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MG Midget and Sprite Technical - Cam shafts
|Hi, looking at overhauling the engine while its out and thought, while I'm about it, I might look at a higher performance cam shaft. |
It's a 1977 1500 and I read on a triumph website (the horror!) that a cam shaft from a TR5 would work to improve performance. Any thoughts from those more aware of these things than I (which is pretty much everyone on the planet!)
Thank you in advance,
|I would like to see how you fit a 6 cylinder cam to a 4 cylinder engine.|
|As you can see the engines a tad longer, heres a photo of mine.
|The TR5 was a superb engine but is was a fuel injected unit and I would suspect that even with a 4 cyl cam ground to the same spec, it may not give the extra desired performance...|
I would contact Jigsaw Racing to see what they suggest.
Or is Deborah still on the BBS...?
|M T Boldry|
|James, it wasn't dumb to ask the question, and we have all done things like that, but it would be if you don't come back and ask more so you can learn.|
|I was wondering if they were correct and that the TR5 camshaft could be used to performance advantage with the application of a hacksaw to the un-needed 2 cylinders of the camshaft.|
It wouldn't be unknown, actually quite common, for a manufacturer to run similar parts down the same transfer line.
I think the phasing might be slightly different for a six-cylinder engine, unless you also used a cut-down crankshaft.
|Dave O'Neill 2|
Good point that, I think that's where my idea falls down seriously. I'll keep quite on the Triumph front from now on.
|The 1500 motor has a relatively mild cam as stock: 256 degree duration with 18-58 timing and .340" lift.|
A better cam is the Spitfire Mk3 profile with 270 degree duration, 25-65 timing and .340" lift. This is as far as I would suggest on an otherwise stock motor - any more and you need a better exhaust, head work and possibly larger valves for the cam to work.
The TR5 / early TR6 cam has 280 degree duration, 35 - 65 timing and .340" lift.
That particular grind was historically (in the 1970's) seen as a good profile for a 'Fast Road / Mild Competition' 1500.
However time has moved on and there are better grinds available. My advice would be the Kent TH5 / 234 profile.
This is a 280 degree duration cam with asymmetric 37-63, 73 - 27 timing and .423" lift.
Moss also market this cam as the 'Triumphtune Fast Road 83' cam (but it is originally a Ford 1300 X-Flow profile that those of us competing with Triumph engines discovered in the early '80s and which idea was 'stolen' by Terry Hurrel of Triumphtune and then marketed as his idea!).
The TH5 works particularly well with judicious head work, and on std inlets and big exhausts will give around 110bhp and 95 ft.lbs torque on twin HS4 carbs.
BE CAREFUL HOWEVER, no one grinding Triumph cams at the moment can guarantee the case hardening, indeed I had a Kent regrind fail within 300 miles which wrote off the bottom end of a customer's motor.
To this end I use camshafts produced by the German company Bastuck. They do a TH5 profile cam and this is available through Jigsaw racing (who are the sole importers I believe). You MUST use new followers and I always recommend the lightweight ones with the oil drain.
I'd also suggest you purchase a 'small journal' cam and fit the early style camshaft bearings into the block.
|I have a 1979 1500 which had some head work work and cam grind about 6 months ago. The mechanic (a specialist MG garage) arranged the cam grind, which was a Wade Cams 1069, apparently with .295" lift and 30-70/70-30 timing. I believe (not confirmed) that it has a 290/285 duration. They also fitted new, larger exhaust valves, port-matched the valves and fitted extractors/headers and straight through exhaust with single rear muffler. The carbs are standard twin SU with ram tubes and sock filters.|
Is that an appropriate cam profile? What advantages/disadvantages would this profile have?
(wishing he had seen this information 6 months ago)
Re-reading Deborah's post and a little arithmetic would allow you to answer your own post, i.e.
Your cam opens (inlet) at 30 before tdc; closes at 70 deg after bdc; and stays open all the time the piston is on the way down - that's 180 deg. So the duration is the sum of these numbers: 30+180+70 = 280 deg.
Similarly for the ex side: 280.
So far, so good.
Next she says 'old' profiles have .34" lift; and newer profile uses .423" (trend in modern cams for Aseries and Triumph engines is for more lift instead of increased overlap).
