CON RODS AND HIGH RPM

[Spike]

Am just in the process of stripping a std clio v6 engine for a refresh and have found someting rather alarming.
having got the motor stripped to the block with the heads off have found that the pistons have been hitting the cylinder head! there are definite witness marks on the top of each piston where the flat of the piston has hit the flat part of the head, i hasten to note that this is not due to a head skim or block skim as it is the flat squish area that is touching.
the squish measurement is correct at approx 1mm and also the bottom end ( bearings etc) are all a-ok, after a chat with the owner have found out this engine has only done a couple of hundred miles since rebuild, and has done a trackday since it was rebuilt.
my opinion is that the std connecting rods are being run well into their elastic region causing this problem, further questioning reveals that a motec ecu is being run on this with the max rpm being dialed up to near 8000 rpm. obviously this is far too high and constant abuse of this kind can only end in one result! of course it could also have been caused by over revving on down changes but am assured by the owner that this is not so.
when you think it is only a std laguna engine with the red line allready raised from 6500 to 7200 rpm this is no suprise to me, you have been warned!
to fix this problem i am having some rods designed to stop this and give a safe higher rpm limit, quote for this is around £250 per rod + vat, anyone with a cheaper option or solution or even having dealt with this before, please let me know.

james, i realise this is probably in the wrong area but felt members need to know the possible dangers of over revving these engines.

 

[Matt]

Interesting read. I thought the 'Laguna' engine for our cars would have a little more play/give - obviously not the case.

 

[Jamesv6]

Sounds like we've all been very lucky so far? or is this unique to this engine you have SPIKE?

 

[Spike]

Interesting read. I thought the 'Laguna' engine for our cars would have a little more play/give - obviously not the case.

I think 'the give' may eventually turn into 'give up' if not carefull.

 

[Spike]

sorry james could not resist that one, maybe yes all lucky or i'm talking s*it, all i'm going on is actual evidence of what i found, and i wasn't impressed.

 

[Matt]

But as you say this is directly related to the MOTEC ECU and the raising of the rev limiter. Maybe that's why Renault only opted for 255 bhp which is quite a conservative amount of bhp for such a large block.

 

[Spike]

But as you say this is directly related to the MOTEC ECU and the raising of the rev limiter. Maybe that's why Renault only opted for 255 bhp which is quite a conservative amount of bhp for such a large block.

quite a reasonable answer :approve:

 

[Brett]

Quite interesting. Can't really blame the Motec software for the mechanical problem though. Would be down to the over optomistic owner or mapper raising the rev limit to the higher limit. 8k is going some in any stock bottom end to be fair let alone a fairly high c/r motor. Have they been fitted with ARP bolts? Might help the problem. Taking a few grams off the weight of the pistons might help too. Like you say though Spike, there is only so far you can go up the rev range until things start to bend and break.

Like the sound of the rods :approve:

 

[k1ano]

I am not too sure but with a Vee engine and the fact that each crank pin has two connecting rods attached and a larger distance between main caps then I would guess it's possible for the crank to flex due to harmonics at certian revs which may ordinarily be above the OEM rev limit. This could allow the piston to hit the head perhaps without it necessarily being the conrod which has stretched.

This is just my leymans 'minds eye' imagining the rotating assemblies inside the engine and mapping these to the symptoms and my basic knowledge of physics.

Martin

 

[Spike]

interesting replies, but as for crank flex this is a no no, i am usually working on ducati vee twins of 1000 cc ish (1/3 of the clio), i can assure you with these the crank flex is negligable even though its got both rods on a single pin and quite a long crank with a lot of weights on each end. however if you want to rev these over 9000 you have the same problems of rod stretch, and have to use a premium rod like the ones i'm having made or something like a carrillo.
i have been using these types of rod since 96 and it does cure the rod elastic problem. ( i prefer not to use the word "stretch" as it implies the rod has got and stayed longer). the air cooled two valve twins i'm revving to 11000, and the four valve water cooled will rev to 12000, but it is costly. power, by the way is around 115 bhp at the wheel for one of these in std trim.
i have also experimented with lightening pistons, but i hasten to add, not stock ones. the only way i could seriously see a way forwards in lightening the v6 piston is by reducing the compression height ( knock the top lump off) problems arise here with compromising strength and also as its a vee you would have to have the crank balanced to the new piston weight. the rods i am looking to have made would be the same weight as std so to avoid if poss this extra cost.
the real answer imho is to forget revving the n*ts off it and modify the engine to make more midrange torque, easier said than done, but i've got a few ideas.

