Maximum RPM with Hydraulic Lifters on Turbo Motor?

[rukman]

Since we seem to be in a more technical mood these days on the forum....


It seems most tuners set the redline at 7000rpm.

In the past i had heard that max safe rpm was 7200rpm for stock S50/S52 cylinder head.

Recently I've come across opinions that a rebuilt head with better retainers and stiffer valve springs could push this limit to 7500rpm. Beyond that your better off changing over to solid lifters.

Has anyone run a setup that revs beyond 7000rpm?

If we are analyzing from just a cylinder head perspective only, i would think that although 7500rpm might be permissable with new retainers and stiffer valve springs, it could be at the cost of longevity and durability....

Even if the cylinder head is capable of 7200 or 7500rpm, we are also faced with increased loads on the piston/rod/crankshaft assembly at these higher rpms. If i remember correctly, the size of the load on the connecting rod is proportional to the rpm of the engine squared. So obviously taking it past 7000rpm on a stock bottom end might not be the wisest choice for longevity either due to the significant increase in forces as the rpms rise.

discuss

[gol10dr1]

wasn't shawnsm talking about building his head to withstand revs around 8000 rpms. he was the one talking about a gt42r setup in his m roadster. last he checked in, he was actually starting on the project. maybe he can chime in.

[96cosmosM3]

Even if the cylinder head is capable of 7200 or 7500rpm, we are also faced with increased loads on the piston/rod/crankshaft assembly at these higher rpms.

Exactly, these engines got a not so good rod/stroke ratio. Maybe some longer rods and pistons with moved wrist pins would be a good idea for someone that want to rev high without punching a rod thru the block.

[Lohe]

You need to look at the pistion speeds at those RPM's I will try and do the math lthis weekend and give the numbers. Plus a down side of going solid lifter is you need to maintaine them ( valve lash) thats what makes the hydro nice. ( e30 and E46 M's have to be adjusted)

But the stiffer springs will help keep you from floating the valve.

[rukman]

You need to look at the pistion speeds at those RPM's I will try and do the math lthis weekend and give the numbers. Plus a down side of going solid lifter is you need to maintaine them ( valve lash) thats what makes the hydro nice. ( e30 and E46 M's have to be adjusted)

But the stiffer springs will help keep you from floating the valve.

this is why i broke it down into a 2 parts...i think even if the head can safely rev to 7200 or even 7500rpm with Hydraulic Lifters, i'm not sure i'd attempt it with factory bottom-end.

Now, if your talking a built bottom-end...ARP main bolts, Forged Rods with ARP bolts, forged pistons...then perhaps a different story. As i've stated before, force goes up significantly as rpms rise...so a 500rpm increase from 7000 to 7500 is going to be exposing the rotating assembly to alot more stress.

On a related note, I've heard Porsche Motorsport were having rod failures at higher rpm not due to the rod bending, but the factory rod bolts failing. However a switch from factory rod bolts to ARP bolts supposedly cured the problem. This was on race motors running to 8500rpm i believe...but then again..how often do they rebuild those suckers.

i'm thinking perhaps built bottom-end as suggested above with rebuilt cylinder head could safely rev to 7200 with perhaps decent longevity and reliability. 7500 might be pushing it...

if you have time to do the calcs...

Perhaps you can explore the idea of built bottom end using stock rod length, wrist pins locations, etc for the calcs. I do know that switching to an aftermarket forged piston/rod combo saves about 60 grams per cylinder i believe...I think factory s52 piston/rod assembly is aprrox 960 grams per cylinder.

I know the move to solid lifters would allow us to rev higher, but this just brings us back to the question of increased forces on rotating assembly again. A closer look at the s54 motor probably is in order to see what bmw has done. Right of the bat, the beefier connecting rods are readily apparant vs the S52.

[DakarE36]

FWIW bimmerworld is going to start selling drop-in cams made by sunbelt for s50/s52 motors that safely rev to 7500. do a search for sunbelt

[rukman]

FWIW bimmerworld is going to start selling drop-in cams made by sunbelt for s50/s52 motors that safely rev to 7500. do a search for sunbelt

just checked it out...from the website...

The high lift profile opens the door to new levels of power - 20Hp more than any other cam we have tested. Extensive attention to lobe shape allows the engine to rev safely to over 7,400 RPM - with stock pistons and valvetrain, street or track

interesting....obviously the Sunbelt cam setup is for a N/A engine...so more lift via altered lobe profile...the real question...beyond increased lift offered by the cams, is there something inherent in the lobe profile design that allows higher rpm capability that perhaps the stock lobe design doesn't regardless of the lift each one generates???

in the BMW FI world most everyone seems to have settled in on 7000rpm...i hear a few upcoming build-ups venturing to the 7500rpm mark...i vaguely remember from my e36 digest days as 7200 being the magic number for max rpm...

of course with the kind of #'s being generated these days we have become kind of like the old big block philosophy...why do you need to rev when you can make so much power anyway...but i recently saw some video of a modded skyline revving close to 9K nothing like the sound of revs + boost

[bmwperson]

just checked it out...from the website...

interesting....obviously the Sunbelt cam setup is for a N/A engine...so more lift via altered lobe profile...the real question...beyond increased lift offered by the cams, is there something inherent in the lobe profile design that allows higher rpm capability that perhaps the stock lobe design doesn't regardless of the lift each one generates???

from what I understand, the "thicker" the lobe the more duration there is, the valve is opened for longer which takes time away from the time that the valves are allowed safely to close... now if you make thiner lobes you have more time to open the valve more (higher lift) Im guessing there is a compromise between those 2 properties that gives you the best performance.

Now ofcourse I have no idea what Im talking about

/back to sleep walking

[ParadigmGuy]

My rev limit will probably be around 8K, where ever the turbo stops making power.