3TG High compression pistons

[jb_22]

Hi,
I'm contemplating rebuilding my 2TG into a 3TG hybrid with high compression 87mm pistons with a 3T crank I have and I was wondering how 12.5:1 CR pistons would run on 98RON fuel with knock/predetonation issues??

Cheers, JB

[NME308]

Hi there JB,
12.5:1 compression is not 'unachievable' on 98RON fuel I feel. It would require careful attention to ensuring no sharp edges in the combustion chamber area and equally careful attention to tune i.e. afr's and timing. The hemi head will be your friend here.

Now the flip side is that unless this is a max effort race motor then is 12.5:1 really necessary? Consider that for each point of compression raised an engine will gain approximatly 3.2% power then will you miss 3.2% or even 6.4% power for the sakes of having a little margin for error in tune and fuel quality?

Lets say - and I'm pulling figures from you know where - the engine makes 210hp with 12.5:1 compression, then with 11.5:1 it will be 203hp and with 10.5:1 compression around 196hp. For the sake of 14hp you could have a far more reliable engine. Cam, port work, and tune will be far more important than an extra point of compression in the overall scheme of things.

My 2c worth from a little experience! Mind you that 15 years ago I was all over this kind of combo just with 4 extra cylinders and I'm still grinning with the memory of it...

Cheers,
Jason

[Radar]

Agreed with jason here, more power can be made more reliably via different paths as in with manifold tuning both on intake/exhaust, electric water pump, longer stroke, oversided pistons, better fuel/spark. Higher compression leads to overly hot cylinder tempretures which is not ideal for thermal effiency, less compression means 98ron will be sufficent even on really hot days or if you want to run really agressive timing.

[jfallen]

Motorcycle engine rune these sorts of static compressions on 98RON with no issue, but their cylinders are significantly smaller and their cam timing much more "aggressive".

BRD can supply pistons in this range for you're engine at a good price.

With a quality tune and ITB's or twin DCOE style carburetors, it's shouldn't be an issue. Also remember though that with such high compression you'd want very aggressive cams (to get the most out of the engine,) beefed up rods (Eagle do a well priced set) to hande the extra power + RPM, upgraded valve springs and a mad porting job.

Or simply get hold of member Starfire and buy his 89mm 2TG/3T hybrid (read 2L spec) that's all built ready to rock and roll with 10.5 pistons. Never been run built with no expenses spared by Chris Miltons in Adelaide. He's chasing ~$5000. Which (trust me when I say this) is an absolute bargain. Includes Twin PPH40 solex's, extractors, auxiliaries, gearbox, flywheel (no clutch from memory.) If you need his details and can't find them give me a PM.

JA

[Hurricane]

i started off down the 12.1 comp path once for a 3tc build (still got the pistions brand new in the box)
plan was to run it on 98 also

had some short talks to some tunners

all said it would be ok to run with EFI with out any issuse given it was tuned corrected
running webbers there was some mixed thoughts on how well it would work.

ether EFI or Carby the motor was to be build with a largelist of mods to sute the high comp.

i ended up not running with it as i went to turbo and never got back to building the motor N/A.

sorry for no real tech info there, i never tryed it so cant offer any real world advice on it.

whats your plan with the car? street driving?

[Radar]

If you really want to go down this path, its mostly a matter of setting your cams correctly to bleed off some pressure. A nice tight LSA will give you good peaky HP&TQ and let you run much higher comp.