49 replies to this topic

[Cooperman]

Maybe we need a bit of basic engine improvement advice here.
First of all the key to improving the power/torque from a 1275 A-series is to fit a properly gas-flowed head with bigger valves. Typically valve size of 35.6 mm/30.5 mm is good. The ports, throats and combustion chambers need to be flowed properly and this included de-shrouding the valves by re-shaping the combustion chambers. As this increases the combustion chamber volumes it is then necessary to skim the head by the correct amount to get a good compression ratio of, say, 10.5:1.
With a decent inlet and exhaust system, including a suitable carb(s), the engine is then capable of 'breathing' better.
To enable this basis to give more real power, a camshaft more able to let the mixture in and the exhaust out is needed. The Kent range is good and either a 266 or a 276 can give good results. A 286 is a great cam, but it will give peak power at around 6400 rpm and you'll need to be able to use around 6700 rpm to get the full benefit and for this a well-balanced bottom end is very strongly advised. Fitting a 286 and not being able to pull 6700 is a bit of a waste as you'll be losing some bottom end and not fully gaining at the top end. In fact, an MG Metro or Kent 266 are great for non-balanced engines at up to about 6200 rpm.
If you take a look at the Kent Cams Catalogue you'll find the power and torque curves for all their cams.
Don't forget that a distributor with a different advance curve is needed as soon as you put the comp ratio up and fit a cam with more overlap.

[ministar]

Thanks cooperman.. Is it really the head not letting me 'rev' over 6k?

All that you have suggested I have done, except the head, it is a mg metro with 35/29's though? Kent information say the 276 should rev to 7k as a max (obviously max power is nowhere near that).

I'm not sure the answer can be found.. Everything has been done right, all new parts.

[The Matt]

Invest in head James! Definitely invest in head. The 276 IS a mild cam for a lot of people, but the 286 isn't that much wilder. People seems scared of these longer duration cams in terms of driveability etc.

Ultimately, a well-flowed head will allow the engine to suck more air in and will exploit the power potential of the other components you use. It depends on whether you want to fit a new cam, but if I were you I'd get a leaded MG Metro head, smooth out the guide bosses, open the ports out a little and fit some nice new guides and roller-tip rockers because I like them!

Or, buy a prepped head from somewhere.

[ministar]

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..


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

[Carlos W]

I'm not sure the answer can be found..

It can, ported and polished head is the answer!

[ministar]

Fanks matthew!!!

It is a MG metro head, with uprated guides and had brand new rimflow valves. double valve springs. Whatever work I do, will be to this head.

[Cooperman]

Thanks cooperman.. Is it really the head not letting me 'rev' over 6k?

All that you have suggested I have done, except the head, it is a mg metro with 35/29's though? Kent information say the 276 should rev to 7k as a max (obviously max power is nowhere near that).

I'm not sure the answer can be found.. Everything has been done right, all new parts.

You really do need a good head and a compression ratio of around 10.5:1, maybe slightly higher. If you only have standard ignition, it may be that you are not getting the correct maximum advance. I've had some good results with a 276, but you'll see from the Kent power curve that it gives max. power at around 6100 rpm, so above that it will start to fall off. Of course, maximum revs are not everything as the torque is also important.
Personally I think you need a better head and a more suitable distributor.
What inlet & ex. system do you have?

[ministar]

It is a recurved electronic ignition from the engine spec, and a rolling road session when the dizzy was standard to give a good basis for the recurve. The inlet system is a mg metro manifold, and exhaust manifold is a maniflow. Like the rest of the exhaust.

Ok, what do you feel when the cam's max power is felt? Is it a wall? or is it felt like a hard cut off?

[ACDodd]

Having dynoed a few engines with stock heads the power you have with that cam is about right. As cooperman said, you need to change the head. If this is a good un you should see in the order of 90 to 93hp @ the flywheel without the use of 1.5 rockers. If you regulary push this engine to 6k plus rpm you will seriously shorten the life of it. The secret to getting the most out of these engines is not by revving the nuts off it. You build an engine to give all it can in the rev range, not at the top end. A good head here will likely give 15 lbft at 5 to 6krpm!
The best way to ruin a road engine is to fit a big cam and fit a stock head, as you loose low down torque and gain nothing up top.

If It were mine I would limit the CR to 10:1 makes the engine less fussy and is easier on ring lands.

AC

Edited by ACDodd, 20 February 2012 - 08:17 PM.

[ministar]

Thanks for all the responses on this, some seriously knowledgable people commenting, appreiciate it.

I wasn't saying I was unhappy with power though, it was about right for the spec of engine, I know this. But question remains, if put a worked head on my current engine, would it let the engine rev past 6 krpm? Because currently it physically wont go past it, i know power is going to fall off but doesnt explain why it wont go past? Sorry if I'm being completely thick...

I guess, I have got to be happy with what I have, and realise I can't take it past 6k.

What benefit would I get from putting the compression ratio to 10:1? Only theoretically though. Would that not reduce power output?

Edited by ministar, 20 February 2012 - 08:39 PM.

[Pigeonto]

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

I would agree,you only have 4-5hp better than a std MG Metro and with a much better cam.If you look at the spec on my profile page I think you'd say you are lacking something in comparison.Dont know what but not conviced its the head(again based on my spec as I left the MG head virtually untouched so I believe it is capable of 90 hp as is).Sorry to throw a spanner in the works but may save you a dissappointment. Twin carbs made a huge noticeable difference immediately I put them on from a HIF 44 tho

[Cooperman]

The 2175 A-series does seem to respond well to a high-ish compression ratio and 10:1 to 10.5:1 always seems to give a good result when a good head is fitted together with an appropriate cam.
I once ran a 1990 Cooper 1330 cc Endurance Rally Mini (standard engine required!) with MG Metro head, slightly gas-flowed, or rather with the chamber and throat 'lumps, bumps and mis-matches removed, the comp ratio at 10.3:1, an MG Metro cam and everything assembled accurately. It gave 82 bhp at 5900 rpm on the rollers and it just went so well. Diff was 3.44:1. Distributor was Aldon Red, exhaust was twin-box RC40 at 1.75"int dia.

[Pigeonto]

Thanks for all the responses on this, some seriously knowledgable people commenting, appreiciate it.

I wasn't saying I was unhappy with power though, it was about right for the spec of engine, I know this. But question remains, if put a worked head on my current engine, would it let the engine rev past 6 krpm? Because currently it physically wont go past it, i know power is going to fall off but doesnt explain why it wont go past? Sorry if I'm being completely thick...

I guess, I have got to be happy with what I have, and realise I can't take it past 6k.

What benefit would I get from putting the compression ratio to 10:1? Only theoretically though. Would that not reduce power output?

Having read the above,properly this time ,you seem to be saying that it will not pass 6000,not just that theres no advantage in doing so??Well my basic 50 hp mayfair would do that before I modified it so if thats what you're saying maybe people are responding slightly off topic and could help with other suggestions? Will it if you slip the clutch? Restricted exh was mentioned earlier.Is the carb piston able to reach the top.Air filter blocked.Is the advance reaching at least 33 deg by 3500 rpm.(check with a strobe)Needle weak at top end(bottom of the needle!!)Have you checked cam timing.

[ministar]

Think you got it spot on there, it wont go past.. It feels like someone has taken it out of gear. Hence me asking what it feels like to hit the max of a cam.

I am pretty sure the carb is opening fully, but will double check. When i remember getting it running it was set at around 33degrees at 3.5k, but might have been altered slighty for rolling road. Needle was optimised for power, profiled BDL. Cam is timed in to 106degrees.

[ACDodd]

Valve float can also cause this.

AC