Pea Shooter Exhaust.

[Wagger]

My 2.5 Vitesse has a straight pipe 1.75 in O/D with a Pea shooter silencer less than 1.5 in bore. It runs nicely and is quiet enough not to draw attention (Unlike a near neighbour's GT6 that annoys his neighbours).

I am not sure if it is hampering power as there seems to be enough of that. However, am I going to blow exhaust manifold gaskets or cause any problems. I ask because it arrived with a 2 in diameter 2000 saloon centre box only. By jove, that was loud.

From my biking days, I remember that so called 'Back pressure' had advantages, so is there any point in me fitting a silencer with a bigger outlet? As Canley's?

[NonMember]

There is a sweet spot size for best performance. Too small is obviously restrictive but too large results in low gas velocity which hampers the exhaust purge at the manifold. I suspect that 1.5in is about at the sweet spot for a 2L Vitesse but may be a little restrictive for a 2.5 unless it's a low-tune TC engine.

[Wagger]

I knew that you would have the right answer Rob. This is a 2500TC engine and the Number accords with 106bhp and 139 lb ft of torque.

I only drove a 2 litre Vitesse once and seem to remember it revving freely just as my 1600/6 did. This one appears reluctant to rev but  using the high gears early gives swift acceleration. In fact, it is possible to pull away in second and shift directly into 4th. It will attain over 5000 rpm, just seems a bit lazy.

 

[cliff.b]

On a related issue, my Spit 1500 has a standard exhaust manifold but all the rest is non standard and about 2" diameter. The silencer has two outlets and appears to have been tampered with. Based on the volume level,  I suspect some of the contents have been removed. 

The car is running well and has plenty of power at high revs but not as much pull in the mid range as I would like. 

Wondering if the exhaust system could be having some impact on this? 

[Pete Lewis]

think in general the 2500 was never a revvy unit due to the amount of reciprocating mass of the longer stroke  and relies on torque to do the work

same reason on the 1500 4 cly  

 

 

[Firefly]

 

      Its surprising what a small increase in stroke can do to a previously sweet engine. 

[cliff.b]

3 minutes ago, Pete Lewis said:

think in general the 2500 was never a revvy unit due to the amount of reciprocating mass of the longer stroke  and relies on torque to do the work

same reason on the 1500 4 cly  

 

 

That's what I thought, but my car comes alive around 5000RPM and still pulling strongly at 5500 which is the highest I have taken it. I would prefer not to have to rev it that high when overtaking etc.

The thing I don't know is if the engine is standard inside. I suppose if it could have been tuned to some extent but how can you know without stripping it 🤔

[Pete Lewis]

1500 has high mid range torque  overtaking should not need to rev it high to get performance   , i would look at advancing the spark a bit 

it should have some good grunt without having to flog it  thats  the plus point of the 1500  all torque  not revvy 

Pete

[Wagger]

You can measure the compression pressures and check how long the valves are open for without doing too much stripping. There are some nice cameras with optics where you can look down plug holes to inspect piston crowns. That would tell you if compression and camshafts are standard or not. Also measure thickness of the cylinder head, that will tell you if it has been ground off.

[cliff.b]

2 minutes ago, Wagger said:

You can measure the compression pressures and check how long the valves are open for without doing too much stripping. There are some nice cameras with optics where you can look down plug holes to inspect piston crowns. That would tell you if compression and camshafts are standard or not. Also measure thickness of the cylinder head, that will tell you if it has been ground off.

Yes, good points. I have a dial gauge so could probably work out cam lift and duration, or at least decide if it appears to be standard or not.

[cliff.b]

10 minutes ago, Pete Lewis said:

1500 has high mid range torque  overtaking should not need to rev it high to get performance   , i would look at advancing the spark a bit 

it should have some good grunt without having to flog it  thats  the plus point of the 1500  all torque  not revvy 

Pete

I have advanced the ignition a bit and it did seem to make the car a livelier. Maybe I will try a little more. Cars not sluggish by any means but, as you say, feel it shouldn't need revving so much and have been wondering if the exhaust fitted may not be optimised for mid range power. 

[Stratton Jimmer]

1 hour ago, Firefly said:

Its surprising what a small increase in stroke can do to a previously sweet engine. 

Torque is directly related to stroke while horsepower is directly related to bore. Hence for an engine of a given capacity, long stroke = high torque while short stroke = low torque but higher horsepower.

[Firefly]

 

                 That exactly why I chose a 2 litre.

[NonMember]

1 hour ago, Stratton Jimmer said:

Torque is directly related to stroke while horsepower is directly related to bore.

That's a very poor approximation, even if it did form the basis of the RAC horsepower rating by which UK cars were taxed for many years.

[Nigel Clark]

The 2500TC engine has a very tame cam profile, probably the main reason it is reluctant to rev. Also very low compression ratio.

