open throttle power with no loss of the XKs higher useful mid range torque. Rob Beere Racing also used to do a timing setting tools with slightly altered cam timing for a little extra go. That's where I'd start if I was in the market for a few more torques and horse powers.

One other thing I might do is investigate the low psi reading on one of those cylinders.

[Heuer]

Means I don't have to take the head off, just need to dial them in.

From the VSE catalogue:

[politeperson]

Thanks boys,

At the moment I am thinking build up a "new " head and swap it all in one. Easier to do the shims on the bench and I have an "in" on some bench flowing maybe.

Hugo, why do you have a owl on your head?

James

[Hugo]

Thanks boys,
Hugo, why do you have a owl on your head?
James

I think he was just warming his feet. Poor little fellow died a while back, from 'bumblefoot' or whatever the technical name is. We adopted him from a sanctuary - built a nice aviary for him out the back, then one October day he came in and discovered the joys of central heating, perched above a radiator, felt the warm air up his bum & never went back. He used to play with the cats. Most amazing creature I've ever met. More pics on my facebook page.

[politeperson]

Very sorry he has now flown away.

I will have a look on f/book.

He looks like a lovely chap in the picture!

[Hugo]

Certainly was - you would not believe how empty the house felt after he had died. If anybody wants more pics please feel free to PM me.