Max safe revs

A question for the engine experts amongst you — what is the maximum safe revs for a standard 850 Commando? I have seen various figures mentioned in this forum, some of which are higher than Norvil's spec on its site of 5,800 for the 850 and 6000 for the 750. For example, some rev their 750s to 7000 rpm. Is Norvil being very conservative, or is there a serious risk of component failure above the Norvil-quoted figures? What max revs do you take your 850s up to through the gears and for sustained cruising? Is it even worth revving a standard 850 over 5000 rpm? This is assuming a bog standard bottom end, standard camshaft and an engine that is in good condition. If you were not sure when the bottom end was last apart, but it seemed to be going nicely, what safe rev limit would you advise?

Or better yet, under what circumstances on the road do you want to be at 7000 rpm?

Dropping it to third and hammering it to get around a truck moving 10 below the limit on the interstate?

I've revved my Interstate above 6,500 and it was still accellerating, although tapering off.

daveh said:

what is the maximum safe revs for a standard 850 Commando?

Click to expand...

The 750/850 Commando owners' handbooks give the following information:


"WARNING: DO NOT EXCEED 7,000 RPM"


Peak power for the 850 is given as "5,800 RPM", and "6,500 RPM" for the 750, so there's not much point revving either model much over its peak power RPM figure.

I have generally found that 6000 is easy. 6000 to 6800 tends to shorten the life of stock cranks, rods and cases and above that you are risking a flywheel explosion which can be really messy and dangerous. Lived through it twice and never want to do it again. James Comstock.

Oh and you might want to check your tach calibration before you wring it out. I have seen many of them off by as much as 1000 rpm at 5000.

Thanks for your advice, guys. Since I bought my Mark 1 850 two years ago, I have been cruising at no more than 4500 rpm, and mostly at 4000 rpm, and revving it to 5000 through the gears. Perhaps I could rev it a tad more, but as LAB says, there's no point in revving it beyond 5,800 rpm. I'll live with that.

Jim, I'm glad you survived the flywheel explosion. No way do I want to experience that.

Dave

I regularly rev my 850 up to 6500/7000rpm. I am running a 4s cam, and so get power slightly higher up the rev range from it, or so I believe. It may not be good for the engine but it sounds great I put new conrods in three years ago, so hopefully they will survive OK, I'll just have to hope that the crank stays in place :wink: Maybe I ought to ease off a bit?

What I have found is that sustained cruising at 70/75 mph drops my oil pressure to just less than 10psi per 1000 rpm and that's running straight 50 oil. If I drop the speed off very slightly, the oil pressure seems to recover very quickly to well above 10psi per 1000rpm. Possibly there is some wear on the big ends, but based on this, I wouldn't cruise long distances any faster than 70ish mph or about 4000 rpm.

hi all,iv,e seen a math somewhere but i cant find it ,it involves piston speeds and stroke lengh to determine max safe revs for a given engine ,im sure from memory its around the 6800 mark for a commando?

Reggie — that's interesting about your oil pressure dropping at 70/75 mph, but you didn't say what revs this equates to. And is this pressure drop common on Commandos at these revs? I run a 21 tooth front sprocket, the clocks indicating a tad over 70 at 4,000 rpm. Glad to hear you're using your bike like the lads who bought them from new!

James, thanks for the tip on calibrating the tacho. I have aftermarket Domiracer clocks, made in Japan, so perhaps they are more accurate than the originals?! I don't plan on wringing her out, but it's good to know. Do you have an oil pressure gauge on your bike? I remember seeing a thread on this subject on this forum. I should find it and read it.

Dave

It is pretty common for the oil pressure to drop after some time at high speed. The oil gets hot and thin and the rod clearances grow so the pressure goes down. An oil cooler helps.
I do have an oil pressure gauge on my injected bike. Many years ago after I had a cam failure I decided to rifle drill the cam with oil delivery holes in the lobes. I ran an oil line to the end of the cam with a small idle jet for an orfice. The extra oil used by doing this made my oil pressure go to zero when the motor got hot. Then I took a chunk of aluminum and some gear steel and whittled out a new oil pump. I made the clearances much tighter and widened the gears by about .080. Now I have 40 lbs at hot idle and 60 lbs down the road. Don't know if it helps anything but it makes me feel better.
I have a medium cam in my motor at 880 cc with a steel flywheel and Carrillo rods. I have the rev. limiter set at 7200 rpm and its easy to bounce off it. It hits the rev limiter at around 130 in high. The present motor has a lot of miles on it and has spent a considerable amount of time on the dyno. It is also getting very noisy so I have a new motor going together on the bench. Hopefully the old one will last long enough to get the new one finished.

daveh wrote;

Reggie — that's interesting about your oil pressure dropping at 70/75 mph, but you didn't say what revs this equates to.

Click to expand...

I am running a 21t gearbox sprocket, so the same revs as your bike.

daveh wrote;

Glad to hear you're using your bike like the lads who bought them from new!

Click to expand...

I like to enjoy it as much as possible, so I ride it reasonably hard, but am always aware of things such as oil pressure/ temp and try to be fairly sensible.