Setting tickover with different camshafts - JAG4 from Guy Broad

[rfs1957]

I'm interested in hearing opinions from people who have hands-on experience in setting up the XK engine with different camshafts, for road use, rather than bar-room hearsay.

This is a subject that I'm reluctant to broach on the Forum, as it's always been apparent that the received - and vocal - wisdom was Do Not Bother Changing The Cams.

I remember a quote that these were the first parts that people took off and tried to flog second-hand !

Quite prepared to concede that this may be me one day.

Nevertheless, I've deviated from received wisdom sufficiently in the past to want to try to find this out for myself, and as the advice I got from Guy Broad was that their "D-Type" cams remained perfectly civilised and flexible at low speeds/revs, without compromising mid-range, whilst adding top-end, I thought I would give this a spin.

These cams are referred to as "Jag4" and they have the same lift as the originals, but with broader shoulders.

They are actually made by Newman Camshafts in Orpington, although you cannot buy them there.

(As an aside, when I used to buy cams from one UK manufacturer in the 1990's, they were made to our own design and were meant to be exclusive to us ......... until one day a consignment turned up that we hadn't ordered, followed by a phone-call from same supplier to ask us to forward them to a competitor whom they also supplied "our" cams to. Nice eh ?!)

There is no data-sheet supplied, or available, especially not when you buy them (out of politeness) from Chesman in Coventry, which is apparently normal by conventional classic-car market standards, and the only advice you get is verbal - max lift at 105° (which means max lift Inlet is 105° after TDC, and max lift Exhaust is 105° before TDC).

Anyway, to cut to the chase. I've not driven the car, the engine is only running on the bench, BUT the tickover is proving problematic, and I'd just like to float the question as to what others' experience has been in this regard.

Needles standard UM, carbs in perfect condition, linkages perfect, you can rule out the obvious ; whether with the overhauled/perfect distributor firing the plugs, or the rock-steady MegaJolt, same result. Standard compression, standard valves, cleaned up ports, standard manifolds.

The engine won't run steadily below 900 rpm.

The air-bleed screws need to be wound out by about 5-6 turns (standard 2), and the main jets need to be between 2 and 3.5mm below the bridge ; a vacuum guage switched across the 3 inlet faces gives a pretty similar level of suck ; and the lambda sensors fitted 1-2-3 and 4-5-6 give plausible and acceptable values that vary predictably with the mixture screws.

All this would be fine, but the funny thing is that the tick-over is relatively insensitive to small variations to any adjustments of air-bleeds or mixure strength ; once you get to 900 rpm, the motor burbles away happily, slight imperfections and hesitations, but none of the conventional tweaks like raising the SU float a small amount with a screwdriver has the slightest effect, and it runs at the same 900 rpm over an implausibly wide range of adjustments that would cause a standard car to react to markedly.

I got no sense from 20+ 'phone calls to speak to anyone at Guy Broad's, being fobbed off endlessly, and to date the most plausible advice I've had was from Rob Beere - to retard the Inlet by 3 or 4° to reduce the overlap and increase the suck.

This is indeed what I'm going to try next, even if this steers us into sub-optimal territory.

Has anyone else experienced this kind of phenomenen please ?

[christopher storey]

My friend with a 2+2 tried what I cannot swear were these cams ( but I think they were very similar ) and they were hopeless and had to be abandoned. Nonetheless, the results were not as bad as yours in that a moderate idle could be obtained, albeit lumpy. However, on the road they gave a massive loss of torque below about 2000 rpm. You mentioned a vacuum gauge : if this is a sensitive one then it is worth connecting it to the servo takeoff and seeing what the results are, because old fashioned though they may be these are in my book one of the most sensitive tuning aids there is . Even with wide overlap ( and I find it extraordinary that no one can tell you the timing ) on a road car I would be aiming for a minimum of 15 inches at idle - and of course ideally it should be more like 20-22".

The other thing which puzzles me is how far down your jets are . 2mm is within the sort of parameters I would expect ( I usually set mine 65 thou or 1.6 down to begin with ) but 3.5mm or 140 thou down is extraordinary. Do you think you may have a vacuum leak somewhere, thus weakening the mixture ? And are your lambda readings in the 12.5 region which they need to be ? 14 or more will not hack it on an XK

[phil.dobson@mac.com]

Just been investigating this exact problem. It’s seems to be a well known issue whet using SU carbs. It’s better with Webber or equivalent whereby the idle is set by opening the butterfly and not using the air bypass valve on the SU
I’ve discussed this with Chris Keith-Lucas, Rob Beere and John Moore. All 3 of these guys gave almost the same description of the issue, with the bottom line being standard cams are the way to go if you want a nice tick over and good torque in the lower Rev range. I’m currently swapping my Rob Beere high lift cams back to standard, which I’ll finish over this weekend.
You can use use the later xj6 injection head cams if you like as these are probably the best in terms of performance and quietness.
Best regards
Phil D.

