I just saw this thread: https://honda-tech.com/zerothread?id=155335 and thought a good conversation could be held about the pros and cons of bore size. Here I am, bored at work again, wondering what people think, non-VTEC, I'm pretty sure you'll be here for this one.

Bore size is a major factor in several aspects of engine preperation and design. The advantages of a larger bore is of course the displacement, but furthermore, there is more room between the valves and the sides of the combustion chamber if the motor is put together properly and the combustion chambers were sized to match the bore of the cylinders. The increase in combustion chamber volume results in a slight drop in compression, but since this is a process that is done when building a motor, this can easily be compensated for with piston and valve design. Another advantage that the increase in combustion chamber volume presents, coupled with the increase in distance between the valves and the combustion chamber walls is the ability to run slightly larger diameter valves. But then the question comes into play of the larger valve effecting velocity, which inturn effects atomization by reducing swirl in the intake track. If the engine is not boosted, this larger valve theory immediately comes into play, where a forced induction car has the velocity created by the postiive manifold pressure to "create" so to speak atomization, the all motor vehicle does not posess this. Running larger valves with the smaller bore is possible in most instances, but then there is the case of disturbing the intake track flow capability since the before small clearance between the valve and the combustion chamber wall is now smaller than before.

Another aspect that is severly overlooked by those seeking the larger bore is the factor of flame travel time. Whenever the discussion of flame travel time comes into play, so does ignition timing. Personally, (and I know many will care to argue this) I believe that ignition timing after TDC is the way to go if you have a motor that can be tuned for such timing, advancing the timing works against the cranks natural movement causing a parasthetic loss with the spark timing before TDC when the piston is moving upward towards TDC the spark starts the combustion early pushing against the piston as it is trying to compress the air/fuel and the compression ratio is not fully taken advantage of either in this process. But back to flame travel time, of course, with a larger bore there is going to be an increase in the amount of time that it takes for the flame to complete its burn. Starting the flame before TDC allows the flame to use the extra movement and compression of the pistons movement toward TDC to burn the air/fuel faster, with the compression of the burn, not just the air/fuel prior to burn. With the burn concentrated at the top of the stroke, the majority of the heat will be concentrated here as well. The pressure of combustion will be striving against the piston at TDC and before, granted the crank's movement can overcome this, or else engines wouldnt even start. But the physics behind this make little sense once broken down. One good side to advanced ignition timing is the reduced chance of detonation, since it is quite hard for an engine to pre-ignite when it has already started the controlled ignition burn. But with the larger bore, it takes longer for the flame to fully complete the burn, now the piston ring and piston are having longer exposure to the heat, which is worse? Longer exposure with less of a sudden increase in temperature or a sudden increase in temperature wit less exposure? To me its a toss up on that one. On the other hand, the longer the burn process, the more force that is being placed on the piston on the power stroke, the more pressure applied for the longer durection of time will obviously aid in power production since the driving force of combustion is applied for a longer part of the durection of the power stroke.

There are other advantages to the larger bore, such as clearance issues for the rods when running a long rod combination and there is also the issue of piston pin to cylinder wall distance and its effect on side loading.

Any opinions?

Damn, if you cant tell, doing tech support is very boring.

 

here we go again.........hehehe

well i quicky skimmed through everything you wrote.......looks good......

about the oversized valves......can a bigger bore engine benifit from oversized valves? depending on what head you are using and for what application. a VTEC head uses 33mm intake valves but the non-VTEC head uses 31mm intake valves. the both have the same 28mm exhaust valves.

now if you were putting a VTEC head a B20 2.0L engine with 84mm cylinder bores for a natrually asperated application. i dont think it's that important to have oversized valves. the head flows pretty good. if using the non-VTEC head i think the engine will benifit from the larger valves due to the fact that the cylinder is much larger than what the head/valves were designed for. larger valves on that non-VTEC head will add to the cylinder volume efficency. you have a bigger cylinder so you should have more air/fuel going into it. porting the head alone will help in the speed of the air/fuel entering the cylinder but the valve size ultimatly controls the accual ammount going in. (of course the camshaft lift and duration also)

i recall seeing a post on another message board about how the rod to stroke ratio plays a part in the ammount of air/fuel the engine needs. the lower the rod to stroke ratio the more volume the engine needs. the higher the rod to stroke ratio the more velocity the engine needs?


as for the ignition timing...........i have no clue........let me think about it for a while........lol.......

