elwilliams13 Posted January 22, 2009 Report Posted January 22, 2009 I know that the stock CR is like 6.5:1. Based on that what compression ratio would require higher octane fuel? Also is CR a better indicator for fuel requirements than a compression test? Thanks. Quote
nigels_world Posted January 23, 2009 Report Posted January 23, 2009 I know that the stock CR is like 6.5:1. Based on that what compression ratio would require higher octane fuel? Also is CR a better indicator for fuel requirements than a compression test? Thanks. Rule of thumb is that over 150 psi you will need more than pump gas. Quote
elwilliams13 Posted January 23, 2009 Author Report Posted January 23, 2009 I am at 155 @ sea level. Looking for a more of a CR ratio response for this one. I am close to stock ratio with my set up. Quote
vulvafan537240 Posted January 23, 2009 Report Posted January 23, 2009 I am at 155 @ sea level. Looking for a more of a CR ratio response for this one. I am close to stock ratio with my set up. got a coolhead? if so, put smaller domes in. if not, get your stock head milled down. Quote
fastrthnu Posted January 23, 2009 Report Posted January 23, 2009 Ive run up to 10.5:1 on 93 with noss cool head, wiscos, engine ice, +4 timing with no problem 40:1 castor 927 @ sea level. I dont know about up in the mountains, but at sea level you can run 93 on higher than 150psi for sure. I think 155psi is somewheres in the 9:0 +/- range. there are some variables though. Quote
elwilliams13 Posted January 23, 2009 Author Report Posted January 23, 2009 I got a coolhead with 24cc on a 4 mil my ratio is at about 6.8:1 with 155 comp. and 8.0:1 with 21cc and 185 comp. I ran the 185 comp on avgas for a short time and did good just wondering how it will do when it gets real hot? Quote
2001Stroker Posted January 23, 2009 Report Posted January 23, 2009 10.5:1 is the highest ratio that you can go at sea level, with 0* of timing, and using 93octane. Your port duration also plays a role in that too. With your setup, you should be completely fine with pump gas. And, AVGAS burns friggin disgustingly nasty. Someone (can't remember who) posted some pics of their engine a few weeks ago, that had been using that crap, and it looked like someone threw oily sand in their motor. Quote
blowit Posted January 23, 2009 Report Posted January 23, 2009 I got a coolhead with 24cc on a 4 mil my ratio is at about 6.8:1 with 155 comp. and 8.0:1 with 21cc and 185 comp. I ran the 185 comp on avgas for a short time and did good just wondering how it will do when it gets real hot? I would like to know how you calculated that? Can you post your formula? Brandon Quote
2001Stroker Posted January 23, 2009 Report Posted January 23, 2009 I would like to know how you calculated that? Can you post your formula? That's why I keep stressing to people that actual compression ratios, port duration, timing, elevation, and air temp have ALOT to do with what octane fuel you need to use. You can have 150psi of cranking pressure and need 110octane, and 200psi of cranking pressure and only need 93octane. Different port durations will change your cranking pressure alot. For piston dome size, enter a negative number. Compression Ratio Calculator Metric Conversions Quote
sheefreak Posted January 23, 2009 Report Posted January 23, 2009 A 250R is a good example. 180 is stock compression and you can run 93 octane. Need to no the Compression ratio. We just all happen to know the cranking compression cutoff on Banshee's. Quote
elwilliams13 Posted January 23, 2009 Author Report Posted January 23, 2009 (edited) I got the ratios from my builder. It is a 4mil 64.5mm bore, 21cc stroker domes, -2mm deck, .0035" piston clearance and 186deg duration. He gave me ratios for both 21 and 24cc domes as posted previously. I run +4 timing, air temp is 80 to 95 degrees high humidity at sea level. Edited January 23, 2009 by elwilliams13 Quote
blowit Posted January 23, 2009 Report Posted January 23, 2009 I got the ratios from my builder. It is a 4mil 64.5mm bore, 21cc stroker domes, -2mm deck, .0035" piston clearance and 186deg duration. He gave me ratios for both 21 and 24cc domes as posted previously. I run +4 timing, air temp is 80 to 95 degrees high humidity at sea level. I am sure there must be a catch in how your builder is accounting for the head displacement. The displacements are most certainly not inline with the quoted ratios. I also have no way of knowing if this is SCR or CCR but I am guessing CCR by the quoted duration. Some guys just make domes differently. Is this 21cc tested volume, 21cc trapped (calculated) volume, 21cc calculated total trapped volume, etc. Things that must be accounted for in a stroker are the above deck piston travel, and the dome piston dome displacement. I am sure with a little testing, you could get to the bottom of it. Just to answer the general question here, cranking compression has "zero" to do with the compression ratio. The ratio "is" what is important when building an engine. You are absolutely on the right track by asking about ratios and I commend you for doing so. The pressure on that little gauge is a "guide" not a "bible". Try this one, go compression test your weed whacker! They generally pump up around 170psi or more yet typically have a compression ratio around 6.5:1. That is a general statement keep in mind.... Point is, your compression gauge don't mean squat for building an engine but your calculator does. You gauge is helpful only to test the "condition" of an engine and that is still only one of many tests. If I only had a dollar for every person that said their compression was like new so the motor must be fine....... Brandon Quote
elwilliams13 Posted January 23, 2009 Author Report Posted January 23, 2009 Supposing the ratios I was given are correct what type of fuels would work with these set ups? I will do more testing and measuring but just want to know what ratios to shoot for for pump fuel or race fuel. I got a couple of domes in the garage if I need them. Quote
blowit Posted January 23, 2009 Report Posted January 23, 2009 Supposing the ratios I was given are correct what type of fuels would work with these set ups? I will do more testing and measuring but just want to know what ratios to shoot for for pump fuel or race fuel. I got a couple of domes in the garage if I need them. There again, I need to know how the CR was derrived. Is this CCR or SCR? Static or corrected? Is the trapped volume actually as stated? See, if I just adjust for that dome, I could have to raise that CR by two full points! See where I am at. VERY tough to ball park some of this stuff. Here is what I need: port height of exhaust, neg piston travel from deck to timing edge, piston series, total "test" volume of head, squish clearance, elevation, squish angle. Now I can help you. Without this stuff, I am going off someone elses data and it would only be a guess from my seat. brandon Quote
bbcmudtruck Posted January 23, 2009 Report Posted January 23, 2009 Very interesting thread to say the least. What is the compression ratio for a stock banshee? Would weisco pistons in a stock bore/port increase the ratio? Quote
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