Turbo conversions and Compression plates

[arseofbox]

Lo.

Im researching ways to lower the CR of my engine as I want to 'blow it (read turbocharge it).

Acc. to what I can find, theres realistically two ways to do it.

1. New low CR pistons - Pros: its neat and reliable. Cons: Hideously expensive

2. Compression plate and 2nd HG: Pros: Cheap. Cons: Apparently get unreliable at high boost pressure

3. Remachining existing pistons - Pros: Real Cheap (I can do it myself ). Cons: Existing items not forged IIRC, so may cause weakness and pistons may need heat treating prior to being safe to use.

I am trying to avoid option 1 at all costs as its far too expensive. Option 2 sounds OK but unreliable and option 3 is option 3....

Anyone got any experience with using these methods and can anyone advise which is the best way to proceed to lower the CR? FYI, Im looking to pull ~110-120BHp from a 8v 1242 running around 0.4-0.6Bar of boost...

I will be able to play with ignition timing...so that may be of use

 

[custard]

right from speaking to VAD even they admitted that the plates cause HG problems
my hi boost cinq has OE spec pistons machined down by "a couple of mm" to quote VAD lol
mines has 90k on clock so i would go with machined pistons personally thoigh is there enough 'meat' on the 1242 pistons?

 

[arseofbox]

my hi boost cinq has OE spec pistons machined down by "a couple of mm" to quote VAD lol
mines has 90k on clock so i would go with machined pistons personally thoigh is there enough 'meat' on the 1242 pistons?

Dunno. I would presume theres plenty meat on them. But calcs would confirm..Its just to do with the increase in volume needed, the cut can be made wider if needed or deeper if needed

 

[custard]

i'll be interested in your progress.if i ever needed to do engine work or replace the engine i would prefer to do the upgrade to the 1242 at the same time

 

[Chas Uno]

The 'compression plate' thing sounds like a real bodge. It's a cheap way of lowering the compression ratio, but I would personally steer clear of it at all costs.

As Custard Boy says, machining the existing pistons should be ok as long as there is enough meat on them in the first place.

One person to ask about turbo-charging FIRE engines is SMOKEME who posts on the forum. You can normally find him in the Panda section. He has turbo charged (and raced!) FIRE engined Pandas and knows all about what pistons to use and where to get them from, plus he's got some obscene amounts of power from the little engines!

If you're spending a fair bit of cash with the turbo conversion, it might well be worth spending a bit more and getting some forged low compression pistons. The last thing you want to do is cut corners and end up having to rebuild the engine twice as a result. Don't ask me how I know this......

 

[smokeme]

Ok let's get answers:

option 1 is the best but expensive (mine cost 425 for a set of 4 pistons, not bad IMHO)

option 2 is a bodge at best, same goes for thicker gaskets and double gaskets. Do not confuse with copper gaskets, these are good for their purpose.

option 3 is a bit iffy since it needs 2.5mm shaved off on the 1242, this may not seem like much but it may cause problems.

Cast pistons are actually stronger then forged, that's their weakness since forged pistons will take a fair bit of detonation to destroy while cast pistons will usually fail before you even notice the detonation!!

option 3 is ok for low boost but i wouldn't use it, if you're going to spend money tweaking an engine the last thing you should do is take shortcuts that may cause an early failure and if the pistons fail well the engine is gone for good.

 

[Chas Uno]

The God of turbo FIRE engines has spoken! :worship:

 

[arseofbox]

option 1 is the best but expensive (mine cost 425 for a set of 4 pistons, not bad IMHO)

Option 3 is a bit iffy since it needs 2.5mm shaved off on the 1242, this may not seem like much but it may cause problems.

2.5mm? That much? What is the diameter of the cut into the piston you are assuming? As its the volume that has to change, couldnt a wider diameter cut be used to reduce the depth needed?

You say LOW boost, but how Low is low? Im looking at ~0.5Bar (7-8psi). Id think 1Bar / 15psi is high....

 

[HP]

Tom, 2.5mm across the entire piston tops is needed to drop to 8.5 CR.

 

[arseofbox]

Ah right.....thats a fair old bit really. Certainly 'a couple of mm' as vAD said. May still be possible, but for sure the measurements of the piston (crown thickness) would need to be known to look into stuff like piston loading and to check that the combustion wont blow it into many tiny bits...

 

[HP]

That would be unfortunate indeed!

 

[smokeme]

One word of advice: Don't measure the thickness of the pistons after you skimmed them, i did and imidiatly ordered a forged set from JE!! It's that thin!! (enough to make me spend ~500 euros plus shipping!!)It should work for up to a bar or so but i wouldn't use it personally, the pistons are already weak stock it won´t get any better.

 

[arseofbox]

One word of advice: Don't measure the thickness of the pistons after you skimmed them, i did and imidiatly ordered a forged set from JE!! It's that thin!! (enough to make me spend ~500 euros plus shipping!!)It should work for up to a bar or so but i wouldn't use it personally, the pistons are already weak stock it won´t get any better.

Scarily Weak then?

Ive done the maths involved and its looking like a skim of around 1.35mm is on the cards ...assuming I havent got it wrong again!

 

[custard]

as ive said my high boost 1108 has done over 90k on skimmed standard pistons.although i shouldnt say it as its got to be tempting fate

 

[GhostWKD]

as ive said my high boost 1108 has done over 90k on skimmed standard pistons.although i shouldnt say it as its got to be tempting fate

dont think i should say that my cinq has the same so i wont *touches wood just incase*

 

[custard]

dont think i should say that my cinq has the same so i wont *touches wood just incase*

is it low comp pistons for sure? i know VAD used both methods on the high boosts.dont know what caused them to use either mothod though

 

[HP]

Scarily Weak then?

Ive done the maths involved and its looking like a skim of around 1.35mm is on the cards ...assuming I havent got it wrong again!

Bringing the CR down to how much, by the calcs?

 

[GhostWKD]

is it low comp pistons for sure? i know VAD used both methods on the high boosts.dont know what caused them to use either mothod though

think they only used the plate for a couple of conversions and kept popping hg's so started to skim the pistons

 

[smokeme]

honestly i would prefer a plate then skimmed pistons , at least the weakest link was the hg