 

On a N/A CRVTEC application, no I wouldn't say that it would be as important for the over sized valves for breathing in that case, cause going too large on the valve diameter can effect velocity, which is one of the most important factors in producing normally aspirated power.

With a higher rod ratio, yes, the velocity would have to increase due to the reduction in piston dwelltime at TDC/BDC the pistons speeds increase and therefore so would the velocity have to to accomidate the engines breathing requirements. In the case of a lower rod ratio, the volume increase is needed to fill the cylinders since piston speeds are lower, this is a bit of a touchy subject as volume increase is a hard factor to dictate. Volume increases can, of course, be aided with a turbo, we all know that. But volume increase on a normally aspirated motor is different, becasue the breathing capability of the head is the major factor in this, with the addition of larger ports, cam profiles, and larger diameter valves the ability of the motor to take in air/fuel increases, but taken beyond reason, velocity is lost, and with velocity goes atomization, with atomization goes the quality of the burn, when quality of burn is lost, ignition timing has to be advanced to overcome detonation. Here is the factor of ignition timing coming into play once again. Velocity is important in both instances, on the low rod ratio motor becasue of the aid in the turbulence that promotes atomization and in the higer rod ratio motor where velocity is needed for increased ability to control the burn process.

 

Good thread.

But I thought it was the opposite way around...with respect to rod ratio. Lower R/S #'s mean that the piston spends less time at TDC and moves away from TDC faster than a comparable motor with a longer rod installed. This information was presented by Team Integra (although now you have to sign up...but still free apparently).

"To explain these features in detail again for those of you who did not catch the first post. Since B18's have a low rod length to stroke ratio:

1.a) our engines have pistons which spend a shorter time at Top Dead Center (TDC) [which is at the very top of the piston travel] compared to the B16A. This is called a Short Piston Dwell Time.

With short piston dwell time, there is less force compressing the air:fuel mix on the compression stroke and the piston quickly changes direction downward as the mix is being ignited. The air fuel mix, with less compression force and rapid change in piston direction downward, does not combust as completely.

Secondly, on the upward exhaust stroke, the piston quickly changes from pushing the burnt exhaust gases out of the combustion chamber to sucking in fresh air:fuel at the start of the intake stroke.

We have fast transitions from compression to power strokes and exhaust to intake strokes. The piston changes direction or flip flops at TDC quickly at each engine cycle or stroke (intake, compression, power, exhaust strokes).

1. b) our engines have pistons which drop down very fast away from TDC during the intake stroke and power stroke. This is called High Piston Speed Away From TDC.

On the intake stroke, high piston speeds away from TDC (travelling down the cylinder) generates high flow velocities through the intake port at low-midrange rpm because the piston has a higher sucking force to draw in more intake air:fuel mix into the combustion chamber at low-midrange rpms.

On the power stroke, high piston speeds away from TDC unfortunately means the piston can run away from the spark flame front travel causing incomplete burning of the air fuel mix.

High piston speed away from TDC also reduces cylinder pressures too rapidly, especially at high rpms. This lowered cylinder pressure reduces the force downward on top of the piston making less power."

There was a lot more than this regarding R/S ratio and its effect on engine breathability/revability.

Forgot to add this too:
Panic's Tech Papers

Has a couple good articles with one of them being on R/S ratio.


[Modified by EE_Chris, 12:28 PM 4/2/2002]

 

just a note for you guys. i have a d16z6 (sohc vtec) and when i got it rebuilt it was bored out to make it about a 1.8L. the pistons barely fit in there and i doubt i will ever have leaking oil rings. anyway, the power gains werent worth the extra money imho. then again, i am running kamikaze 4-1 headers so maybe i could be making more.

