Rb25/30 Na High Comp Build Up

[GTS4WD]

Hay guys..

I got a question..

I want to know.. if I shave off Xmm how much will it increase my Compression ratio by?

Has anybody haved their head? How far did they go?

Will an RB25DE with CVT cope with a 12:1 CR?

What is a safe amount of CR to run?

Any info is greatly appreciated!

Edited April 20, 2008 by GTS4WD

[Eug]

Cris,

in the RB30 thread is a bit of a long read, but in there somewhere there is discussion on achieving desired CR. there is a formula to it, but it involves so many variables.

there's this quench/squish thing to consider, which i'm not 100% clear on. from what i gather, involves a bit of r&d into having the pressures just below the unwanted knock levels. it's far too technical and involves thermodynamics and kinetics and a whole lot of stuff out of my science field.

the gasket used (obviously thinnest possible).

valve clearances becomes something to consider, i'd do valve springs as well due to increased pressure.

in regards the the safe amount, i don't see why 12:1 isn't possible (on ethanol pump fuel, maybe?).

- it does come down to the fuel available, the ecu, the tuner and the actual tune itself

- increased CR = increased pinging

a light skim would be ideal, after that it becomes a case of valve springs, then you may as well get bigger valves, hey while your there port and polish, ahh f**k it why not cam shafts and cam gears.

11:1 would be somewhere streetable i would say... again down to tune & fuel avail.

again there's so many variables; and it's a good discussion topic - so discuss away!

[342Four]

exactly eug it all comes down to r&d and the tune

squish and quench are two very complex topics when dealing with pistons and compression ratio - something that is out of my league (its got something to do with a progressive pressure build in the cylinder so that it doesn't pre-doetonate)

CR is almost as complex - generally (and i mean very broadly...) shaving 1mm off equals increase of 1 point CR

some people say: ohh you cant have anymore then 10:1 max on a street car otherwise it becomes undriveable

bullshit - the Falcon GTHO Phase 3 was (i believe) a very high 13:1 compression ratio, and was very very streetable (and this was on 95RON pump fuel)

tuning the engine for the CR means often a great deal of timing advance and dwell angle with the sparkies etc etc etc

but it is doeable

just think of what honda did with teh ka20 engine in the s2000 - over 200hp from a 2 litre 4 cyl running around 12:1 CR

on another note i would love to hear a high CR rb at high revs....

[GTS4WD]

a light skim would be ideal, after that it becomes a case of valve springs, then you may as well get bigger valves, hey while your there port and polish, ahh f**k it why not cam shafts and cam gears.

Thats the Idea of it..

I just have to make sure that the increased lift out of the new cams will not cause the valves to hit the cylinders..

I just need to gather enough information as to make an educated decision.. I may just stick with 1mm or maby 1.5mm

I Read somewhere that by going NA 25/30 the CR drops to something like 8.5:1 CR? Can anyone confirm this?

Came accross this:

http://en.wikipedia.org/wiki/Compression_ratio

[Eug]

have you got the cams yet? how much more lift do they have over stock?

you'll have to really sit down with your mechanic about it and do some number crunching before even considering shaving.

the NA25/30, depends really on which pistons you are referring to? RB30E? RB30ET? RB25DE? RB25DET (unlikely, but who knows)? also depends on the gasket they're referring to, but scratching my memory i did read something like 8.7 for the twin cam conversion.

the steps now would be:

- check clearance with change in cams only, see how much lee-way you have

- determine what fuel is available, what the tuner is capable of, and the ecu needed.

- determine the ideal ratio that will suit #2

- check what thickness gaskets are available

- is it possible to achieve the desired CR with the gasket + shaving, calculate

- do the valves still clear with the desired CR? if not you might need to lower it a bit.

anything you can add Cris, better to pick everyone's brains for different ideas and angles before throwing dosh at it.

[SKiT_R31]

if your doing the 25/30 thing, just deck the block while tis out of the car, you can go about 2mm and that should push the compression to a nice ~ 10.3:1 (rb30e pistons with RB25 head gives a 8.3:1 CR)

alternatively you could use rb25de pistons, are you rebuilding the block?