The 2500 engine I built for my GT6 has a better cam from Chris Witor, head skimmed for 9.5:1 CR. The engine has been balanced and fitted with a GT6 flywheel, about half the weight of the original saloon item.

The engine revs freely to 5,500rpm, I haven't been brave enough to take it further. It still has all the torque expected of the long stroke 2.5. And it's smooth, as hoped for with a balanced engine.

The 2.5 litre engine in my CP series TR6 is also smooth and revvy, suspect a PO had it balanced.

Nigel

[thescrapman]

2 hours ago, Nigel Clark said:

The 2500TC engine has a very tame cam profile, probably the main reason it is reluctant to rev. Also very low compression ratio.

The 2500 engine I built for my GT6 has a better cam from Chris Witor, head skimmed for 9.5:1 CR. The engine has been balanced and fitted with a GT6 flywheel, about half the weight of the original saloon item.

The engine revs freely to 5,500rpm, I haven't been brave enough to take it further. It still has all the torque expected of the long stroke 2.5. And it's smooth, as hoped for with a balanced engine.

The 2.5 litre engine in my CP series TR6 is also smooth and revvy, suspect a PO had it balanced.

Nigel

My TR6 engine revs well over 6k, too easily in fact. I know it was fully balanced when built in the 80’s. Sounds good at full chat.

when I eventually do untold damage to my 2000 engine, which I will, as with well over 100k it will complain at regularly going to 6k, I am tempted to go maximum bore but keep the stroke, think you can get to 2.3 

 

[Nigel Clark]

41 minutes ago, thescrapman said:

My TR6 engine revs well over 6k, too easily in fact. I know it was fully balanced when built in the 80’s. Sounds good at full chat.

when I eventually do untold damage to my 2000 engine, which I will, as with well over 100k it will complain at regularly going to 6k, I am tempted to go maximum bore but keep the stroke, think you can get to 2.3 

 

These drift I suppose but interesting....

Wonder what capacity would be possible from a 2 litre engine with the maximum over vote to 2.3 litre plus an offer crank grind to increase the stroke slightly. Must try calculating that sometime.

Nigel

[Wagger]

Some of the best motorcycle engines had the ratio bore=stroke. Compromise maybe?

[NonMember]

You local pub "expert" will tell you that long stroke is bad and won't rev, but the Honda 1600 engine of the Rover 416 (a sweet little unit that revs happily to 8000RPM) is very nearly the same ratio as the Triumph 1500 lump. Ford went through a phase of building short stroke engines, most of which didn't rev particularly well and were poor for efficiency.

If you think about it, the combustion chamber of even a long stroke engine is relatively flat, because all the interesting stuff happens at TDC, not BDC. A long stroke engine generally has a much better combustion characteristic than the flat-as-a-pancake space you get on a short stroke one.

So yes, it's a compromise, and a roughly "square" engine is around the sweet spot.

[Stratton Jimmer]

9 hours ago, Wagger said:

Some of the best motorcycle engines had the ratio bore=stroke. Compromise maybe?

For example, the Ariel Arrow - twin cylinder 5.4 x 5.4cm gives a combined capacity of 247cc approx.

[Wagger]

2 hours ago, Stratton Jimmer said:

For example, the Ariel Arrow - twin cylinder 5.4 x 5.4cm gives a combined capacity of 247cc approx.

Velo Venom, 86 x 86 mm 500cc. Also very economical for this capacity.

BSA Gold Star was close. Will have to dig my books out as it's all very scanty in the head now. Internet stuff not too reliable.

[Wagger]

22 hours ago, Nigel Clark said:

These drift I suppose but interesting....

Wonder what capacity would be possible from a 2 litre engine with the maximum over vote to 2.3 litre plus an offer crank grind to increase the stroke slightly. Must try calculating that sometime.

Nigel

When I had my bikes over bored I weighed the original pistons and noted them, I then turned the inside of the replacement pistons out until they weighed he same as the originals. Sometimes it was necessary to shorten the skirts to achieve this. At least I knew that the balance should remain as it was.

[Wagger]

12 hours ago, NonMember said:

You local pub "expert" will tell you that long stroke is bad and won't rev, but the Honda 1600 engine of the Rover 416 (a sweet little unit that revs happily to 8000RPM) is very nearly the same ratio as the Triumph 1500 lump. Ford went through a phase of building short stroke engines, most of which didn't rev particularly well and were poor for efficiency.

If you think about it, the combustion chamber of even a long stroke engine is relatively flat, because all the interesting stuff happens at TDC, not BDC. A long stroke engine generally has a much better combustion characteristic than the flat-as-a-pancake space you get on a short stroke one.

So yes, it's a compromise, and a roughly "square" engine is around the sweet spot.

The first Honda engine that I reconditioned was my brother's pushrod 50cc sports. The internals compared to british stuff looked like the internals of a watch. Lovely smooth parts, no rough castings. I ground and polished all of my British internals. Not yet done this with a car engine.