What are the benefits of boost on a low compression engine VS high compression engine? Is there really a need to lower your compression ratio if you are going to use sometype of forced induction? If so why?
What are the benefits of boost on a low compression engine VS high compression engine? Is there really a need to lower your compression ratio if you are going to use sometype of forced induction? If so why?
Yes, you will severely limit the amount of boost you can run by using too high compression ratio... The cylinder pressure gets pretty extreme.
Can you be more specific.Quote:
Originally Posted by RealMcCoy
wait....aren't you the dude with the supercharged 3/s?
Too much pressure, engine go boom.Quote:
Originally Posted by Blown3000
How to add a turbo to a non turbo car.
Some light reading that should break it down simple enough.
i swear this is the same guy who loudly protested when he was told that superchargers aren't that great of an option for these cars. i don't know if he'd take the advice of people who run turbochargers.
Yes and still running fine.Quote:
Originally Posted by IPD
Have done so now I want to hear from the forum.Quote:
Originally Posted by whitedragon
blown3000, you need to look at the effects of gasoline at a high pressure. as you increase the pressure, the gasoline will ignite sooner. when you increase the pressure high enough, gasoline will ignite with no spark, thats called pre-detonation (or knock). so when you have a high compression engine, then throw boost on top of that, you can cause the gasoline to ignite before the piston is at the top of the stroke. as you can imagine, that is extremely hard on your engine internals, and more specifically, your bearings. this is how you flatten out your main bearings (rod bearings are affected more by oil starvation).
and just so you are aware, what i just explained is how a diesel works. they dont have spark plugs. they also run like 16:1 compression ratio so they can get a whole lot of cylinder pressure built up. also, diesel burns at a lower temp than gasoline FWIW which is why gasoline wont run in a diesel... ignition wouldnt occur as it should. diesel: ~210C, gasoline: ~230C
I think the only person that was loud was you. I will listen to anyone, especially if they are knowledgable and have first hand experience.Quote:
Originally Posted by IPD
I understand the principles of pre-detonation and have addressed that issue. Wondering whether there are limits among other issues. Also like to hear the upside to boosting higher compression engines.Quote:
Originally Posted by futurevr4man
the only upside to boosting a higher-compression engine is that you "peak out" at a lower PSI...which means that you don't have to have your boost system held together with massive amounts of force to prevent boost leak.Quote:
Originally Posted by Blown3000
you will make less power on the same octane.
the limits are your cylinder compression. you can lift heads on these cars readily without taking the proper precautionary measures. fuel is your biggest limit in general though, as it is what is creating the power. obviously if you knock early enough and frequently enough, your limits would be whatever your pistons/rods/crank/bearings can take.
upsides are that you can make the same power at a lower boost level. to be honest, the honda platform doesnt even tune for knock detection, they tune for most power made. i have a lot of honda buddies that are actually knowledgeable and not just honda fanbois, and they have made some impressive numbers out of their little engines. they last as well, but that has a lot more to do with honda materials engineering than anything. they are durable little engines.
there is also a benefit of more torque with higher compression, whereas you can have a higher horsepower with lower compression. higher compression changes quite a few things, and depending on your ecu, it may not like it as well.
im being nice and playing this question game here, but what is your goal? high compression boosted engines are nothing new, not even for this platform. there is no secret to it, its physics and intuition that will tell you what you can and cannot, should and should not do. if you are sticking to super charging, you very well may want higher compression because you probably wont push that much boost. you will need to make sure your intake temps dont get too high because that will set off detonation earlier as well.
are you the older (40's maybe) guy that came to NG with the red supercharged 3k?
I'm not sure what you're looking for...? A lesson in simple physics? how a motor works? Maybe I'm not grasping the question, but it seems pretty simple to me... Do you understand why you can't run 20:1 compression in a gasoline motor? If you increase the volume of air in the cylinder by one atmosphere, you've already compressed it 2:1. Do the math, and you'll find that the difference in compression between our N/A motors and turbo motors is no accident...Quote:
Originally Posted by Blown3000
You raise a good question, maybe I can help shed some light without talking about specifics.Quote:
Originally Posted by Blown3000
The higher the compression ratio (CR) the more power an engine will deliver. With that said The rule of thumb is that a higher compression engine (HCE) will perform much better 'off boost' than a lower compression engine because it has its own natural compression to generate power. Generally speaking a HCE will not have as big a jump in power as will a lower compression engine (LCE) will have as boost is applied. Normally the boost threshold will be much lower in a HCE than in that of a LCE.
