XPAG ENGINE CAMSHAFT

3 Sep

Since early 2010, over about 20,000 miles, I have had experience on my TC of a camshaft purchased from NTG Services with which I am very pleased.

I ordered the “standard” specification cam with a nominal timing of 18-58-58-18, cam lift 0.21 inches, tappet clearance 12 thou inches. The company also offered at that time an alternative “competition” cam with nominal timing 20-65-65-20, cam lift 0.25 inches, and tappet clearance also 12 thou inches.

Recall that the timing figures given above, conventionally refer to, in order:

Inlet valve open (IO) in degrees crankshaft, BTDC (before top dead centre)

Inlet valve closed (IC) in degrees crank, ABDC (after bottom dead centre)

Exhaust valve open (EO) in deg. crank, BBDC (before BDC)

Exhaust valve closed (EC) in deg. crank, ATDC (after TDC)

The valve rockers on my car have the standard cam to valve lift ratio of 1.463, so the maximum valve lift is 0.307 inches (7.8mm).

I have checked my cam on the bench and have found the following results in crank degrees, using a tappet clearance of 12 thou inches.

Note that the timings given below are as a zero velocity (theoretical) valve movement, not simply the timing of the camshaft alone.

(IO) 14 deg. BTDC, (IC) 54 deg. ABDC. Duration 248 deg.

(EO) 47 deg. BBDC, (EC) 18 deg. ATDC. Duration 245 deg.

The valve overlap period is thus (IO) + (EC) = 32 deg.

The inlet valve lobe centre angle (LCA) is 110 deg. ATDC, and the exhaust valve LCA is 104.5 deg. BTDC. The lobe displacement angle (LDA) is 107 deg. cam shaft, note – not crank degrees.

See the calculations set out below.

Calculations

The lobe centre angles (LCA) are the points in the four-stroke timing cycle where the inlet and exhaust valves are at their respective maximum lifts and thus maximum air/fuel mixture and air/exhaust flow rates. There is a complicated relationship between the vibrating gas flow dynamics in the inlet and exhaust systems of the engine and the LCA and LDA values of which the designers had to take account to obtain the best performance in practice on the road of the finished motor. Some modifications (tuning) of these relationships are possible on an existing installation, but are limited without extensive and costly mechanical changes and much experimental effort. Simple improvements (say + 10%) are available to an intelligent owner-driver, but much more than this is usually expensive.

The lobe differential angle (LDA) is rigidly built into the camshaft design and cannot be changed, except by changing the entire shaft. It is the angle between the two lobes inlet and exhaust on the cam shaft itself and is measured in degrees cam, not crank.

The main impact of the LDA upon a functioning motor is in effectively fixing the overlap period for a given valve timing. The extremes of design variation of the LDA are from a minimum of 100 deg. cam to a maximum of 116 deg. A low value of (say) 102 deg. would be for a competition car engine with an overlap of more than 55 deg. crank, while a 114 deg. value (say) would be for a smooth limousine type motor with about 10 to 20 deg. crank overlap.

Inlet cam LCA = (14 + 180 + 54) / 2 – 14 = 110 deg. crank

Exhaust cam LCA = (47 + 180 + 18) / 2 – 18 = 104.5 deg. crank

Cam shaft LDA = (110 + 104.5) / 2 = 107.25 deg. cam

Discussion

In seeking a new camshaft, my aim was to optimise the engine power at mid-range engine revolutions (2000 to 4500 rpm) to give a smooth, flexible and economic cruise performance. The standard NTG cam has supplied this perfectly. I was not interested in all-out speed and acceleration, although when pushed, the car does accelerate well, both in top gear and the intermediate ratios.

The motor has a fairly high compression ratio (8.8 to 1), standard valve sizes, a distributor with 20 deg. crank vacuum advance and a maximum centrifugal advance of 28 deg. crank with a static advance setting of 6 deg. crank. The total advance is thus 34 deg. crank, which is attained at 3,500 rpm and above. The maximum engine power is about 60 bhp at 5,000 rpm.

My TC has a higher rear axle ratio to give a top gear of 18.3 mph/1,000 rpm compared with the standard gearing of 15.8 mph/1,000 rpm. A level road maximum cruise speed of 70 mph at 3,800 rpm is attainable with a little more to come. The flat out maximum on this gearing is about 74 mph at 4,000 rpm (about 50 bhp).

Addressing the LCA and LDA values again from a performance aspect, for a competition type motor a low inlet LCA is normally in the range 102 to 106 deg. crank. For a mildly sporting engine the 110 deg. value is typical.

The lobe displacement angle (LDA) on the NTG standard camshaft of 107 deg. cam is a good sporting compromise in the 100 to 116 deg. cam range.

The valve duration for the standard cam at inlet 248 deg. crank and exhaust 245 deg. crank are, in my opinion, very acceptable for a mildly tuned sporting car, as are the overall inlet and exhaust valve timings and the valve lift of 7.8 mm, especially when taking into consideration the timing retard effect of the 12 thou. inch tappet clearance.

To sum up, the TC performance with this cam is what I hoped for at the start. The car is torquey and flexible in top but is able to rev well when required. At 5,000 rpm in third gear (13.6 mph/1,000 rpm) 68 mph is a maximum, but 60 mph in this gear is only just over 4,400 rpm.

Because of the high gearing and the vacuum advance, the economy is good at around 45 mpg on a run. Overall, I am very satisfied.

John Saunders

Ed’s note:

John has sent me an interesting paper entitled “Combustion Processes in the XPAG Engine”. The typed document he sent me runs to just over 10 pages, but I reckon that its reproduction in TTT 2 would come out at between 5 to 6 pages.

It is quite hard to follow (not due to John’s style of writing, but due to the difficulty of the subject matter to the layman) and I am trying to ‘get my head around it’ and think of ways in which it could be presented so that it would be easier to follow. I’m not there yet, but I’m trying!

Mike Green of NTG Motor Services has sent the above picture of the company’s standard camshaft. It is Part number B284 and retails at £298.80.

https://www.mgbits.comTel: 01473 406031/406032

The arrow pointing to the n/side of the dash reads “Also glove box with St Christopher – very necessary!” (I should think so at 95 m.p.h.Ed). The Note: reads “Birds eye walnut dash; Oil pressure, RPM, speedo, oil & water temperature all grouped together and lit by one bulb externally placed in the middle, throwing a red light (good for night vision) direct on to the B/W dials.

A Mews somewhere in the west end of Edinburgh – TA2243 with other T-Types.

Town of Nevers: camp site on the river Loire –

according to Google, the site is still there.


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