 

I don't know if I can agree with this. Detonation can happen anytime there is unburned air/fuel in the chamber. When the flame front travels, it raises the pressure and temperature of the unburned mixture. That's why engines detonate when advanced too much: The pressure and temperature will have risen a lot due to the flame front, and this is further compounded by the piston compressing the mixture. When the pressure and temperature extremes from these two factors overlap, the chance of detonation is highest. Detonation can and very often does happen after the spark fires. That's why the ignition on turbo engines is retarded.

 

Ahhh, good call, you are correct, higher rod ratio does result in a loss of piston speed. At 8000 RPM here are piston speeds for tome Honda motors...

B16A 20.6 meters per second
B17A 21.7 meters per second
B18C 23.3 meters per second
B18A/B B20B/Z 22.3 meters per second

the faster the piston is moving the less time that it will spend at TDC/BDC and the faster that it can transition into the next cycle. Yes, the lower rod ratio motor produces the torque due to the added velocity that it can create due to a "faster" sucking effect that it has on the ports in the lower half of the rev band, thus the reason that a H23 is so torquey at very low RPM (2000) is that it has even a higher piston speed than any of the B series motors listed above, the rod ratio for the H23 is 1.49:1. If you have ever had a chance to drive one of the 92-96 Si non-VTEC Preludes, you know that the power in these cars falls off way before the 6500 RPM redline. In the higher half of the RPM band, the valves are staying open for such a short time that now the lower piston speed is needed for the "sucking" effect. This is more of the reason that the B18 and LSVTEC make more low end power than the B16 does, the sucking effect created by the piston speed differences, and the LSVTEC, being 1.8 liters, just the same as the B18C, many wonder how it can create more torque than the B18C, piston speed is decreased in those motors, the tradeoff however is side loading and the inability to exploit the rev band, these last two features are the foundations that turbo hondas are built upon. Another fact of the faster moving piston, that you already mentioned, was the lack of time to complete the burn, this relates to what I was saying in the first post.....Which is better the sudden explosion starting before TDC with a high speed piston or the longer burn after TDC with the slower moving piston. I believe in the longer burn with the slower moving piston, but there is still one point that I cant get past that I was thinking about last night after I made my second post on this thread. Think about the differences in heat this way, would you rather run back and forth through a thin "wall" of fire that is 500 degrees but only be in it for a split second or would you rather sit in a 100 degree fireplace for a longer period of time and take "breaks" from the heat? Who is gonna get more burns? Thus the design characteristics of pistons, piston rings, and in cylinder cooling properties come into play here. And keep in mind, maximum cylinder pressure is not always at top dead center.

 

One thing that I forgot to include in that post about advanced ignition timing is that was assuming that maximum cylinder pressure had already been reached at the time of the spark. However, I have a habit of assuming proper tuning will be done on every motor, even though I know this not to be true. To my knowledge, detonation is defined as pre-ignition, when the air/fuel is compressed the un-atomized fuel particles have a tendancy to ignite before the plug fires. Keep in mind that retarded ignition timing on the turbo/nitrous vehicles is present because more air and fual is in the chamber, and it takes longer to mix. Detonation can occur at any point in the cycle that fuel is in the cylinder, yes, that is a fact, octane can be used to over come this, but flame speed itself does not corrolate with octane, the more major determining factors in auto-ignition is compression ratio, stoichiometry, combustion chamber shape and design, piston design, bore size, chemical structure of the fuel, presence of antiknock additives (such as oxygenates) number and position of spark plugs, turbulence, and velocity of the incoming mixture, amongst other things. Assuming proper tuning of all varibles is present, then tuning of the timing and auto-ingition can be compensated for. Bore size can effect the ability for the fuel particles to auto ignite because the particles are spread over a larger surface and temperatures can vary in the cylinder, as well as atomization. One of the largest factors in the ability for gasoline to resist auto-ignition is the boiling point of the fuel. The higher the boiling point the more resistance that the fuel has to heat.

 

Rod to stroke ratio
DON'T MEAN NO THANG BUT A CHICKEN ****!!!