I recently pulled my motor to bits, got the head machined as i thought it might be warped, definitely was, like a banana and it was apparently shaved within 0.0001mm of its life, and you can definitely see that it has, anyway the squash zone didn't look effected, but i don't have a trained eye, i don't know how much they took off, but damn - it was alot!

car wouldn't run on 98 with anymore than -3deg timing - otherwise it pinged, this pushed the compression from 165PSI across all cylinders (cold, throttle closed, 10.5:1CR static) to 195PSI across all cylinders (cold, throttle closed) (low reading could be comp guage used, also other things come into play, like overlap in cams/adj cam gears)

With a 1/4 mix of pure methanol, the car now goes off its f**king head - with +20 degree's timing (by off its head, lights up 2nd gear sideways with locked diff and 215's) haha not a good power estimation, but all i can say is it wasn't a scratch before with 151kw - on what it is now, cant wait to get it back on the dyno

But yeah, moral to the story is, you can shave the head, but isn't the ideal solution - esp for real high compression (for example there is a member with an rb25/30 with 25de pistons and -2mm on the block with 12:1 CR and his car loves 98ron)

I'm going to build another block down the track - and im going to go all out, probably stick with this head and just get some 25det forgies (should drop it about 1:CR)

also for the record, cams are 8.3mm, 255deg, no problems, even with a tooth out (boy that was buckets of fun, no ecu limiter, lag-lag-lag 'what...?' 20det coming on boost 'hmm' 5000rpm sideways, bang 8000rpm in 0.0001 of a second and, bang bang bang bang (thank god i had it set to DUMP fuel at 7800 from -37 to + 50 on the safc!)

haha its all fun and games, just when you put it all together, wind it over with a ratchet to make sure you don't lose your valves

[GTS4WD]

Skit:

well.. someone has done their research

Yeah.. I should be getting it rebuilt:

Balanced crank (anything else needed to crank?)

Shot Peened connies (necessary?)

RB25DET Forgies (laying around)

And then a shave of block ~1.5mm - 2mm

Port and polish head

DET/GTR springs

HOPEFULLY oversized valves (Ideas? DET Valves larger? cheap alturnative? Are RB30 Valves bigger than 25 valves? I could use two sets of 30 valves)

And a set of re-grinded cams

Aj. Cam gears

Im thinking I can re-flash the R33 RB25DET ECU I have to run an appropriate map

P.s Good idea with the ratshat

Eug:

Well there wont be any miss-communication, Ill be doing the work myself.. wish me luck.

- check what thickness gaskets are available I think I will be using a new gasket I already have laying around, perhaps tke a little off that

- is it possible to achieve the desired CR with the gasket + shaving, calculate - I need a maths freak to calculate my combustion chamber volume (top dead center - bottom dead center = Combustion chamber volume?)

Im still in the process of aquiring all the info and parts I need to do it all in one hit.

I will most likely do a write-up.. and most likely ask many MANY more questions (e.i port and polish, C.R's, Valve clreances, re-grinded lift and durations, timeing, fuels etc.)

This will be on a strickt budget, so, anything that I dont need to take to workshop, wont be taken. If there is a cheaper alturnative (i.e, DET springs apposed to aftermarket ones, Regrinded cams apposed to new ones) they will be taken.

[Cubes]

Regarding valves.

No difference.

Rb26 valves are a bee's dick larger; unless you pick them up cheap I don't believe its worth while. Spend the $$ on a usual clean up with a valve deshroud + some cams and you will be miles ahead.

[sh@un]

some people say: ohh you cant have anymore then 10:1 max on a street car otherwise it becomes undriveable

bullshit - the Falcon GTHO Phase 3 was (i believe) a very high 13:1 compression ratio, and was very very streetable (and this was on 95RON pump fuel)

It's all about peak cylinder pressures, and the engines resistance to knock (gets back to squish characteristics, and combustion chamber/head design)

I'm sure the Falc' with it's shitty 2-valve heads wouldn't have had a great volumetric efficiency (didn't make much power for a 5.8L motor), whereas a late model Ferrari or Porsche motor with their 11 or 12-point-something to 1 comp ratios would have huge peak cylinder pressures at high revs, due to the volume density of air the heads flow.

[342Four]

It's all about peak cylinder pressures, and the engines resistance to knock (gets back to squish characteristics, and combustion chamber/head design)

I'm sure the Falc' with it's shitty 2-valve heads wouldn't have had a great volumetric efficiency (didn't make much power for a 5.8L motor), whereas a late model Ferrari or Porsche motor with their 11 or 12-point-something to 1 comp ratios would have huge peak cylinder pressures at high revs, due to the volume density of air the heads flow.

actually it made a heap of power for the time

from wikipedia:

Technical data

The GT-HO 351 Cleveland motor produced over 380 HP (285 kW), and was fitted with a 6,150 rpm rev limiter. With the rev limiter disabled, it would pull 7,000+ rpm, even in 4th gear.

they also were 4v heads

but ur right - the volumetric efficiancy wouldnt have been like that of a ferrari or a porsche

[Eug]

ferrari's head work is miles ahead.. 3d cam profiling if only..