[PeterCrespin]

VVT systems usually retard the inlet cam (less overlap) for good idle, so Rob is right to that extent and you will probably improve your idle.

But then retarding the inlet cam will lower your CR and low speed torque, just as advancing it improves low down torque by increasing the effective CR (i.e. the inlet valve closing point is when your piston begins compressing the mixture). Depending on your induction system and somewhat on the exhaust, the pay-back for leaving the inlet open a bit later would potentially be a bit of ram effect at higher revs squeezing a bit more mixture in just before the valve closes, but generally this only happens over a limited range (like blowing over the top of a milk bottle) and the engine would be a bit more 'cammy'.

But then it's only a mild cam and it would take a lot more radical tuning to make an inherently flexible XK unpleasantly 'peaky. D-types were given the factory 'Endurance' tune, not short circuit:- 3/8 lift as standard and 30/60/60/30 instead of 15/57/57/15. Bikes get away with wild tunes because they are more likely to have separate tracts for every cylinder and get their neck wrung at every opportunity, but cars are more usually driven on a wave of torque. Which is why I've just scored a spotless 1500-mile pair of Iskenderian cams for a pittance from a delusioned E-type owner (the third set of abandoned 'performance cams I have bought). I suspect XK cams are removed even more often than Webers, which is a boon to bottom-feeders like me, who double-dip and fit both sets of cast-offs to the same engines. I cannot believe you weren't given full spec sheet if you bought them new. Are they billet cams or built up flanks? At stock 3/8" lift they wouldn't be base circle regrinds presumably. Did you just assume 0.050 timing clearance then, or just use the timing slots as usual? What running clearances (that can affect idle)? They weren't 4-hole parabolic cores by any chance?

[PeterCrespin]

You can use use the later xj6 injection head cams if you like as these are probably the best in terms of performance and quietness.
Best regards
Phil D.

Sorry Phil, that's an old trope (which I have hinted at when selling S3 engines but been careful not to state directly!).

S3 XJ cams are identical to all others except for quietening ramps. The performance came from bigger ports, valves, electronic ignition and EFI tracts, not cams. Some people try to 'cheat' by running normal clearances on parabolic cams, which on paper opens the valves a fraction earlier using the ramps and closes them later. In practice, the ramps are so gradual (their whole purpose) that the 'extra' valve duration is illusory because hardly any mixture flows through a valve open by a few thou. What that trick CAN do, however, is screw up the idle vacuum like a badly seated valve would do - hence my question to Rory re four hole cores.

[bitsobrits]

I have mild performance cams and a ported S3 XJ6 head, with a large bore exhaust (headers), and alloy flywheel. I spent the better part of a summer getting the idle dialed in. I'm now using UE needles, and a Mallory distributor with a custom advance curve. Also added low profile parabolic air horns in a custom high flow air cleaner which eliminated an apparent mid range torque dip. In my case, I found I needed a bit more initial timing (12), a more rapid advance (all in by 2500), but similar total timing (28 IIRC). Now I have a stable idle at 650 rpm with just a hint of cam 'lope'. I can get it to idle as low as 550, but I get some gearbox rattle (Tremec T5) no doubt due to the alloy flywheel.

No flat spots on acceleration and consistent readings on a tailpipe fuel/air sensor through the rev range as well.

So it was a combination of fuel mixture and timing supported by the air horns that got me there in the end.

[phil.dobson@mac.com]

Sorry Phil, that's an old trope (which I have hinted at when selling S3 engines but been careful not to state directly!).

Agreed peter I should not have included ‘performance’ in my reference to S3 cams.

[abowie]

Were it me, I'd be interested to know.

1. Is the cam new VS a regrind
2. Who actually ground the cam
3. Is the cam grinder reputable and competent
3. What are the specs of the cam
4. Does what I have actually match the specs, and what objective information do I have to confirm that.

[rfs1957]

Nothing wrong with knowing only the max lift figure per se, indeed vastly preferable to opening/closing figures as it obviates the need to know how the valves are shimmed, which is super-critical with the latter.

The cams are new, Newman is a big reputable operation, and Guy Broad (who used to be a shareholder in Chesman) is considered a reference, so ................

I think the best diagram is probably this one





as in, if this is what you use your car for you don't need different cams.



