Stroking/Boring a Jaguar V12

[Pumaracing]

There's a very simple bit of Dimensional Analysis which can be done to see how bhp affects acceleration which should hopefully not be beyond anyone who did maths up to about O level standard.

We know that F = M x A so for a given mass, acceleration is proportional to force.

BHP is measured in units of ft.lbs/s. Force is measured in lbs. To get from BHP to Force we need to divide by ft/s which is speed. So it would seem that acceleration must be proportional to BHP divided by vehicle speed at any given point.

This is indeed the case. At any given speed the acceleration of a vehicle will be directly proportional to the BHP being generated at whatever rpm the engine is then turning. Conversely for a given BHP the acceleration will reduce as the speed goes up and in inverse proportion to it.

[reggid]

I look at it that’s it’s all about "available hp" at any instant (time or speed). With typically 4-6 discrete forward gears having more torque at the lowend and midrange by maximising the cubic inch is almost always beneficial for a hot street engine.

[Pumaracing]

I got the chance to measure a set of Wossner 80mm Peugeot 1600 forged pistons today which are intended for a friend's race engine. Ring land diameter 79.35mm.

O.65mm down on bore size which is right on the money (ok a tad smaller than I would like) of the calculations I posted here earlier. Calcs show that these could cope with a maximum of 400C crown temperature which given the severe usage for forged race pistons might not be too unreasonable.

Hopefully that will knock on the head all the nonsense about similarly sized pistons 1mm or more down on bore size in the ring land area being fit for purpose.

[AdeV]

So, back to the original question then.....

Judging by my calculations, in order to retain a dynamic CR of 7.5:1 (approx.), if I've used the correct figure for the inlet valve closing (and I'm not sure I have, I need to check with the cam manufacturer that I'm not using the angle at 0.020" lift, or whatever), then I'll need to increase the static CR to approximately 10:1 (from 9:1), which - with the bigger bore - means having the piston rise approx 1mm higher at TDC which, given the longer stroke & same length rods, means having the piston-to-pin height approx 3.25mm shorter, which looking at the OEM pistons should be easily achievable with custom pistons, and I may even be able to find something off shelf.

[Pumaracing]

A sensible DCR for race engines such as yours on pump fuel is 8.5 not 7.5.

Edited by Pumaracing on Monday 16th February 15:41

[AdeV]

Pumaracing said:

A sensible DCR for engines on pump fuel is 8.5 not 7.5.

Or, indeed, anything in the range from 7.5 to around 9.0 depending on octane rating, with some engines capable of handling 11:1.

I think I'll stick to asking Google.

[Pumaracing]

I'll restate because my previous post was poorly worded. For your actual 2v engine on the likely pump fuel you're going to be using a sensible DCR is 8.5. If you try and run full race cams on a static CR that generates much less than 8.5 DCR it'll run like a dog at low rpm.

Edited by Pumaracing on Monday 16th February 15:42

[Pat_T]

Arriving late to this thread. It seems to be right old mixture of good information, miss-information and total rubbish!

Some modern production bike engines run up to 14:1 static compression ratio, 12:1 dynamic compression ratio, in my experience low rpm stability & performance is far more affected by the fuel phasing and ignition advance you choose and what driveline stiffness you have.

We've always included the volume down to the first ring in our compression ratio calcs. Why wouldn't you?

Also include the headgasket thickness and depending upon the construction of your headgasket realise that it's compressed thickness might be significantly different to what you measure statically with verniers. Best to do a back-to-back dry build with the block on a marble table and measure the camdeck height with and without head gasket.

Be very careful benchmarking OEM vs aftermarket piston-to-bore clearances. It is probably not as straightforward as it seems. In my experience OEM piston design will have as much to do with maintaining good 'piston posture' and reducing NVH/piston slap during cold-startup as it will anything to do with maintaining a minimal clearance at high load/temperature.

