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[bouncinofftherevlimiter]

I got bored and have been thinking about this a lot and i have done a lot of research on the subject of bore size, and what I'm wondering is, if all things equal, what would be a better choice for a pump gas FI build. I am weighing options here, and I'm kinda leaning toward the smaller bore, for a couple reasons.

First thought is that, being pump gas (which is pretty much shit) is my biggest crutch, the smaller bore would be easier to "control" the combustion in (I'm not able to put into words how i am picturing this....)

Also, there seems to be less back pressure issues with the smaller bore cars that i have seen, vs the big bore.

Lately there have been a lot of small bore cars that have been putting down really good numbers lately....

the downfall to the smaller bore, is well giving up displacement (no replacement for displacement) This would affect spool time, and also out of boost performance. A bigger motor generally seems to allow a more radical set-up and be more tolerable and drivable.

Cost is going to be similar between the two, give or take a few hundred, because all i really need is block and crank, everything else will be replaced for the most part.

Opinions?



Talk amongst yourselves...

[bigdaddyvtec]

smaller bore = more sensitive to flame speed/timing. bigger bore... more forgiving

[sewell94]

Your limiting factor is the pump gas, and the cylinder pressures it will safely support.   

Power is all about how much oxygen you can burn, or how much air you can flow to supply oxygen to be burnt.  Ie typically more air flow equals more power.

With all things being equal the smaller bore (ie smaller displacement) engine will have to run more boost (if this is your future planned boosted lsx camaro we're talking about) to move the same amount of air compared to the bigger bore (ie bigger displacement) engine.  For example with everything else being equal a 1.6l engine with 18.75lbs of boost will flow the same amount of air as 2.0l engine with 15lbs.  The cylinder pressures will be higher on the smaller bore engine compared to the bigger bore engine.

The higher the cylinder pressure the more sensitive the pump gas is to timing and other things that go on in the combustion process.  Remember the pump gas's ability to support higher compression is your limiting factor.  Thats why v8 guys can make 1000hp on pump gas while our turbo 4 bangers wont. Their cylinder pressures at the same power is alot lower.

Unless there are other factors like bore size affecting head gasket sealing abilities, always take the bigger bore (ie displacement) on pump gas.

[bouncinofftherevlimiter]

Thanks guys, This is kinda pertaining to my future project Tom, but its kind of more a general question, with that project being what made me think about it.

I'm just more making conversation, trying to drum up activity around this place with technical discussions, since there hasn't been much lately...

[PhilStubbs]

For a street car my thoughts are to run the largest displacement you can. As you said "No replacement for displacement" you are always going to make more power/torque with a bigger engine. That's one reason I picked up a couple F's.

[ratcityrex]

I agree with the bigger is better.

[jdmhatchracer94]

thats why I'm leaning towards f or h for the next project. Then again I have sleeved ls block that will be 85mm hhmmm.

[HiProfile]

The new problem is comparing bore size between motor types. A good head (decent combustion chamber design, valve size, etc) will do much better against an larger bore block with a shitty head & CC.

At this point in the game, anything "Honda" has already been tested for you. Pick the largest motor you can afford or fit and get the right stuff. A larger F/H2B with decent cams/intake will be just as bad-ass as an all-out modded b16.

[bouncinofftherevlimiter]

The new problem is comparing bore size between motor types. A good head (decent combustion chamber design, valve size, etc) will do much better against an larger bore block with a shitty head & CC.

At this point in the game, anything "Honda" has already been tested for you. Pick the largest motor you can afford or fit and get the right stuff. A larger F/H2B with decent cams/intake will be just as bad-ass as an all-out modded b16.

That is why i emphasized all things equal, of course when you throw variables like head flow and what not, it will affect it.

With my F hybrid, i was honest impressed it made as much power as it did with such a simple set-up, and the drivability was pretty impressive honestly, at least for a 2.2L 4 cyl. The torque it made just driving around allowed the car to get out of its own way rather well without having to rape it everywhere.

The biggest thing i was getting at with this, Tom answered when he was talking about cyl pressure, and the fact that it takes X-air and fuel to make Y-power, and the larger the bore the more area there is to deal with that said X-a/f mixture, and the lower the cylinder pressure.

[JaredKaragen]

Rule of thumb:

The Most efficent bore/ stroke ratio is 4/4. (inches)

[PhilStubbs]

But all things can't be equal so why bother thinking about it? Just look for the engine that does what you are looking for

[bouncinofftherevlimiter]

But all things can't be equal so why bother thinking about it? Just look for the engine that does what you are looking for

equal, such as stroke, heads, turbo, intake, stuff like that...

