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MG MGB Technical - Does an engine plane out?

Its been a few years since the build and just today I pulled her out to blow the cobwebs off as I don't often get the chance in a controlled environment.

I passed the 200 kmh mark which I've never dared to go further than before and the dial finally stopped at 6 O´clock, it did feel as if id actually found the point of no more power and I felt it unwise to push her anymore. It was a lot of fun and I have to ashamedly admit, I did bully a Jaguar in the process to the point submission. It was a Jaguar that prompted the build after a failed main bearing drama in Belgium. So the engine has really come full circle, it also cruises beautifully at 150 kmh and before the cam really rips open. It literally bursts with a twinge on the pedal.

I have a question;

Does an engine plane out? Power wise there must naturally be a point of when max output has been reached. And hopefully that's before the top pops or a piston flies.

Footnote. This is a full spec custom build that I spent several years on and countless thousands. And yes it really does go that fast.

JS Ahlund

It sounds like you have a fairly decent engine, but they are a bit like a chain in that it will only be as strong as it's weakest link
The usual rev limit is affected by breathing and valve springs
If you have a decent cam and valve train it is quite possible that your engine will easily rev past the point where other components can't survive
I suggest you use your tacho and set yourself a safe/sensible limit to suit your engine parts and stick to your limit
William Revit

Plane out?

Horsepower is simply a measure of work done so the faster the engine is revving the HP still keeps increasing, way past the point of maximum torque - in a long-stroke engine like ours anyway - and it's torque that accelerates you.

The trick is to know where the drop-off in acceleration from too many revs exceeds that from dropping down a gear. As well as knowing what a sensible limit for your engine is.

The factory V8 does 125mph/200kph, the gearing is chosen to match the power, and the V8 has a relatively low red line of 5200rpm. That all too easily exceeds it, I have to watch it like a hawk, whereas the 4-cylinder makes so much din I cringe before it gets anywhere near the orange line let alone the red. I think the hydraulic tappets in the V8 make for a safe limiting factor, but I'd never try it. 4-cylinder? Who knows? It all depends on how well it has been balanced as to what point it will blow up.

It depends what you want - acceleration or top speed, you choose your gearing accordingly.

Thankyou. I wasn't quiet sure how to describe what I meant or perhaps what I really was asking, so thanks for your replies!

I should state that its a daily, so rarely sees a chance like today. There may be some days next year if I'm lucky and can get time off. I think I kept it within the capacity bracket today. I do listen to the engine and double de clutch accordingly even daily driving.

The only standard parts are the gearbox and clutch, which will slip if the accelerator is punched. The gearbox is I feel the nature of the beast so I've never been interested in a swap to a five box. Both of these standard parts serve as a method in their own way of not pushing the engine too hard as I do enjoy just cruising within normal limits and don't drive aggressively in normal conditions. I was never decided as to what clutch so it has remained standard until then. Driven calmly within a sensible rev range the clutch does not slip.

Drive train is fully lightened, balanced at all points, top and block fully worked as well, rocker assembly etc. It breathes very well and the reliability and performance is fantastic. My aim was to make it as solid as possible as a fast road engine.

The whole project has been about the build, the vehicle and enjoying it.

JS Ahlund

out of interest, when you are driving aggressively roughly what max revs are you using and is it still pulling hard at that rev.limit
The 6 o'clock position you mentioned would be around 8000 rpm which I tend to think your tacho could be a tiddle inacurate and might need checking
William Revit

Ah,-or was it the speedo at 6 o'clock
I take it you have a kph speedo so what would 6 o'clock be on that
William Revit

With the a highly modified engine build why don’t you put on a dyno and know exactly what you get where?
Mike Ellsmore

If it was the speedo at 6 o'clock, presumably that was the 200km/h originally mentioned. It depends on the year and hence the speedo number but the lowest I have found would be 210 km/h and on the SN5230/14 fitted to 4-cylinder cars from September 74 to June 76 6 o'clock would represent about 240km/h.


