Forward planning for supercharging 1275

[GeorgeHurst]

Hi all,
As some of you may know from previous posts I am tuning up a 1275 Ital engine for my 2dr saloon (HIF44, LCB, Metro cam, big valve ported 940 head etc). I have decided after some research that I am in the future, hopefully next summer, going to supercharge the engine with a homebrew system consisting of an Eaton 45 blowing through the carb with an intercooler.

Next week I am buying all the extra bits I need to get going on the 1st stage (tuning the 1275) and just wanted to check if there are any things that I should be bearing in mind considering the future plan of supercharging...

So, valve guides: do I need to get bronze guides or anything specific other than the standard guides?
Valves: I need to get some new inlet valves, do I need to get particular ones i.e. 'race' valves from minispares?
Centre main strap, yes?
Timing chain: will be fitting dbl valve springs when I super charge it, so does it make sense to go for a Duplex timing chain now? I was going to go for the Metro timing tensioner set up but not sure if this will be up to a supercharger.

I also plan to do basic balancing work to the bottom end - questions about that to come later when I am ready to get on with it

Things like spark plugs, pulleys, head studs I can change when I do it, I am just keen to do all the 'internal' bits once rather than having to do the work and fork out the cash twice.

Thanks for your time
George

[bmcecosse]

The thing about supercharging is - that the power is developed at low/medium revs - there is no need to rev such an engine. So - you don't need double valve springs or centre main strap or special balancing - or even the 'large inlet valves' because the charger pushes the air in. You DO need a good exhaust system - and a low basic compression ratio - say 8:1. Achieved by using big dish pistons - or by machining the tops of the pistons (to achieve greater clearance) - or sometimes by using a steel plate sandwiched between two head gaskets. The MG Metro cam will be fine - although Rovers Turbo engine used only the standard 1275 cam, and Rover deemed it necessary to use special sodium cooled exhaust valves in the Turbo cylinder head - you may want to try to get hold of one of these - although I'm not convinced this is necessary with a supercharger, it could be a wise precaution.

[GeorgeHurst]

Thanks for the reply Roy. I am thinking I will probably buy a decompression plate from VMaxScart rather than getting dished pistons as I think it will work out cheaper (though am I shooting myself in the foot supercharging on normal pistons?)
With the double valve springs I heard that the inlet valves can get stuck open because of the higher intensity and heat of the combustion when supercharged, it's not an expensive upgrade really 8 new springs so if there is even a little benefit of it I think I'll go for them. Duplex timing chain would be needed though right?

The balancing is mainly just for my own satisfaction... I like the idea of doing all that I can to make the engine the best it can be (on a DIY basis - not going to be shelling out hundreds for expensive balancing) - I'm basically planning to do the things explained by you in an old thread on here - balancing the weights of the pistons, big ends and little ends, static balancing the crank/pulley/flywheel by taking material out of the 'heavy side' of the flywheel. The center main strap is cheap and easy not to much of a biggy to install so it makes sense.

I will have a decent exhaust system (LCB and 1.75" stainless straight through), so really the only thing you recommend would be the sodium cooled exhaust guides?

[GeorgeHurst]

just had a look on minispares and it seems the sodium cooled guides are out of stock, but are indeed available - they have an inside diameter of 8mm so it looks like I would have to get specific turbo exhaust valves as well.

[Mogwai]

you really need the crank/flywheel etc dynamically balanced to be effective
I used Basset down balancing to do mine http://www.basset-down-balancing.co.uk/

[GeorgeHurst]

I'm scared by the potential cost of dynamic balancing - if you don't mind me asking, could you give me a ballpark figure for what Basset Down charged for the work?