 

[ray]

this problem goes back to scott and owen between them they decided to do this two engines that i know of i wonder what there thinking was when they did it ?

 

[ray]

spill the beans then spike

 

[Spike]

spill the beans then spike

I had pie for me tea not beans ray.

 

[ray]

humour me ? [smilie=yay.gif] you know no one else will be interested as its to big a project for most

 

[T1RBO]

Here's my experience for what it's worth.

I have two dyno sheets for the Trophy on both MoTeC and Magneti Marelli, from different rolling roads, that both show 30bhp and close to 30lb/ft drops between 7250rpm and 7500rpm. For that reason, we have always had our rev limiter set ant 7600rpm.

On Trophy fuelling, the injectors are flat out and can't flow any more so, my understanding would be that you'd need to be on T/Bs to make sensible use of 8000rpm. I'm not sure that a standard fuel set up could even make 8000rpm but if it could, you'd have to be pretty mechanically unsympathetic to want to persist........ Downshifting into that territory is possible, but it locks the back wheels with pretty dramatic effects (including the most uncontrolled high speed spin I have ever had :rofl

Having stripped the rods from the turbo car (standard and mangled), we were never sure what the Trophy ran, so, we stripped a spare Huger engine - standard rods.

So, I have had two sets of Arrow rods made up, essential for the turbo, precaution for the Trophy, £250 each although that still looks like £3K for a dozen if you ask me [smilie=icon_eek.gif]

 

[ray]

did you ever get in touch with matt hes been waiting for your call

 

[Spike]

Were the arrow rods you had made same weight or heavier/lighter? thinking of a huger crank with shorter rods, what do you think? just go for lightest poss and rebalance the crank. on the subject of fuelling, you say the injectors are at the limits of flow, at what pressure is the trophy run, i would expect it to have been around 5bar so as to keep the injectors within range. obviously this shortens the pulse at tickover but on a trophy, who cares! on a road car this could be a problem.
also, with the correct selection of parts, i cannot see why 8-9k rpm cannot be achieved but also cannot see the point on this motor.
the real point i was trying to make with this post is the risks that are involved with trying to squeeze more out of one of these engines without the correct foundations being laid, there are many unscrupulous people out there who make false promises of big power without the safety factors being paid for, like forged rods and cranks with proper forged pistons. these folk take the money give all the talk then are nowhere to be found when it all goes bang.
dont forget the saying,
SPEED COSTS MONEY SON, HOW FAST DO YOU WANT TO GO?
and,
the fisrt extra ten HP does not cost the same as the last extra ten HP.

i think to build one of these motors properly could easily run to £12k so rods at £1500 is cheap insurance.

 

[Kett]

Can't wait to see the pictures when it's ready Tim... :approve:

...though gutted if they're posted in the For Sale section. [smilie=doubt.gif]

 

[Jamesv6]

I'm hearing you TIm again. Been in your situation, had a wee grizzle about it on a forum once. It's only natural to vent ones frustration a little bit, and of course it's good for people to see these glitches get resolved. They don't feel like little glitches at the time of course, especially when I look back at the bills and realise I threw £40K at it.

I quote your little ditty, but being myself I take a different line. I was behind the steering wheel driving it into the garage that first time, so... Why have I spent so long making it mint again? - BECAUSE I BROKE IT.
(sorry for the capitals shouting everyone)

It's great to find someone capable of sorting our cars. I'm certainly with you there, similar, but in a totally different fashion to you, I've gone back to mostly standard (although not selling up).

Thanks to SPIKE for sharing this technical info on an engine with all of us.

Tim, here's hoping you will fall back in love with the car, and you won't put it up for sale again.



[smilie_santa.gif]

 

[looch]

Cant wait to see it when its done Tim [smilie_santa.gif]