High compression vs low compression

[intheozone]

Hi,

Rather than hijack a thread, I'll ask in a new one.

This will sound possibly like a dumb question, but, what is the difference between a high compression engine and a low compression engine?

Is my 2001 4.6 I believe it is a Thor engine high or low compression? Can it be either? What is the difference? If it is low compression how can I make it high compression? What are the advantages?


Sorry abut all the questions, I just feel a bit confused?


Thanks

Steve

[101RRS]

The compression ratio will be written above the engine number on the tab on the side of the top of engine block on the passenger side between the two centre exhaust pipes.

It should have a 8.3 or a 9.3 (please correct me if the numbers out not quite correct) above the engine number indicating the compression ratio.

The two engines are basically the same except for the pistons - so to upgrade from low compression to high compression you would need to change the pistons.

As a general rule HC will produce more power and more torque.

Garry

[bee utey]

Compression ratio - Wikipedia, the free encyclopedia

[TheTree]

Hi,

Also remember that a higher compression engine requires higher octane fuel to run it.

As with everything, there are tradeoffs ;)

The other thing you may care to read about is power vs torque, motors can be tuned in various ways to provide more of one or the other.

Have fun!

Steve

[redandy3575]

Hi,

Rather than hijack a thread, I'll ask in a new one.

This will sound possibly like a dumb question, but, what is the difference between a high compression engine and a low compression engine?

Is my 2001 4.6 I believe it is a Thor engine high or low compression? Can it be either? What is the difference? If it is low compression how can I make it high compression? What are the advantages?


Sorry abut all the questions, I just feel a bit confused?


Thanks

Steve

Hey Steve

There's a good chance that yours will be a Low comp motor as that was standard that the P38 was set up for the Oz market, however some High comp motor did slip through.

[intheozone]

Thanks guys,

I am just looking to squeeze a few more horses under the bonnet, looking for inexpensive ways do do this.

I think a twin pipe exhaust system may help and make her sound a bit more V8ish

[snowbound]

Staying on topic, I am hijacking! Sorry intheozone. My 3.5 has a psi reading of 120 -125 on all cylinders. I'm told this is low? I went looking for the number on the block but couldn't find it. I will look again.

[101RRS]

Here is a pic of the engine number and compression ratio on the side of a 3.9 block. If only the engine number is there, the block has been leveled at some stage and the numbers removed - the engine number would have been stamped back on.

You can see the compression is CR 8.13:1

https://www.aulro.com/afvb/ (http://s42.photobucket.com/user/gazzz21/media/Forum%20Posts%20Album/P7170085_zps3a5b99a4.jpg.html)

Here is a pic that shows where the numbers can be found - left side between the two middle exhaust ports.

https://www.aulro.com/afvb/ (http://s42.photobucket.com/user/gazzz21/media/Forum%20Posts%20Album/P7170087_zpse712d16e.jpg.html)

Garry

[snowbound]

Here is a pic of the engine number and compression ratio on the side of a 3.9 block. If only the engine number is there, the block has been leveled at some stage and the numbers removed - the engine number would have been stamped back on.

You can see the compression is CR 8.13:1

http://i42.photobucket.com/albums/e334/gazzz21/Forum%20Posts%20Album/P7170085_zps3a5b99a4.jpg (http://s42.photobucket.com/user/gazzz21/media/Forum%20Posts%20Album/P7170085_zps3a5b99a4.jpg.html)

Here is a pic that shows where the numbers can be found - left side between the two middle exhaust ports.

http://i42.photobucket.com/albums/e334/gazzz21/Forum%20Posts%20Album/P7170087_zpse712d16e.jpg (http://s42.photobucket.com/user/gazzz21/media/Forum%20Posts%20Album/P7170087_zpse712d16e.jpg.html)

Garry

Thanks Gary, what does that mean? I was expecting a PSI reading, so not sure how to interpret or convert it.

Just checked, and interestingly I have the engine number but not the compression numbers. Why would the block be leveled?