The engine power does peak and start dropping off.

Depending on the valve springs fitted, valve bounce sets in around 6000 rpm and power drops off rapidly after that.

I have had the speedo of my V8 just past the 6 o'clock position and the sat nav said 137 mph, that's 220 kph.

But because of the 3.3 to 1 MGC axle that was at peak power.
Chris at Octarine Services

I'd love to know the specs of J's engine, if it cruises at 150Ks -before- the cam comes on it must be a monster
Your comment about 6000rpm valve bounce and power dropping off suddenly after, I agree with ,for a std B----I take it we are all talking about 4cyl engines
But the point I was making was that with a decent cam and springs the engine will willingly go past the breaking point of other components if they're not up to the job
I ran a self imposed limit of 7000 on mine for this very reason but ran the same cam grind and springs in a Chev and it regularly span to 8000 without a problem
William Revit

Being long stroke, our 4 cylinder engines are not ideal for high RPM but the full race engines I built were quite capable of 7500 rpm, but that was with everything lightened & balanced, forged pistons and Carillo rods, etc.

Picking up on Paul's statement - yes, torque accelerates the car but horsepower determines the top speed, when the sum of air resistance, friction, gravity, etc equal the available power, there is nothing left to go any faster.

So the engine will "plane out" as J puts it, at different rpms, depending on the circumstances.

Chris at Octarine Services

My 2 cents worth!
Attached is the dyno chart from my MGA 18GB engine showing power and torque curves. Engine is extensively modified (maybe not as much as J's). 1950 cc, cross flow HRG-Derrington head, twin DCOE 45 Webers, JE flat top pistons (11.9 compression), Carillo rods, BMC 770 grind camshaft, APT cam followers, Jet Engineering push rods, Titan roller rockers, 43mm inlet 36 mm exhaust valves, Moss aluminium flywheel, 123/Tune distributor (27.5 deg max advance, 6500 rpm rev limit settings).

You will notice that power starts to drop off around 5750 rpm on this chart - I had a damaged HT lead on No3 cylinder that was breaking down (a result of the earlier Scorcher distributor where No 3 lead hits the steering universal clamp bolt on an MGA, solved with 123 side entry cap). Also as I found out later from Peter Burgess that 123/Tune distributors start to break down the spark 500 rpm below the rev limiter setting! I have since replace the HT leads and will try setting the rev limiter at 7000 rpm to obtain an effective 6500 rpm (I haven't had the time or $$$ to put it back on the dyno to see how these changes effect).

I am fairly confident with these mods, the balancing done and feedback from the dyno knock sensor that this engine can run safely up to 6500 rpm when needed (in practice I normally don't exceed 6000 - controlled by VDO tacho with adjustable shift warning light). In the past before I damaged the engine with a timing setting (ouch!) I have hit 7000 rpm going over the finish line at Collingrove hillclimb!

To answer J's question, yes, B series engines do plane out!

Mike Ellsmore

Thanks for sharing you dyno graph--
Interesting little drop at aprox. 3400, looks like the mixture takes a dip followed by hp/torque--just a thought, the chokes 'might' be a size too small or slightly too early emulsion tubes
Your drop off at 57/58 , I doubt was a plug lead as electrical misfires usually show up lean whereas yours is still nice and healthy there
More likely to be that soft cut rev limiter doing that
With the bits you have listed in your engine I agree it should be able to buzz out to 6500 easily and safely
William Revit

The reason the engine power curve drops off is it runs out of breathing so it can't generate torque, my car will pull o/d top into the orange on the tacho, when visiting Germany, however I don't use more than 5500 and only need that rarely on the road. Racers do a lot of work including strengthened main bearings to review past 7000 often with lower ratio final drives.
Stan Best

This thread was discussed between 08/12/2017 and 16/12/2017

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