I have a question about displacement that I have never figured out. Does a higher displacement number , "bigger" engine, mean that it uses more gas somehow? For example, would a chevy 305 use less gas than a 383 with pretty much the same parts? What about BBC/SBC? Is there a major difference there? Second question, i see all these signatures with the person's car, year, maybe stats, name, etc. What is this? Do you guys make these yourselves or have someone else do them. Any help would be greatly appreciated

If all were optimized running the same percentage of fuel/air, then the larger engine requires more volume.
How it's put to use is another story. The larger engines produce more torque and could be setup to cruise at a lower rpm through different gearing. So it's baically a larger pump, but turning at a lower speed drawing less air and fuel.
There's a bunch more variables, but it's one aspect of it.
As far as the signatures, look at the top of the page where it says user cp. Open it up and fill in the profile section.

What AM was getting at in the first part is what holds true. Many people will say a 300hp SB will use the same amount of fuel as a 300hp BB. Thats actually true to a certian point a larger displacment engine has the power advantage by being bigger in order for a small block to step up to that level it requires more air more fuel basicly more flow. That amount of flow is usually more then the BB has since it must make up the displacment difference this power will be measured at a higher RPM with the small block. an engines basic design is a pump of course a bigger pump will move more air/water whatever at a lower RPM but crank the small pump a few thousand RPM and it will catch up but its now working harder to prduce the same result.

Lets start off by saying that an engine only makes as much power as is being used or produced, no more no less. Given the same engine effeciency, engine size is quite inconsequential.

Maximum engine efficiency occurs where peak torque is made so...

I will write more latter, need sleep, can't think

Hmm, all this info is kinda confusing, but are you saying that horsepower has to do with the amount of gas mileage you get also? Second question, lets say you stroke a 350, would the gas amount used change?

You basicly answerd your own question. Yes! You increased displacment by stroking the engine. By doing that you increased the capability to move air, more air moved more fuel mixed with that air and more exhaust gas produced.

However, by stroking it, you've increased your torque. Theoretically, you could run a different gear ratio, allowing your engine to operate at lower rpms, putting your gas mileage back to where it was. I think this is overly simplifying it, but maybe it'll help some.

also, stroking is a funny thing. A stroked 350 wiill use more gas then a destroked 400 (to around 383) because the 400 uses a larger bore and then 350 uses more stroke. the 350-383 will have more friction because it has more travel, whereas the 400-383 has less.

v8 generally arnt as effecient as 4 bangers because we have more pistons creating friction then the wee pups.

Just go nos. Only use more gas when ur on the juice. Thats the counter to the argument of nos costs more long term because you need to keep refilling it.

Plus, its dirt cheap in reality, its just marked up like crazy. I pay .38 cents the lbs. Just wished my gta was juiced like my old car.So look around for someone to fill you up. And dont pay 5 bucks the pound.

Make it myself, I guess people can have someone else to make it for them as well.

I must say I have an L03 which is not a very efficient engine, even though it is suposed to be the "economy V8 with 2.73's. I make almost the same gas mileage in my carbed(non-egr) vortec 350 with 3.42's. You would think a 350 would use more gas even when just cruising but it is about the same. It does, however, use more gas when you are on all four barrels, but still not THAT much worse than my stone stock L03. I would say when comparing smallblocks, the more effiecient one for a given rpm will be more economical on gas.

Look at the LS1 engine, I bet it makes better gas mileage than just about all the 305's on the board. It is all about efficiency.

mpg is a by-product of efficiency. You can have a highly efficient large displacement engine that will get better mpg than a mismatched small displacement engine (up to a point at least). There are far too many variables to count. If your looking for decent mpg and power, then do the research to get the absolutely best matched engine components that you can. Driving habits and drivetrain components will also majorly affect mpg.

Note: my combo gets me 24 mph on the highway and 356 rear wheel ft-lbs of torque. Very good on gas for the power.