Normally speaking the higher the compression ratio, the more basic or natural torque the engine will produce. By adding boost, regardless of the means, increases the effective compression of an engine. This increase in cylinder pressure will translate into more power and torque due to greater expanding exhaust pressures.
By dropping the compression ratio allows a higher amount of boost to be used, which translates into a greater volume of fuel and air to be introduced into the cylinder. Naturally this generates more hp and torque - so long as the greater volumes of air and fuel are being delivered. Off boost the LCE will perform more poorly than that of the HCE.
As said by others, forced induction devices which increase effective compression ratios can and will cause denotation but that problem and well as other issues, due to increasing cylinder pressures, will have to be dealt with as boost pressures increase.
IMHO, a higher compression engine with moderate levels of boost make a good combination for a street machine.
There's also the benefit of better off-boost performance. Turbo lag is not as noticeable.Quote:
Originally Posted by IPD
As for why you can't run higher compression and higher boost? There's no reason why you can't really. You'll be spending more money to use less boost, and I don't believe the higher comp is going to offset the hp loss from less boost. This chart isn't for our cars specifically (don't think anyone has done one specific to our vehicles) but it's not going to change much:
http://img28.imageshack.us/img28/7950/crchart.jpg
Not taking credit for this chart at all, but I can't find the one I had made for the Starion/Conquest group a decade ago so I have to make do.
Using this chart and knowing how much work 3S guys put into their cars to run specific psi on their cars can give you an idea of how much you will have to do as well. Want to run 15psi on 10:1? Prepare to build the car as if you were going 22psi on an 8:1 motor and you should be safe.
There are other variables that will change this depending on your setup (direct injection motors run higher compression and higher boost due to how the fuel charge is injected), but general rule when not talking about new, high tech motors is that higher boost + higher compression = lot more build money and more things to break.
Nope - not meQuote:
Originally Posted by futurevr4man
Sorry, You may think your answer is pretty simple but I'm not understanding what you are trying to say. Why don't you put it in simplier terms and help me with the math!Quote:
Originally Posted by RealMcCoy
Let me see if I understand, by lowering compression you are able to run more boost effectively by adding more air to the cylinder thereby generating more power. In higher compression engines power is derived not so much by adding air but more along the lines of compressing the air tighter.Quote:
Originally Posted by ibsorgn
Is this why big supercharged dragsters run compression ratios as low as 5 to1?
You're starting to catch on....Quote:
Originally Posted by Blown3000
There are two completely different factors at play. A motor can be envisioned as simply an air pump... The more air you can pump, the more power you can make. The secondary factor is how much energy(power) you can extract from the mixture you have. The more you compress it, the more energy will be released during combustion. This is not only mechanical compression ratio in play, but also camshaft profile and cylinder head design. There is always a limit to the amount of cylinder pressure any given motor can withstand on the fuel used.... The higher you go with the octane rating of the fuel, (burns slower with higher flash point) the more pressure you can get away with...
Boyle's law states that gas volume and pressure are inverse at the same temperature. As one doubles the ether halves for a fixed amount of gas... That can also be interpreted by if you double the amount of gas in a fixed volume, you double the pressure. Let's imagine you have a perfect intercooler, and you get no temperature or pressure drop as you compress the air: If you increase the the amount of air by one atmosphere(14.7 lbs/sq.in) you have doubled the air charge in the cylinder, and doubled the pressure.
At 10:1 ratio, you decrease the volume by 90%, so your pressure rise on a 100% efficient N/A, with no thermal expansion calculated, would net 147 psi.
At 8:1 compression you decrease the volume by 78%, so your two atmospheres of boost would net you a pressure rise to 134 psi.
At the same boost level (two atmospheres) a 10:1 motor would net 291 psi... Seeing a problem with this picture?
Of course those are all theoretical numbers that would be complete bullshit in the real world when you factor in thermal expansion, pressure drop, and fuel mass... But I think it makes the point.