No such thing as a free lunch.

Now, which web-site do bottom-feeders buy their cams from ?

[Tom W]

If you can’t get any specs from the manufacturer or supplier, then you should be able to measure them fairly easily. You’ll need a timing wheel and a dial gauge. I believe timing and durations for aftermarket cams are usually quoted at .050” lift. I don’t know what the equivalent lift is for the factory cams, though again you could determine this by reinstalling one and measuring. Once you have a set of specs that can be directly compared to the factory specs, we’ll be able to see how “uprated” the camshafts are.

Incidentally, I find the marketing of “uprated” camshafts quite fascinating. The implication being that the factory camshafts are somehow inferior, and that there is performance to be gained, by upgrading, with no downside elsewhere. In reality, the choice is just between different camshafts, and picking something appropriate, not that any are inherently better or more clever than others.

Camshafts “work” at a single rpm value (approx max torque point) Either side of that value, the effectiveness will tail off. What you can choose when changing is, what that RPM value is, how well the camshaft works at that one RPM value, and how rapidly the performance tails off either side of it. The 2nd and 3rd attributes being inversely proportional to each other. If the tail off is too rapid, then the engine will feel peaky.

For an XK engine with a 5500rpm rev limit then the factory cams are a good fit. The working rpm value sits at a good point in the rev range, they work well enough to give good performance, and the tail off of performance is so gradual the engine is very tractable and smooth, right down to idle.

The mildly upgraded camshafts will slightly increase the rpm at which the engine makes maximum torque, thus increasing maximum power. They possibly increase maximum torque slightly too. Both these changes will decrease the performance (smoothness) at idle.

[Heuer]

I have trick cams supplied by VSE during the engine rebuild. SU's with UE needles, EDIS/Megajolt and it is only with the latter fitted do I have anything like a stable tick-over but it is still not great. With the previous Magnetronic ignition it was dreadful but once I put my foot down all such concerns were forgotten. Driving in towns and villages is uneasy exacerbated by the 2.88 diff.

My FHC with a stock engine is a delight but then I miss the sheer brutality of the OTS.

[PeterCrespin]

Nothing wrong with knowing only the max lift figure per se, indeed vastly preferable to opening/closing figures as it obviates the need to know how the valves are shimmed, which is super-critical with the latte

No such thing as a free lunch.

Now, which web-site do bottom-feeders buy their cams from ?

Agreed on setting for max lift being quicker/easier [Edit: although of course you only do one shim per cam] but you were chasing an issue where max lift point is immaterial, hence knowing the cam diagrams would help decide on feasibility of potential fixes. Buying a billet cam with only the the max lift degrees is like buying a new car with the Capacities page ripped out of the manual: you don’t need it every day - or ever if you always use dealer servicing - but the time you need it you’ll be annoyed at the censorship.

Last week’s cams came via JL classifieds (with full data).
What have you got against John o’ Groats anyway?

[steve3.8]

Not sure how you have done the setting , but assuming you set the cams up MOP @105 using a long pointer dti on the cam bucket ? , the only suggestion i could add is to put small timing discs on each cam which will show the duration and the overlap. I made some for a lotus tc by using printable cd's , scaling down the print to the needed diameter then cutting the disc to size.

[rfs1957]

Update on the Jag4 cam situation.

I did retard the inlets 4° as suggested by Rob Beere, and it was marginally better, but still unsatisfactory in that it didn't respond logically or proportionately to bypass-screw adjustments, or mixture changes, never mind hoping to understand anything from SU piston-raising.

I will confess to being nervous that there wasn't some major conceptual cock-up regarding how I'd overhauled the SU's, changed the spindle seals, polished the bells, flattened everything, and that in the course of the last few years tinkering there wasn't some air-leak that I had managed to engineer into the set-up.

Having looked again at this map



and reckoning that it is a better guide by far to why I own the car than any performance curve, I bit the bullet, swapped the Broad cams out and put the 1962 originals back in.

NOT, though, without getting Inlet and Exhaust mixed up, starting by fitting the rev-counter drive to the wrong one, installing and shimming them even, before I realised my mistake when putting them back in AFTER shimming and having a heart-attack upon seeing my felt-pen hand-writing on the "wrong" flange-end .................

I can officially confirm that it is possible to fit the cams the wrong way round, rotate the engine by hand, shim everything, without bending the valves.

Beat that.

Much to my relief (this does not do justice to my stress, or my stupidity), the engine now runs like a dream, as low as 600 rpm, responds to the "standard" carburettor adjustments, and I've now got a pair of almost-brand-new-cams to sell.

