| Author | Message | ||
| Paul Newman (Newman)
Member Username: Newman Post Number: 881 Registered: 12-2001 |
You're right but that volume ratio is critical when taking cam selection into account because of the effect it has on the actual (or dynamic) compression ratio that is put to use. Also, your volume ratio would be higher than your static or measured ratio with the engine running and operating above 100%VE. Or far below it at lower RPM's, thats why I feel it varies with cam timing. Probably too many factors to come to a solid accurate answer without the costly measuring equipment needed. Who would have thought an engine was so complicated. | ||
| Mark Eberhardt (Me_k)
Junior Member Username: Me_k Post Number: 227 Registered: 5-2002 |
Paul, dynamic compression is really an effect, it's cylinder pressure. It is a good indicator of VE, but strickly speaking CR is a volume ratio, so I would think it's a constant. | ||
| magoo (Magoo)
Advanced Member Username: Magoo Post Number: 3837 Registered: 2-2001 |
I guess I was wrong I just saw him posting on another thread. | ||
| Paul Newman (Newman)
Member Username: Newman Post Number: 877 Registered: 12-2001 |
Well, maybe he's sleeping but he'll be back. | ||
| Paul Newman (Newman)
Member Username: Newman Post Number: 876 Registered: 12-2001 |
JRV has been blasting away on the Matt L thread. He hasnt gone anywhere magoo. | ||
| magoo (Magoo)
Advanced Member Username: Magoo Post Number: 3835 Registered: 2-2001 |
Sorry, It's JRV. | ||
| magoo (Magoo)
Advanced Member Username: Magoo Post Number: 3834 Registered: 2-2001 |
Where is JVR? Did you guys chase him away????? | ||
| Paul Newman (Newman)
Member Username: Newman Post Number: 875 Registered: 12-2001 |
Mark, with variable cam timing, would dynamic compression not change with it as you are changing the amount of charge captured in the cylinder with different valve events? I would think it would . | ||
| Mark Eberhardt (Me_k)
Junior Member Username: Me_k Post Number: 224 Registered: 5-2002 |
Ernie, If your headed to the track think turbo, if you'll be sticking to the street, a supercharger is probably the answer. Rick, the VE and CR and assumed constant to look at the effect stroke has on the engine. CR is very hard to make variable, although I saw an | ||
| Ric Rainbolt (Ricrain)
Member Username: Ricrain Post Number: 304 Registered: 2-2002 |
"... Since VE and CR are held constant..." Are they? The 360 has dual intake geometry and variable exhaust cam. The Enzo has variable intake geometry and both cams are variable. The newer cars are changing the rules, at least by previous standards. The computers controlling them change parameters based on many criteria (Air temp, cooling temp, Atmospheric press, knock, Exhaust O2, etc.). I doubt that a VE table is a simple one line graph anymore.  A friend of mine worked on a Toyota GTP car during testing one year before he was nearly killed in a freak testing session accident. He said their datalogging had over 200 active channels. Yikes! | ||
| David Burch (Merlyn)
New member Username: Merlyn Post Number: 31 Registered: 8-2002 |
The longer the rod the better as far as torque is concerned due to the fact that the rod angle at the incidence of 90 degrees is less with a longer rod. . Here is a pic of a 049 piston; you can see that the pin is as high in the piston as possible giving the longest possible rod for that stroke. http://members.atlasf1.com/desmo/pistoneS33.jpg There is an effective limit on bore size due to flame front propagation. The bore area is a compromise of valve area and burn rate. Tumble and swirl are very important for this reason. | ||
| Ernie Bonilla (Ernie)
Member Username: Ernie Post Number: 458 Registered: 11-2001 |
So what would be the best way to increase horse power and torque, yet still have a hig revving engine? | ||
| Mitch Alsup (Mitch_alsup)
Member Username: Mitch_alsup Post Number: 263 Registered: 4-2002 |
Torque is a function of displacement, volumetric efficiency, and compression. Since VE and CR are held constant, TQ is proportional to displacement. Big bore short stroke engines have many combustion chamber "shape" problems in achieving both high comresssion and rapid misture burn. Long Stroke engines have problems in reaching high engin RPM levels. The optimal comprimize has been moving from longer stroke engines in the 1950's to the bigger bore engines in the 2000 as technology marches forward. The old 2 overhead valve engines of the 1950 have tollerable breathing, intollerable spark plug location, and poor shape for high compression operation. Modern 4 (and 5) valve heads have compact combustion chambers, excellent breathing, and ideal spark plug location. But even here, compariable displacements+compression will find comparible peak Torque ratings (5%). | ||
| Paul Newman (Newman)
Member Username: Newman Post Number: 842 Registered: 12-2001 |
I checked my book for the gto specs and it says 69mm stroke. It also says 308 drysump engines are mounted lower in the chassis which I think is a crock. I found a 288gto web site and it says 71mm stroke. That goes with what steve said. | ||
| Mark Eberhardt (Me_k)
Junior Member Username: Me_k Post Number: 223 Registered: 5-2002 |
