High compression vs low compression

Hi,

Basically the more variance in PSI between cylinders, the more difference in power there will be between each cylinder therefore the motor will not run as smoothly.

Fortunately this is less of an issue in a big lazy V8 like ours compared to a 4 cylinder engine :p

Steve

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Originally Posted by SteveFarmer

... big lazy V8 like ours...

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Hold on, are you talking about the rover V8 here??? :eek:

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Originally Posted by isuzurover

Hold on, are you talking about the rover V8 here??? :eek:

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Hahaha ... well in comparison to a high compression DOHC 4 cylinder rice rocket our venerable 60's pushrod V8 looks pretty laid back :D

Steve

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Originally Posted by garrycol

So for your 8.13:1 standard low compression rover engine at tdc on the compression stroke you will have 8.13x14.7= 119.5 psi on your compression gauge - it will be a bit less on a perfect engine and a lot less if the engine is old and worn.

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Originally Posted by isuzurover

If it is an 8.13:1 engine those numbers would be as good as you can get!

More likely though is that you have a slightly worn hi-comp engine. However I wouldn't be rebuilding it just yet unless it had lots of blowby or was using oil, etc...

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I agree.

At best with a low compression engine you will get 120psi so anything above that is a Hi Comp engine. The 9.35:1 Hi Comp engine will produce up to 137.5psi - say 130psi for a good engine. So 120-125 is not a bad spread and probably not too bad for an older engine with a bit of wear.

Garry

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Originally Posted by garrycol

I agree.

At best with a low compression engine you will get 120psi so anything above that is a Hi Comp engine. The 9.35:1 Hi Comp engine will produce up to 137.5psi - say 130psi for a good engine. So 120-125 is not a bad spread and probably not too bad for an older engine with a bit of wear.

Garry

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All in all this sounds pretty positive, compared to the negative feed back from the garage. Thank you all for your in-put and apologies for hijacking the thread, even if it was relevant

Now that you have the good "oil", if you have to speak to the garage again, might be worth asking an "innocent" question as to how they reach their conclusions:angel::twisted: then you will be able to judge their competence ...and ethics:wasntme:;)

Just thought I'd throw my 0.02 in.

The Low Compression 4.6 is 8.13:1 (my '99 Thor HSE for example)

The High Compression 4.6 is 9.37:1 (my '02 Thor HSE).

Hope this helps someone!

Cheers
Keithy

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Originally Posted by Keithy P38

Just thought I'd throw my 0.02 in.

The Low Compression 4.6 is 8.13:1 (my '99 Thor HSE for example)

The High Compression 4.6 is 9.37:1 (my '02 Thor HSE).

Hope this helps someone!

Cheers
Keithy

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Keithy can you notice any performance differences between the two?

I'd like to say that it's in favour of the HC 4.6, however I do believe the LC 4.6 has been rebuilt within the last 50,000km. The engine # and CR stamp has been shaved at some point on the LC.

In terms of economy and performance the LC 4.6 I have is better in both respects. It's also smoother at idle. With that said, the HC motor is running Sequential LPG, which may or may not have an effect.

As an example, with the same load on board and running petrol (both run on 95 anyway) the LC P38 averaged 2L/100km better on a 400km trip than the HC.

I'm not sure if there was a camshaft change when the previous owner installed the LPG, or if it's standard, but it's noticeably 'rougher' at idle. According to the installer (who performed pre and post dyno runs), it's making an extra 70nm at the treads on LPG than what it was pre-install. If it is - I can't feel it!

Old blue, with her muddies and suspension lift wins. Smoother idle, more action when the right foot goes down, better fuel economy, unknown internals...

Cheers
Keithy

A high comp will always have more grunt than a low comp of the same condition - of course you get the most out of LPG on high comp as well.

Cheers