Ah, bore and stroke: affect torque?
Discussion
CB-Dave said:
( . . . )
motorbike engines are tiny because of their short stroke, a short stroke engine is crap for torque (that's also why they have insane 14k rpm rev limits etc - there's less harmonic vibration that could damage the engine) and that is also why you have to rev the nads off them to get them to make appreciable power (100bhp at 14krpm etc).
( . . . )
Ah, bore and stroke and torque: Honda 80s / 90s car engines have large stroke compared to bore, relative low torque-high hp, rev pretty high.
What's up? and is this true
For a given capacity, and all else being equal the bore/stroke ratio does not affect torque.
In practice a larger bore favours larger valves, and also less frictional loss (the piston moves a small distance, and peak speeds are lower). The shorter stroke also imparts less stress on the crank and rods, making the RPM (and hence power) capability of such "over square" engines much better than long stroke engines.
Older long stroke engine may develop higher peak torque in some instances, but this is most always down to cam timming. A slow revving engine will be optimised to provide high peak torque, a short stroke high revving engine will more likely be developed to provide higher peak power, at the expense of peak torque. This does not mean that long stroke engine inherrently produce more torque however.
In practice a larger bore favours larger valves, and also less frictional loss (the piston moves a small distance, and peak speeds are lower). The shorter stroke also imparts less stress on the crank and rods, making the RPM (and hence power) capability of such "over square" engines much better than long stroke engines.
Older long stroke engine may develop higher peak torque in some instances, but this is most always down to cam timming. A slow revving engine will be optimised to provide high peak torque, a short stroke high revving engine will more likely be developed to provide higher peak power, at the expense of peak torque. This does not mean that long stroke engine inherrently produce more torque however.
They do affect torque and in many cases an undersquare engine makes for a better road car, something like 83 mm bore by 92 mm stroke on a 4 pot 2 litre.
If you want it to rev a lot you want more like 91 bore x 77 stroke.
Conrod length also makes a big difference to performance, most rods are 5/3 x stroke, as the length gets pushed towards 2 the time to combust lengthens and so you get an appreciable improvement in high rpm power where it becomes more critical.
If you want it to rev a lot you want more like 91 bore x 77 stroke.
Conrod length also makes a big difference to performance, most rods are 5/3 x stroke, as the length gets pushed towards 2 the time to combust lengthens and so you get an appreciable improvement in high rpm power where it becomes more critical.
dinkel said:
I think Valve size has something to do with it as well.
AJP8 has 2 valves per cylinder and gets reasonable torque . . .
But hardly class leading. I cannot see what the fuss is about TVR engines. They're not reliable, they're not that powerful, they're not that light, they don't package that well, they're not even cheap.
Put a mediocre engine in a light car and it will feel torquey.
The best thing TVR could do is ditch their in house engines and copy what Lee Noble does - and get a reliable engine as a base and up the power if required by a company that knows what they are doing.
GavinPearson said:
Alpineandy said:
I think Valve size has something to do with it as well.
AJP8 has 2 valves per cylinder and gets reasonable torque . . .
( . . . ) they're not that powerful, they're not that light, ( . . . )
121 kgs for a 400+ hp 4.2 / 4.5 litre v8: that's awfull light.
GavinPearson said:
They do affect torque and in many cases an undersquare engine makes for a better road car, something like 83 mm bore by 92 mm stroke on a 4 pot 2 litre.
If you want it to rev a lot you want more like 91 bore x 77 stroke.
Can you explain why simply changing the bore/stroke ratio (without changing any other parameters such as rod ratio) on an engine affects torque ?
Interesting question. I can't think of any reason if you keep the rod ratio the same - do you know whether they do keep it the same though?
However, if you have a long stroke engine which is mechanically unsuitable for high revs, then you might be inclined to put a milder cam in. This would give more torque at lower revs, not because of any extra torque inherent in the engine but just because the optimal tune is different. Also if you have a larger piston diameter which accepts bigger valves for a given capacity, this will breath better for a given cam so there would be less need to compromise bottom end torque by putting in a long cam to get breathing at the top end. Don't know if this what happens, but its a theory?
However, if you have a long stroke engine which is mechanically unsuitable for high revs, then you might be inclined to put a milder cam in. This would give more torque at lower revs, not because of any extra torque inherent in the engine but just because the optimal tune is different. Also if you have a larger piston diameter which accepts bigger valves for a given capacity, this will breath better for a given cam so there would be less need to compromise bottom end torque by putting in a long cam to get breathing at the top end. Don't know if this what happens, but its a theory?
I think MR2Mike is quite right. You can make a short stroke high revving engine feel torquey by giving it small valves. Unfortunately this works by robbing power from the top end rather than giving it any more low down.
This assumes that you don't change the cam timing, which of course you would do to make the best power in the rev range available.
This assumes that you don't change the cam timing, which of course you would do to make the best power in the rev range available.
GavinPearson said:
...
The best thing TVR could do is ditch their in house engines and copy what Lee Noble does - and get a reliable engine as a base and up the power if required by a company that knows what they are doing.
TVR can match the power of Noble without forced induction. If TVR didn't do their own engine thing then would the Typhon exist? Well certainly the 580bhp would be trickier to achieve.
I still do think that a simpler cheaper engine for e.g. a new Tamora-like to produce a lower priced entry level model would go down well.
RichardD said:
...
The best thing TVR could do is ditch their in house engines and copy what Lee Noble does - and get a reliable engine as a base and up the power if required by a company that knows what they are doing.
TVR can match the power of Noble without forced induction. If TVR didn't do their own engine thing then would the Typhon exist? Well certainly the 580bhp would be trickier to achieve.
I still do think that a simpler cheaper engine for e.g. a new Tamora-like to produce a lower priced entry level model would go down well.
Skip the AJP8 (due to emissions) and turbo a Speed 6 would give a massive torque / hp monster . . . dunno if the 6 is ready for a turbo.
"new Tamora-like to produce" No no, Tamora will gain value in the future. With the S6 350 hp which is enough.
if you want another lump simply buy another car!
GavinPearson said:
The best thing TVR could do is ditch their in house engines and copy what Lee Noble does - and get a reliable engine as a base and up the power if required by a company that knows what they are doing.
Gav isn't that what TVR did for decades before the advent of the AJP8/6 . IMHO they shoud stick with the route they are already going with the in house lumps 
. They just need to get the hang of putting the develoepment engines in the single TVR make races for at least a year before unleashing it on the public. AFAIK the AJP8 has been well and truely developed in the Tuscans now. I'd like to see them put the AJP6 in it now, I know it is in the GT cars but 20 odd cars throughout a domestic season would certainly sort it out as I said before IMHO
. Harry
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