Compression ? - Dodge Dakota Forum

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Tank Dodge Dakota JOIN HERE 1/06/2004 19:01:09	Subject: Compression ? IP: Logged Message: Which is better, 10.3:1 or 9.2:1 in a quarter mile? Really don't understand compression ratios, can someone explain this subject to me?
both Dodge Dakota JOIN HERE 1/06/2004 19:25:51	RE: Compression ? IP: Logged Message: High compression is better in n/a (naturally aspirated) engines. Low compression is better with a turbo or supercharger. Neither one is better in the quarter mile. you could have a 500 hp motor with somewhat low compression, but have a high compression moter that only has 300. That would make lower compression better in the quarter. Hope this helps.
gen1dak Dodge Dakota JOIN HERE 1/06/2004 20:00:41	RE: Compression ? IP: Logged Message: Another thing about compression. Compression ratio is a mechanical representation of the basic amount of pressure a cylinder will generate. There are other variables, particular related to the cam. A low-overlap cam will generate greater cylinder pressure at lower rpm, while a high overlap cam will generate more cylinder pressure at a higher rpm, so, all else being equal, the 1st will generate more low-speed torque, while the 2nd will come on stronger at high rpm. Then, it gets interesting. Get a high compression set-up for better initial power, then add a moderate to high overlap cam to stretch the upper rpm power. This will also limit the tendency to detonate at lower rpm's with a lot of ignition advance (which you'd need to help on the bottom-end). This is just one variation on a theme. There's all sorts of stuff you can do, but you have to look at what you'll be using it for. Typically, a good street engine (normally aspirated) should stay around 9 to 9.5:1 CR. Again, it can vary some depending on your cam and cylinder heads (pent-roof and hemi heads tolerate higher ratios without detonation). Artificially aspirated engines benefit from a lower mechanical CR, and more boost to generate cylinder pressure. Up to a point, the lower the CR and more boost, the better, but this is dictated by the type of boost and efficiency of the system. Talk about opening a can of worms!
tank Dodge Dakota JOIN HERE 1/06/2004 20:11:05	RE: Compression ? IP: Logged Message: Thanks guys. Greatly Appreciated :)
Merlin Dodge Dakota JOIN HERE 1/07/2004 10:47:21	RE: Compression ? IP: Logged Message: Static compression ratio is the volume of the cylinder at the piston's lowest point compared to the volume of the cylinder at the pistons highest point. Compression can vary when the engine is running with different cam timing at different RPM's as was stated by "gen1dak". I believe "gen1dak" was inaccurate stating that hemi engines can handle more compression without detonation. True hemi engines, unlike wedge engines, have no quench area in the combustion chamber and are actually more prone to detonation.
Watch Out Dodge Dakota JOIN HERE 1/07/2004 17:45:45	RE: Compression ? IP: Logged Message: You are going to get your ass reamed by gen1 for questioning anything he drones on about on this site. You must not understand his concept, and he would explain it to you, but you are not capable of understanding.
gen1dak Dodge Dakota JOIN HERE 1/07/2004 20:52:05	RE: Compression ? IP: Logged Message: http://www.hemidesign.com/history.html http://www.hemi.com.au/hemidefinition.htm http://www.stanford.edu/~bmoses/knock.html http://toyotaperformance.com/ign_combust.htm Merlin, read and learn. You may believe what you wish, but in this particular instance, you would be wrong. It is not my concept, and some may not understand this particular one, but the information is there. As for the mook with the anal fixation that signed in under some other name and complained, all I can say is I can cite no greater example for the statement, "Ignorance is bliss." He must be one happy mother...
Jake Dodge Dakota JOIN HERE 1/07/2004 21:29:28	RE: Compression ? IP: Logged Message: gen1dak There are those that will argue no matter what the facts. My thought to the detractors to your post brings this statement to mind. "Never try to teach a pig to sing. It wastes your time and annoys the pig". As one fellow told me, "I know all about the Hemi. I read about them in a mgazine"
gen1dak Dodge Dakota JOIN HERE 1/07/2004 23:52:45	RE: Compression ? IP: Logged Message: Yeah..very true.
justwondering Dodge Dakota JOIN HERE 1/08/2004 07:36:09	RE: Compression ? IP: Logged Message: gen1, Have you ever been wrong ? man you really kick a$$ with your post.
gen1dak Dodge Dakota JOIN HERE 1/08/2004 21:32:47	RE: Compression ? IP: Logged Message: Yes. It's easy to be wrong. Anyone who reads these things knows that I'm not chiming in on every topic. There are reasons for that. Some have already been answered correctly. Some just do not interest me. The rest are about some areas where I have little or no practical info. I don't know, nor do I pretend to know everything about anything.
