Rb26 Gt-rs Dyno Result

[pnblight]

mark firstly nice work,

Can i ask did you extend the block? as it sounds like you have increase the stoke and allowed for it in the piston design (pin height) which will work but has serious long term issues as the stoke to conrod ratio will be extreme as the rb26 run a low ratio std. What this means is the rods have pivot alot more through each stroke and adds alot more load (wear) to rods, bearings and crank.Guys like sydneykid can explain alot better then me. Please dont take this the wrong way as i know it works and its a great effort to have a go and do something custom yourself much cheaper than buying japanise kit but i believe the reason the japan kits only run a slight increase in stoke is mainly due to this isssue.

pete

Edited June 6, 2006 by pnblight

[r33_racer]

Yes congratulations on that effort. Good to see another aussie thinking outside the square and trying something new. Excellent work.

[Bunta]

Wow that is an impressive curve!!! Would love to have somthing like that some day!

[Mik]

mark firstly nice work,

Can i ask did you extend the block? as it sounds like you have increase the stoke and allowed for it in the piston design (pin height) which will work but has serious long term issues as the stoke to conrod ratio will be extreme as the rb26 run a low ratio std. What this means is the rods have pivot alot more through each stroke and adds alot more load (wear) to rods, bearings and crank.Guys like sydneykid can explain alot better then me. Please dont take this the wrong way as i know it works and its a great effort to have a go and do something custom yourself much cheaper than buying japanise kit but i believe the reason the japan kits only run a slight increase in stoke is mainly due to this isssue.

pete

It will make massive torque though with rods that short. Which is why the jap stoker kits tend to make more torque then their minimal stroke increase would imply.

[CruiseLiner]

awesome work mate sounds like something different

what sort of rpm limit have u set for the engine?

Edited June 7, 2006 by CruiseLiner

[Sydneykid]

There are three things to keep in mind when comparing rod / stroke ratios and power output;

1. Longer rods go slower through TDC, keeping the total combustion chamber smaller, that’s the actual chamber in the cylinder head plus the exposed part of the cylinder. Thereby giving an increase in combustion pressure, this means more torque per firing.

It’s been while since I did the numbers, but from memory, comparing an RB30 conrod length to an RB26 conrod length means that the combustion pressure is around 2.5% higher (with the same compression ratio). This means a noticeably improved efficiency of combustion.

2. Larger rod ratios keep the rod more vertical, reducing the side loads on the piston and reducing friction between the piston and the bore. Using the same RB26 vs RB30 conrod comparison sees a reduction in side loads of around 10%. I don’t know the total friction numbers for an RB but if we assume that they are similar to a SB Chevy where about a quarter of the friction comes from the pistons. This means a reduction in friction of around 0.5%, not a lot but certainly better than adding friction.

3. The maximum piston speed of an engine using an RB30 rod occurs a bit later in the cycle compared to an RB26. Which means it is closer to maximum lift of the camshaft. Hence when the piston is moving the fastest, the valve is more open, causing less of a restriction. This means better filling on intake and better exhaust flow.

There are a number of other benefits from a higher rod stroke ratio such as slower more effective compression to squish zones, longer dwell at TDC means higher preignition tollerance, less detonation etc.

It is also worth mentioning that #2 above means that an engine using an RB30 length of conrod (compared to an RB26) will have less cylinder wall and piston wear. More relevant is that it will have a far less chance of breaking a conrod due to the lower side loads.

The bottom line, given the choice, I would choose the RB30 rod stroke ratio over the RB26 every time.

cheers

Edited June 7, 2006 by Sydneykid

[mark99]

Yeah, what SK said. Basically longer rod to storke ratio is better. However due to me wanting to retain the factory block yet increase the stroke, something had to compromised, thus the low rod to stoke ratio. But i dont cosider it too big a trade off, its not that much worse than other jap brand stokers, yet gives me more capacity at vastly reduced price. The other thing to note is that some the benefits of longer rod to stroke, ie better cylinder filling, are most notable on engines with very poor breathing, ie restrictive induction and heads. Something a turbo charged engine has less to worry about.

So whist in an ideal world i would have preferred a longer rod to stroke ratio, in the real world, for what i wanted, std block, more capacity, affordability, something had to give

Oh and redline is 8000 at the moment, but will be raised once bigger injectors go in.