High performance engines do a better job filling the cylinders, and make more torque than my rule says they should. The Yamaha yzf is a very high performance engine and makes 1.35 x displacement. It's all about the volumetric efficiency. US 2 valve engines are about 85% and the rule works. A full race engine is about 130% - 135% if it's build right. The yamaha is about 120%, so I miss by 35% on that one. HP is torque x rpm, so the same torque at a high rpm gives more hp, so you need to know more than displacement to estimate hp. | ||
| BretM (Bretm)
Advanced Member Username: Bretm Post Number: 3121 Registered: 2-2001 |
An interesting point I came across today is to create more torque, Ferrari enlarged the bore but kept the stroke basically the same with the Maserati engine they just made as compared to the 360's. This goes along with what Mark is saying and the cylinder fill thing. btw, I'm not lecturing about this stuff, I know a bit but not as much as Mark, Steve, etc. I'm just repeating something I realized when reading over specs today. 360 Engine: 3586cc 11:1, 85.0x79.0mm - 395hp @8500, 275ftlb @4750 Maserati Engine: 4244cc 11.1:1, 92.0x79.8mm - 390hp @7000, 333lbft @4500 Here's some math: 3.6L (219ci) is making 1.26x torque compared to displacement (275/219) @4750 4.2L (259ci) is making 1.29x torque compared to displacement (333/259) @4500 To very different engines, yet they make max torque at relatively close rpms and have a very similar relationship between torque produced and displacement. More math: 3.6L (219ci) is making 1.80x horsepower compared to displacement (395/219) @8500 4.2L (259ci) is making 1.51x horsepower compared to displacement (333/259) @7000 So although the relationship in torque is quite close, almost following a constant with engines of the same type with similar sizes, hp obviously reflects the very different approaches to the engines. Might be a coincidence, I found it neat nevertheless that they went more oversquare and it resulted in less hp/liter but torque remained consistent with displacement, not stroke as I had thought it had more to do with. New Jeep wranglers are 242ci and make 190hp @4600 and 235lbft @3200, which follows that rule you are talking about very closely. They're also redlined at around 5250, so realistically they are setup to turn lower than that rpm. The new ones also make 85% of max torque at idle which I find interesting. That's like 200ftlb @700rpm. If only I knew the bore and stroke of them. | ||
| Mark Eberhardt (Me_k)
Junior Member Username: Me_k Post Number: 218 Registered: 5-2002 |
A good rule of thumb is that torque in ft-lbs is about equal to displacement in cubic inches for a naturally aspirated engine. My 1984 308 QV is 178ci and 187 ft-lbs. The new viper is 500 ci and 500 ft-lbs. A yzf-6r (37 ci)sport bike making 120 hp @ 13000 rpm(over 200 hp/liter), 65.5x44.5 (way over square) 50 ft-lb. I think the reason long stroke engines have a reputation for high torque is there redline. The torque and HP curves cross at 5250, so any engine that runs below that has more torque than Hp, above it hp beats torque. In the case of the motorcycle, or an F1 car hp beats torque by a lot, because they can fill the cylinders at high rpm (hp=torque x rpm/5250) Not obvious but true | ||
| Paul Newman (Newman)
Member Username: Newman Post Number: 840 Registered: 12-2001 |
I havent looked in a book recently about the 288 gto but for some reason, again going by memory steve, the stroke was reduced as well. Maybe I should look it up before I continue unless someone else can verify it for me. | ||
| Steve Magnusson (91tr)
Intermediate Member Username: 91tr Post Number: 1237 Registered: 1-2001 |
James S. -- You've brought in a secondary effect related to the length of the connecting rod vs the stroke. Actually, I think the shorter stroke approaches usually get a small benefit from this for the same length connecting rod (i.e., when the crank throw is 90 deg from cylinder axis, the angle between the crank throw and the connecting rod is more reduced from 90 deg as the stroke is increased). Somewhere I had heard that the NASCAR engines move the piston pin up as high as possible in the piston to maximize the connecting rod length relative to the stroke. Mark F. -- No one is arguing that the available torque wouldn't go up if you increased the stroke and hold the bore constant -- my point was that if you increase the stroke AND reduce the bore to hold displacement constant there's no change in available torque. | ||
| James Selevan (Jselevan)
Member Username: Jselevan Post Number: 272 Registered: 6-2002 |