To get very good gasmileage, you need an engine that make good power down low. Meaning that the engine runs very effeciently and make lots of torque down low about 1000- 1200rpm. This way you can run some serious overdrive or double overdrive and about 3.0 rear ratio. Cruise speed at this rpm will be 75 or so and get 30 mpg. This i what the LS1 is all about.

thing is thoguh with the longer stroke you also come into other situations


first
more leverage. with the piston pushing down at a point further then the pivot point then the short stroke motor it is able to use the energy it has more efficiently

second
long stroke motors generally produce stronger intake and exhuast pulses which if used right can actually reduce pumping loss which helps efficiency. the piston speeds on these style of motors tend to create that sucking sensation a little stronger (no badthoughts boys)

third
long stroke/small bore motors also if I remember right also have some interesting things with the burn rates of the fuel
large bore mtoors tend to be more prone to detonation and less efficient burn when compared to small stroke/large bore motors


there was more but feel like crap on this one so yeah

Thats basically the whole idea of using a bigger engine... to use more fuel.

Bigger motor = more fuel/air --> more fuel/air = more power

oh, shudup and go play with your wankel.

Just to add something here...

Thats also why a supercharger (blower) can make big block power with small block inches. For example, a blown 350 can make the same power as a NA 454, because the blower would be forcing the same amount of air/fuel into the 350 that the 454 uses without the blower.

too simplistic to just say "oh it's larger it must use more air"
sorry guys but there is always more then just one thing



read above post by me and you might see

larger engine might actually be able to use less air

once again too many variables to just say larger would use more

huh the engine must flow more air to make up for the displacement loss

again too simplistic
and also just because the motor is larger doesn't mean it is flowing more air at part throtle then a smaller engine

at WOT yes the motor should suck in more air but at part throttle no motor works at 100% Volumetric efficiency
and at part throttle there are many factors that can make it to where teh larger motor uses more/less air then a smaller motor


also the smaller motor doesn't have to flow mroe air just becuase it is smaller

if you take a 400ci motor which at cruise of 2000rpms flows 100cfm (numbers are just htere for reference)
a 300ci motor at cruise of 2000rpms needs to flow like what 150cfm now (again numbers just for reference) just becuase it needs to make up the difference in displacement


TOTAL displacment little bearing on part throttle operationbecause you are not trying to use the TOTAL displacement

your right in saying you increased the capability to use more air

but does that mean your going to use more air?


lets ask this
lets say you are some guy who can bench 500lbs and all that you know mr macho

so you know you have the capability to lift a lot of weight
now lets put you next to some guy who can bench the bar itself if he struggles (again the type of lift is just for example)

does that mean since the big guy has the capability to lift more that he is going to use all his capability to do so in lifting a feather
or a book
or something like that?


he isn't using his full capability so max capability doesn't mean much


now sure if he was lifting 400lbs in a bench sure he is going to use more energy cause it has gone past the point of the little guy being able to bench it and only the large guy can now

but then again that is only cause the small guy has been maxed out

but think of this

more efficient motor = more power/less fuel


but again that is only at total load does it mean that it will use more air/fuel prolly

at low throttle there is no saying for sure oh it is bigger that means more fuel

are you the mod dude of 3go


ANSWER ME BY NOW

Yes Rx, but simple is good.

I dont know any dyno runs made at 1/2 throttle.

Now if we were to talk about efficent engines my new gokart would win, 3cyl subaru justy. I'd safely say it uses less fuel then any 3rdgen.

and being the question was about gas mileage vs displacement as I understood it I don't know anyone who tries to get good gas mileage running around at full throttle


if getting the most power was the key then my top fuel dragster pumping 5000hp would win
well ok maybe not mine other then dreams but hey

:-p

Ok, I believe we can all agree that an engine is nothing more than a self driven air pump.

The question asks for a comparison between efficiency and mpg, not torque, not horspower, not rpm's, not anything else.

The type of car makes a lot of difference as I believe that everyone can agree that a 350 in a Vega will get better mpg than the same engine in a Sherman tank.

So, if you take a bone stock N/A 350, put it on a stand for testing purposes in a climate controlled environment and run it, you will get x% VE.

