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Safe maximum RPM on whiteblock 6?

Turbeam

New member
Joined
Jul 19, 2009
On a modified 6-cylinder whiteblock motor with aftermarket engine management, what are you guys setting as a rev limit?

Higher than stock engine speeds are probably going in the wrong direction for a +T engine, but other than the fact that the stock cams aren't really going to work too well at higher engine speeds I find that these engines are really very rev-happy and I'm prone to keep bouncing off the stock limiter in use. I don't think the stock limit is set for purely mechanical reasons?

Assuming that breathing isn't an issue, what would be recommended limits for either stock internals or with aftermarket rods and pistons?
 
mps

Got A Graham Bell's four stroke tuning book? There's a section in there on the rules of thumb for different types of engine, based on mean piston speed (related to rpm and stroke). That will give you a idea. Modern engines are a lot better made than they were when that book was written though, so probably a conservative estimate.
 
Yes, I do have a copy of that book, and he also did one on more modern engines as well.

I could do some basic calculations, but I don't think the stock rods in these engines are particularly strong either under tensile or compressive loading.

I was really just wondering what other people were setting as a rev limit when they do away with the stock ECU. It may just be exaggerated a bit by my use of a slightly lower included final drive ratio, but the B6304 in my car seems to be into the limiter while apparently still very eager and willing to run faster.

It does feel like the stock 6000rpm or so limit could be a little on the conservative side.... especially with stronger rods in the motor.
 
For all practical purposes, there shouldn't really be any difference in maximum safe rpm between the 2 litre 4-cylinder and 3 litre 6-cylindrer whiteblocks for example...... I wonder what the rev limit on a B4204 is?
 

Really? with aftermarket internals then I take it? You have actually set a new rev limit at 8K?

The valvetrain shouldn't be a problem up to that sort of level, but personally I'd be a bit wary of running a B6304 with stock rods much over about 7K tops even if it was able to breathe effectively at those sort of rotational speeds.

From the feel of how free-revving my engine is and how quickly the revs rise like that even in the taller gears, I think that a limiter is very much a necessity on these motors.

I'm just really interested to hear what other people set these at?
 
I would think the 6 cylinder will do 7K very well provided it has the proper power band and 7K will still suit a more or less stock drive train nicely. The closer you get to a big number on the tacho the more it will become a target point so maybe you should set the limiter to 7100 RPM and shift at 7?
 
For all practical purposes, there shouldn't really be any difference in maximum safe rpm between the 2 litre 4-cylinder and 3 litre 6-cylindrer whiteblocks for example...... I wonder what the rev limit on a B4204 is?



Yes there should. Harmonics for one thing. The long crank of a 6 cylinder is going to hit it's second order harmonic at a MUCH lower rpm than a shorter crank 4 cylidner, and will subsequently shake itself to pieces without a crank damper designed to handle that. And even then.... For reference - BMW's pro teams running S54's in Grand Am were running infrared temp sensors on their TV dampers (and the S54 has a massive damper, held on with 4 M12 bolts because they rev them to nearly 8k from the factory), running them to 8500. I'm not sure what the temp threshold they had, but I know if that temp threshold was hit in a race, those TV dampers were thrown out and not re-used.

That said - I'd agree, 7k would be a good starting point for most euro streetcar I6's of this era, because truthfully they're all pretty damn similar in design.
 
Really? with aftermarket internals then I take it? You have actually set a new rev limit at 8K?

The valvetrain shouldn't be a problem up to that sort of level, but personally I'd be a bit wary of running a B6304 with stock rods much over about 7K tops even if it was able to breathe effectively at those sort of rotational speeds.

From the feel of how free-revving my engine is and how quickly the revs rise like that even in the taller gears, I think that a limiter is very much a necessity on these motors.

I'm just really interested to hear what other people set these at?
I don't know actually:-P My turbo motor (T6, OEM pistons, RSI rods) had a 7500rpm redline that I took it to regularly. Never tracked the thing so I didn't spend a ton of time there, and I was frankly more worried about the transmission. But both worked for the period of time I was doing that.


COMPLETELY UNRELATED but sort of:

Has anyone got into the 960 ECUs yet? Even just rudimentary stuff? I really want to bump the redline on my stock motor for the time being. Cause **** this is slow as hell.
 
I ran to 7600 on stock everything on a regular basis.

Whether or not breathing is an issue becomes mostly turbo related.

With a T67 my power was still climbing at 7600. With much smaller but much more responsive 61mm Borg Warner peak power was at around 6500, but I was getting close to maxing out that turbo (That was @ 540whp and those turbos are only good to 550-600).

They do have a relatively long stroke and poor rod ratio though.
 
Yes there should. Harmonics for one thing. The long crank of a 6 cylinder is going to hit it's second order harmonic at a MUCH lower rpm than a shorter crank 4 cylidner, and will subsequently shake itself to pieces without a crank damper designed to handle that. And even then.... For reference - BMW's pro teams running S54's in Grand Am were running infrared temp sensors on their TV dampers (and the S54 has a massive damper, held on with 4 M12 bolts because they rev them to nearly 8k from the factory), running them to 8500. I'm not sure what the temp threshold they had, but I know if that temp threshold was hit in a race, those TV dampers were thrown out and not re-used.

That said - I'd agree, 7k would be a good starting point for most euro streetcar I6's of this era, because truthfully they're all pretty damn similar in design.


That is true, but I was primarily thinking about the safe rev limit for the reciprocating components.

Although not ideal in some other ways, I believe that one major advantage of the whiteblock motors having been designed for transverse use in many of their applications is that the cylinder bore centres are closely spaced and the overall crankshaft length has been kept down to an absolute minimum.