Steve - if I understand correctly, you are suggesting that the product of piston area and stroke length are constant vis-a-vis torque. Restating, for a given displacement (a function of cylinder cross sectional area and stroke length), increasing one with required decrease in the other does not alter torque. It is not at all intuitive that this is the case. The moment applied to the crank is described as the sum (integral) of the force on the piston multiplied by the angle of the connecting rod at both the small and large ends, and the angular position of the crank journal. Integrating this geometric equation over the length of the stroke will ultimately describe the torque applied to the crank. For example, when the crank journal is at a right angle to the cylinder, maximum "torque" is applied to the crank. At any other angle, the moment applied to the crank is the product of the sine of the angle and the force. Let's continue with the instantaneous position of the crank journal at right angles with the cylinder. At that instant, the small-end rod angle may be 45 degrees (example only), and the large-end rod angle may be 45 degrees (example only), such that the force applied at small-end is piston force multiplied by the square root of 2 divided by 2, and large-end is this fraction of piston force again multiplied by square root of 2 divided by 2. It would seem that this complex interaction of angles would result in a deviation from your premise that piston area and stroke offset each other with respect to torque. I speak without careful analysis or knowledge of this relationship, but only from a mental picture of torque applied to a crank journal and the changing angles as the crank rotates. Your help is appreciated. Jim S. | ||
| Mark Foley (Sparky)
New member Username: Sparky Post Number: 21 Registered: 3-2002 |
I believe that the larger crank throw (and resulting longer stroke) would produce more torque. When I was a tool and die maker I have had experiences with trying-out new dies punch presses and occasionally I would have to try a die out in a press that was too small and sometimes this would �stall� the press. To get over this, you had to make sure the punches were just barely entering the die until the bottom of the press� stroke. At the very bottom of the stroke the crankshaft throw would be at the very bottom of its travel and this point at the very bottom of the throw is the only place a press will develop its full force. This is because the rotational velocity of the crankshaft is basically constant, and the connecting rod is moving the least amount of vertical distance for the same crankshaft movement�the connecting rod has the most leverage. I think the same logic would hold true for a engine�more stroke more torque, but there would be the other limiting variables such as burn efficiency, etc. mentioned by the others. Additionally, from another point of view, in the Chevy Mark IV, big-block (I don�t know about the more recent ones) the truck engines, which are designed for more torque, have the same stroke as the Pass or Perf engines, but the cylinders and connecting rods are 7/16� longer. | ||
| bruce wellington (Bws88tr)
Intermediate Member Username: Bws88tr Post Number: 1281 Registered: 4-2002 |
wow...a thread that i am not even involved with, o, i see my brother paul is doing all the work, ill step back and watch, be careful or ill send horsefly here to boost the thread up to 100 posts and people will not be able to load in time.. hi paul, steve, and matt.. bruce | ||
| Steve Magnusson (91tr)
Intermediate Member Username: 91tr Post Number: 1236 Registered: 1-2001 |
Paul N. -- On the 288GTO only the bore is slightly reduced (81mm to 80mm); stroke is still 71mm. I think they were more interested in beefing up the cylinder liners (and not having special crank/rods) rather than achieving some "better" bore/stroke ratio, but JMO. (From the websites, the 288GTOs capacity had to be limited to 2.8L to meet the Group B rule for turbos of 1.4 x displacment must be less than 4L) | ||
| Paul Newman (Newman)
Member Username: Newman Post Number: 836 Registered: 12-2001 |
I wouldnt think someone would build an engine using that sort of an arrangement, I was trying to isolate the effect stroke has on torque. I know it isnt that simple because of other factors. And yes, an oversquare bore allows for better cylinder filling, I knew that. It has to do with cylinder head/chamber design and more room to work. Now, this may be a little off base but on the 288 GTO, rather than reducing stroke only to achieve 2.8l from a 2.9l engine, they changed both bore and stroke. I wonder if they hit on some magical bore/stroke ratio that they wanted to retain in the 2.8 rather than just destroking a 308 engine. | ||
| Steve Magnusson (91tr)
Intermediate Member Username: 91tr Post Number: 1235 Registered: 1-2001 |
Paul N. (From the other thread) -- Not quite sure how to respond to your question/comments. The force on the piston is the b.m.e.p. in the cylinder x the piston area, and the torque is the piston force x the stroke so the math washes out as equal torque regardless of the bore/stroke ratio (at equal total displacement). I think a smaller area, deeper cylinder would not fill/evacuate quite as well as a larger area, shorter depth cylinder all other things being equal (essentially the gases would be moving up and down a longer, smaller diameter tube). Can't say that I've ever heard of someone commented that they deliberately went more undersquare to get a big performance gain. | ||
| Steve Magnusson (91tr)
Intermediate Member Username: 91tr Post Number: 1233 Registered: 1-2001 |
Same torque -- although the stroke is increased, the b.m.e.p. in the cylinder is acting over a correspondingly smaller bore area. | ||
| Paul Newman (Newman)
Member Username: Newman Post Number: 832 Registered: 12-2001 |
given the same displacement, would increasing stroke and reducing bore not yeild a higher torque output with a lower redline being the drawback? |