Game-Set-And-Match to the nay-sayers who say, what was it, that cams are the first "upgrade" to get put into the small-ads ?

[ralphr1780]

Congrats Rory

[John ball]

When I bought my XK120, it had D type cams previously supplied by I believe Guy Broad from the paperwork.
I could not get the car to tick over properly. The cams were taken out and I took them to Newman, for inspection. He put them on his machine measured and polished them. His comment was they were no good for road use. So, I said why are they sold as fast Road. His answer - that is what we are asked to build and supply !
I sold the cams to to someone who does track days in an E Type. Now I have Rob Beere cams designed for SU’s to produce good torque. They are noticeably different to the look of the D type cam as much sharper cam unlike the very long duration rounded shape of the ad type cams. As far as I know the only difference to the standard cam is a slightly higher lift of .41 instead of .385. But something does not add up is the paperwork duration is described as 235 degrees. Surely this must be wrong, but I cannot get an answer or sensible explanation from Rob Beere ! Have a look at his website and explain to me how that can be so low.
What I can say is that the torque curve on the rolling road is almost totally flat from 1900 to 4500 where it drops off.

[rfs1957]

Yes, there's nothing wrong with the manufacture : mine timed up to within 2° on every lobe.

They're just sold by idiots, to idiots ; I bought them through Chesman (who were massacring my various engine parts at the time) on their advice, through Guy Broad.

Perfectly civilised road cam, with more top-end, they said.

Chesman took my money, and their profit, then sent the cams with no data sheet ; when pressed, they said "ask Guy Broad".

It probably took 10 'phone calls before I got someone at Guy Broad's who knew what a camshaft was, and no, there was no spec sheet, "just time them to 105°".

So I'm an idiot, and they're liars.

But I should have seen it coming ; no such thing as a free lunch.

[PeterCrespin]

If you think about it, car makers - especially minnows like Jaguar up against GM and Ford et al - want to please as many people as possible, as often as possible. When you think that after the first 5/16 th engines, they stuck with their 3/8 th lift cams for forty years straight, you have to accept that it is the best cam for almost every use. A stock engine well put together, and correctly set up, does the business time after time with the best compromises to suit most users most often.

To put it it another way, you have got to be pretty far from the middle of the normal distribution curve to stand a good chance of being pleased with the revised compromises (good and bad) that a modified cam will probably deliver.

I started messing with performance mods on Triumphs, which unlike BSA and Norton twins, also had separate exhaust and inlet camshafts with somewhat similar vernier adjustment capabilities. I can remember the first time I 'hotted-up' a five hundred with some lumpy cams and being underwhelmed at the result. It went a bit better at the top end, but progress to that point seemed stymied by a hole where there had previously been steady torque. Rolling roads were not available to Joe Public and I could never be sure whether the increased rush as the engine finally came on the cam was real, as opposed to simply seeming better because it mostly bogged down before that happened. Standard cams went back in and the bike suited me better as a skint student.

Fifteen years later, I put them back in a 500 Rickman Metisse with a back sprocket the size of a dustbin lid. You don't have time to look at rev counters on scrambles bikes and it was a case of wringing its neck and riding the over-square twin like a two-stroke. THAT was when the Megacycle cams delivered the goods, and I had a grin a mile wide scaring the dayglo ring-ding-ding boys, who sometimes cleared a path for me to pass rather than be flattened by roaring barely-controlled steam roller coming up behind them.

To me a good rule of thumb is that unless you are fitting Webers and an extractor exhaust forget about the cam. Even if you do all three, forget about purring along and admiring the countryside, socialising with your passenger.

Forty years they kept that cam, across three engine sizes, a whole host of models and intakes, and even big-valve fuel injection engines. They knew what they were doing.

[rfs1957]

Cut the crap Crespin, talking down the value of my cams.

I'll pay the shipping, just get them out of my sight.

PS - but Mike Hailwood's 1978 TT-winning bike has the sweetest, most civilised, engine on the road of any bevel twin I have ever ridden ; it's a much nicer road bike than any - er - road bike that they ever made around that time.

[malcolm]

I accept all the above from persons far more capable than I. However, I have to say that my 4.2 E has the Rob Beere "SU Spec" cam, which I'm told is modestly "improved".
I don't know what the car is like with a normal cam, because this was fitted when I got it, but it has plenty of low down torque. It pulls strongly from 20mph'ish in top gear and then keeps revving with no flat spot. Easy and comfortable to drive on normal roads. But performance wise, my car has surprised more than a few lighter (non-2+2) cars, going past most at Goodwood on track days. So can't complain at the cams. As I said though, I haven't driven it with normal cams so.....