Stroking/Boring a Jaguar V12

Stroking/Boring a Jaguar V12

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AdeV

Original Poster:

621 posts

283 months

Thursday 29th January 2015
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Hi,

I've been hunting high & low for a good technical forum to post this question on, and here it was right under my nose all along....

I've got a Pre-HE 5.3 V12, which I'd like to put a 6.0 crank into, and 96mm pistons (up from 90mm). I'll be having the pistons made, so hopefully I can use the original rods & absorb the extra lift in the piston crown to gudgeon pin gap (if not, then I'll have to get custom rods). I also plan to run race cams & ITBs.

My first question is - the CR of the 5.3 is 9:1 if I recall correctly; that seems a touch low to me, would there be any benefit in moving to a higher CR?

Second question - the Pre-HE uses dished pistons with no appreciable "squish" regions, the piston reaches TDC around 5mm shy of the head. Jaguar apparently examined adding squish regions above/below the valves (leading to a sort of fat figure-8 shape cut into them for valve clearance/combustion chamber), but determined this gave lower power than the plain dish. So... should I, too, stick with plain dished pistons & around 5mm clearance (or whatever the cam calls for), or has piston technology moved on to the point where a shaped chamber would now be a good idea - a bathtub shaped cutout maybe? I don't want to mess too much with the heads, if I can help it, beyond a bit of porting/polishing.

Thanks in advance!

BritishRacinGrin

24,602 posts

159 months

Friday 30th January 2015
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AdeV said:
Hi,
the CR of the 5.3 is 9:1 if I recall correctly; that seems a touch low to me, would there be any benefit in moving to a higher CR?
What sort of ignition system do these engines run? will you be keeping it or going aftermarket? if you increase the CR you're probably going to need to increase the ignition advance.

AdeV

Original Poster:

621 posts

283 months

Friday 30th January 2015
quotequote all
BritishRacinGrin said:
What sort of ignition system do these engines run? will you be keeping it or going aftermarket? if you increase the CR you're probably going to need to increase the ignition advance.
I plan to use a Megasquirt in full sequential mode (ignition & fuel) - you're right to ask, the original Jag system would not be suitably tunable...

GavinPearson

5,715 posts

250 months

Friday 30th January 2015
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I would recommend you speak to people like Chesman Engineering in Coventry who have the experience with the V12 to be able to extract the power from it.

This project will not be cheap, but then again most 600+ bhp engines end up being pretty expensive.

e600

1,315 posts

151 months

Friday 30th January 2015
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I suggest Rob Beereracing, they have been there and done that having raced a 750 bhp V12 E Type

PaulKemp

979 posts

144 months

Friday 30th January 2015
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The camshafts dictate the required compression ratio as the more the cam duration the less dynamic compression ratio you get, if your stroking and over boring static compression ratio will increase just by the engineering your about to have done
There are compression ratio calculators and DCR calculators, it would be wise to look at these right now
I would think the static compression will increase quite a bit with these mods so you may need forged Pistons with 98/99 RON

PaulKemp

979 posts

144 months

Friday 30th January 2015
quotequote all
A quick google and I found huge amounts of data
You need to research
A lot
Liners need machining to fit for large bore versions
If the piston is m down the bore and the head is flat you'll get 13:1 ratio
I need more info as it seems do you
Start with bore, stroke, piston deck height, gadket thickness when fitted, any chamber volume in the head
That'll start you off


Pumaracing

2,089 posts

206 months

Friday 30th January 2015
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PaulKemp said:
The camshafts dictate the required compression ratio as the more the cam duration the less dynamic compression ratio you get, if your stroking and over boring static compression ratio will increase just by the engineering your about to have done
There are compression ratio calculators and DCR calculators, it would be wise to look at these right now
I would think the static compression will increase quite a bit with these mods so you may need forged Pistons with 98/99 RON
Seriously? Last week you're asking for basic advice on a Pinto and now you're telling someone else how to build a V12 Jag?

AdeV

Original Poster:

621 posts

283 months

Friday 30th January 2015
quotequote all
Thanks for the further replies...

Gavin & E600 - I'll definitely bear this in mind, my project is a little unusual in that the only restriction on what I can do is "Head and block must be externally identifiable as OEM". I'm also doing it on a relative shoestring, which will rule out buying any serious performance mods (e.g. Rob Beere's £10k heads). On the other hand, I'm half decent at wielding a spanner - and a calculator and a milling machine - so I plan to do a lot myself.

Paul - obviously, I've been googling all around this subject (and plenty of others I haven't even touched on yet), plus I have various books, magazines and so on with additional information. My question is not HOW to increase CR, it's whether it's advisable/sensible or not. The engine has to run on road fuel (i.e. Shell V-Max), and I'm not planning to run forced induction (which would require a lower CR), so the question is, do I change the CR or not? Also - it's not the liners that need machining, it's the block IF you go above 96mm bore. 96mm liners are available OTC for a very reasonable £75/ea in ductile steel, or I could make my own in Ali and have them Nikasil coated for £105/ea.

I'm about to start measuring the piston-to-deck height, the approximate volume of the dish, that will give me the exact volume as it is now. I already know the stroke (70mm), and bore (90mm).


Another question - do I need to take into account the volume between the top ring land and the top of the piston as well, or is that regarded as being sufficiently small as to be irrelevant?

