Inline six. Torque monster? How come?

Come on people. You just read the math involved. In a drag race if both cars are geared about the same so that they are running about the same RPM at about the same speed. The car/engine with the most horsepower will also be the car/engine with the most torque.

 

Thanks Rich,
It's pretty simple, if 2 cars have the same torque at a certain RPM then they have the same horsepower at that rpm.
A dyno actually measures torque and converts it to hp mathmatically. No way of seperating them.
Larry T

 

The reason an inline 6 or 8 will produce more torque is because more cylinders are pushing down on the crankshaft at any given time during a rotation. This is due to the fact that there are no shared crank journals on an inline engine. An inline 4 will have the same inherent torque characteristics as a V8.

 

Am I to understand you are saying that a straight eight has more pistons pushing down on the crank at any given time than a V8? If so can I use this on Things Experts said thread?

 

Lucy, you got some 'splaining to do. Every engine that I've ever worked on had one piston at a time pushing on the crank. The difference is that the angle of crank rotation between firing impulses decreases as the number of cylinders increases.

Bob

 

WOW. Really ?

 

When the crankshaft is at 90 degrees from TDC on the power stroke is the point at which each cylinder develops its maximum torque. Actually, what I typed is incorrect in that an inline engine doesn't have more cylinders pushing on the crankshaft, but rather an inline engine has a bigger percentage of its displacement shoving on the crank journals at a more optimum angle at any given time than a V type engine.

 

I say it depends on who built the engine, no matter what it is.
My six revs high and propels me down the highway at high rates of speed.
My dads v8 would do the same but with out the numb hands and headache.
Wasn't there a fella back in the '50's that had a blown GMC 6 that won a bunch of drags, I think with a little ingenuity they can all be torque and horsepower monsters.
Numbers is numbers.

 

There is no real mystery in this.
Long stroke engines have relatively small bores (for the capacity), and typically must have very small valves and ports. The small ports and valves can create very good cylinder filling at low rpm, which produces relative high torque numbers for the engine capacity. Ideal for a truck or heavy vehicle.

V8 engines typically have much larger bores and shorter relative strokes, which provides much more room up top for a much better breathing cylinder head. As others have already said, the torque is still there, but developed at higher rpm.

Bore/stroke ratio itself has no effect on torque. A long stroke engine may have more crank leverage, but the small area pistons cannot push down as hard with a given combustion pressure.
o A big bore engine with massive piston area and very short stroke pushes down much harder on the con rod, but the leverage to the crank is less favorable. These effects almost completely cancel out making torque independent of bore and stroke ratio, if everything else is kept exactly the same.

What really effects torque is cylinder filling valve timing and compression ratio. Small valves and ports with a tame cam, can give much better cylinder filling at low rpm, regardless of bore and stroke. But power will be strictly limited by the poor top end breathing.

If big torque is your thing, forced induction is always the best solution, regardless of what you have.

 

I think you've got some misconceptions. Regarding maximum torque and rod angle, it occurs when the connecting rod is 90º to the rod journal, not necessarily when the crank is at 90º to TDC. That optimum 90º angle is a function of stroke and rod length. While there may be engines where the rod angle to the rod journal and 90º from TDC coincide, that isn't a constant. With regard to an inline engine when having a larger percentage of it's displacement shoving on the crank journals at an instant, you'll have to specify comparable examples of equal displacement engines and cylinder configuration. When comparing a 6 cylinder and 8 cylinder engine of equal displacement, the 6 will have a greater percentage of its displacement working on each cylinder, but at less frequent intervals (120º vs 90º of crank rotation) so the average torque output (what the dyno sees) is comparable. As to optimum rod angles, there is no difference between inline and V. Both type swing the rod through the same angles regardless of cylinder configuration. Geometry doesn't know the difference.

Bob

 

Maximum peak cylinder pressure is usually always around 20 ATDC, but that just mostly compresses the conrod.

After that, the cylinder pressure rapidly falls off, as the piston descends. Where the rod and crank are at 90 degrees, the crankshaft torque has fallen way off due to the rapidly falling cylinder pressure.
Where the actual torque peak is in terms of crank angle is, depends mostly on rod ratio.

