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MG MGA - What is the max compression ratio on B Series

What is the maximum advisable compression ratio on a B Series engine with 98 pump fuel (Aust. 98) with standard bottom end and strengthened bottom end? What stregthening mods recommended?
Let the opinions fly!
Mike Ellsmore

Wider overlap cams have lower dynamic CR and the engine can take higher static (or calculated) CR than engines with cams having little overlap.

Also depends on the head - alloy heads will take a higher CR.

Generally for a reliable road car I would say 9.75 for cast iron and 10.5 for alloy but you can push that further with a suitable choice of components.
Chris at Octarine Services

Just looked, Australia 98 octane is same as UK and most other places 98 octane (RON - Research Octane Number). We also get 97 and 99 petrol in UK.
Nigel Atkins

By comparison the latest Mazda MX-5 have 13:1 compression ratio using premium unleaded (95RON or higher) fuel - assuming that is a calculated CR?
Mike Ellsmore

If I'm doing an engine(B series)I tend to run the same comp ratio as the fuel as a max.
98 oct. 9.8
95 oct 9.3
and that seems to work out ok for a std or mild cam but as Chris mentioned the bigger the cam the more compression you need to run it
I ran my midget at 12.5 on 98pump fuel with an iron head but that was with a 320deg. cam, I think that is about the max for 98
If a B engine is put together properly to specs the bottom end is fine-A 5 bearing B motor with straight cut rods and a decent set of bigend bolts is plenty strong

The MX5, apart from being probably 60 years younger has a completely different shaped combustion chamber, alloy head,knock sensors, plus the big bonus of phased direct injection, it's that close to being a diesel running on petrol it's not funny
With the injection on them now it's actually timed in 3 stages like a modern diesel with a little taste to start the fire,then a slightly bigger then a third right on where it can develop max power
William Revit

95 oct=9.5
don't know where that 3 came from
William Revit

You are starting to venture into the realms of Atkinson and Miller cycles. The compression ratio we talk about is geometric - whats the volume of the combustion chamber compared to that of the stroke. Valve timing though effects the amount of charge that is trapped. If you have late exhaust valve closing for instance. On engines with carbs or port fuel injection wild cam timing means you can be pushing fuel and air mixture down the exhaust. Generally these cycles are only used on engines with direct injection and variable cam timing (phase usually) Then they talk about expansion ratio. I think the Mazda sited is one of these.
Paul Hollingworth

That got big all of a sudden Mike
What are you up to--
William Revit

My 1.8 J*tt@ is also 13:1. Its a TSI. Tun on regular 87
Art Pearse

North American octane figures, but I have had no problems running stock MGA, MGB and MGC engines on regular (87-89 depending on whether they ping at all on the lower one), MGA/B on approx. 9.75 on premium (91-93) and as high as 10.5 on original HRG heads (just looked back and this is exactly what Chris said too... )

I've run an old Lamborghini (10.8) on premium as well.

All that is a far cry from what we can do today - with knock sensors and computer controlled ignition, one of my naturally aspirated street cars runs 11.5 on premium and my turboed engine runs 9.5 compression and 25 psi of boost on the same fuel!
Bill Spohn

Hi Willie. A friend is just rebuilding his B series MGA with new pistons JE, Maxpeeding rods and 770 cam. Has elected for 10.5 CR. That was the basis for my query. My coupe has similar but with Carillo rods and 11 CR.
Mike Ellsmore

Assuming a 1622 A engine with a 60 thou overbore, 40 cc chamber, flat top pistons 10 thou down the bore and 770 cam - the static CR would be 10.5 to 1 but the dynamic CR would be only 7.28 to 1.

The huge reduction in CR is entirely due to the timing of the 770 cam.

This engine could almost run on cooking oil!

Chris at Octarine Services

Just to add -

General rule of thumb for acceptable dynamic compression ratio to run safely on 98 RON is 8:1 maximum for engines with cast iron cylinder heads and 8.5:1 with aluminium cylinder heads.
Chris at Octarine Services

Had some spare time on my hands today so had a session with the pencil, Went to blurt my findings out on here and there you go you got in before me
But I agree, with that cam it'll be fine--
So I worked on a 89mm stroke engine with a std MGB cam and 8.8 comp to start with so basically a std MGB engine

With a std MGB cam the effective stroke after the inlet valve shuts is 70m giving 7.14:1 ratio

We know an MGB with a bit off the head but otherwise std and a comp ratio of around 9.2 will run on 95 RON nicely-this calcs out at 7.49

9.13:1 works out at 7.4:1

An otherwise std MGB but comp. raised to 9.5 will come in at 7.7 and is borderline on 95 oct but fine on 98

Now---Same engine but with the 770 cam the effective stroke is now 60mm and with 10.5:1 comp. calcs out at---7.4:1 so exactly the same as a std MGB with 9.13 comp. ratio-- it'll run on 98 quite safely

Also, a little note I found at the end of the writeup for Stage 6 (10.4--10.6 comp)in the factory MGB Special Tuning manual states-----
While using 100oct and the 770 camshaft no extra power will be gained by further raising the compression ratio

I haven't got much else to add, but I have used the 770 cam in various engines, some people see them as old school design but really they are a good honest cam and suit these engines well
I did an engine back in the early 80's with one and a Derrington with Webers --- It did a 1.07 in a Magnette at Baskerville, the best 4 cylinder Bs that come over from Australia for the Historics are only around 1.04 now , 40 years later----------------

William Revit

You are right - Miller and Atkinson cycles use late inlet valve closing to limit the trapped charge. They are known as over-expanded cycles.
Paul Hollingworth

This thread was discussed between 01/06/2020 and 04/06/2020

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