If you take that EXACT same 350, in the EXACT same environment and put a 307 crank in it (with proper pistons to maintain EXACT same compression ratio), effectively shortening the stroke by .25", then test it EXACTLY the same as before, you will get a higher VE. This translates to better efficiency therefore better mpg.

The reason behind this is simple. The dwell time at TDC and BDC is shortened (were talking nanoseconds here) which means that even though the shorter stroke means less space to fill, the piston moves quicker at BDC resulting in increased airflow velocity which allows air to keep coming in even as the piston is going back up in the cylinder. This means that the cylinder will be filled at a higher percentage than with the longer stroke.

This is also why a large bore, short stroke engine is more prone to detonation.

This is also why a large bore, short stroke engine will rev quicker.

All else being equal, a 400 will use more fuel than a 350 at any given rpm.

that's all partially true, but you're leaving rod legnth out of the equation of dwell timing for the stroke. a short stroke engine has considerably slower piston speed at the same rpm as a long stroke engine. this is the main reason long stroke engines make more torque down low than an equivillant cubic inch engine using a shorter stroke. the long stroke increases piston speed and along with it increases velocity of the incoming and outgoing gasses. short stroke engines are more prone to detonation because the larger bore engines tend to have less complete combustion. when you fire a cylinder, (or anything for that matter) the flame wants to go in a complete circle first then burn from there. this is the reason the hemi cylinder works so well. from the center of the piston out is the remaining burn which as the piston begins to travel downward it takes the combustion with it and basicly doesn't make a complete burn. anywhere there isn't a complete burn in a cylinder it is going to be a much lower temperature which causes hot spots to develop in the area where the majority of the burn is occuring. aside from the oblong shape of the cylinder on a large bore engine, the only way to properly fill the cylinder with gasses is sufficent valve area and as we all know, the larger the valve the less velocity it'll have at low rpm.

a common 383 has a bore to stroke ratio of 1.0747 to a 305's 1.0736. they happen to be a very close combination with the biggest difference being the cubic inch. here again, you're having to fill a much larger cylinder at the same rpm (assuming same gear/tire/car) and to keep velocity up you'd go with a small valve/small port head which under heavier load would work out great, yet under light load isn't going to fill the cylinder completely and the gasses put to the outside of the bore don't burn as complete. this is the reason the large bore engines of the 70's are gone. not only does the unburnt gasses of a large bore engine cost you in fuel mileage, they also are terrible for emissions. a square cylinder (poor term, but called that by the numbers) such as the 300 inline 6 ford (4 inch borex4 inch stroke) are efficient at lower rpm and are commonly good for decent power to go with the cubic inch. the flame in a cylinder like that is likely to spread to the sides quick and follow the piston as it goes down the bore to finish off the burn making emissions cleaner at the same time. cylinder filling ability is just as important as and part of pistons speed along with burn time which can be affected by piston and combustion chamber shape.

even though the short does have a shorter dwell time will give you that it also has lower avg piston speed.
now sure the less dwell time is allowing a tad more piston speed away from T/BDC but that is quickly and easily overcome by long stroke motor

higher piston speeds also translate into increased air flow velocity
stronger vaccume pulses
stronger exhuast pulses and such

more air flow velocity means more air is able to come in
stronger vac pulses will help with intake pumping losses if setup right
stronger exhuast pules will also help with pumping losses by helping with scavenging

as far as dwell time been a whiel since I thoguht about that so no comment here YET!

well put and shorter than mine rx7speed.

"that's all partially true, but you're leaving rod legnth out of the equation of dwell timing for the stroke."

There was no change in rod length, piston deck was changed to compensate.

"a short stroke engine has considerably slower piston speed at the same rpm as a long stroke engine."

The loss of inertia at BDC more than makes up for increase in drop speed of piston, therefore actually slowing the charge more than speeding it up.

"this is the main reason long stroke engines make more torque down low than an equivillant cubic inch engine using a shorter stroke."

Not applicable in above comparison.

"short stroke engines are more prone to detonation because the larger bore engines tend to have less complete combustion."