[PeterBurgess]

Some interesting observations Patrick, thank you for posting.
Do coatings, either thermal barriers and/or friction reducing ones have a discernible effect? I ask as we are experimenting with the JE FSR assymetric pistons and wondered about the range of coatings they can apply and effectiveness and longevity?
Have you ever worked with low tension rings and gas ports?

Peter

[Pumaracing]

Pat_T said:

Arriving late to this thread. As with everything posted on the internet it seems to be right old mixture of good information, miss-information and total rubbish!

Typo corrected

[AdeV]

Pat_T said:

Arriving late to this thread. It seems to be right old mixture of good information, miss-information and total rubbish!

...

We've always included the volume down to the first ring in our compression ratio calcs. Why wouldn't you?

Do you calculate it cold, or after you've taken into account the piston expansion at temperature? And what temperature do you use, since I assume it's something of a gradient from crown to skirt...

Pat_T said:

Also include the headgasket thickness and depending upon the construction of your headgasket realise that it's compressed thickness might be significantly different to what you measure statically with verniers. Best to do a back-to-back dry build with the block on a marble table and measure the camdeck height with and without head gasket.

Well, the OEM gasket is plain copper, albeit with thicker fire rings. Since this is Al-to-Al (rather than Al-to-Fe), I'm guessing it doesn't really need a complex MLS gasket. Do solid copper gaskets crush that much?

Pat_T said:

Be very careful benchmarking OEM vs aftermarket piston-to-bore clearances. It is probably not as straightforward as it seems. In my experience OEM piston design will have as much to do with maintaining good 'piston posture' and reducing NVH/piston slap during cold-startup as it will anything to do with maintaining a minimal clearance at high load/temperature.

Hence the long skirt? That could make sense. Still, the main reason for the careful measurements is to try to get an accurate CR. Based on what I now know from the Cam manufacturer, I'm likely to need an even higher static CR than I thought to maintain a sensible dynamic CR. It also looks like I'll be running steel liners rather than iron, in case that makes a difference.

[Pumaracing]

What I want to know is where can I get me one of those marble tables? I'm guessing I can measure the compressed head gasket thickness to a squillionth of an inch like that. I can then factor that in to the guesswork every engine builder does to try and decide on a CR which has a huge latitude either way but at least I'll know the head gasket thickness exactly. Or I suppose I could just measure the thickness of an old one like everyone else does.

Jeez this thread gets sillier and sillier.

[AdeV]

Pumaracing said:

What I want to know is where can I get me one of those marble tables? I'm guessing I can measure the compressed head gasket thickness to a squillionth of an inch like that. I can then factor that in to the guesswork every engine builder does to try and decide on a CR which has a huge latitude either way but at least I'll know the head gasket thickness exactly. Or I suppose I could just measure the thickness of an old one like everyone else does.

Jeez this thread gets sillier and sillier.

Now now... admittedly, I did wonder about the marble table (granite would do just as well. ).

[Pumaracing]

AdeV said:

Now now... admittedly, I did wonder about the marble table (granite would do just as well. ).

Good god no! Granite is compressible. You could be a billionth of a micrometre out if you use granite. Steel is even worse. You could be a millionth of a micrometre out if you just use the bed of a handy milling machine. If you don't bother with any of that and just measure the thickness of an old compressed gasket you could be several thou out either way which would be one tenth of three fifths of f**k all which could be 0.1 of a CR point and then the engine will blow up. What to do eh?

[AdeV]

Pumaracing said:

AdeV said:

Now now... admittedly, I did wonder about the marble table (granite would do just as well. ).

Good god no! Granite is compressible. You could be a billionth of a micrometre out if you use granite. Steel is even worse. You could be a millionth of a micrometre out if you just use the bed of a handy milling machine. If you don't bother with any of that and just measure the thickness of an old compressed gasket you could be several thou out either way which would be one tenth of three fifths of f**k all which could be 0.1 of a CR point and then the engine will blow up. What to do eh?

Depleted uranium then! Or, better still, water - my physics teacher back in 19-mumbleteen told me that liquids are incompressible, so that must be the perfect working fluid!