 

Forgive me if I articulated my point poorly...what I intended to state is that bore/stroke ratio has an effect on torque insofar as where in the RPM range the torque (or more precisely, the torque peak) occurs. Oversquare engines (bore proportionally larger than stroke) tend to produce power at higher RPM ranges than undersquare (long stroke) engines.

 

Bore to stroke ratio

 

Bore and stroke ratio can be made meaningless with engineering.Late model Honda S2000 ,the little shit sports car.2200 cc engine 3.4 bore and a longish 3.57 stroke, maximum power at 7800 with a 8000 rpm redline,all the power is at the top end.These engines are know for being soft at lower revs.
By bore stroke ratio theory this engine should be a slow speed 4 cylinder with good torque at moderate speeds .In fact all Honda car engines have longish strokes and hi rpm power curves.

 

Opposite end of the spectrum with Diesels. 6-cylinders pretty much rule the roost for drag racing, with stock-internaled Cummins engines making 900-1100 HP on fuel/spray and producing torque at a 2:1 or better ratio. Duramax and Ford V8's are hard-pressed to produce 1.7:1 Hp/TQ with greater displacement and higher RPM ranges, and they won't hold together without ridiculous investements in parts and machining. The undersquare Cummins produces more torque at less RPM with a much broader curve

 

Sooooooooo.....is an in-line 6 a good engine for a light-weight street rod? Better than a V8?

 

Not if both engines have the exact same cylinder head fitted.

Big block Ford truck engines (460 wedge) have oversquare bores, (bore 4.36, stroke3.85) but where is all the top end power ?

 

Yes indeed, and they also have very efficient DOHC four valves per cylinder breathing.
That is where the power comes from, and high rpm power at that.

 

dollar for dollar - nothing cheaper to make a car go than a SBC - period.

I say that after spending nearly $4k to "almost" make 200 hp with a GMC 270 inline six - looks cool, but can't keep up with a cheap GM crate motor from any number of online resellers.




that said - my next motor will be another jimmy 6

 

That seems counter intuitive...Do you not agree that piston speed as a component of bore & stroke has an effect on the way an engine breathes, especially if all other variables are held constant?

As far as I can figure a short stroke (slower piston speed) functions similarly to longer rod. The effects of longer rods are well documented - longer piston dwell, more complete combustion, and higher compression vs. a short rod.

On the other hand, long rod (or short stroke engines) tend to be less efficient at cylinder filling at low speed due to the slower speed of the piston relative to a short rod (or long stroke) engine.

The question here is - what effect does piston speed have on volumetric efficiency at a given RPM? I think the science is pretty overwhelming that a long stroke relative to the bore - or short rod relative to the stroke (resulting in higher relative piston speed) functions very differently than a short stroke relative to the bore - or long rod relative to the stroke (slower relative piston speed), especially if all other variables are held constant.

 

Don't know about all the ratio's but have ford 150 with 300ci six with fuel inj. Pulls a 8000 lbs trailer , is a torque monster just keep the rev's on the torque peak 2000 to 2200 and it pulls

 

If it inhales forty cubic inches of air during the induction stroke of one cylinder, it does not matter if the bore is two inches, or twelve inches. That amount of air flows in exactly the same way, over the exact same period of crank rotation, regardless of how that forty inches of displacement is made up.

A longer or shorter rod effects the piston motion geometry, mainly around top dead centre. It has more effect on combustion than on induction. The compression ratio depends on the combustion chamber volume and total displacement. Rod length and bore to stroke ratio do not in themselves alter the compression ratio. A 100cc head, connected to 800cc of displacement will give 9:1 compression, regardless of anything else.

Yes, but cylinder head breathing and valve timing have a far greater effect on cylinder filling at any given speed than just bore/stroke or rod length.

Piston speed has no effect on volumetric efficiency. If an engine is turning at 6,000 rpm, it turns 100 times per second. The piston will go from TDC to BDC in 1/200 of a second. The airflow into that cylinder will be exactly the same if the same volume has to fill in the same time. The stroke could be two inches or six inches, but as far as just airflow goes, it still has to fill the same swept volume, through the same cylinder head in the same time, regardless of bore/stroke ratio.
There was a time when I could not believe much of this stuff either. But those computer engine simulation packages take all this into account, and if you read how these programs actually work, you will quickly begin to see things in a totally different way. Much of what is generally believed and accepted is simply not quite what it appears to be.

