A complete waste of time!
A number of people have built 1745cc engines (76mm stroke x 85.5mm bore) in the belief the extra stroke as compared to 1776cc (69mm stroke) will produce more torque. Assuming same valve sizes, cam, carbs, CR etc, average torque will be about the same on both engines due to average cylinder pressures also being the same.
In the real world, based on dyno tests I’ve carried out on a couple of 1745cc engines (not mine), both made LESS power and torque than my otherwise similar specification 1776cc engines.
There is no physical reason why an engine should work so much better because the bore happens to be the same dimension as crankshaft stroke. That makes about as much sense as saying the ideal engine has as many cylinders as the vehicle has wheels.
Yes, there are pros and cons to selecting a particular bore to stroke ratio but that’s more related to piston speed (a function of rpm and stroke) and valve size (bigger bore makes more room for larger diameter valves).
I suspect the reason for 1745cc engines appearing on the scene is a result of people trying to avoid machining costs. Any perceived increase in torque is purely a result of capacity increase as opposed to increasing stroke to bore ratio…
Smaller bore limits your inlet valve diameter, therefore ports tend to be high velocity – exactly what you need for good torque at low rpm. Fit the same small valve heads to a 1776cc engine and you’ll have practically identical performance.
The longer the stroke, the greater the amount of power lost to friction. At 5000rpm, a 69mm stroke engine has a mean piston speed of 11.5 metres per second.
76mm stroke @ 5000rpm… 12.7 mps
86mm stroke @ 5000rpm… 14.3 mps
Many 4 valve per cylinder engines often have stroke lengths close to bore diameter. Not necesarilly because the designers believe ‘square’ engines are best but due to increased breathing potential (2 x small diameter inlet valves per cylinder) they don’t need as large a bore as the single inlet valve VW head to achieve the required cfm value for x amount of horsepower.
One point in favour of the smaller bore… it’s less prone to detonation (assuming good chamber design).
Far better to concentrate on building a high quality ‘conventional’ engine size, where parts are readily available than getting hung up on some theoretical ‘pub talk’, mythical, idealized, ultimate bore to stroke ratio combination.
Not wishing to discourage anyone but I’ve seen a number of ‘I want to build something a bit different’ projects over the years and in most instances they either get abandoned or turn out less than impressive. Maybe this could be the exception to rule….?
There are far more important things to consider when deciding on a particular bore and stroke for your next project than whether the two are close in size to eachother.
[Next followed a question containing the words: “budget build” .. “not afraid of spending a few £££’s” .. “just not too much”]
Depends what you mean by “budget build” .. “not afraid of spending a few £££’s” .. “just not too much”.
Sounds like you want to have your cake, eat it, and not pay for it
Easiest and quickest way to make more torque across the entire rpm range is to increase capacity: “There’s no replacement for displacement”.
If budget doesn’t stretch to a stroker crank, go with 90.5 or 94mm (1776cc, 1914cc).
90.5mm has more cooling fin area so is better suited to very high mileage engines as it dissipates heat more effectively, especially in heavyweight vehicles such as fully laden Type 2. Otherwise 94mm is a good choice when sticking with stock stroke.
Going the long stroke route you’ll find little difference in price between the various strokes. 82mm is a good choice if you’re prepared to buy H-beam rods. The case needs clearance work but the increase in capacity over using 74m or 76mm is worth it in terms of increased torque and power. I see no point using anything less than 78mm.
However, capacity isn’t the only determining factor in the amount of power and torque an engine makes. Heads, cam, carbs/EFI, balancing, quality of build, attention to detail etc etc all play an important part. There are plenty of 1776cc (and smaller) engines out performing much bigger motors because the builder has optimised ALL parts of the combination rather than allow one aspect of the build determine everything else.
Increasing bore and stroke to any worthwhile level requires machine work. Accept that as fact! Going to great lengths to come up with some Mickey Mouse bore/stroke combination because it saves machining the case and heads is wasted effort.
Check torque figures for a number of engines (regardless of make) and you’ll find a close relationship between peak torque and engine capacity…. a well built n/a mild street engine will make around 70 to 75ftlb per litre, whether it be a 1776 or 2387. Power outputs can vary substantially but the torque per litre figure holds up pretty well.
Peak torque occurs at the rpm the engine is most volumetrically efficient i.e. the engine speed at which the maximum volume of air is drawn (or forced) into the cylinders.
To make more torque, you must flow more air through the engine. Several options here… bigger valves, bigger ports, higher lift cams, more duration, etc etc but these mods push peak torque higher up the rpm range – not necessarily the right way to go for a super drivable street engine. Alternatively, add a turbocharger or supercharger and ‘force’ more air into the motor – loadsa torque!
Assuming forced induction doesn’t fit the bill (or £1k budget!!), by far the easiest way to pump more air and therefore make more torque at reasonable rpm is to increase the size of the pump i.e. IF YOU WANT MORE TORQUE, BUILD A BIGGER ENGINE!
(remember ‘torque per litre’ formula)
RPM at which peak torque occurs will be influenced by cam choice, valve size, ports, head work, carbs etc. The trick is to maximize power within the rpm range the engine spends most of its time. That’s where experience plays a part in selecting the right ‘combination’.
Fredster says his 1745cc engine felt more responsive at low rpm than his 1776 motor.
I seriously doubt the 1745 makes more torque than the 1776. It’s likely they both have similar peak torque per litre figures. The 1745 will make it’s max torque at lower rpom than the 1776.
Fredster then came to the conclusion the performance difference was attributable to choice of cam.
Spot on… the stock cammed 1745 will reach peak efficiency at lower rpm due to less lift and duration, therefore max torque will be lower in the rpm range than the 1776 with 110. The 1776 has more power because peak torque is at higher rpm, even if the PEAK torque value is the same as the 1745. See formula…
BHP = (torque x rpm)/5252
Now fit the 1776 with a stock cam… you’ve improved drivability in the range that was lacking before, but sacrificed higher rpm horsepower as a result.
For a given engine size you won’t succeed in making a whole lot more torque by opting for some oddball stroke/bore ratio. All you can do is adjust where in the rpm range that torque occurs. Bore diameter and crank stroke play a part in the equation but are less influential than other parts I’ve mentioned.