4XDAK Dodge Dakota JOIN HERE 1/09/2004 07:30:14	RE: Compression ? IP: Logged Message: Thank You !
Merlin Dodge Dakota JOIN HERE 1/09/2004 22:12:40	RE: Compression ? IP: Logged Message: gen1dak check this page out http://www.streetrodstuff.com/Articles/Engine/Detonation/Page_5.php BTW the new "Hemi", which is not a true hemi, now has a combustion chamber with a QUENCH AREA.
gen1dak Dodge Dakota JOIN HERE 1/10/2004 10:31:23	RE: Compression ? IP: Logged Message: Merlin, regarding the Hemi head design.... As far as I know, the only one we're discussing is the 5.7 from Dodge, so what's with the somantics over the "Hemi" design? It's the closest thing to the definition of a Hemi. If you, and others wish to opine as to the identity, and seem to feel it's not an engine deserving of the "hemi" designation, go hump someone else's leg, K? If you do not feel the 5.7 qualifies as a Hemi, why are you discussing the detonation tendencies of a Hemi, and referencing the 5.7? Now, as for the quench area, this is the saving-grace for the ???..."Almost Hemi" 5.7. That's a saving-grace from an emissions standpoint. For anti-detonation tendencies, having a centrally-located spark plug (two for the "Almost Hemi"--hey, let's call it the AH5.7!). This cuts burn distance (and time) in half. If the spark plug was on one side, and it had to burn over a 4-inch bore, the results would be different. The AH 5.7 isn't the only engine with central, or near-central plug location. Look at the LA/Magnum series. Funny, I've seen open chamber (high quench area) and closed chamber (low quench area) LA's rattle from detonation...all pretty close to the same degree of advance relative to ignition vs compression ratio. The AH 5.7, like all "Almost Hemis", and "true" Hemis for that matter, have that nice rounded chamber that evenly distributes heat (this reduces hot spots). The dome shape acts essentially like a shaped-charge as well, which directs the forces of combustion. The exhaust valves are more efficiently cooled as well. With this in mind, there's less tendency to have hotspots that causes detonation. Don't confuse the two. Quench is a major factor in emissions, minor in regards to detonation. The Hemi's head surface area controls heat spikes, and the central plugs compensate for the shape by cutting the burn time nearly in half by virtue of 1/2 distance in burn path. The fact that the new AH 5.7 has domed pistons to reduce quench area (as well as modern electronics, MPFI, and cat cons.) is why it can PASS EMISSIONS. The domed pistons have been around for ages to increase compression, by the way, so reduced quench is also nothing new. HOWEVER, coupling it with the aforementioned emissions equipment and adding a second spark plug is why it's now clean enough. Now, for the link you posted....It would appear to make my point very clearly. At first glance, it would seem to undermine my statements, but read it again and look at the example. "Racers put a dome on the piston to increase the compression ratio. If you were to take that solution to the extreme and had a 13:1 or 14:1 compression ratio in the engine pistons had a very tall dome. The piston dome almost mimicked the shape of the head's combustion chamber with the piston at top dead center. One could call the remaining volume "the skin of the orange." When ignited the charge burned very slowly, like the ripples in a pond,, covering the distance to the block cylinder wall. Thus, those engines, as a result of the chamber design, required a tremendous amount of spark advance, about 40-45 degrees. With that much spark advance detonation was a serious possibility if not fed high octane fuel." Okay, look at the example: "If you were to take that solution to the EXTREME...". 13 or 14:1 compression? Okay, how about 40-45 degrees of ignition timing along with that 13+CR? Anyone? Bueller? Bueller? Look at the last sentence...." With that much spark advance, detonation was a serious POSSIBILITY if not fed high octane fuel. Now let's think about that for a minute. Ladies and Gentlemen of the Jury, show of hands. Who out there is running 13:1 compression with 40+ degrees ignition on pump gas. Even on premium with booster? Probably nothing less than Race gas 105 octane would do. Read it again....""serious POSSIBILITY" Not even a definite case of detonation, but a "possibility if not fed high octane fuel".....in that setup, which was obviously race-oriented. This would also seem to infer that running an even slightly milder setup would not require high-octane fuel. Say, 11:1 CR and 38-42 degrees total advance? Yeah, that's do-able on a street-Hemi with 92-octane pump gas. It's done regularly on the 426. That's a daily-driver, not a weekend special. Most of the detonation complaints I see on this forum are from guys with 9.5:1CR Dakotas that have a MP computer installed and are now getting a few degrees more ignition advance, with 92-octane gas, and not because of a bad belly pan gasket. Well Dammit! That Hemi head design is just a rattling/detonating piece of crap isn't it? Merlin, it doesn't have to evolve into some sort of hostile deal. I doubt I can be much more clear in this matter. If you choose to persist in your beliefs, fine. I'd rather not continue debating the matter. These things inevitably devolve into an arguement I'd just as soon skip. Now, I suppose I've "droned on" long enough, too much for some.
