Thread: 3.5 V8 High comp or Low comp?

If it is a high compression from a 2door, it will be from an 81-85 and originally had air injection. I had one in my 77. These were an interim motor to meet Australian leaded fuel emission standards. Most cars of that era were slugs.
I can assure you these were the worst 3.5 made from a performanc epoint of view as the cam was a very poor one.
I later drove a 2 door with a fresh low comp and it killed my high comp 3.5.

Regards Philip A

G `day Tony .

your 84 was std 9.35:1 any 2door std won`t be any higher .

If its 10:1 it`s from a Rover car and you would need to check which heads it wears because if std , the valves are smaller than your 84 but would still run ok .

Depending what it is it may also have a smaller oil pump but as long as it pumps it would be ok .

If you 84 is stamped 8.13:1 it`s not the std engine .

What are the stamped Comp ratios and the engine # prefixes of yours and the one or ones your looking at , if you`d like to know where they originated or just the engine number prefix will tell ?



Peter

Just to elaborate a bit, The only High comp engines in Range Rovers were 9.35:1 as Peter said.

If you are looking at buying a second hand engine then avoid 9.35:1 RRC engines as they were the worst performers.

If you are BUILDING a 3.5, hi comp is a good idea , up to a point as each 1 in comp ratio is supposedly worth 10% in fuel economy,so a 9.35 "should" be 10% roughly more economical..
However many reports suggest that RV8s need 98 octane to run a 9.35.1 ratio and that more than wipes out any economy gain.

If I were to rebuild my 3.9 , I will put in hi-comp pistons but grind volume from the combustion chambers to reduce the compression to under 9:1.to use either E10 94 or hopefully 91.
Regards Philip A

Go with a 9.35 motor if you can find one and fit a decent cam. What you lose cost-wise using premium fuel you gain in range and available power. I have a 9.35 motor from a SD1 in my ute and it sings nicely, especially on gas.

Hi Comp all the way, waste of time using 8.13:1 pistons Build yourself a nice EFI cammed 9.35:1 long engine, small valves and ports will give great low down torque. make sure you play with carb needle profiles because slightly richer can be the difference between good bottom end and rubbish bottom end. The best outcome is to use some genuine SU carbs from a P6B Rover or early 110, these will give you really good performance when compared to the 175CD Strombergs. Don't skimp on a decent distributor either, your original '84 should be electrnic, if not, fit one.

JC

I have a set of 9.35:1 20 Thou Oversize if you need them all rings and pistons never used.
Bought them and never used them

Originally Posted by PLR

G `day Tony .

your 84 was std 9.35:1 any 2door std won`t be any higher .

If its 10:1 it`s from a Rover car and you would need to check which heads it wears because if std , the valves are smaller than your 84 but would still run ok .

Depending what it is it may also have a smaller oil pump but as long as it pumps it would be ok .

If you 84 is stamped 8.13:1 it`s not the std engine .

What are the stamped Comp ratios and the engine # prefixes of yours and the one or ones your looking at , if you`d like to know where they originated or just the engine number prefix will tell ?



Peter

Peter I will grab the comp and engine number in the morning some time and publish them but im pretty sure its an 8.13:1 stamped although currently it is much much lower.

I seem to recal finding a list of engine numbers and applications on an english web site years ago and it matched up with my car and chassis number range but we will see what transpires in the morning.

Id love to rebuild the engine that's in the vehicle now and bought an injected 3.5 5 years ago to replace the original engine while it was being rebuilt but sadly it had been mucked around with and the bottom end was all loose and had been apart and not properly put back together.

So I sent the block to be tidied up and never saw it again as the shop shut up and my efforts to retrieve the block came to nothing.

Id do it myself but its been 20 years since I built an engine and im a bit scared I'll bugger it up which was why I went looking at a Chrysler 6 or SB 8 transplant.

Thing is this engine has been so damned good although underpowered and these Rover 8's seem to rev well.

Indecisions a bugger.

Tony

Originally Posted by PhilipA

Just to elaborate a bit, The only High comp engines in Range Rovers were 9.35:1 as Peter said.

If you are looking at buying a second hand engine then avoid 9.35:1 RRC engines as they were the worst performers.

If you are BUILDING a 3.5, hi comp is a good idea , up to a point as each 1 in comp ratio is supposedly worth 10% in fuel economy,so a 9.35 "should" be 10% roughly more economical..
However many reports suggest that RV8s need 98 octane to run a 9.35.1 ratio and that more than wipes out any economy gain.

If I were to rebuild my 3.9 , I will put in hi-comp pistons but grind volume from the combustion chambers to reduce the compression to under 9:1.to use either E10 94 or hopefully 91.
Regards Philip A

Hi Philip,

Fuel and Rangies seems to be a funny thing and i have a log book I'll check in the morning but the old girl seemed to run really well on BP Ult 98 and equally well on E10 95, put 91 octane in her and she ran ok ish but really chewed through the juice which is understandable considering her compression is so low due to extreme wear.

She does however have a Stromberg 2 bbl downdraught instead of the zeniths and id probably grab 2 SUs from a Jag if I refitted twins and get them rebuilt to suit.

Economy isnt a huge issue and I went so far as to remove the LPG system fitted shortly after buying the old girl 10 odd years ago.

Numbers etc to come in the AM tomorrow.

Cheers,
Tony

Gday phillip I have just purchased a 76 2 Door Rangy non detox 3.5 what advise can you give me on making this motor perform better it has extractors already a non standard cam bought from rover tech standard carbys standard ignition this engine drinks fuel lacks power. prevoius ford v8 owner now wont to learn the v8 rover stuff by the way i am a mechanic of 30 yrs exp..cheeers Mike