 

How do you figure? Rod ratio is the foundation of the geometry of the internals of every motor that has been in every car that you have ever driven. And I think it should be readily apparent since every conversation about the internals of a motor falls back on the topic of rod to stroke ratio.

EDIT- and non-VTEC, got any thoughts on the spark timing/combustion chamber heat yet? I'm still thinking....or at least trying to....

EDIT #2-another thought.....does anyone think that piston pin distance to the cylinder wall has that large of an impact on rod ratio, cause the larger the bore, the more distance there is from the piston pin to the cylinder wall, and if you picture a side angle view of a connecting rod at maximum thrust angle and carry the line of sight past where the connecting rod ends at the wrist pin and to its imaginary impact point on the cylinder wall you can see how the rod ratio can be comprimised by the bigger bore and its increasing the distance between the piston pin and cylinder wall. Although it is appearent that this can increase side loading, I know of know way that it could effect the rod ratio numericially, and thus piston speed stays the same, I would think that this would have more to do with the angle that the thrust is applied to the crank rather than anything else......still thinking......

[Modified by riceboy, 5:54 AM 4/4/2002]


[Modified by riceboy, 6:03 AM 4/4/2002]

 

---Email me when a reply is made to this topic---

 

i think having the ignition timing BTDC is better no matter what bore size you are running. if you move it to TDC or ATDC you are effectivly loosing the momentium of the combustion cycle. the engine will run with the timing at TDC or ATDC but will be lazy and not very responsive. if you can some how prove tuning the engine ATDC will be a better performer without dumping raw fuel out the exhaust please tell us. spark it at TDC or ATDC and there is some unburnt fuel still in the cylinder the exhaust stroke will just dump the excessive fuel out the exhaust.

by the time the piston moves up the cylinder and the spark plug fires the air/fuel BTDC the piston will be at the top of the cylinder where maximum cylinder pressure will force it's way down.

hope this makes sense??

 

with a set of jun 3's or skunk2 stage 2 cams you have an issue with valve overlap. with both cams set in an optimal location the intake and exaust valves are .006-.010 apart if you have oversized valves (which hurt flow, unless the seats and ports are scaled up) you are not able to get the overlap correct. in short terms the benifites from cams far out weigh the beifites of over sized valves. we still run stock sized valves in the integra drag car to this day.


Straight from the mouth of Steve "OmniMan" from Group A.

If the Skunk2 drag car is still running stock valves, I doubt you need them.

 

ignition timing should always be <u>before TDC</u>

as the spark ignites the fuel mixture and this continues to compress and teh energy is released - pushing the piston down...

this takes place in milliseconds...

no timing should ever be after the piston has reached the top of its compression cycle, otehrwise the compression does less work...

this does make sense.

t..

 

That's true.

riceboy, this statement is valid, but all the energy should get rereleased when the piston moves back down. The pressure of combustion will be striving against the piston before TDC, but it will strive that much harder with it after TDC. Ultimately there should be no losses (should).

 

Piston dwell time at TDC is in my opinion not a good thing, especially when the mixture's burning. Smokey contends that we want maximum cyl. pressure at TDC. Well, rotate an engine's piston to TDC and try to push it down....I'll give you a sledge hammer, and you will not rotate that crank because at TDC the rod's in line with the crank journal. I prefer to dictate when the engine will see maximum cylinder pressure, and I typically achieve it at about 25 degrees past TDC....the piston pushes real easy there due to the angle and mechanical advantage you now have achieved with the crank.

straight from the mouth of TOO...the man knows his stuff

 

If it dwells at TDC for long it might not be good, but it means that it will dwell at 5, 10, 15, 20, and 25 degrees longer, too.

 

Yes, all valid points, but....intentionally forgetting about the effects of side loading, compare the higher rod ratio/slower moving piston to the lower rod ratio/faster moving piston.

