compression ratio for 4.6

Seight-V8 · Post by **Seight-V8** » Tue Jan 19, 2016 9:50 am

Hello all,

So a few thoughts going through my mind today....

My compression ratio will come out at 11.1 to 1....for the rebuild of my rover v8 4.6 engine.....does this sound like to much for the spec below.

4.6 block with top hats
4.0 pistons (13cc bowls)
stage 4 heads
piper 285 cam
45mm trumpet base & manifold
vems efi (like megasquirt)

The old setup was standard 9.35:1 and I've had the opportunity to grade slightly, just thinking is it too much before I spend my money.

cheers

scott

sidecar · Post by **sidecar** » Tue Jan 19, 2016 12:36 pm

I run a 4.6 with a Piper 285 cam, I run at 10:1. I think that you are right on the boarder line at 11:1, actually I think that its too high! The old 10.5:1 P6 lumps do not even cope with todays fuel very well and you are another 0.5 up on them. (OK an old P6 maybe full of carbon which won't help but you get the idea).

Also people bang on about running a high CR and then just backing off the timing if it detonates but an engine setup like this will never run as well as a similar engine with a lower CR and the correct ignition timing.

I have 'scooped' out chambers before now in order to lower the CR, it is possible with the right tools, it just takes ages as you have to do the buretting several times of check chamber.

Anyway AJMHO!

DaveEFI · Post by **DaveEFI** » Tue Jan 19, 2016 12:53 pm

I ran a 10.6:1 P6 happily for many years on 4 star petrol. But you do need to make sure the timing and mixture are spot on.

However, mapped ignition should make the job easier.

Triumph-V8 · Post by **Triumph-V8** » Tue Jan 19, 2016 1:00 pm

Agree to that, although the Rover P6 cam is a bit smoother
compared to the 285 Piper and from that can not handle higher
CRs. The hotter the cam the higher the CR ratio can/must be!

As suggested it is a good chance to bring the combustion chamber
to the size of the 94mm bore what allows to avoid some wall
shrouding around the valves and brings things closer to 10:1

Darkspeed · Post by **Darkspeed** » Tue Jan 19, 2016 3:43 pm

Are all of the figures you have used actual figures measured in the head and the block?

If it is a UBV head then it should be all the way out to the bore anyway - Mine is and I am only using the biggest valves on the stock seats. - even on standard valves there are gains to be had taking it to the bore line.

What have the heads and block cc'd at?

Before machining to get the CR we wanted the heads on mine were 32.5cc Ave. We had the block machined a fair bit to bring the pistons up the bore and with the big cut outs the block cc'd at 20.

I am using Omega pistons (cast) and an M248 and we have settled on 10.75. Super only. 45mm ITB's, MS2

I would think with a reasonably soft cam like the 285 H and the stock cast pistons you would want no more than 10.25. maybe a little less.

The old P6 had very poor heads so a very low VE and probably never managed to fully fill a chamber anyway.

DaveEFI · Post by **DaveEFI** » Tue Jan 19, 2016 5:19 pm

I'd love to see some accurate figures for the original high compression engine against the last of the carb SD1 units. Despite the better heads on the SD1.

Darkspeed · Post by **Darkspeed** » Tue Jan 19, 2016 6:22 pm

DaveEFI wrote:I'd love to see some accurate figures for the original high compression engine against the last of the carb SD1 units. Despite the better heads on the SD1.

Accurate figures for what?

Seight-V8 · Post by **Seight-V8** » Tue Jan 19, 2016 9:02 pm

so, a decision to be made...

Checked my head chamber volumes at 29.5cc.....and this is also stamped on side the head.

checked the piston bowl volumes at 13cc.....

height of piston to deck 1mm..

but I've have had an idea....

My current 4.6 pistons are +0.020ins, as my block was re-bored, but looks like a liners gone, so I am to get top hats fitted....at chesmans...

if I use my current pistons with these values

4.6 pistons (22cc bowl)
94.5mm cylinder diameter
94.0mm gasket 1mm thick
82mm stroke
29.5cc combustion chamber

I end up with a compression of 9.75:1

Looks like a better option......current pistons have only done 475miles...

Just a waste to re-bore new top hat liners at +0.020ins from the start, but engine wont need to be re-balanced either....

For what I have read on the forum over the last few days, I think its best to go with this idea.

cheers

scott

minorv8 · Post by **minorv8** » Wed Jan 20, 2016 5:46 am

My 4,6 engine has a CR around 10,5-10.6 figure, factory pistons, the cam is Real Steel Tornado. It runs 95 octane fuel (the lowest rating), don´t know how it compares to UK fuels) with total advance of 30 degrees. No problems whatsoever. Pistons are factory 4,6 ones.

DaveEFI · Post by **DaveEFI** » Wed Jan 20, 2016 9:37 am

Darkspeed wrote:
DaveEFI wrote:I'd love to see some accurate figures for the original high compression engine against the last of the carb SD1 units. Despite the better heads on the SD1.
Accurate figures for what?

BHP and torque.

SimpleSimon · Post by **SimpleSimon** » Tue Jan 26, 2016 10:55 am

Seight-V8 wrote:so, a decision to be made...

Checked my head chamber volumes at 29.5cc.....and this is also stamped on side the head.

checked the piston bowl volumes at 13cc.....

height of piston to deck 1mm..

but I've have had an idea....

My current 4.6 pistons are +0.020ins, as my block was re-bored, but looks like a liners gone, so I am to get top hats fitted....at chesmans...

if I use my current pistons with these values

4.6 pistons (22cc bowl)
94.5mm cylinder diameter
94.0mm gasket 1mm thick
82mm stroke
29.5cc combustion chamber

I end up with a compression of 9.75:1

Looks like a better option......current pistons have only done 475miles...

Just a waste to re-bore new top hat liners at +0.020ins from the start, but engine wont need to be re-balanced either....

For what I have read on the forum over the last few days, I think its best to go with this idea.

cheers

scott

Just to add stock head gaskets are not 1mm thick (well Elring are not) 1.28/1.33 and the opening is not 94mm (95.5 approx) every little matters when calculating CR

Seight-V8 · Post by **Seight-V8** » Tue Jan 26, 2016 7:04 pm

The compression ratio 9.75:1 was made on the provision I will use cometic head gaskets with a 94mm bore and 1mm thickness....

I don't actually know this yet, till I have worked out how much was removed from the deck height when the block was top hatted last week.

I need to take the block to work so I can measure it correctly, then finally decide the compression ratio I want to try an achieve.

Used elrings last time, and wasn't impressed with the sealing....I think cometic are expensive but maybe worth it in the long run.

regards

scott