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MG MGB Technical - which cam

hello i would like some advice, i have a pritty standard rebuilt engine . im fitting a fast road head which will give around 30% more power ,i have a aldon dizzy ss 4 branch man and su air filters with upgraded needles. i cant make a decision on the cam, do i not bother or go for eather a piper 270 or 285 is there a lot of difference. ive been told that the car will not iddle very well with the 285 thanks daren
daren

My car idles "OK" with an Oselli 271. If I was building a daily driver I would go with something tamer. For a fun road car 270 seems good. Car flies once it gets over 3000. Anyting wilder I looked at gave less power low down than the factory cam , which is well chosen.
S Best

Look at your cam grinder's characterization of the cam.
A cam that has a power band from 2500 to 6000 is no fun to drive in ordinary use.

One that says from 1800 or 2000 and up, is ok around town and gives extra power in the mid range where it matters in road driving. To get max horsepower at 4500 to 6000 rpm you loose a lot of mid range power. The few extra ponies at the top end, is not worth the loss of power at the mid and low end.

A little bit is good. Too much is not.

Barry
Barry Parkinson

thanks barry,ive decided to go for the 270 better for the road daren
daren

will i still beable to use the vacume advance when the fast road head is fitted,ive been told that iwill have to block the pipe and advance the engine a few degrees.i might have a problem with pinking iff the vacume is connected thanks daren
daren

I modified my distributor by putting in a lighter spring which brings the spark advance in earlier. It helps at the low end when the overlap in the cam causes a loss of power and compression. The vacuum advance works very well on light throttle cruising and helps economy.
Barry
Barry Parkinson

barry thanks for that i would like to ask you a million and one questions but i would be here all night thanks daren i would be interested to know your engine spec and the bhp
daren

If your cylinder head is not ported, don't bother with a cam bigger than the 270. The stock exhaust ports max out their flow at way too low a lift to justify it.
Even with a moderate port job, the 285 won't create more power. It will just give you less low rpm power. Your cylinder head will still have a maximum flow rate that can't realize the potential of the cam.

Jeff Schlemmer

In practice, the diz vac issue depends on how well the cylinders fill at midrange, 3-4000 on a road car. Compression, fuel, fuel ratios and cylinder filling affect rate of burn; better filling needs less advance and less filling needs more advance.

A hot cam and big valve engine may not fill as well at low speed but will fill better at higher speed. Hence, as Barry says, allowing the mech advance to run up quicker at the bottom may help pick up. If you then get to cruising and level the throttle the vac advance may be too much for the better filling at high speed and it may pink. Or not.

I actually have the reverse case at the bottom end. My engine's mildly tuned with a flowed head, stage 1 cam and standard bore exhaust with header. It gives higher than standard compressions at cranking of 180 and runs 95 unleaded.

I bought a timing light with rpm/advance and tweaked the heavier diz spring to hold advance off a little more than standard at low speeds (15 BTDC total instead of 20 at 1000rpm). This has made the thing super-tractable right through the range from 1500rpm in top gear without pinking, full advance at 3000rpm instead of 2200 and the vac doesn't cause a problem. (Bear in mind the vac has 20° which is inter-related with the mech advance so any change in the mech will be diluted in overall effect.) This small tweak would probably help a lot of standard-ish engines running on unleaded these days as pinking is usually on load at lower revs.

A compression test on your finished engine may give an idea how well it fills at low revs, then set up for high speed advance on the road and work back with that info in mind. Rich.
Rich

I have a piper 285 (Burgess recommended), a fastroad head, K&N's, richer needles (a lot richer). I have my idle set at 900 and it idles ok in my opinion. My engine gives me more than enogh tourque on low revs driving around town. Even 1500 rpm works in hills not very steep.
I do have my timing set 2 degrees (advanded or retarded I dont remember) on the tourqier side.

/Jesper
Jesper Akesson

Rich
Nobody talks much about vacuum advance differences. For instance, the later catalytic converter models have a large vacuum advance. The spring inside a vacuum advance can be stiff which requires hi vacuum to advance, or soft which obviously requires less vacuum to advance. The travel or amount of advance is also variable. I have adjusted the travel on another vehicle by wrapping wire around the rod from the vacuum actuator, to reduce the amount of vacuum advance.

I'm sure someone can advise of different compatible vacuum advances available. - and where?

I became very skilled (on a backyard basis)in these issues messing with and trying to get just ordinary performance out of Calif smogged vehicles back in the 1970's. Many had both vacuum advance and vacuum retard. My most dramatic and satisfying messing around, took a 2.3 turbo mustang and increased the horsepower from rated 125 to close to 180. (admittedly there is a lot more you can mess with, with a turbo)The engine still had all stock parts and still passed smog. (at that time a test made at idling).

