rztmartini Posted September 13, 2006 Share Posted September 13, 2006 ok i tried using the boost compression ratio calculator here and i was getting some depressing numbers. http://www.turbofast.com.au/TFcompB.html it says that with 16 psi boost, 7.4 static compression ratio i would need better than 96 octane. i am using 70% as a best/worst case scenario but would this change dramatically if i had an intercooler, efficient turbo, etc? im tryin to see how much boost i can run on 91 octane w/o water/alky injection. will be using megasquirt on a N42/P90. sorry if this has been gone over already... Quote Link to comment Share on other sites More sharing options...
spork Posted September 13, 2006 Share Posted September 13, 2006 yes, you want an intercooler. Without one you will be severly limited on how much boost you can run on only 91 octane. As far as the calculator goes...its a fun little toy, but not that accurate IMO. There are way too many variables that it doesn't cover such as port and combustion chamber shape and so on. Quote Link to comment Share on other sites More sharing options...
TimZ Posted September 13, 2006 Share Posted September 13, 2006 As far as the calculator goes...its a fun little toy, but not that accurate IMO. There are way too many variables that it doesn't cover such as port and combustion chamber shape and so on. Agreed - I get so sick of seeing the "Boost Compression" myth perpetuated in various web calculators. All they are doing is multiplying your CR by the boost ratio - which gives a number that has no grounding in the underlying physics. Boost pressure has an effect on your tendency to detonate, but not in the way that this implies. Your CR dictates the temperature increase that occurs during the compression cycle (among other things). Basically, the temperature delta is directly proportional to the end cylinder volume divided by the starting cylinder volume. This number doesn't change when you run boost. The boost pressure affects the temperature at the beginning of the cycle, not the temperature delta. Since the name of the game is to keep the final temperature from exceeding the point where the air/fuel mix ignites itself, you can see how each can contribute to your tendency to detonate, but they aren't equivalent - there is no "equivalent CR" that corresponds to your boost pressure - this simply has no real meaning. Sorry for the rant - pet peeve. Anyway, what you should take away from this is that it's very valuable to keep your manifold air temps as low as possible, as you were initially guessing (i.e., high quality intercooler, more efficient turbo, etc. will all help) Quote Link to comment Share on other sites More sharing options...
thehelix112 Posted September 14, 2006 Share Posted September 14, 2006 Nice post Tim! The incorrectness of such web calculators had occurred to me but I hadn't spent any time thinking about why. My neurons thank you. Dave Quote Link to comment Share on other sites More sharing options...
defrag010 Posted September 14, 2006 Share Posted September 14, 2006 I think where you're getting the bad numbers is in the VE. Under boost, your VE is way above 100% Quote Link to comment Share on other sites More sharing options...
rztmartini Posted September 14, 2006 Author Share Posted September 14, 2006 thanks for your posts everyone. i think i may have mis-stated my question. I know that when pressure increases, so does temp. i meant to ask maybe for some personal experiences as far as octane/CR/boost. i think the calculator meant before boosting VE, as much more air than the orig displacement is being forced into the pistons. Lesson learned: online calculators are junk. got it! Quote Link to comment Share on other sites More sharing options...