Merlin Dodge Dakota JOIN HERE 1/10/2004 22:27:32	RE: Compression ? IP: Logged Message: gen1dak, I have no desire to get into a pissing contest with anyone or hump anyone’s leg. You claimed, originally, that a hemi engine was less likely to have detonation problems at higher compression ratios than a wedge engine. I claimed that a TRUE hemi chamber was actually more prone to detonation at high compression ratios. We were not then and are not now talking about the 5.7, which as I have said, ad nauseum, is not a TRUE hemi. First of all, the only way to obtain very high compression ratios in a naturally aspirated hemi engine is to use a very large dome on the piston. There are several disadvantages to doing this. First of all, the pistons are relatively heavy; the large dome interferes with the flame travel making it much less efficient; and the large area of the piston dome is conducive to higher piston temperatures which, in itself, can promote detonation. You also have to run a lot of spark advance and/or run dual plugs with a large chamber head to obtain more complete combustion. The old iron Harley Sportster engines, which had hemi chambers and large piston domes and with the spark plugs offset, used to run 45 degrees total advance as a normal tune up spec. On the opposite end of the spectrum, Nascar engines of several years ago, before a maximum compression ratio was mandated as a cost savings measure, ran almost diesel like compression (16 or 17 to 1) with very small volume, fast burn, wedge combustion chambers and basically flat top pistons. These engines produce over two horsepower per cubic inch on 110 octane gas with ignition timing in the 30 degree range. Believe me, I’m not knocking (no pun intended) the Hemi. Nothing breathes like hemi heads. It was one of the greatest internal combustion engines ever designed. It does, however like anything else, have its limitations. BTW, if you think that a quench area in an engine is only for emissions and has little to do with detonation, I suggest that you read up on Sir Harry Ricardo.
gen1dak Dodge Dakota JOIN HERE 1/11/2004 19:15:05	RE: Compression ? IP: Logged Message: I know Jake, I know. I should've listened to you.
gen1dak Dodge Dakota JOIN HERE 1/11/2004 20:23:21	RE: Compression ? IP: Logged Message: Wait. You're comparing the tune-up specs of an air-cooled Harley to a water-cooled auto engine? Whaaa?? Okay, flat top pistons in a Hemi, with a blower. That means light pistons, no domes. Now what, just out of curiosity, what happened to the alleged detonation-inducing factors? Gonna ping or not? As for NASCAR several years ago....several years ago they were running leaded high-octane racing gas. I'm guessing you mentioned their 30 degree ignition timing to point out that they make 2hp/cubic inch with mild timing. BINGO! You don't run a lot of advance on an engine that's running wide open 95% of the time. Granted, in a Hemi, you'd run more ignition timing than non-hemi. You also retard the cam timing. This doesn't change anything I've already said, and cited in your own examples. Side-by-side there are Magnum/LA's pinging away with 9.5:1 CR and small quench areas, but Hemis (331/354/392/426/ and given time for hop-ups, the AH 5.7 too) sitting right next to them(or will be, in regards to the AH 5.7), on the same gas and running equal or even more ignition advance, running 10.25-11.0:1 CR and they ain't pinging. As for the 5.7, any quench that it has was put there for emissions control, NOT detonation control. It's the difference between theory and reality. Deal with it.