An engine with a cam that is weak on the low end and strong at mid and top, produces it's own spark timing challenges.

Barry
Barry Parkinson

Yes indeed Barry.

Mine's 25D4 40897 diz which has one of the highest amounts of vac advance; 10° diz = 20° engine. I've kept an ear for pinking and it's fine but I too thought of looking for a vac unit with less advance if it was a problem, rather than taking it off completely.

Mallory do a diz with adjustable mech and vac but I wouldn't like to try and set it up! Mine's 5-13 10° but there are 7-11 7° and 5-11 8° units on other diz',

http://www.mgbmga.com/tech/mgb21.htm

I've plotted the new mech curve and laid it over the original MGB and ST curves, all using their static setting and working to 30°BTDC full advance for a meaningful comparison,

http://homepage.ntlworld.com/richard.maile/MGB/diz%20curves.bmp

Tightening the heavy spring has stopped it advancing too soon and pinking, that 600 rpm point was around 20° and it was fully advanced soon after 1500 before.

However, looking now at the curve, it seems the light spring may also be a culprit so I've ordered some springs to try and get more to the original spec. I'm thinking a stronger, or new, light spring and a little more slack in the heavy spring may do it, what do you think?

Rich
Rich

Rich,

You said "I actually have the reverse case at the bottom end. My engine's mildly tuned with a flowed head, stage 1 cam and standard bore exhaust with header. It gives higher than standard compressions at cranking of 180 and runs 95 unleaded."

Several years ago there were some lengthy discussions on this board on a concept called "effective" compression ratio. The theory on 'effective' CR is that, as you increase the valve overlap, the 'effective' compression ratio will decrease at lower rpm because there will be less filling. As I remember, there never was a definitive answer as to whether this was a real effect or not - mainly because no one had any empirical data.

Now, you may just have some data that enables us to determine whether the efect is real or not.

Do you happen to know what your current physical CR is, i.e. the actual volumes of the combustion chamber, head gasket, cylinder, etc.?

From your compression measurements, it appears that, either the concept of 'effective' CR is poppycock or your physical CR is VERY high.

Could you please shed some light on this?
Richard Smith

Effective CR is real effect , the good filling at moderate revs with a tamer cam proves this .With lots of overlap at lower RPM what you pump in the inlet goes straight out of the exhaust, at higher RPM the scavenge effect from the higher gas speeds gives better effective CR and power or economy . My car can return 40 mpg on motorways and autobahns .
S Best

Here is an article about effective compression ratio.
http://www.kb-silvolite.com/news.php?action=read&N_id=30

Clifton
Clifton Gordon

Richard, S,

Effective CR is real and accounts for an engine's power band. We should think of it related to engine speed. A stock engine may well have more effective CR at low speeds than a race engine but the race engine will have more at higher speeds. It's also possible for a mildly tuned engine to have more than a stock engine at all speeds if it doesn't have too wild a cam which loses the charge at low speed. Indeed, just skimming the head would do this, hence first stop for tuning!

At low/cranking speed effective CR is influenced mainly by the closing angle of the inlet valve ABDC as compression can't start til the valve closes. In some engines this is then compounded by the inlet tract being larger which reduces gas speed and hence ram effect. (Think 16v versus 8v in modern engines). The net result is a lower charge in the cylinder at low speed which shows as a lower psi at cranking.

The effect is sometimes referred to as overlap but overlap isn't involved on this revolution as both valves are closed at the top of the compression stroke and there will be little gas inertia to affect filling. Overlap causes low speed fuelling problems on the next rev, during induction, when the inlet opens BTDC before the exhaust closes ATDC so fuel goes out the exhaust as S says. Here's a good pic for anyone getting to grips with suck squeeze bang blow if we're still allowed to say that,

http://www.pipercams.co.uk/NewPiperWeb/redesign/TechAdvice.html

I've no data for my engine as the PO had it built for him. The cam's Kent or Piper stage 1. The head's a professional job, nicely worked but again quite mild. I fitted a new header along with a standard bore straight through stainless system and carbs are standard SUs.

It has 11000 miles on it and at cranking all cylinders are 180. I checked this after setting the tappets but still thought it a fluke. What's happening I think is a higher theoretical CR from the head (smaller chamber), flowing letting it fill a little better, and a mild cam which isn't closing too late to compress it. Net effect is a bit more than stock at low speed.

Rich
Rich

This thread was discussed between 13/04/2005 and 15/04/2005

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