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MG Midget and Sprite Technical - How many HP from higher CR?

I am trying to decide if I can justify for myself the cost of new flat top pistons to replace the dished pistons that are now in my 948 midget engine. The difference will of course be that the compression ratio with the dished pistons is 8.3:1, with the flat top pistons it will be 9.0:1 as it originally was.

Vizard and other sources state clearly that a BMC A engine "wants all the compression that it can get". But the factory data show surprisingly little difference between the original Frogeye and the Midget Mk 1. 42 HP for the Frogeye, and with a different cam, bigger carburetters and valves AND compression increased from 8.3 to 9 the Midget engine gives 46.5 HP.

I have a Peter Burgess Econotune cylinder head, which I believe gives a slightly higher CR than original, so the combination with dished pistons may be close to original factory spec anyway.

What do you think? What difference will the higher CR make, and will it be worth the cost?

Tore

If your not into originality then using Mini 998 rods and pistons is a worthwhile upgrade that opens up a whole new world of tuning bits.

Rob
Rob aka MG Moneypit

Hi Tore,
You need to calculate the real compression ratio of your engine and then adjust the chamber volume to get the compression ratio that you need. Have a look at this video -
https://www.youtube.com/watch?v=ow5cGV7bXCw

In it Vizard explains the real compression ratio not the swept volume/chamber volume ratio. The piston doesn't start compressing anything until the inlet valve is closed with is some way up the compression stroke.
The video explains it all around 48:10 in.
9:1 is the compression race engines use related to intake valve closure.

Good luck

Dave
Dave Brown

Very interesting video. Thanks - I will have a closer look tonight!

Tore
Tore

Tore

You do not say what other work you are planning to do to the engine and need to do.

If you need oversize pistons because you have rebore the block due to damage/corrosion then you may find that the availability of pistons and rings may move you to more easily available 998cc Mini pistons and rods (and over bore sizes in those if you wish). This also opens up options as the others have said.

Flat top 988cc Mini Cooper pistons and 12G202 head were a common way to upgrade ordinary 998cc Minis towards Cooper spec, along with twin SU carbs and a suitable exhaust (and overbore sizes of pistons if your Mini engine was tired). You have the advantage over 998cc non Cooper Mini of a suitable head to start with and twin 1 1/4 carbs.

Pistons and con rods, see: www.minispares.com and other suppliers.

Also what is the bottom end of your engine like? Time for a refresh with new shells (bearings), bolts nd a centre main strap?

Cheers
Mike

M Wood

Sounds expensive

What about sourcing a stock 1275 and freshing it up

In stock form, no mods id think the.1275 would pump out a good 15 to 20 extra HP over the 998

I want to say the 1275 from the factory bealched out 55 to 60 HP

Prop
Prop and the Blackhole Midget

Drave browns link now clickable

http://www.youtube.com/watch?v=ow5cGV7bXCw

Prop
Prop and the Blackhole Midget

from the original special tuning data:

8.9, 29.0 cc is 60 BHP,
9.5, 26.5 cc is 62 BHP,
10.5 23,2 cc is 64.5 BHP
Flip Brühl

Flip...

I think one of us is watching to much porn,

60ish + HP for a 998 ???

Prop
Prop and the Blackhole Midget

Hi Tore

We had a Mini Moke in on the rollers July 2014 it gave standard sort of figures but seemed very soft with a torque dip associated with a tuned cam. The owner thought he had ordered a stad engine. His Mini 850 van made same top end power but seemed sharper. This year he asked us to do an Econotune head job on his Moke and re tune it. We removed the head and found it was quite a work of art, all ports bigger, largest of the cooper inlet valves and very opened up chambers a la Vizard style. The valves had not been modded and the seats were 45 degree only. The bad news was the chamber vol was massive giving a CR of 8.4:1. Turns out it had a mild road cam too. Engines thrive on compression especially with a tuned cam. The head modder/engine builder had fallen into a trap of losing CR by maybe gaining flow in the chambers. We skimmed as much as we dared off the head, found a thinner head gasket and less dished valves and ones with thicker margins. We got the CR up to 9.2:1 and cut three angle seats, lead free inserts and modded the backs of the valves. Power at the wheels went up from 24.6 to 27.4 and torque up from 32.9 to 36.9. Engine power up from 36.9 to 39.7. Higher CR would have been even better.
The owner is pleased with the new found crispness and go. try and get your CR up to say 9.75:1 if you can Tore, it will fly.

Peter


Peter Burgess Tuning

Thanks everybody!

My Midget has been off the road for some years, for a very slow and relaxed rebuild. The engine pushed out a lot of oil, indicating high pressure in the crankcase, despite having a rebore to + 0.030 not long ago. For that reason I suspect some mistake has been made during the rebore. The piston ring gaps are also quite large. Everything else is in perfect condition.

I have dismantled the engine and will now have cylinders and pistons measured to see if they are within spec, or if the bores are too wide. That is where the opportunity to get flat top pistons comes in, either in the existing bores or if I need to take it out to + 0.040.

The pistons in the engine now is the Morris Minor type, with five rings. So in addition to low CR they do of course make a lot of friction.

Tore

Flat top three ring Tore.

Peter
Peter Burgess Tuning

But when chasing high CR combined with high lift cams/rockers you do need to check valve piston clearance.

Love the 11-1 CR on mine but now with slightly less lift ;)
Onno K

Remember to take into account what fuel octan you have avialable to you, as well as tunablity of the timing in those high CR specs

Prop
Prop and the Blackhole Midget

I use only Shell V Power 98 octane for our MGB, and so do practically all our MG friends in Norway. Most oil companies here offer only 95 octane, and I have often wondered how long 98 will be available to us.

Driving home from the MGCC European Event in Denmark last year, I noticed that most stations in Denmark and Sweden have 95 and 92 octane.

If 98 disappears from the market, I suppose it will be more practical to have a lower CR in the Midget engine?

Tore

98 octan... ah, those where the good ol days, now I cry tears of joy just finding a pump that has ethanol free 91 octan

Btw... when reading forum about octan... the usa octan rating is figured differantly and is a couple of points lower then the uk rating... so if you have 98 out equivalent it 96... of which I can assure no longer exist in the wild nature of the usa convenice store gas pumps...its extinct

Prop
Prop and the Blackhole Midget

Octane ratings are more complicated than I thought. I guess 91 octane in the US is not that bad after all?

From Wikipedia: Difference between RON, MON, and AKI

"Because of the 8 to 12 octane number difference between RON and MON noted above, the AKI shown in Canada and the United States is 4 to 6 octane numbers lower than elsewhere in the world for the same fuel. This difference between RON and MON is known as the fuel's Sensitivity,[4] and is not typically published for those countries that use the Anti-Knock Index labelling system.

https://en.wikipedia.org/wiki/Octane_rating

Tore

My 998 has 10.5:1 CR with 276 cam and runs happily on 95 RON. Currently using 10º static advance.
L B Rose

This thread was discussed between 14/08/2015 and 19/08/2015

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