Jump to content
SAU Community

Recommended Posts

Is my engine under more stress if i run the same boost (possibly less), the same rpm, the same approx A/F values but make more power. This also assumes that my radiator and oil cooler are suitably sized to remove any additional heat input.

Reason i ask is that i was told my engine may not handle the extra power that my turbo setup makes if i installed some cams. If im increasing the effeciency of a motor am i stressing it more?

Link to comment
https://www.sau.com.au/forums/topic/51165-stresses-on-engines/
Share on other sites

ooh ooh a classmate :D

 

Was Roy a real nerd then? ;)

I was always too dumb to be a nerd :( And surprise, surprise too busy talking about the weekends pi55 up :D

I can do the mechanics of it all, but basically ; more air, more fuel more bang = more power.

The more bang is the concern. :Oops:

But if i keep revs sane, ie std at 7,200rpm, oil and water temps under control and only run 1.2bar, would having a set of cams that take power from 235-240 to 245-260rwkws really hurt a std RB20 that much more then running the extra 0.2bar of boost (i have my scramble set to 1.4bar...so when its pointed in the right direction i give it hits from 1.1 to 1.4 bar for the straights) :Bang:

Just added up what this whole RB26 cam farkup has cost me, :)

There is probably an energy equation in there related to each cylinder i'm guessing.. and the amount of time that energy remains in the cylinder (which would be controlled by the duration of the cams amongst other things)

Unfortunately I forgot my maths nearly 10 years ago :D freakin computers...

it would be easy to say there would be less stress on the engine when gaining power from cams then rasing boost as they both increase air flow but cams dont increase heat, like adding more boost.

I think that hold's true because what i have read of SK's posts being able to run reliable 250kw with cams and stock injectors but going about making 250kw with a fuel reg and boost the pistons burn and die. So 200kw becomes the reliable power limit.

SO GO THE CAMS!!! :D

Thermal stress, or mechanical stress ? they are quite different.

You can quite easily blow up a stock engine by leaning it out, as we all know. Thermal stress is probably the most dangerous with a mildly modified engine, if it is badly tuned or not tuned at all. Detonation damage is another very quick killer of turbo engines. None of this has anything to do with the basic strength or integrity of the engine itself.

Mechanical stress is a completely different thing. Rpm and inertia forces are what most likely are going to break things and that has little to do with power output. If it is properly tuned and you stay within the factory redline it can make significantly more power without any additional MAXIMUM mechanical loading. The average loads will be up, and the torque be higher, but that will not break things, except maybe the clutch.

Bigger cams and valve springs might be dangerous if you then expect to wind it out to 10,000 Rpm all the time, or buzz the engine by dropping back to second gear at 160 Kmh. An extreme and silly example, but you get the idea.

LOL, its an internal combustion engine, and ppl never meantion cyclic loads:)

I may just wait 3 months and 10-15,000kms to see how the wrecker engine goes before i get the courage to lean on it a bit more with cams. Want to eliminate as many variables as possible so if the engine goes bang i at least have an idea of why.

But the power cannot go up at the same rpmif the torque doesn't, which means that there has to be more load on the piston, conrod and crank (as well as all the inbetweens) and so it goes. If it was RPM increase alone then sure, inertial forces increase, but more power at the same rpm means there is more air going in, therefore more fuel required for a stoich mixture, so a bigger bang must result.

I'd run a little extra power until you are able to get an oil change/oil analysis done, then plan the next power increase. Fuelling is not an issue for your setup though mate.

Your call.......

This is a interesting topic

Whats everyone's general opinion about the safe power level (assuming all the support systems are up to scratch) for each RB motor?

my understanding is -

RB20 - 240-250rwkw with around 7500 as a 'safe' RPM

RB25 - 290-300rwkw with around 7300 as a 'safe' RPM

RB26 - 300-320rwkw with around 7800 as a 'safe' RPM

I think revs and detonation are the real killer of RB's, keep both of them under control and I reckon there is no reason Roy your engine would be under any great deal of more stress

Chris

This is a interesting topic

Whats everyone's general opinion about the safe power level (assuming all the support systems are up to scratch) for each RB motor?

my understanding is -

RB20 - 240-250rwkw with around 7500 as a 'safe' RPM

RB25 - 290-300rwkw with around 7300 as a 'safe' RPM

RB26 - 300-320rwkw with around 7800 as a 'safe' RPM

I think revs and detonation are the real killer of RB's, keep both of them under control and I reckon there is no reason Roy your engine would be under any great deal of more stress

Chris

Hi Chris, I would be inclined to believe that the RB20 safe rpm limit is more like 8,000 rpm. Ours has been at 225 rwkw for over 2 years and we run it to the rev limited 8,250 rpm all the time. I would add heat to your list of RB killers, especially oil temperature. But by far the worst thing for R's is poor tuning.

