TVR 500 piston options ?

TVR Beaver · Post by **TVR Beaver** » Tue Jun 19, 2012 9:24 pm

I have a slight chatter on overrun at about 1400 RPM and its been suggested that it may be piston related?.. not that its causing any problems and I know at least 50% of the 500's make the same noise after a certain number of miles.....
But, I was thinking this winter I may have the engine out and have a play...
I waswondering what options are available as I don't want to stray too far from std but wondered if forged pistons for example would be better??... if so, will the existing con-rods fit them etc etc...
Any advice appreciated as yet again the search does not want to play... Cheers..

kiwicar · Post by **kiwicar** » Wed Jun 20, 2012 9:01 am

Hi
Are TVR 500s 3.7" bore or do they resleeve and use a bigger bore piston to get the extra capacity?
Best rards
Mike

TVR Beaver · Post by **TVR Beaver** » Wed Jun 20, 2012 11:51 am

they are the std bore size (what ever the biggest was) but with a longer stroke... so it does mean you have to crop the skirts a tad....

kiwicar · Post by **kiwicar** » Wed Jun 20, 2012 12:14 pm

Hi
Depending on your budget, I would take the bore out to chevy 305 size, (3.736) and use a chevy flat top piston, they have a much shallower skirt than the rover, you would need chevy rods (the smaller big end) to go with them, but choose carefully and you could save quite a bit of weight.
http://www.summitracing.com/search/Depa ... -aluminum/
Othewise it is bach to the old favorite of the ford mod piston for 3.7" bore and bush the little ends on the rods, again a much more modern design with a lot less skirt and weight.
Best regards
Mike

Wotland · Post by **Wotland** » Wed Jun 20, 2012 12:52 pm

Main problem with TVR500 is the rod/stroke ratio very poor.

The bottom of piston skirt suffer more because rod angularity at bottom.

TVR Beaver · Post by **TVR Beaver** » Wed Jun 20, 2012 4:20 pm

I think its the piston thats the problem on the chatter... however it only does it when the engine is hot so may be not piston slap.. more like a bit of play in the small end.. and only on over-run so no load on the piston.. soon as you load it, it stops....

DEVONMAN · Post by **DEVONMAN** » Wed Jun 20, 2012 6:34 pm

TVR Beaver wrote:I think its the piston thats the problem on the chatter... however it only does it when the engine is hot so may be not piston slap.. more like a bit of play in the small end.. and only on over-run so no load on the piston.. soon as you load it, it stops....

As you probably know, the chatter on over-run on the 500's has been discussed on many forums but I don't think anyone has come up with a conclusive reason.
A variety of reasons have been put forward such as small end play, piston slap, camshaft resonance at say 1400rpm, timing chain chatter,( (even the chain hittting the cover).
Some of these engines are externally balanced on the front pulley and flywheel and this may contribute to the resonance.

On the subject of pistons, I have forged pistons in my turboed 500 and the piston slap noise drowns most other noises.
I got them made in the US by Probe to my design.
They are noisy due to the initial running clearance of 2.5 thou and the short conrods of the 500.
If noise is you concern then forged pistons will be noisy and you may not like them. They get quiet if you thrash it for a few minutes.

The 500 conrods are unique to the crank so any other you fit will need to be narrowed and standard width bearings only just fit. I tried to use Clevite 77 bearings and they fouled the journal side radius.

The tvr pistons are Rover 4.2 pistons with a shortened skirt and some valve cut outs added so choice off the shelf is limited. The only pistons I have found are Acralite forged (10.5 comp ratio) but you need to use 3.9 rods and narrow the big end to match the standard 500 rod width.
Re balancing would be essential.

Hope that helps.
Denis

TVR Beaver · Post by **TVR Beaver** » Wed Jun 20, 2012 10:02 pm

Thanks Denis... yes that helps a lot and is clear enough for me to understand LOL. Although I'm an Engineer, pistons, rods and the nuances that go along with them are all new to me when talking about deviation from standard.
It is indeed the chatter at 1400 rpm that drives me made... when I got the car it would just start making the noise if it got up to 90 deg or so. But over the last 10K miles, it now starts at 50 deg so does it at normal operating temp.
The strange thing is that I changed my oil before last weeks trip to Le Mans from a 10/40 Carlube semi synthetic that I was using to flush the engine a tad, to a better, thicker Millers CFS 10/50 fully synthetic and guess what... the noise is worst?
That said, the fully synthetic is slippery as hell, you can tell the difference between your fingers so it's obviously something that moves sideways made be as opposed to film breakdown.
At least 1/2 of the 500s I know do it.. some with 15K on the clock (some have done it from new)... but others with 85K on the clock are as quiet as a mouse?.. so indeed its a strange one.
Rob at V8D says it’s the small ends.. and I can see that as the Ali piston will expand more than the pin at temp... but I’m sure its not the gap in the piston that makes the noise, more its ability to move sideways relative to the con rod as the gap opens up?.. Where as Power think it's Con rod play on the crank or a liner moving (the last one I can't see at all)

So with that in mind I thought I may swap out the pistons... Its a shame really as I've just done 1200 miles to Le Mans and back and the oil level has dropped about 4mm on the dip stick from full... so it's using nothing in comparison to some motors?

