2500 Engine Red Line
Discussion
Hi,
Does anyone know what the red line is supposed to be for a standard 2500 engine? I had previously thought it was 5,500 rpm but recently saw an Autocar road test of a 2500S, where they state it is 5,800 rpm. The 1968 Autocar road test of the TR5 suggests the red line is 5,500 but they took it to 6,000 for their performance tests.
Perhaps I'm being being cautious (and pedantic) but with the long stroke 2500 engine, could 6,000 rpm do some harm?
Chris
Does anyone know what the red line is supposed to be for a standard 2500 engine? I had previously thought it was 5,500 rpm but recently saw an Autocar road test of a 2500S, where they state it is 5,800 rpm. The 1968 Autocar road test of the TR5 suggests the red line is 5,500 but they took it to 6,000 for their performance tests.
Perhaps I'm being being cautious (and pedantic) but with the long stroke 2500 engine, could 6,000 rpm do some harm?
Chris
used to have my rev limiter hard cut at 7k,but like you I thought better for it being a long stroke engine it may not like it in the long run so pulled it back to 6250rpm.i don't really hit it that much anyway especially on the road and it is a bit futile to carry on revving passed the engines peak power output when ideally you should be changing up a gear(if you have another one
)could get a dyno run to see where peak power is occurring as even with a known cam it may not be accurate.
ETA my engine isn't standard so the 7k limit wasn't too sky high to start with.
)could get a dyno run to see where peak power is occurring as even with a known cam it may not be accurate.ETA my engine isn't standard so the 7k limit wasn't too sky high to start with.
Edited by garagewidow on Monday 26th June 17:05
A limiting factor for engine speed is the mean piston speed. It should be the acceleration of the piston as it move up and down the bore, but the mean speed is much easier to calculate, and 20meter per second is he usual limit for a production engine.
Triumph six-cylinder 2L engine stroke = 2.992”(76mm)
At 6000rpm, the piston travels from top to bottom of the bore in half a revolution:
Duration of stroke = 60/6000 x 2seconds = 0.005 seconds
For the 2L engine, mean speed of piston = stroke/duration (in seconds)
= 76/0.005
=15200mm/s
= 15.2 meters/s
Extend the revs and the numbers look like this:
2L, 76mm stroke
RPM Mean speed in meters/sec
6000 15.2
7000 17.7
7500 19.0
2.5L, 95mm stroke
6000 19.0
6500 20.84
So a 2.5L breaches the 20m/s rule at 'only' 6.5K, while the 2L can go up to 7.5 and stay below.
An estimate of the acceleration shows why this is important. A standard 2.5L piston weighs 346grams on the bench, but when accelerated by rapid revs as above would 'weigh' nearly 900 KILOgrams!
No doubt garagewidow has lightened his pistons, strengthened the conrods and even gone for a steel crank, all mos that can allow the 2.5L to rev safely. It'll never get near F1 levels though, such as the 2014 Renault F1 engine, that with a stroke of only 53mm but revving to 15K had an MPS of 25.6!
Stick to a limit of 6000rpm in your 2.5L!
John
Triumph six-cylinder 2L engine stroke = 2.992”(76mm)
At 6000rpm, the piston travels from top to bottom of the bore in half a revolution:
Duration of stroke = 60/6000 x 2seconds = 0.005 seconds
For the 2L engine, mean speed of piston = stroke/duration (in seconds)
= 76/0.005
=15200mm/s
= 15.2 meters/s
Extend the revs and the numbers look like this:
2L, 76mm stroke
RPM Mean speed in meters/sec
6000 15.2
7000 17.7
7500 19.0
2.5L, 95mm stroke
6000 19.0
6500 20.84
So a 2.5L breaches the 20m/s rule at 'only' 6.5K, while the 2L can go up to 7.5 and stay below.
An estimate of the acceleration shows why this is important. A standard 2.5L piston weighs 346grams on the bench, but when accelerated by rapid revs as above would 'weigh' nearly 900 KILOgrams!
No doubt garagewidow has lightened his pistons, strengthened the conrods and even gone for a steel crank, all mos that can allow the 2.5L to rev safely. It'll never get near F1 levels though, such as the 2014 Renault F1 engine, that with a stroke of only 53mm but revving to 15K had an MPS of 25.6!
Stick to a limit of 6000rpm in your 2.5L!
John
Edited by tapkaJohnD on Tuesday 27th June 12:04
Edited by tapkaJohnD on Tuesday 27th June 12:05
Edited by tapkaJohnD on Tuesday 27th June 12:10
you may be right but the main problem with the 2.5 is that it uses the same rods as the 2.0 so you have a bad rod to stroke ratio which increases the thrust loads on the piston against the cylinder wall.
if only they had increased the block height by some 20mm in conjunction with longer rods it may have revved almost the same as the 2.0,but we know it wasn't practical and too expensive for triumph to do this.
if only they had increased the block height by some 20mm in conjunction with longer rods it may have revved almost the same as the 2.0,but we know it wasn't practical and too expensive for triumph to do this.
I owned a few 2500 saloons and I can tell you that the red line on the tachos was 5500 rpm, if that helps.
Frankly, the ordinary carby engine is running out of puff by then anyway.
If you've got hot cams, big valves and decent PI or big carbs then it's a different story.
Frankly, the ordinary carby engine is running out of puff by then anyway.
If you've got hot cams, big valves and decent PI or big carbs then it's a different story.
Edited by RCK974X on Thursday 29th June 06:32
RCK974X said:
I owned a few 2500 saloons and I can tell you that the red line on the tachos was 5500 rpm, if that helps.
Frankly, the ordinary carby engine is running out of puff by then anyway.
If you've got hot cams, big valves and decent PI or big carbs then it's a different story.
Thanks again RCKFrankly, the ordinary carby engine is running out of puff by then anyway.
If you've got hot cams, big valves and decent PI or big carbs then it's a different story.
Edited by RCK974X on Thursday 29th June 06:32
Chris
Just spotted a 2500S rev counter on eBay.
https://www.ebay.co.uk/itm/Triumph-Mk2-2-5S-rev-co...
It looks like the red line is about 5750.
Chris
https://www.ebay.co.uk/itm/Triumph-Mk2-2-5S-rev-co...
It looks like the red line is about 5750.
Chris
https://m.youtube.com/watch?v=3Bizy2V5EMY
I say six, maybe 6 1/4 if blue printed and a cam. You need a lot done to go beyond that 😉
I say six, maybe 6 1/4 if blue printed and a cam. You need a lot done to go beyond that 😉
jellison said:
https://m.youtube.com/watch?v=3Bizy2V5EMY
I say six, maybe 6 1/4 if blue printed and a cam. You need a lot done to go beyond that ??
I guess if you really want want revs the 2000 crank is the way to go, I think Kastner ran these up to about 8000rpm. (That can't be right though can it?) I say six, maybe 6 1/4 if blue printed and a cam. You need a lot done to go beyond that ??
I sometimes wonder if a well sorted 2000 wouldn't be better in a TR than the 2500.
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