Tony D Posted September 14, 2006 Share Posted September 14, 2006 I'm with Tim on this one. Take a look at general industry: NOBODY does multiple stages of compression without intercooling. Why? Temperatures would quickly get out of hand. And the temperatures (as well as combustion chamber ddesign) will play the majority of the parts in what octane you require. Many people (myself included) have run 92 octane pump gas to some stupid pressures (in my case 17+psi) without an intercooler. BUT THAT DOES NOT MEAN IT WASN'T INTERCOOLED! At a point above about 10-12psi on pump gas (or about any gasoline for that matter) you end up enrichening the mix quite a bit past what you need for combustion, and retarding timing to keep it running without detonation. Gasoline becomes your anti-detonant. In my case it flashed off in the turbo and dropped outlet temperature of the turbo. No matter which way you try to sort it, PV=nRT doesn't change. it's a physical law. You have to drop the temperature between stages of compression or the temps will soar exponentially. For instance, (ballparking here) you have 80 degree day, and run 2:1 Compression ratio in your turbine. You are running just under 15 psi in your intake manifold. That air without intercooling will be (again, ballparking, offthe top of my head, no calculations done) anywhere from 180 to 270 degrees depending on your compressor efficiency. No biggie, but you got another rise coming. (without boost you would go from atm to 87psi, in this case you will go from From 29 t 188 psi) in both cases the delta is exactly the same, thereforethe temp rise will be the same. BUT, without cooling that 180 to 270 degree air, that 7:1 cr bump will jump that easily to auto-ignition range. now drop that turbine outlet temperature from 180 to say 100, or even better, 80 degrees (through anti detonant injection or proper intercooling) and for all that pressure in the chamber, the temperature will remain the same. if you can drop your intake charge to near ambient temperature, boost will really not be the issue. Blowing the spark kernel out of the plug gap, or combustion chamber design and hot spots will become the focus of your ire... I don't know if this is answering your question, which I think is "how much boost before intercooling on what octane"---and the point I'm trying to get across is that one way or another you HAVE to drop the temperature in between the turbo and the cylinder. While you may not "intercool" it with a radiator, you end up with a defacto intercooler by dumping gasoline into the chamber or turbine inlet to flash off and cool te works before it compresses. in the end anti-detonant is not as an efficient way of doing it, though you can run a lot of boost by injecting it right at turbine inlet. If you monitor that temperature, it will dictate what octane you will need. Most Z-Cars with agressive spark timing (even N/A) will start spark knocking when the temperature is above 110 Ambient under any sort of load. So any time your intake charge is appreciably above 100F with today's gasolines, you start taking timing out, or not being able to load it up really well. Quote Link to comment Share on other sites More sharing options...
veritech-z Posted September 14, 2006 Share Posted September 14, 2006 Tony D, you were injecting fuel in front of the compressor like a draw through? Was that supplemental to the regular fuel injection, or did you just have a draw through carb setup of some kind? Quote Link to comment Share on other sites More sharing options...
rztmartini Posted September 14, 2006 Author Share Posted September 14, 2006 Thanks for your reply, but again i think i mis-worded. i already have an intercooler and would not even think about boosting without it. the questions is: how much boost (with intercooler & megasquirt), 91 octane and 7.7:1 cr is safe? sorry for the confusion! Quote Link to comment Share on other sites More sharing options...
MrFancypants Posted September 14, 2006 Share Posted September 14, 2006 Thanks for your reply' date=' but again i think i mis-worded. i already have an intercooler and would not even think about boosting without it. the questions is: how much boost (with intercooler & megasquirt), 91 octane and 7.7:1 cr is safe? sorry for the confusion![/quote'] Every motor is different (and I cant tell what you are running), but I ran 12-14 psi (intercooled) on a daily basis with a 9.5:1 SR20DE on 91 octane gas. Even in Arizona summer heat (110*F), though admittedly on really hot days I would back it down to 10-12 psi. I had it as high as 18 psi on the dyno without any issues of pinging or knocking. Your fuel maps and tune will play a huge role in what you can run, of course, and I was running an arguably rich setup with a JWT ECU (15*BTDC). The general rule of thumb is never boost more than 5-6 psi without an intercooler. - Greg - Quote Link to comment Share on other sites More sharing options...
LamboZ Posted September 14, 2006 Share Posted September 14, 2006 Thanks for your reply' date=' but again i think i mis-worded. i already have an intercooler and would not even think about boosting without it. the questions is: how much boost (with intercooler & megasquirt), 91 octane and 7.7:1 cr is safe? sorry for the confusion![/quote'] theres so many factors to that question that its very hard to answer it. all depends on your fuel system, running any type of alky injection? timing, etc etc. theres alot of factor then just how much boost can you run. Im running 20psi on a T3/T4 mildish turbo, no intercooler, meth injection, timing is pretty conservitive.. right at 20* (i think), 55lb injectors, big fuel pump, standalone. I do all this on 93pump. My older setup i ran 20psi without meth injection on pump gas all day long. Was intercooled, standalone, 720cc injectors. never had a problem. Quote Link to comment Share on other sites More sharing options...
Clifton Posted September 14, 2006 Share Posted September 14, 2006 Thanks for your reply' date=' but again i think i mis-worded. i already have an intercooler and would not even think about boosting without it. the questions is: how much boost (with intercooler & megasquirt), 91 octane and 7.7:1 cr is safe? sorry for the confusion![/quote'] You won't have a problem running 16-17 psi. Basically timing plays the biggest part in what safe is if the AFR is isn't too lean. Adding extra fuel (gasoline) will help cool some but not as much as most would like to believe. Running lower than mid 11's is unnecessary and will just cost HP without any further cooling benifit. I think where you're getting the bad numbers is in the VE. Under boost, your VE is way above 100% Manifold pressure doesn't change VE. Quote Link to comment Share on other sites More sharing options...
