Max rpm

[steveww]

What is the general feeling on where to set the rev limiter? IIRC the standard rotor arm limiter was 6500? I know it all depends on what has been done to the bottom end but would a standard twinc manage 7000 for the occasional peak?

[iain.hamlton]

Isn't the rev limiter is 6750 for the big valve engine? I think these have the same bottom end as other late engines: square main caps, good con-rods and big-end bolts.

Maybe 6500 is safe for the earlier ones.

best regards, iain

[rgh0]

I thought the rev limiters were all nominally 6500 rpm but variation was often around up to 200 or 300 more (or less) as they were a fairly crude device. The parts manual only lists one rotor arm and thus one cut out setting for all models

The bottom end is safe to around 7000 from catastrophic failure provided all the components are in good condition (and whats "good condition" is a fairly long discussion in itself). The standard rod bolts are the weakest link and wise to replace with ARP bolts or equivalent if planning to set the rev limiter to 7000 rpm.

The other challenge is ensuring you dont go over 7000 rpm as catastrophic failure can happen around 7200 rpm so you want to ensure your ref limiter is accurate and effective and can stop overreving if you miss a gear change near the limit.

Rohan

[MintSprint]

Lotus used to supply road test cars without the rev-limiting rotor arm and period road testers regularly ran well into the red zone on the tacho in search of maximum performance.

One road test that sticks in my mind is John Bolster recording a top speed of 127mph on an Elan that should have red-lined (at 6,500rpm) at 119mph... a quick calculation suggests he must have been pulling over 6,900rpm for long enough for the car to peak at that speed.

As Rohan says, it's generally acknowleged that 7,200 - 7,250 rpm is the point where you start running a serious risk of the standard bottom end throwing a rod, but apart from that I guess it's down to how much risk and accelerated engine wear you're prepared to accept?

[steveww]

Thanks for the info. As I hope to have the car resprayed this year, I will save the cost of a new engine and stick with 6500 as my rev limiter setting on the omex. With the installed cams the extra 500rpm does not give much more power.

[msd1107]

The Road & Track test of a '67 S3 SE Coupe (with 155-13 and 3.55) got 7000, and they complained they couldn't get more! That extra 100 rpm is worth about 2 mph.

David
1968 36/7988

[steveww]

Not a problem with mine, it easily hits the limiter in top @ 6500rpm.

[elanmac]

The rev limit cut out can be changed with the original rotor arm by adding/removing the screws in the spring mounted earthing bar. It would be by trial and error to get the correct revs.

[steveww]

I am using an OMEX electronic rev limiter with shift warning light. IMHO a bit more reliable than the standard mechanical type.

[elj221c]

Steve,

When I bought my Elan back in '75 the PO cliamed that the engine was all steel. I wasn't much bothered either way as the car was a steal(!) but with this erronious information I reved the thing regularly to 7500 and sometimes to 8000.

I did this for about two months befor taking it apart because it was obvious that the cam timing was out and I wanted to find out what was inside.. It went like stink over 4500 but was a serious fire risk at low revs. (serious blowback and lots of petrol out of the carb trumpets)

When in pieces I found an ordinary crank, which I expect was had been toughened, 125 rods, short jackshaft, Cosworth forged pistons and BRM phase 3 cams timed on the standard sprocket timing marks. The big end bearings were crazed so I don't suppose the crank would have lasted long.

I rebuilt it with new rings, bearings plus Cosworth bolts and properly timed cams and it managed another 45k befor I changed to a crossflow blocked 1700. The original engine was very sweet and free reving to 7000, occasionaly to 7500, much nicer than the 1700.

Just for interest, I have the spec sheet for the Cosworth Mk 12 which quotes 140 bhp at 6500 on L1 cams, 83.5mm bore (1594cc) oil pressure 60-70psi running and 20-25psi tickover when hot. Max revs 7000 but 6500 in first and second gears. This on a cast iron crank and flywheel, balanced but no mention of toughening.

Oh, they also quote 95-100bhp as standard so the 85 at the rear wheels in another posting doesn't sound too bad.

Roy
'65 S2

[elj221c]

Steve,

When I bought my Elan back in '75 the PO cliamed that the engine was all steel. I wasn't much bothered either way as the car was a steal(!) but with this erronious information I reved the thing regularly to 7500 and sometimes to 8000.

I did this for about two months befor taking it apart because it was obvious that the cam timing was out and I wanted to find out what was inside.. It went like stink over 4500 but was a serious fire risk at low revs. (serious blowback and lots of petrol out of the carb trumpets)

When in pieces I found an ordinary crank, which I expect was had been toughened, 125 rods, short jackshaft, Cosworth forged pistons and BRM phase 3 cams timed on the standard sprocket timing marks. The big end bearings were crazed so I don't suppose the crank would have lasted long.

I rebuilt it with new rings, bearings plus Cosworth bolts and properly timed cams and it managed another 45k befor I changed to a crossflow blocked 1700. The original engine was very sweet and free reving to 7000, occasionaly to 7500, much nicer than the 1700.

Just for interest, I have the spec sheet for the Cosworth Mk 12 which quotes 140 bhp at 6500 on L1 cams, 83.5mm bore (1594cc) oil pressure 60-70psi running and 20-25psi tickover when hot. Max revs 7000 but 6500 in first and second gears. This on a cast iron crank and flywheel, balanced but no mention of toughening.

Oh, they also quote 95-100bhp as standard so the 85 at the rear wheels in another posting doesn't sound too bad.

Roy
'65 S2