49 replies to this topic

[ministar]

I want to extend the power band of my modified 1275. At just over 6k it looses all power and really falls off cam.

1275 A+ block,
MG Metro 29/35 Cylinder Head (unleaded),
Kent 276 Fast Road Megadyne Camshaft,
MG Metro Inlet Manifold,
Hif44 Carburettor,
Maniflow exhaust manifold,
Maniflow 2 back box,
Refurbished Helical Gearbox,
Verto Clutch,
3.1 Final Drive.
10.5:1 compression ratio
NGK BP7ES Spark Plugs
High octane petrol..

What modifications would see an increase in top end revving? Balanced bottom end or a cam change I guess will be the most obvious thing to say, but lightened flywheel, 1.5:1 ratio rockers? Don't really want to take the engine apart if I can avoid it...

I will be changing to a 3.4 final drive as it falls off cam too easily, and fitting a cross pin diff in the coming months.

Thanks,

James


Edit. for anyone thats interested 77 bananas at the fly, and 88 torques.

Edited by ministar, 20 February 2012 - 12:54 PM.

[The Matt]

A 286 will give you a little more power higher up, but sacrifice a bit of grunt lower down. Really not up on cam specs but have you looked at the Swiftune jobs? I've heard they've got good cam selections.

I guess the other thing that's going to restrict things is yer head.

[ministar]

I wish I had gone for a 286 now, but when it was built up I didnt know what I would be using it for Hence the ******* choice of camshaft!! haaha. Would head work increase the power band? after more than 6k with my current cam? I mean it gets to 6k preeetty nicely!

[Sam Walters]

You could try advancing the cam timing just a shade, but on the 5 port this doesnt seem to affect it too much.

There isnt a lot you can do. The 276 is as tame as a kitten. the 286 and 296 are much the same.

Beyond the 296 is where it starts to get interesting. But to be honest on a standard bottom end 6500rpm is your max.

try a 296sp and get a good well modified head to go with it.

Edited by Sam Walters, 20 February 2012 - 01:21 PM.

[MiniLandy]

77 is quite low for an engine of that spec, sounds like the head is restricting things quite a lot.

[ministar]

77 is quite low for an engine of that spec, sounds like the head is restricting things quite a lot.

Agreed, would porting be the most effective way of freeing up extra power, or valve largening? Perhaps together would be the best. I think if I did go down working the head it will be done at Peter Burgess.

[MiniFTW]

Your could try the 1.5 rockers. I rang up Swiftune about getting more power out my SW5 (similar to the 276 according to the man at swift) and they said about buying some as the cam was designed for them . Also some decent head work. E.g a port + polish, uprated valve guides, and better flowing valves. The head could be restricting you of the power at the top if its not able to breathe enough. A lighter fly wheel would allow it to rev and pull off the line faster, but you loose the inertia effect so i presume the top end would suffer. Cam is an obvious, you could go for the SW10 or the 286 from kent. The will pull harder later on but again suffer at the lower end. But if you are changing the diff to a 3.44 you will find your acceleration is better so the power you have lost at the bottom from fitting hotter cam wont be as noticeable. I would personally go for a hotter cam and have some head work done. And then fit 1.5 rockers if your still not happy. What are filter are you using?

[MiniLandy]

77 is quite low for an engine of that spec, sounds like the head is restricting things quite a lot.

Agreed, would porting be the most effective way of freeing up extra power, or valve largening? Perhaps together would be the best. I think if I did go down working the head it will be done at Peter Burgess.

If it's an MG Metro head it'll have the larger valves, and these aren't the most restrictive bit of the inlet/exhaust tracts. The most restrictive bits are the casting around the valve guide and the short side radius, as shown in the head porting chapter of the yellow bible.

As for higher ratio roller rockers, the gains i've heard of are negligble. Never tried them myself, but on a friends car they gained minimal top end, and lost a bit of low down (dyno proven). Up to you there really.

[ministar]

Thanks for the responses guys.

So, it is really a case of increasing the cc's within the head? What effect would that have on compression ratio? Minimal?

The valves are the normal mg metro ones 35in/29ex. It has uprated valve guides and rimflow valves. along with double valve springs.

I would have also said 1.5:1 ratio roller rockers are pointless on my engine, and only really suited to increasing the effect of they cam on a hairy cam, but wanted a few more views on it.

And also Im using a K&N filter for HIF44.

[MiniFTW]

Hmm that blows me out the water Your engine is nicely setup! if you increase the cc in your head, your compression ratio will become lower. Cooper man is good to talk to about CR (he helped me out) could always go for a higher flow carb like a webber? but your MPG will suffer.

[ministar]

hehe. thanks!

Hmm in terms of a weber, I would, but it would be a 40 dcoe (most suitable?), and would have to be on a real manifold, as I dont want a silly swan neck thing!! Does nothing for flow. So that will mean cutting a box into my baulk head. Not convinced...

I do have a 'vizarded' hif44, on an mg metro inlet. And im not too worried about mpg.

[MiniLandy]

Thanks for the responses guys.

So, it is really a case of increasing the cc's within the head? What effect would that have on compression ratio? Minimal?

No no no, not that bit of the head! Porting the head doesn't mean increasing the size of the combustion chamber (which would lower the comp ratio and cost you power, as has been said), it means increasing the flowiness of the PORTS (the bits between the valves and the manifolds).

Hope this clears things up.

Also, twin carbs might not net you any more top end power, but they made a massive difference to the foot-down responsiveness on my car.

Edited by MiniLandy, 20 February 2012 - 03:16 PM.

[ministar]

Ok, I understand thanks re porting inlet and ex ports. :)

If I was the increase the combustion chamber (metal shrouding the valves) that would increase power. Correct? About 28cc as standard if iirc?

But then I would then raise the CR up again (as a matter of course.)

Edited by ministar, 20 February 2012 - 03:26 PM.

[MiniLandy]

If I was the increase the combustion chamber (metal shrouding the valves) that would increase power. Correct? About 28cc as standard if iirc?

But then I would then raise the CR up again (as a matter of course.)

I think 28cc is the volume of the 998 cooper heads (dome top pistons), not sure what the 12g940 heads are...

increasing the combustion chamber vol. wouldn't make a noticable difference, beyond a little bit of smoothing, the big power gains are made at the casting around the valve guides.

[MiniFTW]

I really would look at a new cam, its the 276 that is restricting your engine, and little things on the head (if it was a standard engine there would be lots to change, but yours is a good spec) you could up your comp ratio but I thought for a road car 10:5:1 was pretty high, if you want power in the higher revs your gonna have to buy a hotter cam, its really frustrating that a compromise usually follows a benefit its just weighing up what you really want I guess, if its a road car you are gonna loose driveability if you go with a lairy cam but it obviously takes power higher up the revs... Or you could have a naughty NOS injection come in at higher revs to give you some extra GO! Paha! (joking!)