Yes. Of course you can boost higher compression engines for performance improvements but to build for significant boost the CR should come down to be effective.Quote:
Originally Posted by Blown3000
I would like to hear more details concerning your statement. Please be specific. Thanking you in advance.Quote:
Originally Posted by IPD
based "unscientifically" on me never having seen any 10:1 3/s run above 350awhp on pure pump. 8:1's can break 500 without breaking much of a sweat on pure pump.Quote:
Originally Posted by ibsorgn
Pretty simple statement... You will reach the knock limit of your fuel with less boost. Therefore you will move less air, and make less power.Quote:
Originally Posted by ibsorgn
Was looking for a technical explanation. Might want to quantify your statements so that folks know when you're writing from a point of knowledge or just making an off-handed remark.Quote:
Originally Posted by IPD
Yes that's a pretty simple statement, however I thought IDP was equating octane to energy not resistance to ignition. Actually I still don't know, maybe he was. Many times these off-handed remarks are very nebulous while others are very direct and informative such as your answer in post #21.Quote:
Originally Posted by RealMcCoy
i put in my $.02 from what i've observed. i'm not the resident expert in fluid-dynamics, physics or engineering. i'll let everyone else fill in the blanks as to why something is the way it is--which mccoy did nicely.Quote:
Originally Posted by ibsorgn
Here are my opinions from driving both a stock TT and now having a boosted 10:1.
At 6-8 psi the 10:1 TT felt stronger in the bottom end and had more punch to it that a stock TT. This is due to the higher compression giving it a better bottom end and helping the turbos spool faster. About 6-8psi on a 10:1 TT would probably be about what a stock TT runs HP wise.
I was able to get to 12 psi before having to add a FMIC to keep the intake charge temps down as the stock SMIC were not up to the task.
At 14psi I had to add 50/50 meth to keep knock under control. I was able to get it to 16psi and stable before started having knock issues again. I relate this to running out of injector where even bumping the base fuel pressure to over run the stock 360's would not help.
I now have a set of 450's I'll be adding come spring and re-tuning. Hoping to get to 18psi but I'm pretty doubtful. At the 16psi limit I hit I'm guesstimating 360-380 hp. Never was able to get it on the dyno as I don't have one close by me.
I'm basically testing the limits of boosted 10:1 to see what my limit will be. This is on a complete stock cast crank bottom end and stock heads.
Don't want to hijack Blown's post but out of curiosity did you try any high octane fuels (racing fuel)? What type of Air fuel ratios (if you monitored) did you hit at 16psi? Very nice to have a fellow that has actual experience in this matter.Quote:
Originally Posted by DrGonzo
I have only ever run 93. No race gas around me and the few times I went to the track they only had leaded which I don't want to run.
I'm really trying to see what she is capable of on pump gas. So staying away from the 100+ & E85.
AFR's tuned to about 11.0 with the meth on full.
Once I get the 450's installed I'll have to do a full re-tune and mess with the timing more to squeeze the last power out of it.
lots of people have 10:1 tt experience. Gonzo's just gone a bit further with meth, etc--whereas most everyone else throws in the towel and goes 8:1 when they hit the wall way below 400whp.Quote:
Originally Posted by ibsorgn
i don't think ethanol/race gas is relevant. compare pump+meth numbers are available on several dr750 cars. the gap between thier whp and gonzo's on a 10:1 is almost 300whp. no need to compare multiple fuels when 93+meth already shows the contrast nicely.
http://www.3sgto.org/f2/dyno-results...750s-2102.html
now i'm assuming that gonzo is running 9b's. even so, if he upgraded to, say, 13t's...the limit would be the compression ratio of the engine, not the capability of the turbos (and we know that 9b's start to fall down after 14-15psi). my hypothesis is that on a good day (50 degree temps) and pump + meth-on 13t's--gonzo might be able to scratch 400whp...but that's it; larger turbos wouldn't add any additional benefit. and 400whp can be done without breaking a sweat on basic turbo upgrades for an 8:1.
You make a very good point, but leave out what the pressure actually does in the cylinder. It is (mostly) not the pressure that makes the gas preignite, but the temperature. By severly increasing the cylinder pressure, you exponentially increase the temperature of the air charge. Which is basically what an intercooler tries to prevent. By chilling the air, you make it denser. Which increases the in-cylinder pressures....but at the same time, significanly reduces the temperature that can be generated FROM that pressure (if the air had 0 energy, compressing it wouldnt create any....but there was energy, compression causes all the molecules to bounce around in a smaller space, which causes friction and an increase in temperature. there is a simple equation for how pressure and temperature relates but i can't seem to remember it right now.) Basically, the lower the temperature going in, the less hot it can get, which reduces pre-ignition potential.Quote:
Originally Posted by RealMcCoy
All race gas does is resist pre-ignition at a higher temperature than an equivelent fuel at a lower octane. So it can be safely injected into a higher temp environment with no worries (or less worries) of preignition.