OK, so, help me out here..... if my calculations are correct, in order to get a dynamic CR anywhere NEAR 8 (let alone 8.5), using a cam with the inlet valve closing 105 degrees ABDC, then I'm looking to have to build to a static CR of around 17.4:1. That seems bloody excessive to me, but is static CR actually relevant to anything? Would I be better looking at a milder cam, given those numbers?

[AdeV]

Umph, I just worked out how much all this is going to cost me.... and there's no guarantee it's going to work.

Back to plan "F" (use original engine, with slightly uprated cam, and work on the heads & inlet.

[Pumaracing]

AdeV said:

OK, so, help me out here..... if my calculations are correct, in order to get a dynamic CR anywhere NEAR 8 (let alone 8.5), using a cam with the inlet valve closing 105 degrees ABDC....

You're avin' a larf right? Or someone is having one with you. IVC at 105 ABDC would indicate a cam duration of something mental like 360 degrees. That's extreme drag race territory, coughing and farting at anything under 6000 rpm and then a brief surge of all hell breaking loose. The only place you'd want something like that in your car is maybe in the boot for ballast if the weight distribution is wrong.

[AdeV]

Pumaracing said:

AdeV said:

OK, so, help me out here..... if my calculations are correct, in order to get a dynamic CR anywhere NEAR 8 (let alone 8.5), using a cam with the inlet valve closing 105 degrees ABDC....

You're avin' a larf right? Or someone is having one with you. IVC at 105 ABDC would indicate a cam duration of something mental like 360 degrees. That's extreme drag race territory, coughing and farting at anything under 6000 rpm and then a brief surge of all hell breaking loose. The only place you'd want something like that in your car is maybe in the boot for ballast if the weight distribution is wrong.

That's what Kent Cams have quoted for their full race cam...

[Pumaracing]

AdeV said:

That's what Kent Cams have quoted for their full race cam...

Twaddle. You're making a right bleedin' meal of this. The specs for all their cams are on their website. The JAG8 race cam has 310 degrees duration and closes at 85 ABDC.

http://www.kentcams.com/product-details/223/Camsha...

It's also clearly a sh11t cam with the wrong lobe centreline angle and not much lift.

Stick a JAG7 into it which has a better lobe centreline angle and not much less duration or lift,

http://www.kentcams.com/product-details/222/Camsha...

set the CR to 11.4:1, DCR 8.0 and get on with the job. It's not rocket surgery.

[AdeV]

Pumaracing said:

AdeV said:

That's what Kent Cams have quoted for their full race cam...

Twaddle. You're making a right bleedin' meal of this. The specs for all their cams are on their website. The JAG8 race cam has 310 degrees duration and closes at 85 ABDC.

I have it in an e-mail just yesterday (erm, last week I mean), from the horse's mouth:

Kent Cams said:

On 11/02/2015 08:03, Info wrote:

Hi

Yes that was a regrind price, new billets are £1600.00

Inlet is fully closed 105° ABDC

Regards
Del

From: AdeV
Sent: 10 February 2015 20:46
To: Info
Subject: RE: KC - Web Enquiry - 13/01/2015

Hi,

Thanks for answering that for me - is that a re-grind of my original cams, or is that a brand new cam? If the price is for a regrind, can you tell me how much it would be for a new cam to be manufactured? I understand there is approx. 12 week lead time on such an order.

Also - for the JAG8 product, can you tell me how many degrees ABDC the inlet valve is fully closed please? Trying to determine my dynamic CR.

Cheers,
Ade.

Pumaracing said:

http://www.kentcams.com/product-details/223/Camsha...

It's also clearly a sh11t cam with the wrong lobe centreline angle and not much lift.

I'll let you tell them that.

Pumaracing said:

Stick a JAG7 into it which has a better lobe centreline angle and not much less duration or lift,

http://www.kentcams.com/product-details/222/Camsha...

set the CR to 11.4:1, DCR 8.0 and get on with the job. It's not rocket surgery.

Of course, there's always Piper cams....

Edited by AdeV on Tuesday 17th February 12:34