Which Pistons

[T04GTR]

e85 with the 13cc aries would be a nice option. couple that with some little combustion chamber mods and the comp will come down to 9.5 or lower.

[tm_r33]

not looking at switching to e85 just yet. would love to, dont get me wrong, but its not within the budget at this stage.

[Supa Steve]

Yeh weve learnt over the last 5 years of in/outs its just easier to disconnect everything and just take motor/gbox out in one hit. Do what you need to do, then chuck it back in!

Comment about your piston choice mate. Your going for higher comps to help response? Weve run both cp's flat top and domes and its makes bugger all difference. What does make the difference is manifold choices. Or so we found when at the track.

On a road car running 9:1 might be well and good, but for 95% of the time whilst on the race track you never fall off boost so much that you would really need that extra comp to help get you back on. Whilst racing you more then likely wont notice the difference. I couldnt.

Yeah i also find all this talk lately about high CR combo's being more responsive etc. is more theory than anything. Ive owned several VL turbo's both standard and hi-comp conversions with neither being superior than the other unless camshaft profiles and manifolding differences were brought into play. The bore and stroke still stay the same so swept volume doesnt change just the space its compressed into. The engine has to work slightly more to compress the mixture and does receive a slightly larger increase in velocity when exiting the exhaust port. If the supporting hardware is not inplace to support this small increase its lost before it reaches the turbine housing. This small advantage is negated as soon as boost comes into play as the air is forced into the cylinder not drawn in under vacuum. Id rather search for a little headroom in CR so on a hot day if tunes a little off or fuel quality is down the engine doesn't get a hard time. Id say driving an identical car with an 8.0:1 or a 9.0:1 the seat of the pants dyno would be hard pressed to tell any difference and in some cases would actually think the low CR car is the high CR if the placebo effect was introduced.

Edited July 18, 2010 by Supa Steve

[JonnoHR31]

you cant lower CR to allow for a bad tune. if theres too much timing it'll knock regardless. the only solution for that is to get it tuned properly

under light load with the same timing map a higher CR motor will be more responsive

[r33_racer]

point taken. i have done a lot of research and talking to people over past week including people who have rb30det with lower CR and i feel that staying close to the 9:1 ratio will suit my application. i guess time will soon tell. so you have played around with a few different exhaust manifolds to get the correct result?

I didnt mean to be negative about your choice if it came across that way. Just wanted to share my exp with the matter. It maybe me, but on the track I couldnt tell the difference. The other possible issue is if you run bigger cams with more lift, you just need to be aware of your piston to valve clearance. We had the 9:1 CPs,1.2mm HG and 10.2mm lift and we were around .120" or slightly less.

We went from a gcg china exhaust manifold that cracked after every second race meet to a 6boost mani and we picked up something like 300rpm spool and made more power on the same boost. But we also tried greddy inlet manifold and currently have a plasmaman and we picked up abit of down low power from the that switch. It was the manifold changes that I felt made the most difference, atleast on the dyno and on the track in comparison to the slightly higher CR. Lol at the moment we are just about to switch over to a gtr inlet/throttle bodies. So we will be able to see what difference that may make too.

But everyone is different, you or others may notice things I dont or cant. When your done and out racing you will be able to see for yourself. At the end of the day so long as you make the power you want and you get the response your chasing thats all that counts. Doesnt really matter how you get it! Hopefully it will be enough to help you win!

[tm_r33]

you didnt come accross negative at all r33_racer. All the input i can get is very helpful. Ive only owned imports as street cars previously and much of my racing experience is go karts and a couple of track days.

Your right in suggesting that it doesnt matter how i get my power as long as i get it. Many suggested running lower cr and that way allows more room for timing and more boost.

But on the same note just as many people say stay at around the 9:1 mark because im not planning on heading past about 17 psi and not planning on dialing a pile of timing into it.

[T04GTR]

at the end of the day, a engine with a higher comp will be more eficant. better off boost and run cooler under light load.

[T04GTR]

im pretty sure the latest di bmw's are around 11:1 9:1 seems to be at the lower end of the scale for factory turbo cars with mpfi.

[r33_racer]

Yeh it does seem that way nowadays....

Edited July 19, 2010 by r33_racer

[James_03]

Wiseco rb25det pistons in my rb26/30.

11cc dome with 0.8 off the deck.

[tm_r33]

engine is partway through the build. gone with the aries 10cc dome top. at 0 deck with 1.2 head gasket they should give close enough to 9:1 which i what ive been trying to stay close to through my research and build.

james_03 did u nip 0.8 off the deck because it needed it (ie. the face was damaged) or simply to lift ur CR ?

[T04GTR]

the 25 pistons sit lower in the bore due to the differant pin height.

[Gerg_R31]

im using a +40 thou 10cc dome JE piston, made from using 2 molds, which has eliminated the extra piston to deck height that comes with using rb25/26 pistons

[mattis96]

What would the standard rb30 na pistons with 1.2mm head gasket give you in cr? I remember reading it somewhere but can't find it now.

Edited July 27, 2010 by mattis96