Rev her up to what?

I seldom take the revs above 3600 because the torque curve peaks in that region…but recently I took it up to 4500 and thought the engine sounded like a threshing machine…see no reason to do that again…its not exactly a free reving machine…what do you all do?

I never much liked that sound either, TBH. Mine will happily go there, and has no particular vibrations or anything. I just don’t like the sound, plus it completely overshoots gears. I do it twice a drive once warmed up and then a few miles from home just to burn off carbon.

'Bout the same: a good engine is in no danger at 4500, and even up to 5000, but, overall, on the street, 3500-4000 is optimal.

With Tweety’s essentially open exhaust, the sounds from 4000 to 5200 were wondrous!

My view is that as long as you’re confident in the engine, high revs are good. It will stop ridging in the bores, and glazing to a degree.

And as Paul says, the noise between 3k and 5.5k is amazing.

And if I’m wrong and your engine blows up, well then you get the fun of rebuilding it!


Mine is a 4.2 and doesn’t like higher revs. Over 4000 it sounds or feels like a bag of hammers in an oil drum. This is OK as torque below 3000 rpm means I never use high revs in lower gears and over 3000 in top looses licences quickly.

One day I’ll rebuild the engine, then I’ll discover why it won’t rev smoothly. I expect to find mismatched or bent conrods, imbalanced pistons slapping about in poorly honed cylinders, out of balance crank, and most other components out of tolerance. And a severely depleted bank balance, regardless of how much work I do personally.

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Over about 3800rpm mine gives the impression it‘s about to burst or explode. I always thought it should be smooth up to 5000, like everone else‘s. Didn’t know there are more having similar feelings :wink:

Looking at a rebuild to make this work.

To be fair to both Martin and Chris, I have personally rebuilt the engines in all of my cars, with supervision from someone who has built hundreds of them. The machinist I use is very precise, and everything gets balanced to the nth degree.

Having done it right and spent the money I feel confident to push the engines to the maximum safe limits that Jaguar suggested. If ,as you say, your engine doesn’t feel good at higher revs it is sensible not to push it.

If it is mechanically sound there is no reason why it wouldn’t run to 5000 and stay up there. I have fuel injection but it did not do harm and it sounds fine up to where it shifts and pulls strong. Martins’ clearly has a problem. It sounds as if the timing chain is all over the place, we are curious if we can find the source.

I expect that how the engine sounds has a fair bit to do with the acoustics management in individual models. Our '72 XJ6 4.2 sounds great and fairly quiet at 4000 plus after about 10,000 miles since rebuild. Our E 3.8 (4,500 miles) is definitely noisier at the same rpms but I don’t have any concerns, just different intention and design, I rather revel in it.

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In my experience, 4.2s are not “happy revvers,” whereas 3.8s are.

Pete, my 4.2 sounds really good going to 5000. It sings up there - tempting me to go into the red zone. Thankfully the torque signs off at 4k, making it less attractive to do. I sense it’s running out of “breath” over 4.5k. “Turns gas into noise” up there but it’s good music!
I can only imagine that a XK race engine, ported, cammed, and carbed sympatically would be fun to experience…for a while.

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Hate to idly speculate about any problem, but “noise” may be a reduction in smooth acceleration or sudden perceived lack of power. Rather than jump to a conclusion that you have valve float or something I guess I’d look into ignition advance (or lack thereof) or some carb issue, with ignition more likely I’d guess. Straining to rev past what should be well within an engine’s range could seem like excess noise or mechanical issues where it may be strangling from some fuel or ignition problem. All of us can tell the difference between noise and strangling, but when we’re all tied up in driving our cars and spending so little time above 3 800 rpm we may imagine noise where just crappy running exists.


Well, I’m not one to push my cars to their limits. I don’t think I’ve had the 4.2 above 3800-4000 in the short time I’ve owned it (500 miles). It feels like I should shift by then. It has so much grunt up to those rpms I haven’t felt the need to go further, and, as others have pointed out, it doesn’t really “sing” at those rpms. But, I think I’ll give it a try now that I’ve read this thread.

All that said, here are some of my other observations. I have a '61 MB 190 SL (4) and a '67 MB 250 SL (6). They don’t sing above 4K either. They’ll go beyond that and are fine, but they seem much happier below 4K. I also have a '64 Porsche 356C (4) and a 70 911E.(6) I have to be careful on the 356 not to hit the 5K redline, it revs up there so freely. Other than the increase in volume, you’d not know if the engine is turning 2K or 5K. And, it never misses a beat. The 911E, well, the higher you go, the more she loves it. 5500 is when she starts to sing, I think 6800-7000 redline. This car does not like low rpms, keep it at 3K or above at all times. Putting your foot into it and getting on a barriered on ramp at 5500 rpm is a very special sound, all Porsche.

So, i guess it may boil down to the brand. MB is luxury, boulevard cruisers that like 4th gear and cruising, tight feeling engine. Porsche wants to fly, use all the gears, keep them moving, and step on it, very free-wheeling engines. E-type seems to fall somewhere in between for me, and I love driving the E.

Except for that stiff clutch of mine.

4.2 Doesnt rev?
40-95mph in third no dramas, then again my piston and rods are with a gram of each other on a balanced crank assembly with a lightened flywheel…


I refer to it as a “thrashing” sound. It makes me feel like the tach is wrong and I am at much higher revs.

From memory:
I notice a great difference in backing off the timing just a a degree or two when low octane gas is around and putting the timing back up. Much more willing.

a lot of this is psychological, I can “feel” those heavy con rods and flywheel.

I hope I get to experience this again.

Norman Dewis in his video interview about the Belgian high speed run said he “ran it up to 5500 every shift and at the end of the timed distance had 7400 RPM and, somehow it stayed together”.

For really wild revs, no car other than Italian exotica quite matches the banshee wail of the V-Tec engine in my S2000 as the revs hit 6750 and it STARTS to fly. Red line 9000rpm.

But for a real rev happy engine you need to be sitting on the engine with handlebars to steer. Try 200bhp at 14000rpm in a typical one litre crotch rocket. Now if that sort of engine exploded you can say goodbye to your wedding tackle/

It might be: have you checked the tach’s accuracy?

That’s what led–partly–to Jerry Mouton grenading an engine.

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yeee haaa… I love it. I drive mine like I stole it. life’s too short.

You might check your cam timing. My 4.2 was very unhappy (and gutless) above 4000 RPM until the cam timing was dialed in. Now it revs freely to the red line. That said, there isn’t much reason to rev it above 4000 in normal driving conditions.


I hit 50 odd mph in first gear (around 5500 rpm or a bit more) in my 4.2 coupe recently. Wanted to keep going.

I had to back off in 2nd pretty quickly to keep below 70 mph then just short shifted into 4th. This engine has a bit over 2000 miles since I rebuilt it.