I've had 3 cams in the motor.. the original 435.. a Stealth.. and now a Power 885... and each time the noise has not changed so I'm guessing its not cam / lifter / top end related... so pistons / rods it must be.. just wish I could stop it doing it as it drives me mad...

I did wonder if you could get pistons and rods where the bearings are in the small end instead of the piston.. may be this would help....

If you have any more info on the 'noise' it would be appreciated to either support or dismiss my theory so far....

Cheers. .John

DEVONMAN · Post by **DEVONMAN** » Fri Jun 22, 2012 11:35 am

Hi John,
Another theory I have is :-

The 5.0 TVR pistons have very little stability when they are at the bottom of the bore. Due to the long stroke, the pistons almost pop out the bottom at BDC. Given that the pistons are modified 4.2 items, they have offset small ends. This has the desired effect when the piston is pushed down on the firing stroke but I'm not sure the offset pin works well on the upward stroke and the piston may be chattering as it re-enters the bore at certain revs.

By shortening the tapering skirt of the 4.2 piston, the skirt o/d must be smaller and I assume this automatically gave some extra running clearance in a standard bore. This may have been deliberate in this high performance application, or have TVR used specially prepared u/s blocks?

I look forward to any comments

Regatrds Denis

TVR Beaver · Post by **TVR Beaver** » Fri Jun 22, 2012 12:39 pm

Yes.. that is something I thought of as the skirts are wider at the bottom than say 10mm up the piston....
But I'm not sure how much they machine off them... and I was not aware how far out the bores they come? (I assume the more std ones don't come out the bores??)
That may explain the chatter getting worst when the oil is improved as it may move more / quicker when the friction is reduced (fully synthetic)...
It is strange however that it gets worst as the temp goes up......
Piston slap noise usualy works the other way... louder when cold??
and why only start after 45K miles??

DEVONMAN · Post by **DEVONMAN** » Fri Jun 22, 2012 1:48 pm

Hi again.

The 4.2 pistons are 72mm tall.
TVR 500 pistons are either 60mm (early production ) or 64mm (late production. I'm not sure of the changeover date.

However, it's what height of skirt that remains in the bore that is important.
So, piston height 60mm and liner bore is 135mm long, stroke is 90mm therefore pistons pops out 15mm.
The skirt height below the ring land is 40mm so only 25mm of skirt remains in the bore at BDC to give stability.
This dimension is 37mm for a 4.2.

If the theory is correct then I think the thinner oil when hot may be allowing a louder chattering.
The original 4.2 piston skirt expansion design criteria may well have been altered significantly by chopping 12mm off the lower skirt and who knows what expansion the 500 items go through given the good drowning of oil at BDC.

Regards Denis

TVR Beaver · Post by **TVR Beaver** » Fri Jun 22, 2012 2:42 pm

Hi Denis

Its the thicker, better oil that makes the most noise for some reason... But I agree with the expansion bit... The pistons are oval and grow along the Small end pin dim when hot... If the bottom of the pistons have been removed (as we know they have), this could result in the accross skirt dim getting reduced even further when hot resulting in a rattle... may be it starts like on mine at 45K as the BDC turning puts a small indent into the piston side over the many hours of use... making it more pronounced with age?... And I guess the reason some do it and some dont could be down to piston to bore clerances

My mate has an older car 96 I think and his is okay... may be a sorter piston as stated above.. or may be another pistion supplier and differing expansions??.. who knows....

DEVONMAN · Post by **DEVONMAN** » Fri Jun 22, 2012 3:12 pm

I have just examined some pistons that were removed at 40k due to a liner problem and they all have a slight wear line mark at 15mm up the skirt on the thrust side. This would be consistent with rocking due to a push upwards by the rising pushrod, the offset pin increasing the effect slightly.

I believe normally pistons expand overall when hot but moreso along the pin line, hence the initial oval shape.

Regards Denis

TVR Beaver · Post by **TVR Beaver** » Fri Jun 22, 2012 3:25 pm

Mmmm... may be this is the area where the noise comes from... Obviously not a problem as everyone has said... but a tad annoying...
.
I though all cast pistons were oval (wider over the skirts) and they expanded more round as they grew over the area that had the thickest section.. IE along the pin so expansion side to side.... I'm no expert but this is what I've always thought to be correct??
.
Who knows what goes on...??

Rob at V8D says the only way he's got rid of the noise was to replace pistons, but as I said before, he was thinking more small end play as he's put some into an oven and the small end can become a bit loose at temp.... but when cold they are okay... but you can't get them tighter than a realy good clearance fit when cold or they would start to pick up... so where do you go on this one?
I think your theory on skirts and slap could be the answer and a slight indent / localised flat spot being the cause..

DEVONMAN · Post by **DEVONMAN** » Fri Jun 22, 2012 3:40 pm

I agree that the pistons become more round as they get hotter but I believe the whole thing gets bigger and tighter in the bore.

On the subject of the chatter, I think it's QED on either small end noise (although 4.2's don't suffer from it ) or piston rattle at BDC.

Regards Denis