 

Thanks for the reply...this is interesting stuff!

What's your take on the relationship between rod ratio & dynamic compression?

 

Hay, LittleTimmy, is that a RAMBLER I see in the background??? COOL. Dare to be different!!!

 

Sure is interesting stuff !!

Long rods cause the piston to move slower around TDC, and faster around BDC. So the piston dwells for less time stationary at the bottom of the bore while the big end swings sideways on the crank with the piston stopped.

So I suppose to get the same dynamic compression, you would probably need to close the inlet valve a bit earlier, as the piston come charging back up the bore slightly sooner and slightly faster.

I believe the advantages of longer rods are mainly for getting higher combustion pressures at high engine speeds. Not sure they have any significant advantages for induction.

 

Hay Beatnikpirate, Are you glad you asked??? Bet you you can't remeber it all to take back to your buddies at the cruise in. Print it all out and let them figure it out. Damn this HAMB is SLOOOOOOW tonight.

 

There's a lot of very good information here. I'm always impressed by the knowledge of many of our fellow HAMBers. I find this discussion really interesting and enlightening......having wondered about this for a while. Thank you to everyone who responded.
I will carefully re-read through all this again, trying to grasp as much as I can, and then at the next cruise-in I can dazzle everyone with my brilliance....or maybe I'll just baffle them with bullshit.

 

Antny asked:
"Sooooooooo.....is an in-line 6 a good engine for a light-weight street rod? Better than a V8? "

Good question. I always thought a Ford 300 or Chevy 250/292 would be cool in a <2000 lb lakes style roadster for street use. What do you think?

 

Tis a miyth. They tend to have decent torque due to a long stroke. Tis also true that that could be equaled with a large bore but it rarely is.
They are certainly a lot of fun. If you pick an 6 cyl engine of choice and work with it to see what you personally can get from it you will have a long enjoyable career and will probably spend a lot less money than the competition.
I enjoy playing with my slant six rail and can see my self still fooling with it 10 years from now.
As for the SMALL BLOCK CHEVY IS THE BEST BANG FOR THE BUCK crap. BALONEY!While a decent good engine it is, there are lots of good motors that will match or beat it $ FOR $ I would personally be glad any day of the week to run a $for$ contest with a SBC with a BB Mopar. There is a engine that can be built very powerful for so little it isnt funny. The 360 is another "huge power for little dough" engine. I have never once with either engine felt disadvantaged against a SBC. I just did a SBC and a week later a 440 Mopar (my specialty. ) Cost was within pocket change of each other and I know the 440 will be well over 500 HP and the chev will not be. The Mopar is a street motor to boot. However both customers would have a lot more fun running six cylinders. Nothing tears a V8 guy so bad as being whoomped by a good working six. All the theories are interesting but inside the cylinder the engine does not know it is a six an 8 or even a 10. It just knows it has this bore , this stroke and if it can breathe properly it will make this power. Wether you put 6 together or 8 or 16 even is what makes it all add up. The V configuration is best for balancing which is why Henry liked it. Intake manifold design is also much smaller than an inline and easier to make. Still I have enjoyed the 6s I have been involed with over the years. Lots of fun for low bucks and decent power. If you are inclined to go that way do it. it is real hotrodding at its best. BTW any six can be built for less. 250 chev is probably one of the best deals(307 v8 pistons) and a ford 300 can be whipped together easily as well. (390 v8 pistons work great) The slant six is more fun than a barrel of monkeys.
Don
The photo is of my Wife Linda at Desoronto (Mohawk Dragstrip) in the mid seventes. She just put away a V8 Chevlle for her big win using the 66 cuda behind her which had a 1975 198 cubeed /6 i built for her in it. Mild cam (marine cam) offy intake adapted to 500 Holley 2 bbl. head and block severely planed and head ported with reworked 360 mopar valves. And she really is a little ol lady school teacher. The smile tells the whole story. The Chevelle people were not smiling btw. This car ran high mid 15s and gave 25 MPG Imperial. We drove it as a daily driver in warmer weather and also made two trips t Florida as well. Drove one morning from Jacksonville FL up through NC never under 85 MPH. The little six just hummin like an bumble bee on steriods. This car was fun. Darn near perfect we thought.