Curious Tech Question

[GhostRyder Concept]

I got a question cuz I would really like to get in depth with the engineering point as far as the powerplant of a engine.....What is the relationship between Bore Stroke and the Compression rate and how does this make a car go faster???? Any kind of insight would help.

[drdisque]

stroke highly affects engine speed and where torque is on the rpm curve.

compression ratio has been highly discussed on other forums, basically, higher compression is good unless you use forced induction where lower compression is better, there's always a limit of how high you can go with either boost or compression for a given octane of gas before detonating.

Bore is a bit more complex, in general more bore is better, but as bore gets larger, engines tend to make less hp, especially low in the powerband and more torque.

[TatII]

a engine with a large stroke ( under squared ) will tend to have a lower rev limit due to the piston speed at a given rpm is extremely high. since the tq of the rod and crank is greater from a longer leverage arm ( longer stroke ) it will have higher tq.

for a engine with large bore ( a RB26DETT ) it allows to engine to rev extremely high becasue the piston has less distance to travel per stroke. this is not good for tq but very good for top end as it will have a broader powerband becasue it can rev higher.

for compression ratio. the higher you go, the more power you will make, whether its n/a or forced induction. however you can only go so high with forced induction before the engine temp goes way up as your compressing a boosted engine. this heat will cause pre ignition ( detonation ) and will cause engine failure. so generally you can get away with turbo charging a high comp engine on low boost and it will make more power then a low comp engine running the same boost. however there is a low limit of how much psi of boost you can run before it starts to detonate. ( you can get away with it to a certain degree if your running a stand alone fuel management system taht allows you to tune individual cylinder's igntion maps, and fuel maps. ) but your cylinder temp will still be waaay up like that of a diesel engine.

for low comp engines, power off boost will be sluggish becasue your not drawing in as much air per intake stroke, and your not compression the a/f mixture as much on the compression stroke. the less dense the charge is, the less power the power stroke will be. however this allows you to run more boost becasue it has a lower temp. you can easily make up the lose in power by running more boost saftly.

general rule of thumb. for n/a higher comp is better. for forced induction, lower comp is safer which leads to running more boost which leads to being faster.

[Neutrino]

TAT II is correct however I would like to make two notes:

1. Oversquaring will generally make it easier to increase the rev limit, however the bigger the bore the more inertia the piston will have which will become detrimental to high RPM.

2. Preignition is only one of the forms of detonation and not the most common one - caused ussually by hot spots. More often you'll have an uncontrolled explosion after the spark with a big shock wave traveling in ahead of the flame front.

[958Rocky]

I've never heard the terms oversquaring and undersquaring before. Can you guys explain that?

I think the K20as have the same stroke length as the old B series motors but since their .4 liters larger they have a larger bore, right? So that means they should be able to support higher rpms.

[Neutrino]

Originally Posted by 958Rocky

I've never heard the terms oversquaring and undersquaring before. Can you guys explain that?

I think the K20as have the same stroke length as the old B series motors but since their .4 liters larger they have a larger bore, right? So that means they should be able to support higher rpms.

an oversquare engine will have acheive most of its displacement trough a large bore

an undersquare will acheive it trough stroke

generally an oversquare engine will rev higher because its rods are shorter and have to travel less. But that is only a generalisation because a large wide piston in an oversquare engine can be equally detrimental to high rpm.


But those are not the only facts that affect RPM. The cam profile is equally important. A high lift cam will breathe perfectly at high RPM but will lose TQ at low and cause a nasty idle. Honda and other manufacturers use VTEC type cams that give you the equivalent of two cam profiles good for both high and low RPM.

[Alastor187]

Originally Posted by 958Rocky

I've never heard the terms oversquaring and undersquaring before. Can you guys explain that?

In the simplest form the definition is:

Bore = Stroke :: Square

Bore < Stroke :: Under-Square

Bore > Stroke :: Over-Square