Edited June 7, 2006 by mark99

[Sydneykid]

Yeah, what SK said. Basically longer rod to storke ratio is better. However due to me wanting to retain the factory block yet increase the stroke, something had to compromised, thus the low rod to stoke ratio. But i dont cosider it too big a trade off, its not that much worse than other jap brand stokers, yet gives me more capacity at vastly reduced price. The other thing to note is that some the benefits of longer rod to stroke, ie better cylinder filling, are most notable on engines with very poor breathing, ie restrictive induction and heads. Something a turbo charged engine has less to worry about.

So whist in an ideal world i would have preferred a longer rod to stroke ratio, in the real world, for what i wanted, std block, more capacity, affordability, something had to give

Oh and redline is 8000 at the moment, but will be raised once bigger injectors go in.

Did you consider the OS Giken 3 litre solution?

Longer cylinder liners.

But using the RB30 length conrods and a 38 mm spacer.

Instead of the RB26 length conrods and an 18 mm spacer.

Then you could have used a standard RB30 crank and the oversized pistons for 3.1 litres. I believe the liners and spacer plate would have cost less than a bespoke crankshaft.

cheers

[mark99]

You mean copy an OS or buy one? Buying one no, too much, but copying yes. Thats my next project, sort of a copy of both OS and Kakimoto high block... Already got the first few things cut.

But gotta finish this first.

[Cubes]

I still think this setup would be awesome for a street gtr running mild say up to 400rwkw maybe more, depends where it proves to be reliable.

Being able to run 2.9ltrs and retain the stock rb26 block has huge advantages for those that wish not to run the rb30 block.

[Beer Baron]

thanks for sharing the project mate. it sounds awesome and it's nice to see someting a little different.

[Gav]

I still think this setup would be awesome for a street gtr running mild say up to 400rwkw maybe more, depends where it proves to be reliable.

Being able to run 2.9ltrs and retain the stock rb26 block has huge advantages for those that wish not to run the rb30 block.

Mark99 I take my hat off to you for going down this route and sharing the experiences - as Cubes says, an interesting alternative and certainly has the potential for a good streetable GT-R.

I'm not convinced of the cost benefits, however as I have a similar power curve with more headroom potential (i.e. rpm) using an HKS stroker crank and offset pin pistons. This crank has the advantage of being counterweighted and less prone to inducing destructive harmonics at high rpm. A good aftermarket damper is obviously part of the package.

The HKS "entry level" stroker cranks and pistons are now in the order of $6k to $6.5k in Australia. It would be interesting to see what the true cost of modifying the RB30 crank and making custom pistons is compared to the HKS kit.

Again, not meant to be a critisism but just some personal experience I'd like to add to the discussion. Will certainly watch your progress with interest!

[Mik]

There are three things to keep in mind when comparing rod / stroke ratios and power output;

1. Longer rods go slower through TDC, keeping the total combustion chamber smaller, that’s the actual chamber in the cylinder head plus the exposed part of the cylinder. Thereby giving an increase in combustion pressure, this means more torque per firing.

It’s been while since I did the numbers, but from memory, comparing an RB30 conrod length to an RB26 conrod length means that the combustion pressure is around 2.5% higher (with the same compression ratio). This means a noticeably improved efficiency of combustion.

2. Larger rod ratios keep the rod more vertical, reducing the side loads on the piston and reducing friction between the piston and the bore. Using the same RB26 vs RB30 conrod comparison sees a reduction in side loads of around 10%. I don’t know the total friction numbers for an RB but if we assume that they are similar to a SB Chevy where about a quarter of the friction comes from the pistons. This means a reduction in friction of around 0.5%, not a lot but certainly better than adding friction.

3. The maximum piston speed of an engine using an RB30 rod occurs a bit later in the cycle compared to an RB26. Which means it is closer to maximum lift of the camshaft. Hence when the piston is moving the fastest, the valve is more open, causing less of a restriction. This means better filling on intake and better exhaust flow.

There are a number of other benefits from a higher rod stroke ratio such as slower more effective compression to squish zones, longer dwell at TDC means higher preignition tollerance, less detonation etc.

It is also worth mentioning that #2 above means that an engine using an RB30 length of conrod (compared to an RB26) will have less cylinder wall and piston wear. More relevant is that it will have a far less chance of breaking a conrod due to the lower side loads.