Doing this helps get around some of the crankshaft issues related to the rocking couples balancing each other out along the length of a typical straight six engine crankshaft. Failures of just this kind are almost a tradition on some older big I6 motors like the old Jaguars.

The down side to this of course is that the close bore centres are starting to show problems of their own due to having thin liners and limited amounts of supporting material around them at higher levels of boost.

I don't think that I have heard of anyone suffering a crankshaft failure to date on a B6304 yet though? My main concern was not having a rod let go if the stock rpm limit was exceeded by a thousand revs or so.

Next question, can the stock ECU's inbuilt limiter be overcome or its onset delayed by some means, or does this require an aftermarket system?
 
Do you have a torque curve for the engine? What transmission?

My current engine is still very stock, only really fitted to set up the chassis in preparation for a forced induction motor going in (which should already have happened by now) :oops:

It also has a stock M47 gearbox and a 3.54:1 final drive, but typically uses 15" low profile road legal competition tyres.

It just always seems to be running into the limiter with ease and often at a bad time. Having another 1K rpm or so up my sleeve would be very useful at times.


At least from what you guys say I shouldn't have any concerns about buzzing the stock limiter setting.
 
That is true, but I was primarily thinking about the safe rev limit for the reciprocating components.

Although not ideal in some other ways, I believe that one major advantage of the whiteblock motors having been designed for transverse use in many of their applications is that the cylinder bore centres are closely spaced and the overall crankshaft length has been kept down to an absolute minimum.

Doing this helps get around some of the crankshaft issues related to the rocking couples balancing each other out along the length of a typical straight six engine crankshaft. Failures of just this kind are almost a tradition on some older big I6 motors like the old Jaguars.

The down side to this of course is that the close bore centres are starting to show problems of their own due to having thin liners and limited amounts of supporting material around them at higher levels of boost.

I don't think that I have heard of anyone suffering a crankshaft failure to date on a B6304 yet though? My main concern was not having a rod let go if the stock rpm limit was exceeded by a thousand revs or so.

Next question, can the stock ECU's inbuilt limiter be overcome or its onset delayed by some means, or does this require an aftermarket system?

Ahh. Well, thats cool. FWIW, I don't often see BMW crank failures either due to this. It's everything else that comes apart. On S52's and M54's especially (same crank), some people try to rev them to 7500 or higher and I've seen sheared keyways, sheared oil pump shafts, oil pump nuts backed off, flywheel bolts backed off, etc. The vibrations it causes thru the rest of the engine are amazing. Whats even better were the morons who thought it a good idea for awhile to remove the TV damper for a ltw crank pulley :run:


So - sorry. Was just trying to point out that a longer crank 6 has much more to be considered re: max rpm's than a 4 cylinder. If there's concern with rods coming apart at a certain rpm, are we talking about the bolts backing out and the cap coming off? This is apparently 'common' in LS engines that try to rev out past 6500 or so, the stock rod bolts can't take the additional stresses.
 
So - sorry. Was just trying to point out that a longer crank 6 has much more to be considered re: max rpm's than a 4 cylinder. If there's concern with rods coming apart at a certain rpm, are we talking about the bolts backing out and the cap coming off? This is apparently 'common' in LS engines that try to rev out past 6500 or so, the stock rod bolts can't take the additional stresses.

No need to be sorry, that was a good point, its just that as far as I'm aware it isn't normally an issue on the 6 cylinder whiteblock?

I've thankfully never had a rod failure yet either, but just on visual appearance alone they don't look to be particularly strong. This being Turbobricks I guess most people are going to be more concerned with them bending under extreme compressive loading rather than them tearing apart under tensile loading at higher than normal rpm.

I would have thought that most people planning on running at rotational speeds much in excess of 7000rpm would have switched to something like the RSI H-beam rods as a matter of course.

Ironically, these sort of revs on a mildly boosted turbo engine are probably considerably less stressful to the rods than on an equivalent N/A engine at the same engine speeds due to the extra cushioning effect of the end gas pressure around TDC on the exhaust stroke?
 
Ahh. Well, thats cool. FWIW, I don't often see BMW crank failures either due to this. It's everything else that comes apart. On S52's and M54's especially (same crank), some people try to rev them to 7500 or higher and I've seen sheared keyways, sheared oil pump shafts, oil pump nuts backed off, flywheel bolts backed off, etc. The vibrations it causes thru the rest of the engine are amazing. Whats even better were the morons who thought it a good idea for awhile to remove the TV damper for a ltw crank pulley :run:


So - sorry. Was just trying to point out that a longer crank 6 has much more to be considered re: max rpm's than a 4 cylinder. If there's concern with rods coming apart at a certain rpm, are we talking about the bolts backing out and the cap coming off? This is apparently 'common' in LS engines that try to rev out past 6500 or so, the stock rod bolts can't take the additional stresses.

Meh. These motors have a 90mm bore spacing. The crank's not really that long.

Piston speed and/or rod ratio become an issue before the crank will.
 
Aren't the hydraulic lifters more of a concern at sustained higher rpm? Kenny - did you switch to solid lifters on yours?

I run my T5 to 8500+, but I have solid lifters. I also switched to the longer rod (147mm)/short skirt piston setup.
 
Aren't the hydraulic lifters more of a concern at sustained higher rpm? Kenny - did you switch to solid lifters on yours?

I run my T5 to 8500+, but I have solid lifters. I also switched to the longer rod (147mm)/short skirt piston setup.

I kept the hydraulic lifters, because with the 6 cylinder, the hottest stock cam profile is the early 960 cams.

That said it's worth noting there's a big difference in valve train between the 960 motor and the T6, and the valve train continues to get better as the motors get newer.

If I were building one of these motors today, I would use the latest head I could, with solid lifters and the smaller valve stems.
 
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