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MG Midget and Sprite Technical - Valve gear

Last week, I submitted a request for advice regarding potential modifications as I rebuild my 1971 1275 for road use. The engine is now stripped and the head and block are being checked by the machine shop.

I especially appreciate Prop's recommendations regarding the Swiftune cam, valve guides and roller tip rockers.

I have consulted with several suppliers and am receiving conflicting information. I have been told by Swiftune that their SW5 cam will supply increased performance between 3000 and 4000 RPM but that the cam is best used with 1.5:1 roller tip rockers. They recommend the "purple" type with roller tip only and center bushings, not the more expensive needle bearing type.

A U.S. supplier recommends an Elgin 101 cam with stock rockers for a mile upgrade unless compression ratio is increased from the original 8.8:1 and comments that the high lift rockers would be counterproductive unless there is an increase in compression ratio.

This kind of makes me want to use something like the Elgin cam and keep the rockers stock as it would much less expensive.

If everything else is kept stock, is it really worth importing the Swiftune cam, roller tip rockers and such or should I just stick with a mild road cam and be done with it? I am about ready to order pistons and bearings so I need to decide on the valve gear.
Glenn Mallory

Higher compression ratio is an essential ingediant to having horse power,...think of a ballon, fill it 10% (1 to 1 CR) with air and hit it with a needle, probably wont even pop, fill it 50% (1 to 7) and it will make a nice little bang, fill it 500% (1 to 73 CR) stick it in your mouth and pop it ...and your gonna loose some teeth, aka compression ratio

when you order the pistons get the 21253's these are 9.75 to 10 CR without any mods to the deck and head still okay with pump fuel

the 21251s are stock 8.3 - 8.8 in that area...the differance in price is around minimal for a setof pistons between the 2, If you go this direction...Id stay strictly stock.

On the cam situation, Camshafts have many differant profiles, to achiv differant things like gas milage vs high performance...most likly the elgin cam has a taller lobe and a fatter duration then the SW5, meaning the valves open wider and stay open longer...but this will have less torque, on the SW5 the cam lobes are .280 inches high and the duration is avg....meaning its got some torque but no midum to high end meat/power/musle...because the lift is short, but you add an extra .5 ratio or inches to the highth and now you got something, because the valves stay open longer on a narrower duration...on a 1.5 ratio So a lift of .280 inches devide by .5 ( 1.5) if it was 1.3) devide by .3 any way .280/.5 = .140+.280 =.420 of lift remember the head can only flow so much, once you get to .430 to .450 your done, if you increase to .750 it wont do anything more then .430, because you have maxed out your valve to seat distance


as to roller rockers vs roller tiped, this is one of those things like brake fluid, I think the roller tiped are better, because you have fewer moving parts, the tip is the only thing that rolls and it does it at the valve tip...just like the orginal rockers....otoh the roller rockers some how roll on the shaft and the tip is solid...when you load that much pressure/stress that far away form the valve tip, something bad is gonna happen...the argueent for something going bad on a roller tip is ...the roller tip breaking off

One option you might look into is a scatter cam, if you didnt want to do a roller rocker...Id look at the kent 266...it comes in various kit forms as well like the SW5

Another idea is to have another posting to the crowd here and ask there opinion saying "I dont want to use a roller rocker but want a cam that will kick in around 1800 rpm and hold till around 4000rmps" and see what people say

the other thing to consider is the head...this really is the heart and sole of the engine, if it dosnt breath, all the Cr and cams, and hot ignitions will do vary little. At the vary least you need bigger "penny on a stick" valves and you need to grind out the blobs of metal that hold the the valve guilds...those 2 things alone are huge

Prop..I hate my brain, it never lets me sleep...Prop

Prop and The Black Hole

Forgot to add,

About CR...the thighter you compress the air/fuel mixture in the cly. compression chamber the bigger the bang...just like fire crakers and pipe bombs...if you compress the gunpowder mix super tight in a hard shell it will be a big bang, pour it out loose on the ground...your lucky if it will even smoke, never mind go pop.