:cheers:

Ok cool

mine currently has a rev limit of around 8200 with my re-programmed ECU

What would be a sensible every day rev limit for a RB20 running about 18 psi boost

Agree re oil cooler, I put one on early on, perhaps thats why mine has survived as long as it has

Theoretically with a set of larger cams the dynamic compression ratio is lowered. Reducing the thermal stress and detonation issues.

The motor 'should' be a little more forgiving.

If a rod is going to let go it will let go. Not because of the cams making a little more power.

A good set of cams help the combustion process become more controlled and less likely to detonate.

It may explain why Steve's old R33 held 320rwkw without the usual issue of cracked ringlands.

Lowered comp 8.6:1 and a set of cams.

EDIT: Cam selection for NA is fairly straight forward being the larger the cam the higher the static comp needs to be in order for the DCR to be in a good range to suit the fuel. (otherwise it will make less power)

With Forced induction it throws another variable in to the equation.

Joel is spot on. Have a look at the Mazda Miller engine, it will amaze you. Although supercharged not turbocharged, this engine has a static compression ratio of 10.2 :1 and runs 18 psi boost from the factory. And they don't detonate either.

The reason being the inlet valve closes 72 degrees after BDC. The piston is almost half way up the bore before the inlet valve closes, meaning only half the actual piston stroke compresses anything.

Think about it. If the theoretical compression ratio is 10:1 and if only HALF the stroke length is used it would only actually be 5:1. It is not quite that low, but you get the general idea. It is not that simple because of con rod angles and piston motion, but late inlet closing does reduce compression. The cranking pressure with a compression gauge will give a pretty good idea of this effect as well.

As Joel says, some of the normally aspirated racing engines run 16:1 compression ratio, but the cams are pretty extreme too.

With a turbo, what stuffs things up is exhaust back pressure, which is nearly always more than boost. On a stock factory engine it might be twice boost pressure. If you have turned up the boost yourself it might be closer to three times boost pressure.

A large overlap cam simply is not going to work with a small turbo. Small turbo, small cam. Big turbo big cam is the general trend.

