Bigger Bore for better gas mileage?

[Lazy JW]

I'm going to ask this question here using the air-cooled VW engine as an example, but it may apply to other makes also.
The air cooled VW is notorious for overheating, especially when todays regular unleaded fuel is used with the stock compression ratio (7.5:1). The late Gene Berg did exhaustive testing and demonstrated improved longevity by reducing compression ratios down to about 6.6:1. Gene did most of this work based on altitudes near sea level and high speed freeway driving in southern California.
What would happen if rather than lowering the compression, we put in bigger bore cylinders? Stock bore (1600cc) is 85.5mm, 88's and 90.5's are readily available (the engine cases and heads need to be machined to fit)
For part throttle cruise (stock heads, cam, and carb) the increased working are of the piston would require lower effective PSI to achieve the same force. Could this same force be accomplished by using a slightly smaller charge of air/fuel mixture at the higher compression level without risking the dreaded detonation/overheating? Or would the gains be offset by additional thermal losses? I'm not talking about crazy levels of compression, and just need to make enough power to get me and my lunchbox to work and back. Or would a set of NOS (New, Old Stock) 83mm jugs for a 1500 be better? I have access to a set of these also.
Any thoughts, comments?
Joe

 

[rbohm]

8) fuel economy is affected by many things, bore, stroke, cam duration, valve lift, bearing clearances, etc. depending on the engine design a larger bore can improve fuel economy by adding to the torque produced by the engine. the reason it likey works in the vw is the low cam timing numbers, and short stroke conspire to kill low speed torque. if you increase the bore on said engine, you gain low speed torque. that said, on large bore engines, good comparison would be the ford 302 and the chevy 305, the 4" bore of the 302 helps the engine make torque when combined with the shorter stroke, but the 305 with its smaller bore and longer stroke will get better fuel economy as the piston pushes down on the crank for a longer period of time even though the rod/stroke ratio is smaller than the 302. what it comes down to is, can you bore the engine enough to make a difference? in the vw yes you can as the cylinders are removable, and any bore cylinder, with in reason and the limits of the case, can be installed at anytime. in a one piece block, like our sixes, you are limited to what can be done with overboring by how thick the cylinder walls are, unless you want to make some radical mods to the block, meaning installing thick sleeves to allow a larger bore(very costly to do).

 

[Anonymous]

"piston pushes down on the crank for a longer period of time"
No.

 

[Anonymous]

Bigger cylinder = more volume needed @ same vacuum level = more mixture used = more pumping loss. Doesn't work.
Smaller cylinder with higher compression uses less gas more efficiently.

 

[Lazy JW]

Sometimes I am not very good at articulating my thoughts with words. The point that I am pondering is if there is a benefit in using higher compression in combination with a bigger bore to improve the combustion efficiency of a lightly loaded engine at part-throttle cruise. It seems to me that under those conditions, the EFFECTIVE compression pressure will be rather low, and if the static compression ratio were raised slightly in conjunction with a larger bore that more force would be applied which would require a smaller amount of fuel to accomplish said force without detonation. I realise that there will be more thermal losses through the greater surface area, but we also know that as long as we don't get pinging or detonation, that more compression = greater efficiency. But higher compression in a stock VW= Meltdown This engine would need to be restricted by using single port heads and small carburetion to avoid having too much volumetric efficiency which would lead to certain death at WOT. I also realise that most folks here are more interested in stuffing more air/fuel mixture in so as to get more power out. That is a pretty well understood process. But improving the thermal efficiency of an existing engine seems to be pretty much overlooked. Or maybe I am just nuts.
Any other thoughts?
Joe

 

[Anonymous]

But improving the thermal efficiency of an existing engine seems to be pretty much overlooked. Or maybe I am just nuts.

there is truth in that...for example a stock coil over high performance coil...people that have upgraded just this small part notice the difference in power...a small bit in mileage..but that is just a hotter, bigger spark...so the coild is a step toward thermal efficiency...

I also realise that most folks here are more interested in stuffing more air/fuel mixture in so as to get more power out.

i think people do this because it is an easier process for power...there is usually more factors involved when trying to improve the thermal efficience...i think something liek this has been covered before...either that or i read it somewhere...something like how even a 25% gain of thermal efficiency would give such and such increase in power and mileage...but to reach that stage of thermal efficiency there would be many problems in the engine since things cant handle the heat...one thing i have done is to add a small amount of hydrogen...please no comments on the hydrogen..think i've had enough....but that certainly increases the thermal efficiancy of the engine...since in the process both hydrogen and oxygen enter the chamber with the fuel the hydrogen/oxygen first ignites which then ignites the fuel quicker and the mixture is quite a bit hotter...i think about 215,000 BTUs of energy are being used if i remember...gas uses about 125,000 and hydrogen uses 90,000 BTUs so hydrogen itself sucks but combined it certainly gives better results...and i'm pretty sure that once i get another high performance coil in my engine...if my seats havent been hardened in the rebuild it had before i bought it...i can probably expect burnt valves in a period of time...i hope to get a new head before that so i dont have to find out...anyways....hope something i wrote has some meaning..lol

 

[BB6Mustang]

Thermodynamic efficiency is directly related to compression ratio. Increase CR and the engine will get better mileage (assuming it doesn't run into detonation).

As for the bore/stroke question. In general (ie. this may not apply to your air cooled engine due to cooling issues) a small bore, long stroke motor will get better gas mileage than a large bore, small stroke motor. Reducing the bore size improves the flame propagation because the flame front doesn't have to travel as far radially to reach the sides of the bore. I may not have explained that very well. Let me know if you would like it re-explained more clearly.

 

[Anonymous]

Thermodynamic efficiency is directly related to compression ratio

oh...thanks..ok...

As for the bore/stroke question. In general (ie. this may not apply to your air cooled engine due to cooling issues) a small bore, long stroke motor will get better gas mileage than a large bore, small stroke motor. Reducing the bore size improves the flame propagation because the flame front doesn't have to travel as far radially to reach the sides of the bore. I may not have explained that very well. Let me know if you would like it re-explained more clearly.

makes sense....but what if u were to increase the bore size and get pistons that allowed for better flame propagation...i know the stock pistons dont have a very good flame propagation.....what would be the best pistons for that? dished? what do u call that area...the quench or something...cant remember

 

[Anonymous]

RE: efficiency and compression: typically, the higher your combustion chamber pressure before ignition, the more efficient the burn is (you get more work out of the same amount of fuel). Probably because the molecules of fuel and oxygen are in closer proximity and can complete the reaction easier.

To me, a VW sounds like a natural for ceramic coatings EVERYWHERE, i.e. piston tops, valve faces, exhaust stem below the seal, exhaust port, combustion chamber face on the head. a well tuned intake/carb/cam/exhaust would probably help quite a lot considering how under-powered these things tend to be.

Probably the biggest improvement would be thermal coatings, though.

Ern

 

[Anonymous]

Regarding detonation issues: often these are caused by poor spark performance, particularly on points-ignition systems. I have cured several engines that knocked or pinged (with CR in the 8 range) by simply switching to electronic ignition. This is especially true on older engines, as the combustion chamber technologies used were largely a 'best-guess' design and were highly inconsistent from one mold to another (or even within the mold that cast my 200's head!). A longer duration spark goes a long way toward curing several pinging ills; you get this for free with most electronic ignitions.