Prop
Props Black Hole

It all depends how far you want to go with engine mods & what your driving style / roads are like.
Perhaps a better head would be more suitable for your tuning needs?
I would suggest you read up on Vizzard.
Brad (Sprite IV 1380)

It might be because I'm jet lagged after an even more grindingly long flight to the UK than usual, but the link between compression ratio and roller rockers is one that eludes me completely - I'd say there isn't one.

Higher ratio rockers are good for street cams, as they give a relatively bigger cam for any given seat duration. The seat duration has a lot to do with how tractible the engine is, so this means you get better performance while retaining drivability.

Not all high ratio rockers are looer tipped. The roller tip is a definite plus as it greatly reduces sideways thrust on the valve and prolongs valve guide life.

On one of our cars we have a Kent 266 with 1.5 roller rockers, and it's great for all-round driving and is fully tractible. Another is almost identical but has the 286 cam. It's very driveable on the street, but has a lumpy idle and is tetchy under 1500 RPM on full throttle.
Paul Walbran

It is a matter of effective CR to static CR. On long duration cams the effective CR is lower than the static because the intake valve is still slightly open when the piston starts its compression stroke. Talk to an experienced A-series engine builder. I suggest Hap at acmespeedshop.com. NFI.
Trevor Jessie

My personal vote for a warm rebuild would be to use HC pistons 21253? and a VP6 cam from aptfast.com , agian NFI
Trevor Jessie

I don't see a vote for the Kent 276.
A good street cam IMHO. Plenty of power thru the ranges, and an acceptable idle. I use mine with stock lifters and Isky pushrods.
Phil

Phil, what is your static CR?
Trevor Jessie

I thought the kent 276 was the pure stock camshaft

prop
Props Black Hole

A question regarding roller tipped rockers,... When setting the gaps, do I set the same gap as for a solid tip or should the roller be touching the valve?
graeme jackson

Graeme you use the same valve clearances as you would for solid rockers OF THE SAME RATIO (whatever specified for your cam grind).

Be careful, with roller rockers you need to insert the feeler gauge from the end if the rocker in parallel to it. If you go in from the side you can misread the clearance.
Deborah Evans

Send your head or a spare to APT for a mild grinding of the ports and combustion chamber. Dave can suggest the best cam, piston, CR to get the results you are looking for. I've stopped by to visit and get head work done. They are very knowledgeable and helpful. Dave Anton (spelling might be incorrect)was on the team that developed all of the information in Dave Vizzard's book.
A word of caution, the octane of the gas you want to put in the car should be the determining factor of how much you want to modify your car. 93 octane is $3.00 plus a gallon around here. If you only drive it occasionally it won't be a problem.
J Bubela

Trevor

I agree entirely about static and effective CR. However, higher ratio rockers don't increase the seat duration - they do increase the effective duration a bit in that at any given lift the valve is open a bit longer. However, this effect is more marked at higher lifts rather than lower lifts, so it will not be enough to alter the effective CR significantly.

So I wouldn't worry about CR not being "right" for higher ratio rockers. Worry about the $$$$, yes, but then extra power always costs more!
Paul Walbran

Correct Paul, I did not address the high lift rockers. I was trying to explain why piston choice and cam profile are related to effective CR. But now comes the question... are high lift rockers worth the extra $$$ and setup hassle if the head is not ported?
Trevor Jessie

Trevor

Verry dependant on the cam choice.

The sw5 is designed to work with them other cams might not be.

i am curious about the elgin 101 profile can not find it on the website

if you want yo be finished as cheap as posible rebuild fit an sw5 and increase CR substantialy.

But then your stuck with expensive fuel so in the long run it is cheaper to have it done proper.

Onno Könemann

A ported head would be high on my list. Even earlier would an LCB manifold, especially before I changed the cam profile. I recall one notable occasion years ago when friends full of enthusiasm fitted a "3/4 race"camshaft, weber and head job ... but retained the standard exhaust manifold. They got dragged off by a standard midget. They also got out paced by a dog, so the car for ever after was known as the "3/4 race dog" - even after they'd seen the error of their ways and fitted an LCB.
Paul Walbran

This thread was discussed between 21/08/2009 and 23/08/2009

MG Midget and Sprite Technical index

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