[bmcecosse]

The balancing work is only necessary for high revs - do the simple stuff yourself (pistons/rods etc) - but I wouldn't bother with the 'dynamic' balance since you won't be doing high revs, and doubtful if you should lighten the flywheel - but no harm is doing so IF you can do it absolutely evenly thus not losing the balance. It's the exhaust valves that are sodium cooled (in the stems) - and so yes - they need larger bore guides. Can't imagine the inlet valves getting 'stuck open' at any sensible road-going supercharger pressure! Heavy springs only absorb power and increase wear. You don't really need a Duplex for an MG cam - the Mini type with tensioner is best for your purpose. DO get a decent oil cooler ! I would suggest the Turbo oil pump (not inexpensive!) - but not sure if it will go under the Minor backplate - you would probably have to modify it. The crank should ideally be hardened - and the crank bearings should be Leaded Bronze, not Aluminium/Tin alloy. On the Turbo engine Rover eliminated the oil groove in the bottom half of each main bearing shell for greater bearing area and strength (and it was so successful they continued it for all later engines) - if your engine is not so fitted - I think it will be worth doing. The tangs of these shells are offset to avoid mistakes - not too hard to make the necessary indents in your block and caps to take these shells.

[GeorgeHurst]

Thanks Roy, I feel confident doing the piston and rod balancing, but not entiterly sure if I could do the flywheel entirely evenly. If you will indulge me, here is the plan I had for the jig to do so...

A piece of 18mm stiff, straight ply, with four threaded rods bolted into it. The rods would be spaced as far apart as the two end main journals of the crankshaft. A piece of 20mm straight box section with a hole drilled in each end would be slotted over the threaded rod, i.e two rails. There would be a nut under each piece of box so that each end could be wound up or down to make it dead level on all four planes. This would then allow me to place the crank assembly on the two rails and find the heavy side of the flywheel as it rolled level. Does that make sense?!

I noticed your comments about balancing your Mini engine and you said you trimmed the pistons right down to just below the pin, I was thinking about doing that too - but would the counterweights on the crankshaft then be out of balance with the weight of the pistons?

Presumably if I were to only balance the pistons and rods then I couldn't rev much over 6000rpm like a normal engine? I know supercharger applications don't require high revs but I want to try and build a solid engine, I can't bear the thought of working hard on it and spending loads on it and then tearing it to pieces with a supercharger (or does that just happen anyway?!)

Thanks for the info on the Turbo bearing shells. Will probably have to do that on round two, as funds won't permit too much right now, but that's good to know.

Forgot to mention that i will definitely be fitting an oil cooler cooled by a leccy fan also. Minispares do a 'cheap' turbo oil pump for about £30 - I need a new one for the engine anyway so will get that, thanks for pointing it out.

On Mogwai's word dynamic balancing doesn't seem ridiculously expensive, not cheap, but doable - what exactly would I achieve by doing so? Will I achieve a noticeable increase in rev limit or engine longevity?

Jeez, sorry, long post!
Cheers
George

[bmcecosse]

The rails idea is pretty much how we did my 'racing' 998 Mini engine - many moons ago now! Except we used large diameter silver steel bar - so the rolling was steel to steel and point contact. The crank/damper/flywheel assembly definitely wanted to settle at one point - best of ~ 10 rollings. We then drilled a hole or two - with much experimentation - until it would settle anywhere. We revved that engine to well over 9000 - but it was a short stroke 998 !! It never broke. The 1275 is never really happy revving much over 6000 - so unless building a competition engine - don't bother! balance the pistons and rods by all means - but you also need to check and equalise the compression heights. Don't worry about the crank balance weights - they are just for the crank. Wouldn't bother with blading or wedging either! Most of these mods are for high revving - so not relevant to a road engine - although balancing does of course make for a 'smoother' engine - the 1275 is never all that smooth. My short pistons were for the racer - leave them pretty much standard for a road engine. Trim off a little if you really must - but it's not going to make a huge difference. Get the biggest oil cooler you can find - 19 row if possible -at least 13 row. Be aware the Turbo pump will be thicker - no problem on a Mini - but you will likely need to modify the backplate for in-line use.

[RobThomas]

I've got some alloy 28cc chamber big valve heads for the 1275 supercharged engine. Bog ports, bronze guides, great cooling, thick deck and alloy. There are some Turbo heads and Turbos in the shed, too. Any good?

[GeorgeHurst]

ok, well I'm definitely not aiming to build a competition race engine so will probably just balance the pistons and the rods in that case. Perhaps then when I come to put the supercharger on I will then have the money to get the bottom end professionally dynamically balanced so that I have piece of mind that the engine is 'done properly'. If the 1275 is generally no good over 6000 then there is no point spending loads of time and dosh to get it to go over 6000 then I guess!