Detonation is caused by an uncontrolled flame front. This is why combustion chambers are designed to control the direction of the flame. Detonation will occur also when the air/fuel mixture is denser as is what happens when you pack more material in the same amount of space. This is why timing needs to be advanced and/or higher octane (slower burning) fuel is used.

"a common 383 has a bore to stroke ratio of 1.0747 to a 305's 1.0736. they happen to be a very close combination with the biggest difference being the cubic inch. here again, you're having to fill a much larger cylinder at the same rpm (assuming same gear/tire/car) and to keep velocity up you'd go with a small valve/small port head which under heavier load would work out great, yet under light load isn't going to fill the cylinder completely and the gasses put to the outside of the bore don't burn as complete. this is the reason the large bore engines of the 70's are gone. not only does the unburnt gasses of a large bore engine cost you in fuel mileage, they also are terrible for emissions. a square cylinder (poor term, but called that by the numbers) such as the 300 inline 6 ford (4 inch borex4 inch stroke) are efficient at lower rpm and are commonly good for decent power to go with the cubic inch. the flame in a cylinder like that is likely to spread to the sides quick and follow the piston as it goes down the bore to finish off the burn making emissions cleaner at the same time. cylinder filling ability is just as important as and part of pistons speed along with burn time which can be affected by piston and combustion chamber shape."

Now you have added far too many variables and rendered the comparison useless for it's intended purpose.

This is a semi interesting thread with talk about bore/stroke ratio, dwell time, VE etc.

For REAL LIFE driving, the things that make the BIGGEST difference are:
*Set up (heads, exhaust, intake, timing etc.)
*tuning (not necessarily ECM only either)
* MOST importantly, the way you DRIVE!

MY CAR'S EVOLUTION:
*Stock 305, CFI, Headers, exhaust, 3.23 gear, 5 speed = 24 mpg on the higway @ 75 mph

*Switched to a 3.45 gear = 24 mpg on the higway @ 75 mph

*Same as above w/a STOCK LO5 long block installed = a best of 25.5 mpg on the higway @ 75 mph (I attribute the better milage to the swirl port head)

*Same as above, but w/a tired sbc 400, ported intake, bored TB's, 224/234 cam, 90pph injectors, roller rockers, electric fan = 24 mpg on the higway @ 75 mph


You'll notice here in this comparison, that over the years of changes, I picked up about 140 hp (at WOT obviously) over the stock 305, with NO penalty to highway fuel economy! Obviously at WOT, the 400 will use about twice as much fuel, but you ain't using ~320 hp while cruising at 75 mph, and talking about VE is pointless since it sucks on any engine during that mode of operation... the throttle is almost closed!! lol. The bigger engine required less throttle angle to achive the same ~25 hp to make the car go down the road at 75 mph. The net result was the SAME fuel economy as the wimpy, stock 305. Around town driving was a different story...

Maybe I am reading this wrong, but are yousaying that the LS1 makes GOOD power down low? A TPI 305 has way more punch down low than an LS1, and so does a LT1. LS1's are dogs on the bottom end. I know, I have driven plenty.

lol didn't even know you where making a reply

must have just made it while you where

Did I just read that correctly??? Wow. I sure hope not, 'cause it looks like you just said that a 305 TPI make more low end torque than an LS1.

I'm sure that I must have mis-read that though, seeing as ther LS1 is pushing out 300 lb-ft of torque at 1000 RPM! And 330 lb-ft by 1500. I KNOW you aren't going to try to tell us that a 305 wheezer does better than that! lol.

http://64.81.147.40//albums/album50/P4237722.jpg

eh not going to say anything on last one

What he said!

i'm just not seeing all of the logic to back up his statements are you rx7speed?

Tom 400 CFI, i also agree with you.

first the ls1 no low end torque


pass the pipe :-)



second your right zippy I don't understand for sure what he is saying

I'm surprised that nobody has tried turbo-compounding one of these engines. That technique was good for 20% more power with the same fuel burn, or the same power with 20% less fuel burn.

Check out the Wright R-3350TC. There's even a few still in operation.