Your cam only opens the valve with .295" lift - signficantly less than ideal.
So it sounds like a dud to me, unless you can fit higher ratio rocker arms to compensate.
Over to the experts...
|Maybe S Travis is quoting cam-lift rather than valve-lift?|
I'm not familiar enough with Triumph engines to know what the standard rocker ratio is and I don't have a w/s manual to hand, but I'm assuming it's not 1:1
|Dave O'Neill 2|
|Could be ... need to be consistent:|
- the timing figures appear to be valve-related
- so I assume lift was as well...
|The Triumph rocker ratio is generally taken to be 1.5 : 1 (in actuality it is nearer to 1.45 : 1).|
So if he is quoting cam lift as opposed to valve lift then he is in the ballpark. If not then the engine won't make what it could.
With regards to cams:
There were a number of 30-70 symmetric timing profiles tried in the early '80s including the Newman with 0.390" valve lift and the Triumphtune 'Fast Road' (really a Kent Profile) with 0.380" valve lift.
I would guess the Wade 1069 is a copy of one of these.
Neither are particularly BAD profiles, but then again you can do better with a bit more inlet timing, which this engine likes. However, in order to get this you do need to go to asymmetric timing to limit the overlap through flow area because the 1500 just can't rev like its smaller brethren. Indeed, increasing the overlap only really works on the small crank 1300 and the 1147 motors.
Dependent upon engine spec these profiles will give a maximum of about 90 bhp in a 1500.
For example, last year I worked on Kate's Tifosi Rana (a Frogeye look alike based on a 1500 shell and fitted with a 1300 large crank motor). This was fitted with the Kent TH6 profile cam, extensive head porting, stock inlet (ie 1500 size) and large exhaust valves.
This cam is an asymmetric 42-68 78-32, 290 degree duration, 0.445" lift.
On a 1300 small crank I would expect a minimum of around 110bhp with this spec - her motor produced an estimated 102bhp at the rolling road. So, nearly 10bhp down compared to a small crank motor. This is because the increase in overlap is negated by the fact that the engine won't (safely) rev as high (because it is having to rotate a greater mass). Taking this further, the engine produces nearly 10 bhp less than a 1500 on a TH5, a cam with less duration but, significantly, less overlap.
The upshot of this is that, while her car is quite rapid (about the same as a Class C racer) owing to its very light weight, it is very, very, 'cammy'. Do this in a std weight car and it would be horrible to drive. Do it in a 1500 and it would be a waste of time (and money) because you won't get anymore power than on a TH5 (because it won't pull the rpms) and you'll lose the bottom end torque that the 1500 is noted for
With regards to rockers:
You can get high lift (roller) rockers for the Triumph 4-pot(eg TITAN) but these have their pros and cons.
While they are way stronger and stiffer than stock (and thus somewhat more reliable in a race motor), every set I have come across are at a 1.65 : 1 ratio. This is completely pointless with a wilder cam grind because once you get above about 0.460" valve lift there is no increase in flow, so you are just battering your valve train for no advantage.
The only way I can see these to be beneficial is if you were regrinding a Triumph cam for race use - the base circle is such that the maximum you can get is about 310 degrees duration and 0.440" lift. So if you wanted (say) a BL Special Tuning 70-90 cam you could 'put back' the lost lift.
However, for the reasons stated earlier I do not recommend regrinding Triumph cams.
All this is moot however, because anything more than about 280 - 290 degrees duration moves the power band into an area where the 1500 will shake itself apart without some serious $$$$$ being spent, and even then it won't make the sort of output than you can get from the small crank 1300 motor.
|Thanks for the information. I will try to get some more specific information. I guess the best idea is to buy a new cam with ideal profile as suggested above.|
I am happy with overall performance however I am concerned that I have lost low end power and torque. If I hit a steep hill at say 35 mph, the car loses speed and I need to down change through the gears, sometimes as low as first gear by the time I reach the hill top. Is this likely due to an inappropriate cam, or another likely cause?
nb I have not driven a standard midget (for comparison). My car needed new valves when I bought it, so I had performance mods made straight after purchase.
This thread was discussed between 24/09/2010 and 02/10/2010
This thread is from the archive. The Live MG Midget and Sprite Technical BBS is active now.