Tony D Posted September 15, 2006 Share Posted September 15, 2006 Tony D, you were injecting fuel in front of the compressor like a draw through? Was that supplemental to the regular fuel injection, or did you just have a draw through carb setup of some kind? The answer: "Both"! Quote Link to comment Share on other sites More sharing options...
Tony D Posted September 15, 2006 Share Posted September 15, 2006 Thanks for your reply' date=' but again i think i mis-worded. i already have an intercooler and would not even think about boosting without it. the questions is: how much boost (with intercooler & megasquirt), 91 octane and 7.7:1 cr is safe? sorry for the confusion![/quote'] Mr Fancypants said it again, and I'll reiterate it: It depends more on manifold temperature and spark advance than just the octane and boost level! If you have a GOOD intercooler, you can run MORE boost for the same situations. "Safe Boost"? No such thing! I have seen intercooled engines blow their pistons out at 8psi. Run lean and all bets are off. Same engine, running properly ran safely over 20 psi, and it was an 8.5:1 CR. Boost is not the issue, temperature and spark advance, in conjunction with combustion chamber condition are! Quote Link to comment Share on other sites More sharing options...
TimZ Posted September 15, 2006 Share Posted September 15, 2006 Boost is not the issue, temperature and spark advance, in conjunction with combustion chamber condition[/i'] are! Ditto. Quote Link to comment Share on other sites More sharing options...
veritech-z Posted September 15, 2006 Share Posted September 15, 2006 So you were using the old single SU draw through on top of the stock fuel injection? That's genious... Quote Link to comment Share on other sites More sharing options...
rztmartini Posted September 15, 2006 Author Share Posted September 15, 2006 Boost is not the issue, temperature and spark advance, in conjunction with combustion chamber condition[/i'] are! Got it...hopefully i can figure out how to install MSnS for this... Quote Link to comment Share on other sites More sharing options...
defrag010 Posted September 15, 2006 Share Posted September 15, 2006 Manifold pressure doesn't change VE. It most certainly does. Volumetric efficiency is a ratio of the engine's airflow to the engine's displacement and rpm. The more air the engine flows, the better the VE. Adding boost (positive manifold pressure) increases airflow to the engine, thus yilelding a higher VE. The formula is VE= 3456 x cfm / displacement x rpm Quote Link to comment Share on other sites More sharing options...
Tony D Posted September 15, 2006 Share Posted September 15, 2006 So you were using the old single SU draw through on top of the stock fuel injection? That's genious... no, on one setup I ran a holley 750 draw through, and has hobbs switches and jets to dump fuel in the linet (egads, that I even ADMIT I did this...) On another there were the ubiquitous Hobbs Switches dumping fuel in the manifold, as well as additional injectors BEFORE the turbo. And FINALLY on a triple blowthrough, again, Hobbs switches with a spearco Injection system using carb jets, solenoids, and a big tank of "anti detonant".... After Spearco Tech Rep got very uneasy finding out I was dumping fuel into the turbo inlet under boost using their non-fuel rated components. So I switched at their insistence to "anti-detonant" which is open for further discussion. On that final setup, I ran up to 22psi without an intercooler, but with injection before the turbo of fluid (take your pick: ethanol, methanol, water, isopropyl alcohol, windshield washer fluid...I ran all sorts of crap through that setup....) BELIEVE ME, MEGASQUIRT IS FAR BETTER! There is also a code available that controls an additional injector for anti-detonant injection before the turbo....... Uses the Fast Idle PWM circuitry I believe. Quote Link to comment Share on other sites More sharing options...
veritech-z Posted September 15, 2006 Share Posted September 15, 2006 Huh. I was thinking of using a full size injector in place of the cold start injector and using megasquirt to deal with it, but I really haven't gotten that far with my MS install yet. And since I haven't built my turbo manifold yet, I guess it's all armchair academic at this point in time anyway. I'm hoping to make some more progress with my wiring harness tomorrow, and MAYBE have it running this tuesday if I'm lucky... Quote Link to comment Share on other sites More sharing options...
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