Reducing CR by skimming Pistons or new con rods

Reducing CR by skimming Pistons or new con rods

Author
Discussion

MaxRothery

Original Poster:

201 posts

111 months

Tuesday 19th May 2015
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I've been looking into rallying and getting into the high teirs with a 1.6T and the engine I've got lined up has a CR of 11.1 which is quite high so it would be better with a CR if 8.5-9. I've done the calculations and to reduce it that drastically would require either 2.2mm taking off the piston or the con rod. So which would be easier and stronger to shorten? There is a turbo kit for this engine but it only runs 4 psi of boost and I want to take it up to the 2.5 bar the S2000 regulations allow

andyiley

9,106 posts

151 months

Tuesday 19th May 2015
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I might be missing the obvious, but is there a reason why you don't just buy a set of forged low compression pistons?

MaxRothery

Original Poster:

201 posts

111 months

Tuesday 19th May 2015
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The only set I can find involves boring the engine out to 1.9lts and I don't particularly want to spend £730 on Pistons I couldn't use. The stock internals are already strong, but it's just too high of a CR to use

stevieturbo

17,229 posts

246 months

Tuesday 19th May 2015
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MaxRothery said:
I've been looking into rallying and getting into the high teirs with a 1.6T and the engine I've got lined up has a CR of 11.1 which is quite high so it would be better with a CR if 8.5-9. I've done the calculations and to reduce it that drastically would require either 2.2mm taking off the piston or the con rod. So which would be easier and stronger to shorten? There is a turbo kit for this engine but it only runs 4 psi of boost and I want to take it up to the 2.5 bar the S2000 regulations allow
I'll be impressed if you manage to shorten a conrod.

And without seeing the piston, no idea whether you could safely remove material from the crown. Some you can, some you wont.

But if you've bought a high compression n/a engine and have a notion of running 2.5 bar boost...how you achieve the desired compression really is the least of your worries.

It's about time you sought an engine builder and tuner who can do that sort of thing reliably, and expect to be well into 5 figures to achieve it.

PaulKemp

979 posts

144 months

Tuesday 19th May 2015
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Or a thicker head gasket.
The likes of Cometic will do gaskets of differing thickness

PaulKemp

979 posts

144 months

Tuesday 19th May 2015
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Or a thicker head gasket.
The likes of Cometic will do gaskets of differing thickness

Stiggolas

324 posts

146 months

Tuesday 19th May 2015
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As above really, look for a decompression plate. They're out there for most engines.
Machining pistons is a headache and as for shortening con rods....

davepoth

29,395 posts

198 months

Tuesday 19th May 2015
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what engine is it?

anonymous-user

53 months

Tuesday 19th May 2015
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MaxRothery said:
I've been looking into rallying and getting into the high teirs
Then you need to put up a decent budget for engine dev, or you'll just be pootling round at the back!


Bodging off a few points of CR ins't going to get you a competitive rally engine i'm afraid, let alone in the "High Tiers" of the sport.........

Pumaracing

2,089 posts

206 months

Tuesday 19th May 2015
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stevieturbo said:
I'll be impressed if you manage to shorten a conrod.
Ha! What do you know? Five minutes with an angle grinder. The trick is knowing whether to shorten it at the top, the bottom or in the middle.

MaxRothery

Original Poster:

201 posts

111 months

Wednesday 20th May 2015
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davepoth said:
what engine is it?
. I'm looking into using a jimmy with a 1.6 internals. The M13A with M16a crank, rods and Pistons, creating a 1.6 in a 1.3 block. Only reason for doing this is that the regulations state that it must have a production variant but the engine can be modded to increase engine size. Otherwise I'd be swapping engines over

AndyS2

869 posts

257 months

Wednesday 20th May 2015
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I'd be impressed if you took 2.2mm off the piston and it was still strong enough to withstand 2.5bar of boost!

stevieturbo

17,229 posts

246 months

Wednesday 20th May 2015
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AndyS2 said:
I'd be impressed if you took 2.2mm off the piston and it was still strong enough to withstand 2.5bar of boost!
There are plenty of pistons I've used in the past where I would have no concerns.

But a 11:1 CR n/a engine piston ??

Not a chance.

Really, this guy is looking for a cheap bodge...for a build that is in the £20-30k range by mentioning the turbo rally cars

Either that, or his goals/intentions are completely screwed up

Boosted LS1

21,167 posts

259 months

Wednesday 20th May 2015
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OP, decide how much power you want and how much boost will deliver it. Then buy some custom made forged low compression pistons. This is the proper way to do it. Forget about decking plates, thick gaskets etc. Also, you'll never get the con rods the same length or weight if you shorten them ;-)

Crafty_

13,248 posts

199 months

Wednesday 20th May 2015
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2.5 bar is an awful lot of boost, I would suggest the piston will need to be changed any way and probably the rod for steel H beam rods too, this gives you a chance to order the rod size accordingly and/or alter the piston. You have to consider a motorsport engine is going to get far more abuse (high revs etc) than a road car.

I think you need some advice from an expert in this field to find suitable items. I believe you can have both items made (pistons certainly), but cost is likely to be £1500, maybe more.

I agree with whoever posted above about £20-30k engines, you're going to need quite a good budget, a set of £700 pistons is only the start.

Far as shortening con rods, it is possible - I've seen a set where the small end was machined and moved up the rod to shorten the stroke, only a small amount mind.


Boosted LS1

21,167 posts

259 months

Wednesday 20th May 2015
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He he.

Huff

3,141 posts

190 months

Wednesday 20th May 2015
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I'd have thought (as a layman in these matters) if you bolt the thing together as-is at 11:1 and run it in at 2bar+ the rods will very quickly be adjusted to a more suitable length automatically...

mtrehy

87 posts

146 months

Wednesday 20th May 2015
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Crafty_ said:
Far as shortening con rods, it is possible - I've seen a set where the small end was machined and moved up the rod to shorten the stroke, only a small amount mind.
No you haven't.


Crafty_

13,248 posts

199 months

Wednesday 20th May 2015
quotequote all
mtrehy said:
No you haven't.


I forget how much the one I saw was done, but there you go, thats an MR2 rod apparently.

mtrehy

87 posts

146 months

Wednesday 20th May 2015
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And you think that changed the stroke?