Speed Six General Information

Contents -

1. Torsional Vibration and the Speed Six Engine 

2. Boring and Stroking - how big can you go?

3. TVR Speed Six Oil System Upgrade - why you need it, how it works

Read about our work on the Speed Six as spotlighted by TVR Sprint magazine in their July 2011 issue, available on our website by kind permission of the TVRCC.

The October 2011 issue of the same magazine also carried a history of the AJP/Speed 6 engine's development written by ourselves,  which  makes for interesting reading. Again reproduced here by permission of the TVRCC.


TVR Sprint magazine feature Origin of the Species - brief history of AJP/Speed 6 as Adobe .pdf file - click here to download

Origin of Species - AJP/Speed 6

TVR Sprint magazine feature Plan B as Adobe .pdf file - click here to download


TVR Sprint feature

1. Torsional Vibration and the Speed Six Engine

One of the problems with the Speed Six engine is torsional vibration, The crankshaft is 660mm long from the rear main bearing to the front main bearing, and the  nose is another 127mm on top of that. Total length is 787mm. This makes for a long crank - 31 inches - in real money!

Now looking at the two engines, the 3.6 has a stroke of 83mm - or 3.268 inches - and the 4.0 has a stroke of 92 mm - or 3.622 inches. Both engines suffer from torsional vibration, the 4.0 litre more than the 3.6 because of the longer stroke .

The cause of this is the rotational mass of crank rods and pistons, which is in two parts: the crank and the bottom half of the rod (big end) is the rotating mass, and the top half of the rod small end and piston being a reciprocating mass.

For an example of what can happen when things get out of hand, see the pictures below. Here, a 4.0 litre  engine was run up to 7500 rpm on a dynamometer, and then the throttle butterflies inadavertendly snapped shut by the operator..... exactly what happens should you miss a gear on a downchange.

Little end showing weak point in rod where breakage occurred.

Damaged components laid out for inspection.



The rod - or what's left of it, after being dug out from the crank journal

The cylinders are in pairs 120 degrees apart and are firing as 1 & 6,  2 & 5 and then 3 & 4. As each pair reach tdc one cylinder will be exhausting, then once over tdc it will be on induction, whilst the other will still be on compression, followed by its power stroke once over tdc. On one 360 degree rotation of the crank each pair of cylinders  will go through the above cycle.

At the bottom end, the Speed Six's main bearing caps are however only retained by two nuts and studs with locator dowels.
 
If you ever have a chance to look at the main bearings from a disassembled engine, you will see that there are marks on the outer edges These are caused  by the crankshaft moving and twisting under load, where the caps move under the torsional stresses of crank shaft rotation at each pulse of an individual cylinder's compression/ power stroke, plus the 'braking' effect on the crankshaft by other cylinders on their exhaust / induction strokes.

Early TVR Speed Six engine without crankshaft damper

Early TVR Speed Six re-designed engine without crankshaft damper:

To try to counteract this 'twisting', TVR followed standard automotive design practice by fitting a fluid damper to the front of the engine, (actually a small block chevy part). However, we are still of the opinion that the standard bottom end remains a weak point of this engine.
 
Accordingly,  we have developed a revised main cap support modification, comprising add-on bracing components installed with additional machining so as to positively locate the caps in horizontal as well as vertical planes. This both cuts down vibration and strengthens the block in this critical area at the same time.

2. Boring and Stroking - how big can you go?

Whether it is a Speed 6 or the Briggs and Stratton 4-strok on a ride-on mower - there are only three ways you can increase the capacity of an engine. By over-boring the cylinders and fitting matching larger pistons. Using a longer stroke crankshaft to increase the distance the piston has to travel along the cylidner bore. By a combination of both of these

However, there are a number of things you should know before going down any of these routes to gaining greater power.

To start with, if you are just overboring your engine, providing there is enough bore wall thickness to fit the bigger pistons and you have a gasket to for the larger bore, you will need to adjust ignition and fueling. If your over bore is over 1mm or .040" thou., how much extra capacity you will get depends on how long a stroke you have. You can check this out by using the following formula - 

Bore x bore x 0.7854 x stroke x number of cylinders = capacity
To convert mm to inches divide by 25.4.
To convert Inches to mm x 25.4.

When stroking an engine you have to take into account the following factors: stroke length, rod length, and piston pin height. The first step after fitting your stroker crankshaft is therefore to check the position of the piston at top dead centre - or T.D.C., as it is known. If you stick with a standard piston and rod, you will find that the piston will stick out the top of the block - so you will need to either machine the piston (only if there is enough crown thickness to skim it down enough to give you the correct compression ratio or valve clearances) or else:

Fit new pistons with the correct pin height for the new stroke
Use a shorter conrod to bring the piston down the bore
Do both of the above

Remember, if the stroke is increased by say 11.8mm or 0.300" thou, you will only need to move the piston pin nearer the crown by HALF the extra stroke. Alternatively,  you would shorten the rod by 2.95mm or 0.150" - or use a combination of these two methods to get the compression ratio you need. Now, by increasing the stroke we also increase piston speed  for a given RPM - because the piston now needs to travel a longer distance, but in the same time.