I was going to match the compression in each cylinder by placing a piece of perspex with a hole in it over each combustion chamber in the head and then syringing in some oil to measure the volume of each. I will also check the compression heights and get them milled down if uneven - how accurate should I be looking - to the nearest mm or to the nearest 0.1mm?

With the turbo oil pump, will it mean cutting the 'lid' of the oil pump cover , extending it outward and then rewelding the lid on, or is it width rather than depth that is the issue?

Thanks very much for the offer of the heads Rob, are alloy heads a great advantage over the cast iron ones? What do you chaps think would be better for my purpose... alloy head with bronze guides or turbo head with (presumably?) sodium cooled guides?
Do the heads have valves and springs etc Rob?
I'll say no to the turbos thanks as I want to go super rather than turbo, but am possibly very interested in a head.

Cheers
George

[bmcecosse]

I can't imagine how expensive the 'alloy' heads are!! The piston heights relative to the top of the block should be spot on to each other - as close as possible. Ideally the stroke of the crank needs to be checked too - to ensure each crank has the same stroke....... There really is no end to how far you can go with 'blueprinting' an engine. 1275 can certainly be revved above 6000 - but that's when it starts to get expensive and fragile.

[GeorgeHurst]

Yes I suppose it is one of those things that once you start walking down the road you could just keep on going! Definitely going to stick to balancing rods and pistons, compression height and combustion chambers. Depending on how my saving goes I may then dynamically balance the crank etc and then do some further blueprinting/balancing work when I come to fit the super charger in a year or so.

Rob, PM on it's way to you about the heads...

[RobThomas]

The heads, as mentioned, are expensive. £600 for a brand new head with guides and unleaded seats. They are light enough that you can lift one up with your little finger without any trouble. Try that with an iron head!

Their ability to dissipate heat allows you to run a higher comp ratio which is ideal on a Turbo/Supercharged engine.

[Kevin]

Their ability to dissipate heat allows you to run a higher comp ratio which is ideal on a Turbo/Supercharged engine.

Excuse my ignorance on the subject but I always thought when supercharging a lower compression was desirable.
Not that I have direct experience of supercharging or alloy heads, that's why I wondered just out of interest.

[bmcecosse]

Are these 8 port - or 7 port heads Rob ?

[GeorgeHurst]

Goodness me! £600! That is definitely out of budget I'm afraid, but sounds like they are serious bits of kit... one day maybe!
I think I will stick with the large valve head that I have and fit bronze guides.
Thanks anyway Rob.
Cheers,
George

[minor_hickup]

What pistons are you going to use, the most common failing on a supercharged engine seems to be the pistons used breaking up or becoming holed under relatively small amounts of detonation which seem inevitable when using a carb and dizzy and setting up using trial and error. I would have thought only the late A+ type would be suitable for lower boost applications but would possibly give too higher compression. I know at least 4 people who have tried home brew superchargers or Jonspeed kits and the only real 'success' was with a really well thought out car blowing through a turbo carb and using megasquirt ignition. The best 'blown' a-series I've ever seen or had anything to do with was a turbo setup using a T2 turbo. This car used a lot of parts sourced from Avonbar who seem to have a lot of experience in forced induction.

[GeorgeHurst]

Cheers for the Minor Hickup. I'll keep the details of Avonbar to hand for when I come do the supercharger, good to know of a forced induction specialist.
With regards pistons I was going to go keep the standard pistons and then lower the compression with a compression plate, but when I do get the money to do it I may look into some aftermarket pistons that are specificaly made for the purpose. I'll be upgrading to electronic ignition when I do it also as I hear that the standard ignition setup isn't up to it.
Thanks for the input, cheers
George

[bmcecosse]

Indeed you will need to keep out of detonation - usually by modifying the ignition advance curve - so it advances LESS. And also with water injection at heavy throttle openings. I don't think blow through Turbo carbs are used very much....... someone on here tried that about a year ago with great non-success. GAS on here has a very nice suck-through system on his Traveller - talk to him!