The bottom line, given the choice, I would choose the RB30 rod stroke ratio over the RB26 every time.

cheers

Average cylinder pressure over time is actually greater with a short rod than with a long rod. The reason for this is that maximum leverage on the crank occurs when the rod and crank are at right angles. This condition happens quicker with short rods leading to more torque.

That said, longer rods will always be preferable (a rod ratio of about 1.75) for the other reasons sydneykid mentioned.

Edited June 7, 2006 by Mik

[pnblight]

mark99 do you have the data on the internals bore size ,stoke length now destoked and compression ratio? And what do you think the limits of the combo are in rmp, rwhp and boost.

pete

[Sydneykid]

Average cylinder pressure over time is actually greater with a short rod than with a long rod. The reason for this is that maximum leverage on the crank occurs when the rod and crank are at right angles. This condition happens quicker with short rods leading to more torque.

That said, longer rods will always be preferable (a rod ratio of about 1.75) for the other reasons sydneykid mentioned.

Is that in reponse to my #1?

If that is the case, then I am not sure that we are talking about the same thing.

With the longer rod, the piston is around TDC longer for the same number of crankshaft rotation degrees. Hence “around longer” means while the combustion is actually occurring, That’s why I say “Thereby giving an increase in combustion pressure, this means more torque per firing”.

The angle of the rod at ½ stroke is something altogether different. Where the longer rod is also an advantage as the right angle to the crank occurs earlier in the combustion process. Therefore imparting more torque at that time.

Or have I missed something?

cheers

Edited June 7, 2006 by Sydneykid

[mark99]

Thanks for the interest guys. Yeah really broad power was what i was after for street use.

Gav, for that price is that a fully counter weighted or semi counter weighted crank? While i agree horses for courses, im hoping the cost to be sustantially less than that.

Pete, at this stage, the same as any rb30 crank, so far what i have done seems to have no adverse effects on the crank, but iil keep you posted.

Edited June 7, 2006 by mark99

[Swiper the Fox]

Thanks for the interest guys. Yeah really broad power was what i was after for street use.

Gav, for that price is that a fully counter weighted or semi counter weighted crank? While i agree horses for courses, im hoping the cost to be sustantially less than that.

Pete, at this stage, the same as any rb30 crank, so far what i have done seems to adverse effects on the crank, but iil keep you posted.

Address of where to send my crank to ??

[Scooby]

Address of where to send my crank to ??

[Gav]

Gav, for that price is that a fully counter weighted or semi counter weighted crank? While i agree horses for courses, im hoping the cost to be sustantially less than that.

I believe it's for a "Step 2" crank and I believe fully counter weighted (unless someone can say otherwise?) as shown here:

HKS website states these are good for 9,000 rpm and 588 engine kw, but I suspect this is somewhat conservative.

[rob82]

There are three things to keep in mind when comparing rod / stroke ratios and power output;

1. Longer rods go slower through TDC, keeping the total combustion chamber smaller, that’s the actual chamber in the cylinder head plus the exposed part of the cylinder. Thereby giving an increase in combustion pressure, this means more torque per firing.

It’s been while since I did the numbers, but from memory, comparing an RB30 conrod length to an RB26 conrod length means that the combustion pressure is around 2.5% higher (with the same compression ratio). This means a noticeably improved efficiency of combustion.

2. Larger rod ratios keep the rod more vertical, reducing the side loads on the piston and reducing friction between the piston and the bore. Using the same RB26 vs RB30 conrod comparison sees a reduction in side loads of around 10%. I don’t know the total friction numbers for an RB but if we assume that they are similar to a SB Chevy where about a quarter of the friction comes from the pistons. This means a reduction in friction of around 0.5%, not a lot but certainly better than adding friction.

3. The maximum piston speed of an engine using an RB30 rod occurs a bit later in the cycle compared to an RB26. Which means it is closer to maximum lift of the camshaft. Hence when the piston is moving the fastest, the valve is more open, causing less of a restriction. This means better filling on intake and better exhaust flow.

There are a number of other benefits from a higher rod stroke ratio such as slower more effective compression to squish zones, longer dwell at TDC means higher preignition tollerance, less detonation etc.

Can you explain how a longer dwell at TDC gives more tollerence to preignition???? The longer the time it spends at TDC the more heat combustion chamber consumes meaning more chance of pre ignition. You definately cant get the same timing numbers into rb30/26 compared to a rb26 both with similar dynamic compressions.

fixed ur quote up rob (R31Nismoid)

Edited June 8, 2006 by rob82