Regards

Jez

[TheTree]

Thanks Gary, what does that mean? I was expecting a PSI reading, so not sure how to interpret or convert it.

Regards

Jez

Hi Jez

It is a ratio so it is expressed as 8:1 or 7:1 etc, this is different from compression pressure, which is measured in PSI.

You can do a rough conversion, as detailed here
How do you convert compression ratio and psi (http://wiki.answers.com/Q/How_do_you_convert_compression_ratio_and_psi)

But as it says there are other factors involved in the actual pressure you finish up with

Steve

[101RRS]

What Steve said - atmospheric pressure is 14.7 lbs per square inch. If you had an engine of compression ratio of 10:1 then the engine is compressing the air 10 times so 14.7 would give a theortical compression psi of 147 on you compression meter but as the links say they are reasons it may be a lit less.

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.

Maybe it has been rebuilt or had a over heating episode at some stage. Does your engine number have the dot matrix style of engine number or one that has been stamped on?? I guess 80s engines were all stamped and maybe not the did not stamp the CR on it. Not sure about very early v8s.

Cheers

Garry

[snowbound]

Wow! so this old bugger is on a learning curve! The engine number is stamped on, and thge area where it is stamped is very shiny, it is not the original donk so goodness knows. I am just trying to ascertain if the PSI reading I was given as "poor" is actually poor or whether someone was trying to get workout of me! Call me a cynic!

[loanrangie]

Chances are its had a new block, the number is stamped on an angled flange so is not removed during deck machining.

[101RRS]

Chances are its had a new block, the number is stamped on an angled flange so is not removed during deck machining.

Certainly was on the 4.0 block I have.

[TheTree]

Wow! so this old bugger is on a learning curve! The engine number is stamped on, and thge area where it is stamped is very shiny, it is not the original donk so goodness knows. I am just trying to ascertain if the PSI reading I was given as "poor" is actually poor or whether someone was trying to get workout of me! Call me a cynic!

Hi

The motor can still run with lower PSI than when new, whats more important is that there is no more than about 5PSI difference between the cylinders

You may wish to read this thread, it suggests 135 PSI

http://www.aulro.com/afvb/technical-chatter/31103-v8-compression-test.html

Steve

[snowbound]

Hi

The motor can still run with lower PSI than when new, whats more important is that there is no more than about 5PSI difference between the cylinders

You may wish to read this thread, it suggests 135 PSI

http://www.aulro.com/afvb/technical-chatter/31103-v8-compression-test.html

Steve
Awesome Thanks Steve. It seems that my PSI is down but apparently down all around and within the 5 psi range, so that's not so bad... I hope! Actually it is going pretty good on petrol, just need to sort the gas situation. Next job!

[101RRS]

135psi would suggest a high compression engine (135/14.7= 9.18:1).

Garry

[TheTree]

135psi would suggest a high compression engine (135/14.7= 9.18:1).

Garry

Garry

I reckon you are on the money there !

Steve

[TheTree]

Awesome Thanks Steve. It seems that my PSI is down but apparently down all around and within the 5 psi range, so that's not so bad... I hope! Actually it is going pretty good on petrol, just need to sort the gas situation. Next job!

Hi,

In an ideal world they would all be the same, but especially on an old motor, there will be variances

As Garry noted, 135PSI is a high compression engine, what sort of number were you getting?

Steve

[isuzurover]

Staying on topic, I am hijacking! Sorry intheozone. My 3.5 has a psi reading of 120 -125 on all cylinders. I'm told this is low? I went looking for the number on the block but couldn't find it. I will look again.


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...

The most important thing is all cylinders are close by the sound of it.

[TheTree]

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

[isuzurover]

... big lazy V8 like ours...

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

[TheTree]

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

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

[101RRS]

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.


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...

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

[snowbound]

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
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

[Hoges]

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:;)

[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

[Hammer H]

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

Keithy can you notice any performance differences between the two?

[Keithy P38]

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

[ozscott]

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