What dictates the compression ratio of a Rover V8 engine? Is it the design of the piston crown?

mrtask

Well-Known Member
I hope everybody will go easy on me if I'm asking stupid questions. Last year I helped a good mate of mine to yank the V8 engine and automatic gearbox out of his P5B saloon. It is currently languishing in a corner of my garage while he further modifies his front subframe, which he has previously adapted to take independent dual wishbone suspension and a steering rack from a Granada.
I was surprised to discover the engine has a low compression ratio of 8.13:1. Going from the engine numbers list over on the V8 Owners Forum we think it may originate from a '70 - '83 Range Rover. Whatever, it is what it is, but could it be made to be "a bit more"?
I am quite literally "asking for a friend"! At one point his plan to was to ditch the old Rover engine altogether and fit a substantially more powerful V8 from a more modern car. In the meanwhile he's gone and blown a bunch of money on a very rapid 'modern', and apart from having seriously depleted his bank account, he is realising even if he fitted a brand new Corvette engine his old P5 is never going to handle like his new vehicle, despite all the suspension mods and braking upgrades he's had done to it! So he's now thinking about sticking with the RV8 after all, but perhaps refurbishing it before refitting it.
Soooo, I was pondering. What exactly dictates the compression ratio of a Rover V8 engine? Is it a matter of the top of the piston having a deeper dish shape than a higher compression engine? Or are there other differences as well? I suppose what I'm wondering is if it would be possible to increase the comp ratio of this old 3.5 litre V8 'simply' by fitting different pistons? I (we) welcome your thoughts, wise ones!
 
The pistons dictate the CR, but it would be possible to fit later heads with the smaller 28cc (?) combustion chambers to an early engine to acheive the same result.
 
As Harvey says, later "ten bolt" heads with the smaller combustion chamber would up the compression if used with tin gaskets, but then you'll be into shimming the rocker pedestals or using adjustable pushrods to get the preload all correct
 
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The low compression engines will run perfectly well with low octane fuels. They normally came with more advance in the distributor and lighter springs which can actually give you more power and better economy than the HC engines. Replacing the heavy spring with a light one and timing for 34 degrees max advance (without vacume unit working) and about 10 degrees initial advance will make it perform very well on 94 octane or less. Higher octane fuels are generally less powerful than lower octane types so they really need higher compression to make more power; hence the need to tune to the octane rating. Note the figures above a pretty much the maximums. The manual gives figures for all the factory distributors and some are close enough to leave them alone. P5 and P6 came with the low compression engine from the start if requested.
 
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I have found that the combination of CR and ignition advance has a big effect on an engine's ability to rev nicely, and where its torque peak is. not to say the cam has no effect, but its harder to change. in my experience the 8.13:1 cr has a great low end, but doesnt rev well; 10.5:1 has a great top end, but poor bottom end , revs well - ideal for an auto; 9.25:1 is the all-round sweetest - decent bottom torque, and revs well. similarly the 9.35 CR in S2 Discos with 3.9 engines , with R380 5 speed , is very nice to drive. preferred engine now is 3.9L, small chamber heads with composite gaskets.

BTW, to calculate the volume of the static combustion space, use the following;-

Static vol = Swept volume / (CR-1) , static being head space + piston crown space.
eg for a 3.5L, 8.13 CR, static =441/7.13 = 61.85cc. Early engines had a 36cc head chamber, so the piston space is 25.85cc
for a 3.9L, 8.13, static = 493/7.13 = 69.14cc. With 28cc head space, piston space is 41.14cc
for 3.9L,9.35cr , static = 493/8.35 = 59.04cc. less the 28cc head, piston space is 31.04cc.
 
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Static vol = Swept volume / (CR-1) , static being head space + piston crown space.
Don't forget the gasket volume and the height above the piston to the top of the block. The gasket thickness is significant in the CR calcs, E.g. 40thou @ 89mm dia = 6.3cc.
G
 
Assumed that was obvious, its always there. also it varies depending on tin or composite gasket. Whats with mixing units - thou and mm???
 
Thanks for all the input, gents. Quite a lot to think about!
I am particularly heartened by @mikecoombs' observations that "low compression engines will run perfectly well with low octane fuels", and that the "P5 and P6 came with the low compression engine from the start if requested". If the latter was the case, perhaps it wasn't a later swap, as we had assumed?
Presumably this 8.13:1 engine could be set up to run on E10 then? Before it was removed from the car the engine had provided plenty of 'oomph!', or at least it seemed plenty quick to me! Certainly made naughty noises with one less exhaust silencer than standard, in fact it sounded like a NASCAR when pushed!
 
Most of the work to make the engine E10 compatible would be replacing the rubber in the fuel system, I think...
 
There isn't much in a standard P6B fuel system which will cause trouble due to a reaction with the Alcohol. the fuel fuel tap gasket, pump diaphram and valves and mabe some carb seals but Mike Shaddick ran his P6B for years on straight or nearly straight ethanol without issues. Tuning is different however as it will cause the engine to run lean as the correct mixture for alcohol is significantly richer than petrol. However this applies right across the range so its just a matter of adjusting the jet down a bit further.
 
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