High Comp And E85

[discopotato03]

I think some of the problems with these DE+Ts is that the NA compression ratio and cam combinations don't suit turbocharging . Really they are set up to have good part throttle performance because thats where production cars are intended to be used most of the time . The higher static CR gives a higher dynamic or effective CR and thats where these engines are tuned to run efficiently - consumption and emissions considerations too .

OE turbocharging is generally sized to make a small amount of boost at lowish revs provided the throttle is open far enough . Also its comon for the ignition mapping to run a bit more advance because the lower combustion temps/pressures allow closer to best torque timing . Also many more current engins have variable cam timing on one or both cams and this which allows them to effectively alters the cylinders "trapping efficiency" and literally the dynamic or effective CR .

I think given a choice a turbo short block is always a better basis because of piston oil squirters and turbo plumbing oil/water holes . Pistons you have some say over and cams are open too .

With todays pump fuels I don't think I'd built a turbo engine under 9.3 to 9.5 to 1 particularly since everything has some form of intercooler nowdays . In the bad old days things like 280ZX turbos/VLTs/Z18 Bluebirds/early DR30s had no intercooling and had to live with lower static CRs so they didn't detonate to death on boost .

Often the last revision of any turbo engine ie last of the RB25s and SR20s is worth looking at because with tightening emissions laws the manufacturers had to find ways to run leaner and hotter whilst reducing detonation as much as possible . To reduce detonation they often looked at combustion chamber and piston crown designs to get the most energy from lean mixtures and add cooling in areas that made earlier engines detonate . It can be as simple as water jacket redesigns to pull a bit more heat from around the exhaust side of the chambers and exhaust valve seats .

I suspect this is what Nissan did with RB25 Neo heads and having a more compact combustion chamber probably ment less surface area for the head casting to absorb combustion heat . You'd think they tried pretty hard to get the quench working well with a revised piston crown and with a more compact chamber you can have close to a flat or even slightly dished piston which is ALWAYS better than having a mountain across the middle of the piston to get the static CR high enough with the larger volume chamber ie R33 spec RB25DET heads . If I was serious about building an efficient RB25 it would be a Neo headed one for sure because the factory spent millions developing a more detonation resistant engine . Not that we would have it as lean as Nissan did but less detonation allows closer to best torque timing and that makes all the differenece in a performance engine .

I'm not up on SR engines so I'm not sure if the last or S15 spec heads and pistons were different to earlier ones - the VCT ones I mean . I know the S15 spec GT2860R turbos had higher temp spec turbines and turbine housings than the S14s version so you'd think they were running leaner and hotter (combustion temp wise) than earlier versions .

Lastly that GT2860RS was designed by JKav for a specific purpose and that was not high end performance .

Back in a sec .

[discopotato03]

It was intended to be a club spec turbo that had more hot side flow potential flow for its compressor side than your usual GT2860R type turbos . The idea was to have a better pressure balance across the head (boost vs turbine inlet pressure) so that typical 1800 odd cc fours could run for extended periods on boost without melting anything . Actually JKav mentioned that his original one (he worked at Garretts in the US years ago) had a non gated 0.9 something AR turbine housing and an external gate and his Miata (MX5) did quite well with it .

A .

[Super Drager]

Pretty sure S15 turbos are GT2560R's........