Compression ratios - I've got a headache.

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Captainbeaky
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Compression ratios - I've got a headache.

Post by Captainbeaky »

OK, I'm rebuilding my 4.0 litre Defender 50th - currently a bog standard High Comp 4.0 running GEMS.
Engine was fine until it was run with the oil pickup detached, so mains are scuffed up, but top end and bores fine.
The mint condition 4.0 HC pistons went into a customers D2 4.0 engine rebuild.

Plan is to build it as 4.6, as I have a brand new 4.6 crank and 4.6 rods here. Unfortunately, all the pistons I have are scrap, so I'm working out the engine spec.

Here's where the headache starts, and please feel free to chime in if I get a number wrong, or if I've missed something.

I'm thinking of putting 4.0 pistons in, but have been told and read online conflicting stories that this gives me 11.1 CR which is too high for the RV8 (too high for me anyway)
So - I decided to calculate this.

Std published CR is 9.34 for both engines, yes?
4.0 is 3950cc, the 4.6 is 4554cc.

And CR = Total volume / unswept volume - or more correctly, CR = (Unswept volume + swept volume) / unswept volume
From our o-level maths, we can get the unswept volumes = Swept volume / (CR-1)
So, per cylinder, the unswept volumes (per cylinder) are:-
4.0 = 59.20cc
4.6 = 68.25cc
(This also sort of matches my later calculations too)

Now, the 4.0HC piston has a 13.3cc bowl, and the 4.6 HC piston has a 22cc bowl.
So, a 4.0 HC piston in a 4.6, using 4.6 rods should be easy to calculate.
Unswept volume will be 8.7cc less (22cc - 13.3cc) = 59.55cc.(or 476.4cc's for the whole engine.)
Putting this into the CR equation,
CR = Total Volume/Swept volume = (4554cc + 476.4) / (476.4)
We get a CR of 10.55 - this seems pretty good - and about what I'm looking for and have had success with before.

Now, this assumes that the compression height of the pistons is the same, which it is at 35.5mm.
And the rods are 5.5mm shorter on the 4.6, so give an identical piston compression height when installed (11mm increase in stroke)

Now, when I plug in the specific dimensions into a spreadsheet I designed, I get something a bit different...
Assuming the following:-
94mm Bore
71mm or 82mm stroke (4.0 and 4.6)
Cylinder heads - 28cc volume
Piston to deck is 1.7 on both engines
Composite gasket is 1.3mm on both engines, and 95.5mm dia.
I have assumed the 22cc and 13.3cc volumes for the 4.6 and 4.0 pistons as before.

I get swept volumes of 3941cc for the 4.0 and 4552cc for the 4.6

I get CR's of:-
8.9 on the standard 4.0HC
9.0 on the std 4.6HC
10.12 on the 4.6 with 4.0 pistons.

Please someone have a quick look, and confirm I'm on the right track, and anyone know where this 11:1 CR comes from??

TIA,

Mike.



garrycol
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Re: Compression ratios - I've got a headache.

Post by garrycol »

When I built my 4.6 from a 4.0 block - I used a 4.6 crank, 4.6 conrods, 4.6 gudgeons (the are thicker walled than the 4.0 gudgeons) and used low comp 4.0 pistons - this gives a compression ratio of just under 10:1 - I see you calculated it as just over, I calculated it as just under - in both cases similar and no where near 11:1.

Garry

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Re: Compression ratios - I've got a headache.

Post by ChrisJC »

So when I did it, my numbers were pretty similar apart from the piston top to deck height. I back-calculated this to give me the correct 9.35:1 compression ratio on standard parts, and I made it 1.2mm.

Then I worked forwards, and with 4.0 pistons in a 4.6, I reckon the compression is 10.8:1.

For ref, here is my calculation:

Bore: 94.0mm
Stroke: 82.0mm
Therefore swept volume: 4552cc
One cylinder swept vol: 569cc
Head gasket volume (thickness 1.2mm): 8cc
Piston top to deck height is 1.2mm, giving a volume of: 8cc (this is a fiddle factor to get the 9.35:1 calculation to work)
Piston bowl volume: 13cc
Head chamber: 29cc
Piston down volume: 569+13+8+29+8 = 627cc
Piston up volume: 13+8+29+8 = 58cc
Compression ratio: 627:58 = 10.8:1

Chris.
--
Series IIA 4.6 V8
R/R P38 4.6 V8

Captainbeaky
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Re: Compression ratios - I've got a headache.

Post by Captainbeaky »

Thanks for that - your numbers seem to match mine - I’m not losing my marbles.

I’ll fully check in the morning in the office.
I had to apply the same fiddle factor to get the CR to match the spec too.
But I measure the piston to block clearance at 1.7 or 1.8mm from memory...

So, it’s 4.0 HC pistons then....

garrycol
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Re: Compression ratios - I've got a headache.

Post by garrycol »

Captainbeaky wrote:
Wed Apr 21, 2021 10:05 pm
So, it’s 4.0 HC pistons then....
In my engine build mentioned above I used 4.0 Low comp pistons hence my calcs of a bit less than 10:1 - to me this is more usable than 11:1 with HC pistons from the point of view of fuel used etc - yes a little less power but hardly noticeable.

Dont forget to put in 4.6 gudgeons (well maybe 4.0 HC gudgeons are the same as 4.6).

Garry

Captainbeaky
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Re: Compression ratios - I've got a headache.

Post by Captainbeaky »

Where does 11:1 come from?
I can’t get anywhere close to this with my calculations.

The 13cc is for the 4.0 HC pistons, which gives me about 10.1:1 CR...

4.0 LC pistons come out at nearly the same as 4.6 HC pistons (actually a little more at about 23cc)
So 4.0LC pistons would give a slightly lower CR than 4.6 HC pistons.

( headache returns)...

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Re: Compression ratios - I've got a headache.

Post by SuperV8 »

When I built my 4.6 I measured (Burette'd) the combustion chamber and noted it was different to the advised 'spec'
would be best to actually measure 'your' engine to get an accurate compression ratio figure. Even if brand new there is a tolerance from the factory.

what cam are you using? as a cam with a higher overlap/duration can lessen the effect of a high compression ratio.
Dax Rush 4.6 supercharged V8 MSII

Captainbeaky
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Re: Compression ratios - I've got a headache.

Post by Captainbeaky »

I’ve got several heads to choose from, and I will be using a higher lift cam - not sure which yet.

I’m just trying to get my head around which pistons to get, and I keep hearing that the 4.0HC pistons give 11:1, but I can’t get anywhere near that with my calculations!.

I’ll be measuring everything, to check the actual measurements, of course.
(As soon as I track down where I put the valves when I stripped the heads down)!
I’m also going to do a quick dry build, so I can double check my piston to deck height.

At the moment, though, I’ve got to get a Bimota 600cc engine back together, a Jag4.2 fully rebuilt, and a Smart bottom end put together. It would be OK if it wasn’t for bloody customers!

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