It is finding the balance of compression ratio / intake air temp / octane used that is the key to turbocharged, supercharged, and even naturally aspirated very high compression engines.
Yeah I'm running stock 9b's. There is no need to upgrade turbo's as the limit of the 9b's is matched to the limit of the 10:1 from what I have done. I have a good feeling that I have hit my max with the 16psi not only with the 9b's but with what the 10:1 can handle. I still want to see if I can get to the 18psi mark even though I know he turbo's will not hold it to redline....
I'm currently building a stock TT block and once I have it completed I'll be swapping out the 10:1 block. Then I start tuning it with the 9b's, get it stable and maxed, then probably look at dr650's. It will probably be 2 years before I get to the dr650's as Iam in no rush to build the engine up.
That statement is completely true and it was one of the walls I hit multiple times. Hence the reason for having to add the large FMIC and the water/meth injection. At a certain point, it was all about keeping the air intake charge cool with the high compression. The stock SMIC are just not efficient enough to keep the charge cool. Even just regular driving if I got on it I would get knock. Just not enough area in the SMIC to keep the charge cool enough.Quote:
Originally Posted by Keyan
Keep in mind that I have enough bolt on's to properly support a 600hp TT "Minus turbos" and I am not to the 400hp mark. But I also know that all of those bolt on's will be moved over and help support the next 8:1 engine I am building so the money spent was not in vain. After I finish the block my only cost will be the DR650's since I have all of the supporting mods already.
I really appreciated the numerous points you have all made. From your inputs forced induction on a NA, in my case supercharging, has its benefits although somewhat limited by comparison to motors using lower compression ratios. From what I've read and your posts, combining a higher compression engine with forced induction gives you better low end throttle, which as a street engine and a daily driver is where I live. From what I gather, my combination was a good choice and I'm happy to have learned that.
Glad to see you probed this issue. You will have no regrets supercharging your NA. I've been running my Magnuson MP90 for 4 years without a hitch. These little motors respond well with the low rpm boost provided by the supercharger, and your right about the power being provided in our normal low throttle DD rpm range. I'm can be at full boost at 2500.Quote:
Originally Posted by Blown3000
Long time getting back to this forum. 4 years problem free! That statement really knocks a hole in those that nay say supercharging. I've been into my mods for about a year and haven't experienced any problems to speak of. I just don't see any drawbacks on supercharging these little motors other than it's not the normal power adder route.Quote:
Originally Posted by ibsorgn
I am a bit late to this thread, but perhaps this will help the OP. There are a couple of things to keep in mind when comparing two engines with differing static compression ratios and boost.
Temperature and pressure are both factors and they are interrelated but it is not necessarily as straight forward as it may seem. Doubling the air/ fuel charge in the chamber (through boost) results in an increase in peak cylinder pressure by approximately 20%, but average cylinder pressure throughout the cycle is much higher, resulting in significant power increases. The static compression ratio will raise peak pressure as well, but will result in a much smaller increase in power due to much lower average cylinder pressure throughout the stroke.
Compressing the air prior to entering the cylinder through forced induction creates heat, the amount of additional heat is determined through various factors, one of which is the efficiency of the device used to compress it. Intercoolers are used to reduce the temperature of the incoming charge, thereby increasing the air density and allowing higher boost levels. Reducing the charge temperature through proper intercooling significantly reduces the chance of detonation, which is what allows you to run slightly higher boost.
Because the higher static compression ratio increases peak cylinder pressure without the corresponding significant increase in average cylinder pressure (which is where the real power is made), you give up some power because you have to give some boost-- assuming of course that all else remains the same.
Part of the problem in this thread is that most people in the 3s community have not ever written a timing map. A 9:1, and 8:1 engine can run the same boost, but the 9:1 engine will need less ignition timing, and the room for error between mbt, and detonation will be much smaller. The new BMW turbo's run 10.2:1 compression, and can make plenty of power.