First, the lower rod ratio/faster moving piston. The faster moving piston can have the ignition timing advanced more easily without gaining parasthetic loss because not only is the piston moving through the cycle and the transition from before to after top dead center takes less time than that of the slower moving piston, but the piston also has more thrust energy toward top dead center since the piston speeds approacing top dead center are greater. This allows it to have the force to compress the already igniting mixture. With the faster moving piston the combustion cycle must be completed earlier, or like stated before, the piston starts to move away from the combustion.

Now on the flip side of the coin, we have the higher rod ratio/slower moving piston, the piston spends more time at top dead center, and like was said it spends more time at every point before and afterwards in the cycle. If maximum cylinder pressure is delayed until after top dead center, then so can the spark event. And since now we have a engine cycle where maximum cylinder pressure is after top dead center with a slower moving piston, the decreased piston speed gives the combustion cycle longer to complete its burn while at the same time taking advantage of the optimum crank angle that is after top dead center.

While we have all been talking about spark timing (as I thought that we would) I have been thinking more and more about the advantage of increased piston pin to cylinder wall distance. The thrust angle upon the crank is increased, and therefore, in the event of the slower moving piston with maximum cylinder pressure after top dead center and the spark event at the same time, the angle of thrust on the crank that is preferable is reached earlier in the cycle, thus the combustion can be started earlier in the cycle while still remaining after top dead center and the crank angle will still be preferable for the least amount of restriction againt the combustion cycle. The importantce of this is neglidgeable at best, but its importantce is still valid for the ignition timing that is after top dead center, since the preferable crank angle can be achieved close to top dead center.

Hopefully this makes sense........


[Modified by riceboy, 10:10 PM 4/5/2002]

 

just put a turbo on it, it will go fast!

hehe jk, im enjoying this thread, trying to learn stuff..

 

I think the B18c and B18A/B B20B/Z values are swapped.

 

Great thread.

just two things have been bothering me bout this though. wouldn't a smaller bore make more power because there would be a greater pressure over a smaller area? the other thing is that wouldn't an engine make more torque (and hp because hp is calculated from torque) with a longer crankshaft? because isn't torque measure in lbs per ft?

just my $.02

 

On the bore, there are two things that are main benefits of the larger bore, flame dispirsement area flame travel time. Flame travel time directly correlates with the duration of the downward force on the piston, and the larger bore also has more surface area meaning that a cyliner head and piston combo could be designed to promote the burn on the exhaust side, one for heat dispertion and two, that side having more burn takes advantage of the cranks natural movement.

And as for the longer crank, yes, you are right, but you will sacrifice rod ratio and everything that corrolates with it.

 

I think you are right

 

Here my $.02 on the timing issue.

I do agree that the flame front will travel more and need more time for it. My friend had a lude hybrid and we ran it at 24 base timing. It made 195 all motor on 92.

The thing you failed to mention is cylinder pressures. Although the spark and explosion all start before TDC, it finishes just after, right when the crank turns past TDC if your good. That is where I personally would want all of the explosive foreces being produced pushing down on my pistons. Thats when there is the most Dynamic compression. Static compression and Dynamic compression are different. Static is when the engine is not moving, measured top to bottom. Dynamic compression is actual compression at a given point in crank rotation during combustion. Now think about when its all moving. If you spark it too late and your explosion finishes with the piston further down your bore, your DYNAMIC compression will be less than it is at TDC or closer to the top. You may as well just lower your compression and advance your timing.

The way I see it, your going to get that heat to your rings either way.Too far retarded ignition timing will create too much heat. Now you have an explosion near a piston already on the way down from interia. At this time the pressures acting on the rings to induce their sealing is less too, so the heat radiating through the rings to the cylinder wall isn't as good either. Id rather have sudden heat with sufficient pressure, and horsepower.

Also, advancing timing will not help "reduce the chance of detonation". Preignition, maybe but its different from detonation. Preignition is when it all burns before you meant it too. Before the piston ever reaches TDC, it can put a hole in the piston from the bottom up. Detonation is the collsion 2 (or more) uncontrolled flame fronts, it will crack pistons from the top down.

 

Team Integra's website has a huge amount of information on this subject [timing, peak cylinder pressure, R/S ratio, etc...].

Its still free....just sign up for an account.