GavinPearson

5,715 posts

250 months

Saturday 31st January 2015
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I would start looking at what parts are available for big block Fords and Chevys and adapting those for the valvetrain and bottom end.

e600

1,315 posts

151 months

Saturday 31st January 2015
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I would speak to Rob at Rob Beereracing. he is most helpful, even if you don't buy something

PeterBurgess

775 posts

145 months

Saturday 31st January 2015
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Yes you should take into account the piston bore ring depth annulus volume for the sake of accuracy and completeness. On the range of engines and piston combos we work on we get a range of 1-1.7 cc, enough to want to include it in the CR calcs. I suppose on the other hand, if you ignore it you are always on the safer side of CR smile

http://www.mathcaptain.com/geometry/annulus.html

Peter

Pumaracing

2,089 posts

206 months

Saturday 31st January 2015
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PeterBurgess said:
Yes you should take into account the piston bore ring depth annulus volume for the sake of accuracy and completeness. On the range of engines and piston combos we work on we get a range of 1-1.7 cc, enough to want to include it in the CR calcs. I suppose on the other hand, if you ignore it you are always on the safer side of CR smile

http://www.mathcaptain.com/geometry/annulus.html

Peter
Utter nonsense. For a 90mm bore engine, piston crown diameter the normal 0.5mm down on that (89.5mm) and a top ring 8mm down from the piston crown the annulus volume at ambient temperature is only 0.5cm^3. However this diminishes to an even smaller value as the piston crown expands at running temperature and would only be a tiny fraction of a cm cubed and utterly inconsequential for CR calculations for that or any other engine.

PeterBurgess

775 posts

145 months

Saturday 31st January 2015
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I thought you were a professional engine builder Dave. If you don't calculate it you will not know whether the particular application is of much relevance to the given CR or not. We run around 1mm crown to bore difference on many of our pistons, some top rings are 9/10mm down on our applications.. We calculate it because we like to make sure we get it right. Am I right in thinking you don't bother to measure it, if so that seems a little slack to me. Most folk seem to use the annulus volume when they phone in to tell us what ccs they want.

Peter

Pumaracing

2,089 posts

206 months

Saturday 31st January 2015
quotequote all
PeterBurgess said:
We run around 1mm crown to bore difference on many of our pistons,Peter
Then you are nuts. For a normal sized road car piston to expand by 1mm it would need to reach 600 degrees C at which point it is near as dammit molten and the piston has disintegrated anyway.

PeterBurgess

775 posts

145 months

Saturday 31st January 2015
quotequote all
Pistons expand as do bores with heat so the bore to piston gap does not close down as much as you seem to reckon. I am talking about the relationship between OD of piston at ringland with the bore size with the depth to the top ring.
We also measure the chamber vols with the plug to be run as we have found different make and heat range plugs can vary 0.4 ccs. Gaskets vary too so need to be measured and not generic guesstimates.In the end Dave all these little volumes add up and have an effect on the CR. If you choose to ignore these it is up to you but doesn't seem very professional to me.


Peter

Pumaracing

2,089 posts

206 months

Saturday 31st January 2015
quotequote all
PeterBurgess said:
We also measure the chamber vols with the plug to be run as we have found different make and heat range plugs can vary 0.4 ccs. Gaskets vary too so need to be measured and not generic guesstimates.In the end Dave all these little volumes add up and have an effect on the CR. If you choose to ignore these it is up to you but doesn't seem very professional to me.Peter
Please stop squirming. Plugs and gaskets have nothing to do with the ring land issue you posted gibberish about. Anyone who tries to measure CR with different bits in the engine to that which it is going to be built with is bonkers to start with. You've been caught out so many times on here in the last few years posting nonsense about engines one would like to think you'd give up and stop exposing yourself to ridicule.

chuntington101

5,733 posts

235 months

Saturday 31st January 2015
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If the comp ration is only 9.0:1 the why not add a couple of turbos or a supercharger. smile

AdeV

Original Poster:

621 posts

283 months

Sunday 1st February 2015
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chuntington101 said:
If the comp ration is only 9.0:1 the why not add a couple of turbos or a supercharger. smile
That idea is being held in reserve in case I can't make enough power out of the NA version biggrin

(I'm trying to avoid it because the championship rules mean I'd need to change from the "modified" to the "specials" class if I add forced induction - at which point I can go mad with the bodywork & suspension as well, which starts to get REALLY expensive and complicated...)

AdeV

Original Poster:

621 posts

283 months

Monday 2nd February 2015
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Pumaracing said:
PeterBurgess said:
We run around 1mm crown to bore difference on many of our pistons,Peter
Then you are nuts. For a normal sized road car piston to expand by 1mm it would need to reach 600 degrees C at which point it is near as dammit molten and the piston has disintegrated anyway.
I measured the existing kit tonight, workshop temperature is around 5C, so that might affect things slightly. The piston crown is 0.75mm smaller than the bore - but this is an aluminium piston running in a cast iron liner, so I imagine there is some differential expansion at working temperatures.

Measured (slightly estimated) CR, excluding the bit between the piston and the bore down to the top ring comes out at 9.2:1; some git broke my burette so I can't CC the piston dish accurately, estimating it using trig gives that figure.

All very exciting, however, reading various bits of "literature", would suggest the dynamic CR is of more interest than the static; and that a figure of around 7.5:1 would be a good compromise between power and detonation likelihood of detonation.