Here's an example.  Using the formula -

Piston speed = stroke in inches x rpm divide by 6 

- we apply this to an  F1 engine with a 40.64mm or 1.6" stroke, which gives us  - 

Stroke of 1.6 inches  x  18000 rpm
Divide by 6 = 4800 ft per minute
Divide by 60 = 80 ft per second (or if you want that in metric, divide by .3048 = 24.38
meters per second)

These engines use some pretty exotic materials to run at these speeds - which interestingly enough,  would also be needed if you increased the stroke to 3.2 inches but halved the rpm, since  the piston speed would in fact remain  the same. Though the rpms may have now dropped to 9000,  the inertia shock loads each time the piston reverses direction at the end of its stroke, plus the need to cope with and dissipate heat that is built up within the engine through friction and combustion, will require similar engineering solutions, if the motor is to stay together. 

Talking of friction and heat, lets turn for a moment and look at rod ratios. The maths is simple enough - simply divide the stroke into the rod length, measured centre to centre from little end to big end (crank to piston pin).

So, for a 5.7 inch rod, we divide by 3.75 which gives a rod ratio of 1.52.

Which In our opinion, is just the right side of acceptable. Why? Because once you go below a ratio of 1.5, the following sorts of issues can all start to raise their heads - 

Side thrust on the piston is increased because the rod angle is more acute, leading to increased wear on the  rings and bores.
Since the piston speed is faster at top dead centre, cylinder filling is compromised on overlap.
The pistons can be noisy - not an issue on a race car, but perhaps a bit wearing on the road.

How does the TVR Speed Six  compare with all this theory? Here are the factory specifications  - 

3.6 litre conrod length is 150mm, with a stroke of 83mm, giving us a rod ratio of 1.8 
4.0 litre conrod length is 144.5mm, with a stroke 92mm, yileding a rod ratio of1.57

For the record, the early 4.0 litre had a shorter conrod, at 142mm long
, with a rod ratio of 1.54.

NOTE. Elsewhere on this  site you should find the technical  information on the conrods.

3. TVR Speed Six Oil System Upgrade

First we need to understand the system we have at the moment. The oil is fed from the header tank to the oil pump, then from the oil pump to the oil filter on the inlet side of the block through the filter to the main oil gallery that runs along the inlet side of the block. From the main oil gallery the oil is then fed to the 7 main bearings to lubricate the crankshaft, and finally through the crankshaft itself to the big ends.

If you look at the main bearings you will see one half has an oil  groove in the part that fits in the block the other half that fits in the main cap is plain. This means that in the standard engine, as the crankshaft rotates, oil feeding the big ends is cut off for 180 degrees of crankshaft rotation. To improve the oil flow, the main caps can be modified by fitting a grooved bearing in the cap - this allows a constant oil feed to the big ends and aids cooling as well lubrication.

Turning to the oiling system in the cylinder head, the oil travels from the main gallery up the two oil feeds to the cylinder head on the inlet side, then  goes through the follower shaft on the inlet side to the back of the head and stops. The oil also flows forward to the front of the head, where it crosses over and travels back down the  follower shaft on the exhaust side. This means that  the last part of the head to get oil is the back of the head. Along the way, the follower shaft feeds oil to followers and  sprays oil on the camshaft lobes, as well as  lubricating the camshaft bearings. If you look at each follower, you will see that there is no oil groove in the centre - so as the follower moves off centre the oil is cut off momentarily which compromises not only lubrication but the effects of oil spray cooling to the inside surfaces of the cylinder head.

Speed Six rear head oil modification

Additional oil feed to rear of head, as fitted by RND Engineering to its Speed Six heads

To rectify this, RND fit an additional oil feed to the rear of the head on the exhaust side. When we first looked at this in 2005, we spoke to Al Melling direct and his suggestion was to up the oil pressure as well, which goes along way to rectifying the oil feeds that were removed by TVR in their redesign.  Using Al Melling's original design of finger followers - which are are not only lighter but are the correct shape to follow the camshaft  - which now have a 360 degree oil groove added,  constant oil flow to the camshaft lobe is achieved, and cooling to the cylinder head ehanced . We have redesigned the oil pump its an all new pump with stronger main shaft and deeper rotors to increase oil flow by 30% and also increases oil pressure by doing all this we go along way to getting things back to the original design. To get things even closer to the 'what should have been' engine specification, you can fit our Al Melling designed valves with 8mm diameter valve stems and reduced pressure valve springs.

To summarise these changes, we now have a new oil feed to the back of the head, 360 degree oiling to the mains and big end bearingss, and redesigned finger followers with 360 degree oiling and constant flow of oil to the camshaft lobes. All this aids cooling, as well as ensuring oil is always present where and when it is needed.