E85 + Rb30det + High Comp = ?

[XKLABA]

Static comp ratio means very little in the grand scheme of things, its the dynamic that makes the difference for example

My 2630 has a static comp of 9.5:1 and 250 cams given the timing of the cams iirc I have about 8.2:1 dynamic comp ratio, where Daves mate with 11.7:1 static comp with 290 cams (depending on how they are timed ) probably has around 7.8:1 purely because he has 40 crank deg per cam less stroke then I have which is why he gets away with/needs such a high static comp

I believe its a balance act between the comp ratio and timing, the higher comp ratio makes the engine more efficient which not only give more power off boost but encourages the boost to come on sooner, the trick is getting the comp ratio high enough but not so high as it limits timing below its optimum point

[GTSBoy]

Agreed. But ignoring the effects of cam duration, static comp and boost are multiplied by each other to work out the total cylinder pressure - same as when you use twin-charging. A bit of extra static actually buys you a bit of extra static multiplied by the boost. To reach the same total pressure (to hit a given power target) you can use less boost, which is better in a number of ways, as I said before.

Add cams back into the equation, if you plan to lean hard on the knock resistance of E85, now you need to either use less duration than you might on petrol, or use even more static compression to overcome the loss of pressure from big cams than you might on petrol.

[discopotato03]

My take is that static compression ratios are worked out from physical volumes but they're never achieved real world because rings and sometimes valves never achieve a perfect seal . Also its virtually impossible to have zero loss throttling though GTRs are better off than other RBs here .

I would look into engines that run really high boost pressures and see what they do to get around detonation issues . I think you'll find that a lot rides on the hot side because that has a lot of say how an engine rejects heat which is mostly out the exhaust . Note how some competition engines seem pretty big on exhaust manifolds turbines gates exhausts etc , not necessarily huge but big in relation to the cold side in some areas .

To me the fact that higher tech turbos like the EFRs have quite large turbines for their compressor size tells me that less restriction is part of why they can make good power .

Actually thinking about it big truck diesels often use large turbines and turbine housings and diesel compression ratios are higher than the petrol engine norms . This would be about have a good pressure balance across the engine from the torque peak to maximum power / revs , and keeping it up for long periods of time .

I think the only reason more conventional car turbos have smaller turbines is to increase their response with less revolving inertia . Manufacturers want heat through the cats ASAP for cold start and running emissions compliance .

If you could get RIPS in NZ to talk to you he should know about super power RB30s and exotic fuels , may not want to give his hard earned experience away because he had to learn through trial and error .

Lastly if spending big you could consider going a bit larger to 32-3400 ccs and having a less extreme state of tune .

A .

[jet_r31]

Its better in a petrol motor built for power to have a hotside that outflows the coldside period really. Motor is much happier/safer and runs cooler that way.

with advances in comp wheel design now it makes it a bit better to achieve without as many flaws as befores aswell

a lot definitly rides on the hotside as in most modern motors it really is the biggest restriction in the end

cheers

darren

[AngryRB]

Is there a relationship between the amount of squish area and the comp ratio? I read somewhere that if going from say 9.5 upto 10.5, the squish area/size would determine if it knocks. so larger squish would create hotspots and knock compared to smaller squish with a higher comp?

Also heard that a higher comp ratio reduces combustion temperatures because the increased pressure clears out the cylinders more efficiently.

Edited August 29, 2014 by AngryRB

[discopotato03]

The usual practise with competition engines , not Hemi POS designs , is to have as compact a combustion chamber as possible so more of the volume is in the piston crown for better combustion efficiency . The idea is to compress the charge into a small area near the spark plug to have shorter flame paths .

Part of the reason why dated designs detonated was because the end gasses , last to be burnt , were further away from the intended ignition point and the The longer it takes for the intended burn to reach the end gasses the greater the chance of heat and pressure igniting them .

AFAIK there was SFA development in RB26 head castings from beginning to end of production . RB25 heads went through a number of revisions from R32 GT25 to R34 GTt , I think it's significant that the chamber size shrank and the piston crown was redesigned to give the same 9.0 static CR as a 33 spec RB25DET .

Granted RB26s have different dynamic compression characteristics but I think the Neo is a better design .

A .

[GTSBoy]

The Neo design is primarily about reducing fuel consumption and emissions, enabling it to be rated as a LEV in Japan. The best way to do that is to reduce the surface area of the combustion chamber in the head, thus reducing thermal losses to the coolant.

Has a side effect of being a better "performing" combustion chamber design somewhat per what Adrian wrote above.

More importantly - squish area is absolutely key to good combustion chamber design. Getting rid of the squish pads in a combustion chamber is not the best way forward.

[bigmikespec]

More importantly - squish area is absolutely key to good combustion chamber design. Getting rid of the squish pads in a combustion chamber is not the best way forward.

Squish and quench (different things) is a whole different discussion. The RB26 does not have a lot anyway (squish area that is) and by the time you fit over size valves and relieve around the heads of the valves for flow there is even less.

What you don't want is high squish velocity which creates hot spots where flow is forced around sharp corners. Big power, big boost, high rpm less squish is the way forward! A little around the perimeter of the chamber is probably good to quench that area to help ring seal and blow by.

[discopotato03]

My opinion is that the longer it takes the spark plug initiated flame front to reach the end gasses the grater the chance is that heat and pressure will light them up .

TC RB heads are a production compromise so things like valve angles will never be ideal in a competition application . To be ideal a TC head needs to be tall enough to have a narrow included valve angle inlet to exhaust and to have high inlet ports and an ideal port shot at the backs of the valves . To see stuff like this look at the old Nissan LZ twin cam heads or Cosworth BDA ones . Narrow valve angles means shallow compact chambers and you don't need lumps of aluminium on the piston crowns to get high compression ratios .

From what I heard GMS used higher than standard CRs on their RB26s particularly when boost levels were dropped . Are there any pics available of the piston designs they used ?

A .