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now


  • Similar Content

  • Latest Posts

    • So, I started this repair and got as far as "fixing" the holes with some fibreglass. God all those years working on boats came back quickly. I decided I'd reach out to some rust guys just to see what they would say about it. I came across a guy about 40 mins away and went to see him. He said the windscreen needs to come out, that there might be some more bits around the windscreen and he'd quote them at the time. But his quote was $300 to remove and replace windscreen and $3k for the damage he can see. He said he could respray the roof for $1200 and the bonnet for another $800 (somebody has previously rattle canned it, its horrendous). This is $5300 + any small additional bits. It's a lot, I get that and the name of one of my fave youtube channels 'Not Economically Viable' comes to mind.  I'm not being financially rational, but I've taken him up on the quote. He's opening a new shop in November with more room, so we're waiting for that. I'll leave the currently missing headliner out until then. I'm looking forward to it being fixed and having the paint looking nice again (lots of clear coat issues on the roof too). / flame suit on.
    • Oh and some up-and-comings; New rear drivers wheel bearing. I'll do that this weekend while the diff is out. The car is already up and the d/c axles and missing exhaust will help with space. This is the last bearing for me to do and I've been dragging my feet on it. I also have some new EBC blue stuff pads for the car and some new brake fluid. I haven't ever flushed the fluid in this car and looking forward to it. I have 600 degree fluid to put in. Not exactly "race fluid" but better than the typical stuff I have been using.
    • A proper clutch/plate type mechanical diff with quite a lot of pre-load and high locking % is better for drifting. Much more consistent in its behaviour. A helical can be annoyingly vague and inconsistent in how it responds under the sorts of abuse found in drifting.
    • Some updates here. I pulled the entire interior out, minus some trim to respray the seats with Colourlock dye. It turned out really nice though I accidentally let the dog in the car after and she scratched up the front seat.  This is what it looked like before, the colour was just washing out everywhere but thankfully the leather was in good nic. Then after the respray   And after the bloody dog jumped in The headliner is out waiting to be retrimmed, but it will stay out now until Nov - see why below. I replaced the stereo/headunit with a period-styled Android headunit. I have no after pics, but I'll get some. This is because of the missing pixels. I tried to fix this twice with replacement ribbon cables but couldn't. Also the bluetooth interface I'd bought for this was crap. Then there's the rusty roof. Pics and info in this other thread. I have decided to get this repaired professionally, but I'll update that thread. This is why the headliner will stay out for now. I'll be getting the roof and bonnet resprayed at the same time the rust is fixed. I also had an interesting issue with my drivers door lock.  For a small period I was having issues getting any 12v power to the car - I mean *any*. It would have no dash lights, nothing. It happened while I was at the shops and I couldn't get in the car. So, we had 2 problems. The most pressing here is that I was locked out. I have only a single physical key hole on the car, the drivers door and no amount of turning would unlock the car. Surely it doesn't need power for this? The second issue is why am I losing all power periodically, The battery isn't dead, its almost like the battery isn't even there. Two issues that were surprisingly easy to fix. You fellow BMW over-engineering lovers will appreciate this. The lock in the door has 5 states; mechanical lock, electric lock, neutral, electric unlock, mechanical unlock achieved at -90 degrees, -45 degrees, 0 degrees, 45 degrees and 90 degrees. Although, the unlock is towards the front of the car, so opposite for LHD countries. Sticking the key in and turning 45 left or right is what is used 99% of the time. It activates the central locking etc. 90 degrees is for dead battery access and, obvs, only un/locks the one door. But because the mechanical lock is never used and is 27 years old, it seizes up. I was totally unable to turn the key far enough to get to the mechanical unlock (At the time of locking myself out, I didn't even know this was a thing). I eventually did it with some vicegrips and teflon spray.  I made a quick vid for other E39 peeps.   The battery issue is totally new to me also - It wasn't making sufficient contact between the post and the terminal. The terminal was bolted on tight, but the car wouldn't have power. After checking the battery with my multimeter I accidentally contacted the terminal and the battery post and the car got power. The battery was only a few years old and in good condition. I cleaned the post and the terminal with a wire brush, bolted it back on tight and never had the issue again. I'm still surprised that despite having solid contact it didn't work. Also, the car was getting Warragamba sized pools of water in the back when it rained. My initial concern was another rust problem. But when I went out on Weds while it was raining and while I had no headliner in I could see a steady stream of water coming through the roof mounted aerial. As this aerial is for the (now removed) car phone I pulled it out and whacked a blanking grommet in the hole. It seems fine now. I'm thinking I might get the hole permanently filled when the rust is fixed. Moving forward and things in progress; The tailgate needs some attention. I have taken all the trim off to clean it all and address some small rust spots. I have partially done all of this but I'll finish it up hopefully this weekend. As all the trim are now entirely devoid of trim clips I have bought a heap of strong velcro and I'm hoping it does a good enough job as any of this trim in good condition is super expensive and usually in Europe as we dont have many of the wagons here. Suspension and brakes!!! This is exciting. In the front; New control arms New sway bar links New lower Eibach springs (the only modification I'm planning on this car) M Sport shocks (these came with the car and will replace the longer shocks in the car) New top mounts Used 540i calipers (stock brakes suck!) New 540i disks and pads (22x296 mm for 528 and 30x334 mm for 540i) New front wheel bearings (thank all that is holy for bolt on bearings!) Annnnd in the back; New control arms New sway bar links Adjustable air suspension arms (fool the car into what the current height is so the self levelling suspension can match the new front ride height) New ball joints I'll also be doing a brake fluid flush while I'm in there. I'm planning on switching the car over to the 16's that came with it so I can clean up and respray the M Sport 17's. They've lost a lot of colour over the years and have some gutter rash. None of this will start until the E90 is back.
    • You mean you will regret it for drifting duties? I don't quite follow.
×
×
  • Create New...