Merlin Dodge Dakota JOIN HERE 1/12/2004 12:03:15	RE: Compression ? IP: Logged Message: gen1dak: you seemed, not surprisingly, to have missed the whole point. The reason that I mentioned the Harley engine, which had a hemispherical combustion chamber, was to point out the difference in a 50 year old, not very efficient, design compared to a modern, state of the art, pushrod race engine. Remember, I said opposite ends of the spectrum? I tried to get the point across about ignition timing. Ignition timing is a very good barometer of what’s happening in a combustion chamber. If you use more ignition timing than you actually need to completely burn the fuel/air mixture, you are wasting power. The aforementioned Harley engine needed 45 degrees to get the job done, due to the large chamber, the spark plug not being centralized, and the large piston dome. The Nascar engine needed only 30 degrees because of the efficiency of the very small, swirling, fast burn chamber and the flat top pistons, and a properly placed spark plug. BTW, the gas that Nascar used then is the very same gas that they still use, 110 octane leaded. As I said, they mandated a maximum compression ratio as a cost savings measure for the race teams, not because of the gas. It takes a lot of money for R&D to build engines with extreme compression ratios. I believe the max is now 12 to 1. As far as the quench area in the new “Almost Hemi”, you are right. But don’t you think there’s a reason why a quench area produces better emissions numbers? Gee, could it be ignition efficiency? Incidentally, Harley now uses a much smaller, redesigned combustion chamber with a quench area in their new engines. I’m sure that you’ll be delighted to know that. The quench area or squish area, as it is known to some, was developed many decades before emissions problems were even thought of. Remember Harry Ricardo? It was discovered that an engine could run with a higher compression ratio without detonation or pre ignition when there was a quench area in the combustion chamber. Even your lawn mower with a Briggs&Stratton has one, check it out.
gen1dak Dodge Dakota JOIN HERE 1/12/2004 22:48:44	RE: Compression ? IP: Logged Message: Merl, I'm fairly sure I have not said anything negative about having small quench areas. It is a commonly accepted fact that having small quench areas allows for greater combustion control and quick burn times. A more complete burn produces cleaner emissions. I haven't disputed this in any way. My focus has been on anti-detonation qualities of Hemis. You stated the Harley needed 45 degrees to get the job done. No dispute there. However, the simple fact that it can handle 45 degrees of advance would tend to validate anti-detonation tendencies. How many non-Hemi style engines do you see run that much advance without detonation. As I have repeatedly cited, Hemis routinely run higher compresion with more ignition without detonation. How are they able to do this? The design of the head. The basic shape. Is it the cleanest from an emissions angle? No. That's a different topic. Anyway, my questioning of the Harley head example (and the new heads pretty much suck) was in regards to the thermodynamic qualities of an air-cooled head, which tends to run hotter, and comparing it to a water-cooled design. Come to think of it, the Harley is another good example that backs me up. Hotter heads, but not detonating. Without quench? How can that be? (By the way, it's a rhetorical question). Oh, and high rpm power is better with less ignition advance, Nascar or Bonneville, even a high-speed street engine. This is a reality, regardless of the head design. Low-speed power--more advance. High speed power--less advance. You really need to go back and re-read the posts. I never said NASCAR limited CR because of the gas they were using. The reduced CR came from Supertruck engine rules. As you said, it's cheaper, and as they found, they were still very close in power. Maybe you're missing the point from the angle of it being two different methods. Small quench, quck burn. Large quench via piston dome, slow burn. Slow burn? Compensate. Cut the burn distance....quicker burn. There's a lot that can be done with either style. Also, you keep diverging to quench and it's anti-detonation qualities. I haven't disputed this. I'm talking about hemi-style heads. You have yet to dispute any of the real-world examples I've cited. Why is it the Magnum A engines (for example) rattle with such mild CR and timing (and small quench compared to Hemis, but the Hemis, with more CR and timing do not? Same gas/air quality/ambient air temp. Why?
R/TBlues Dodge Dakota JOIN HERE 1/21/2004 20:42:56	RE: Compression ? IP: Logged Message: Gen1, Detonation does increase emissions. I have not read up on it in a long time, but it is increased Nitrogen and something else that is a result of detonation. It is one of the pollutants that depletes the ozone layer. Aluminum heads help reduce detonation because they don't retain the heat like the cast iron heads (fewer hot spots). That was the main reason Ford was the first to put aluminum heads on all their V8's. Ford had the worst combustion chamber design of the big3. The EPA was tightning up on the emissions of the trucks back in the early 90's. The cheapest and easiest way for Ford to fix their problem was to go to aluminum heads. Dodge was the last one to give in and put aluminum heads on their truck engines. It was easy for Dodge to put it off because Dodges were less prone to detonation (better design). Ford had the worst design and Chevy was in the middle. Consequently, that's the same order that the 3 changed to aluminum heads. It was all based on emissions.
gen1dak Dodge Dakota JOIN HERE 1/21/2004 21:15:57	RE: Compression ? IP: Logged Message: Where is it that I said detonation doesn't increase emissions? Detonation interferes with normal combustion, which will increase emissions. As for aluminum heads, the fact that they dissipate heat more quickly than cast iron means greater compression can be run (typically a full point more-it depends on the head design....here we go again) on the same gas and other settings without detonation. Nothing new here. Higher compression makes for a hotter burn, which helps improve emissions and increase power output.
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