Rule of thumb:

The Most efficent bore/ stroke ratio is 4/4. (inches)

Really?? Thats interesting.... Ill have to remember that...

[PhilStubbs]

That's my point, you can't find 100% identical engines other than bore. Why concern yourself with something that isn't possible? Just pick the engine that you see similar results to what you want.

[bouncinofftherevlimiter]

That's my point, you can't find 100% identical engines other than bore. Why concern yourself with something that isn't possible? Just pick the engine that you see similar results to what you want.

I get your point, but I guess I'm not grasping how technical you are getting with it being "100% identical" because I can think of a few different examples where you could have 2 motors built with exactly the same components other than bore, and the one that made me think of this is the 5.3 vs 6.0. Obviously they won't be "100% identical" if the bore is different...

With those motors, you could put together a complete running motor, remove block and pistons, substitute for other, put back together and run... regardless, they will both easily give me the "similar results" I want, so therefore the question was asked as to which would be "better."

[PhilStubbs]

with this site being 97% honda, i ASSumed thats what you were referring to. in that case...... i would still go big bore if you can afford it

[Foowee]

Are we really asking if over or under square is favorable with pump gas?  I'd say quench n low r/s ratio are your friends when it comes to knock resistance.  I'd be inclined to say a larger bore is also more knock resistant than a small bore, if all other geometry is the same.

[bouncinofftherevlimiter]

Are we really asking if over or under square is favorable with pump gas?  I'd say quench n low r/s ratio are your friends when it comes to knock resistance.  I'd be inclined to say a larger bore is also more knock resistant than a small bore, if all other geometry is the same.

Technically they are both over square...

But the larger bore being more knock resistant would be what i was most curious about. Ill have to look into quench, but i don't want to have to deck block too much, depending on how it will affect C/R. I know on an old 406 SBC i built, i had that motor "zero decked" to promote quench, and it raised C/R so that was a bonus, in that application..

R/S ratio is going to be tough to change without going with custom pieces, I'm going to just use shelf parts and keep it as simple as possible

[Foowee]

From what I gather about quench, running a tighter clearance close to the intake valve helps to shove the mixture across the chamber.  One fellow I previously worked with was a dyno operator for gm, he mentioned trying to squeeze the mixture to one area of the chamber significantly increased knock resistance.  He did not say where.  Everything I'm finding leads me to believe, it's around the exhaust valve, but I'm not sure yet.

There are other ways to increase knock limit...  Ceramic coating the intake valves is beneficial.  Preventing heatsoak in the intake passage(insulating gaskets, re-routing coolant away, etc.).  Eliminating crankcase gasses from the mixture... 

Is this mainly about how to select a gm truck motor for pump gas n boost?

[bouncinofftherevlimiter]

From what I gather about quench, running a tighter clearance close to the intake valve helps to shove the mixture across the chamber.  One fellow I previously worked with was a dyno operator for gm, he mentioned trying to squeeze the mixture to one area of the chamber significantly increased knock resistance.  He did not say where.  Everything I'm finding leads me to believe, it's around the exhaust valve, but I'm not sure yet.

There are other ways to increase knock limit...  Ceramic coating the intake valves is beneficial.  Preventing heatsoak in the intake passage(insulating gaskets, re-routing coolant away, etc.).  Eliminating crankcase gasses from the mixture... 

Is this mainly about how to select a gm truck motor for pump gas n boost?

What was done on the SBC was to run the piston at "zero" down in the hole, then the head gasket thickness, which was in the .030 range, was used for the stretch of rod and whatnot, and what it did was shove the mixture from that dead spot of the head, which is the flat area on the head around the chamber that was in the bore area, and turbulate the mixture into the combustion chamber, which helped re-atomize it. Some of the builders i remember hearing used to say that on the best "zero deck" motors, you could actually see where the piston would polish a spot on the head where it just kissed it.

On the full open chamber heads like our honda motors have, I'm not sure how it works, it probably has something to do with what you were alluding to above.

I have always interpreted "squish" as the motor having a place between two objects, generally piston and something else, wether valve or flat spot on the head, that was tight enough to displace the mixture in that area, and re-turbulate the mixture in the chamber to swirl the mixture and help re-atomize it, and also possibly moving the mixture to a desirable location, which would prevent detonation.

Im liking where this discussion is going, anyone want to add to this, I really would like to learn and absorb some information on topics like this.

It loosely is, i just basically wanted to start a conversation about some technical stuff, because discussions around here have become rather stale lately, and my next project will be loosely based on that type of build, so it was kinda of the inspiration for the question.