[sh@un]

actually it made a heap of power for the time

from wikipedia:

Technical data

The GT-HO 351 Cleveland motor produced over 380 HP (285 kW), and was fitted with a 6,150 rpm rev limiter. With the rev limiter disabled, it would pull 7,000+ rpm, even in 4th gear.

they also were 4v heads

but ur right - the volumetric efficiancy wouldnt have been like that of a ferrari or a porsche

There are lots of fanciful claims for the Hoey's power output. The original figures were in SAE (Say Anything Exciting). The modern figures in DIN put it somewhere closer to 220-230kW. Not a whole lot for a pre-emissions 351c.i V8.

And '4V' heads in clevo speak doesn't mean 'multi-valve'. The heads definately only feature 2 pushrod operated valves per cylinder.

[GTS4WD]

So should I be bothering with DET springs wich are stiffer?

Any clues on on whether RB30 Valves are bigger than 25's? Or if there are valves bigger than the 25's? VG's?

Has anyone gotten their cams re-grinded? How would it compair to a new cam? Its a shit load cheaper to get them re-grinded..

Would it be necessary to balance the crank?

[Eug]

So should I be bothering with DET springs wich are stiffer?

Any clues on on whether RB30 Valves are bigger than 25's? Or if there are valves bigger than the 25's? VG's?

Has anyone gotten their cams re-grinded? How would it compair to a new cam? Its a shit load cheaper to get them re-grinded..

Would it be necessary to balance the crank?

i could draw up a 5k build for you right now if you want seriously though, just work around your budget.

2nd hand DET springs would be the cheapest and most effective upgrade while you're there. just be sure to get them checked out so that they have even spring loads.

can't confirm the valve compatibility sorry

someone here had their stockies re-ground mildly by Sure Cam

stocko specs are 240º & 7.8mm

reground to:

inlet 255º & 8.2mm

exhaust 258º & 8.2mm

$170 per cam

ph 02 4736 1742

a significant improvement over stock i would say, and would be a decent budget upgrade.

is the block coming out too? balancing the crank would be ideal, but not THAT important. while your there knifeblade it

[GTS4WD]

Well it will be a 3L block.. So yes.. Id like to balance and get bearings.. even look into shot peened/Cryo treated (???) con rods..

I want reliablility..

Budget will be about 2K hopefully.. with all the work (except the work where professional tools are concerned e.i re-grinded cams etc. is concerned)..

I just need the appropriate information to know what Im looking for..

Ill be doing alot more reading on this..

Its a shame there arnt many people looking for NA RB power..

A full lost of parts and details will be up sometime in the near future so I can sus everything out..

I still need information on air/fuel mixtures.. I need to read more into that..

and on that topic.. put down 6new injectors and perhaps a fuel pump too ( there goes more money)

[Cubes]

The stock rods are good for 400-500rwkw providing you keep a cap on the rev limit.

Treating the stock rods really is a BIG waste of $$.

Size the injectors and fuel pump 'just' incase you want to strap a turbo on to the side of that highcomp RB30DET when you discover it still gets beaten on the roll from a stock turbo'd mild r33 rb25det's/20det's.

I'd imagine the high comp rb30det would work quite nice with a gt30 1.06 or gt35 1.06; pick up a cheap 1k xr6t turbo, bolt it on to a std manifold, little bit of ic piping and a dump pipe and your up just under 300rwkw with a high comp fuel efficient responsive rb30det

[Eug]

$2k in my personal opinion isn't enough.

shot peened & cryo treated rods is correct, something you should look into. i take it you're going stock rods? might be a bit of a waste. what's your predicted rev limit?

it is a shame, i'd love to get stuck into it - unfortunately, i'm not one to do things by halves. if i were to do it, i'd sink an easy $10K. hello 9,000rpm.

injectors, just use RB25DEt ones, fuel pump = gtr.

[Cubes]

Are you saying he should look in to treating the stock rods?

Speak to multiple engine builders and you will soon see its a big waste of $$.

Forged rods are the only way to go if you want to rev the thing. Especially when they are so cheap these days.

Price up shotpeening and cryotreating and you will definitely be looking at those forged items.

[Eug]

i dont know what internals he's decided on, the $2k budget doesn't really stretch far.

if he's going to see revs then i'd agree with the forged items.

but for a 'stock' build up, then i personally will look into whatever fits into the budget.

[342Four]

crank is (i believe) nitrided from factory so it would only need a linish and a balance

like cubes said the stock rods are good for a lot of kw so dont bother there

cryo as much as you can afford

valve springs, valves etc