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Underbody braces, turbos and more!

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PostPosted: Sat Oct 05, 2013 7:28 am 
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Joined: Thu Jan 08, 2009 9:14 am
Posts: 19
Location: Croatia

Ok, I'm in the process of building overbored G13B with 75mm Honda P29 pistons... so I started to do some calculations using online compression calculator... and I'm asking you guys for help...

Am I doing it right?

Calculator that I'm using is from site --->

I've searched forum and read some topics and everyone is talking about 14:1 + compression ratio with honda pistons... so now I'm confused... ok, in my calculations piston to deck height is probably wrong...but not much??? :huh:
my goal is 12 (12.5):1 compression ratio... because I don't want to use racing fuel... which is difficult to buy here in Croatia.... and it's not cheap...

One guy here in Croatia did this conversion, and he wasn't having any problems with untouched p29 pistons in G13B engine running on 100 octane fuel (probably about 96 real octanes, because here in Croatia fuel is crap...)

sorry for some writting mistakes (if there are any), english isn't my domestic language... :D

PostPosted: Sat Oct 05, 2013 9:25 am 

Joined: Tue Jan 24, 2012 7:54 pm
Posts: 42
Location: Sydney
I have done this to my engine, I would check the following -
My previously untouched cylinder head had a chamber volume of appox 30cc. After giving it a clean-up cut of 7 thousands of an inch it measured 29cc.
If you are using the preferred multi layered steel head gasket from the Suzuki van then it has a compressed thickness of 50 thou (1.25mm). From the factory the pistons usually sit down the bore about 10 thou (0.25mm).
For best performance I suggest you deck the block so the clearance from the piston to the head is about 30thou (0.75mm), mine is cut to 25thou. For this you will need the pistons out of the bores about 20 to 25 thou. If not decked the squish clearance will be 60 thou (1.5mm) which is leaving a heap of power on the table. Just remember the compressed charge in this dead zone does not contribute to the power stroke and is wasted.
Once the above is factored in your calcs will come in at about 14 to 1.
My engine is at 12.3 to one with P29's, 25thou squish clearance and 29cc chamber. The P29 crown is 7.2cc and approx. 3.5mm high from the factory, I cut mine down to 60thou high (1.5mm) for the 12.3 to one as above.
Good luck with it and remember a low dome promotes a faster flame propagation which is good.

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