Agreed: stock pistons, of almost any car,were never designed for those kinds of stressors.
From Don Miles in the UK (supplier of XJR-# race bits) my understanding is that the bottom end has a weak spot in the main bearing caps. Getting into the 450 to 500HP range and having the engine last means doing a very simple modification, but higher horsepowers warranty replacement with steel mains.
What happens:
Where & Why:
VS steel mains by Don’s co:
Apparently Lister had problems with this happening in the 7.0L car and several have been corrected since they left the factory doors.
~Paul K.
Those steel caps sure are pretty! Interesting design. I probably would have gone to longer bolts, squaring up the top of the cap to make it thicker in the stressed area, and avoided “recessing” those bolt heads in that manner. But that’s just me.
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I guess the call there was for lightness.
Historically this is what has been fitted (Borrowed from Don’s TWR Facebook Site):
Another of their new style steel caps | Late group 44 caps | Std caps
Notice the fitting for a puller at the center top of the replacement caps:
Removal tool:
~Paul K.
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Don’t these aftermarket caps have to be line drilled on the block? Or was Jaguar’s machining precise enough that the bearing caps can be pre-machined and the bearings will line up?
Gee, all of those aftermarket caps look like they have stress risers around the bolt face! Is there some reason I’m not seeing why they didn’t go with a longer bolt to get rid of the stress risers? They clearly don’t care about weight, as those caps look massive.
That’s a very good question Kirby. Donald Miles would be the best to answer that. It may also be that these are going into high dollar race engines anyway, and a certain amount of checking/machine work is implied.
I would also assume that when building your own V12, you might want to re-consider the factory clearances - and maybe go for something tighter. Easy enough to do on the rod big-ends… but I don’t know how you’d easily do that on the mains… other than just adjusting bearing clearances to the best of what’s available.
He keeps saying he’s going to put together a website, but so far it’s only Facebook that he’s been maintaining… so its where you have to go to see all the magic that they’re doing, or inquire about parts.
e.g. the only source for TWR Group A intake manifolds (new castings / remanufactures):
And many many many other magic bits mostly targeted at XJR 6-9-15 owners and or V12 race engine builders.
~Paul K.
I’m guessing here… that it’s a material thing… since these are billet forged bits you don’t get the same sort of possibility of cracking STEEL as you do a cast iron? Sure I suppose both could be polished/shot peened.
He’s mentioned that they don’t like the OEM studs and use something stronger… ARP I suppose. Another commenter mentioned something about using hollow locating dowels [around the studs?] An interesting idea, but supposedly there’s never been a problem w/ these caps walking at all.
I can see with the small (short) lands that there’s not much point in going higher VS locating against the block, there’s just no block further up to mate with unless the block was machined and the cap was made into more of a "V shape… Maybe external bolts could be passed through perpendicularly then… and maybe it would just be overkill.
~Paul K.
I think it’s safe to assume, the people who designed these steel caps had a fair bit of engineering expertise, beyond the suppositions of us who don’t.
I didn’t understand any of that.
I may have misinterpreted what you meant by use longer bolts. I assume you meant longer studs… and when you said square up the first thing that popped into my mind wasn’t adding a few thousandths to the nut seat lands. It was more like this:
Longer studs on all the way around presenting the opportunity for bolts (in red) to stiffen the block/cap location perpendicular to the existing studs. The lower land only has the area in purple to fix against UNLESS the cap was made wedge shaped on both sides and the block machined so that the fit was perfect. That’d be a BIG honkin cap!
I guess you meant “why not raise the area in the green semi-circle there to make it possible to machine that whole land flat?” shrug I guess possible. but that would mean custom machining the large top nuts there otherwise the relative position of the windiage try & I suspect pickup would change slightly.
I guess its easier to keep that distance in the blue double headed arrows fixed.
~Paul K.
Still waiting for the ITB’s to arrive… long travel from US to Sweden
I will not use the ITB’s on the turbo engine, those are for a other project
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How will you control ignition? Adding 150 hp of NOS without removing advance will kill your engine…
I was actually talking about using longer studs for the long studs, the inner ones, thereby allowing the lands for those nuts to be up on top of the bearing cap rather than notched into it. But your photo illustrates that the nuts used here are tall and presumably used to fasten the windage tray. So making the lands 1/4" higher or so would involve nuts that are 1/4" shorter in order to keep the windage tray in the correct spot.
Hi tobmag
In reply to your question
How will you control ignition? Adding 150 hp of NOS without removing advance will kill your engine…
I did ask this question to Treaver langfield who runs the Wizard of Nos Company about retarding the ignition; this is the answer I got
My Question
(At what power level boost do you think I may have to retard the ignition if at all on this engine?)
Answer
On such a relatively low powered NA engine for its size and inherent strength, it’s unlikely that you’ll have to retard the timing at any power level the system can deliver. I’ve never needed to do so on Jags I’ve personally worked on and I don’t know of any customers who have done so either.
What I have found out
I have read that for every 50bhp of Nitrous ignition needs to be retarded 2 degrees if I need to do this I will fit an ignition retard module upgrade to the fireball ignition I already have. Also the Wizard of Nos Company claims there system injects liquid Nitrous and not gas this has a massive cooling effect on cylinder temperature
Peter
I have never used NOS my self but know that it is an instant killer if done wrong.
If oly adding 50-75 hp I wouldnt bother about ignition either but at 150-200 you are looking at massive cylinder pressure hence the need of retard the ignition… its same as on a turbo car. at 1 bar you have probably removed about 10-20 degrees depining of your engine set up.
Getting back to the OP’s (hey, that’s me) idea…
I think I’d like to go with TWR style pistons with 10.5 or 11:1 compression, and coat the pistons and head.
On coating the head, would you coat the entire head, or mask off and just coat the portion of the head that tops the combustion chamber?
I would think just the combustion chambers, thats where you want to keep the heat in.
Yes…I was thinking the same thing. I don’t see a marked increase in depth over the original at this point…so if the small cast piece needs to be removed on the original then the spot facing certainly needs attention. I would also be nervous introducing an extractor hole in the location shown…not a good idea in a highly stressed component…helpful to the mechanic but detrimental to the fatigue life of the part.
Engineering textbooks are full of such examples…
Wouldn’t being billet steel make them quite an improvement over the stock items?