Hulio-mv Posted March 28, 2008 Report Share Posted March 28, 2008 I was looking for a guide to get me close on overall octane vs compression. I was thinking I had seen one here in the past but maybe not. I tried the search and didnt find what I was looking for so if anyone has a website or can show me what to type in the search to get the info I am looking for thanks. Quote Link to comment Share on other sites More sharing options...
rtcc Posted March 28, 2008 Report Share Posted March 28, 2008 theres one on the noss cool head page. I dont know the address but he has a banner on here somewhere. Quote Link to comment Share on other sites More sharing options...
Hulio-mv Posted March 28, 2008 Author Report Share Posted March 28, 2008 (edited) I will look thanks I see the compression chart but nothing for octane at any given compression, I know there is alot of thing that affect this I was just wondering if there was a chart somewhere to get close to what is needed Edited March 28, 2008 by Hulio-mv Quote Link to comment Share on other sites More sharing options...
fun_on_the_sand Posted March 28, 2008 Report Share Posted March 28, 2008 Evey fuel suplier would have different requirements for different fuels, is not directly related to the octanes One thing you need to calculate first is the Compression Ration (CR) CR = Crank pressure / 1 atm Atmospheric pressure varies with altitude, but is safe to assume it is 14.5 As an example: The crancking pressure on my cylinders is 197 psi CR = 197psi/14.5psi =13.58 => CR = 13.6:1 Now with this number you can go to a race fuel supplier and ask for a fuel that satisfies your CR Go to www.vpracingfuels.com and you can find the specifications for different fuels I use VP-110 since that is rated for CR up to 14:1 I hope this helps Quote Link to comment Share on other sites More sharing options...
gregrob Posted March 28, 2008 Report Share Posted March 28, 2008 ^^ Good info. For us hight altitude guys you would also need to consider that in the equation. Baro is right around 11 here instead of the standard 14.7. My numbers would be 135 / 11 = 12.28CR Right on the edge of pump gas I guess. Quote Link to comment Share on other sites More sharing options...
Hulio-mv Posted March 28, 2008 Author Report Share Posted March 28, 2008 (edited) that is compeession divided by atm correct? M y new domes and o-rings should arive today and I will put it all togrther and see what fuel I will need Basicly I know I will neeed higher than 93 so I am trying to figure out if I willneed to go 50/50 or if i can go a little less thanks Edited March 28, 2008 by Hulio-mv Quote Link to comment Share on other sites More sharing options...
fun_on_the_sand Posted March 28, 2008 Report Share Posted March 28, 2008 that is compeession divided by atm correct? M y new domes and o-rings should arive today and I will put it all togrther and see what fuel I will need Basicly I know I will neeed higher than 93 so I am trying to figure out if I willneed to go 50/50 or if i can go a little less thanks Yes that formula is correct :thumbsup: What domes are you getting? use the chart shown on www.nossmachine.com as a reference, your actual pressure reading may differ from that due to multiple factors (wear on rings and cylinder, temp on the engine, etc) Just to let you know, mixing race fuel with pump fuel doesn't help to prevent detonations due to high pressure on your combistion chambers. Octaneage is a property of the fuel molecules that does not mix. In other words, if you mix 90 octane with 110 octane fuel, you won't end up with 100 octane fuel. instead you will have a very expensive 90 octane fuel. The molecules of the 90 octane fuel will preignite due to the high pressure and that will cause detonations. Quote Link to comment Share on other sites More sharing options...
Hulio-mv Posted March 28, 2008 Author Report Share Posted March 28, 2008 (edited) getting a new set of 19cc. So is this wrong? 91+91+115+115=412 divide by 4 = 103 Would that give you a guess at octane? EDIT I see you already said that is wrong sorry. So why do reople run 50/50 mix? Edited March 28, 2008 by Hulio-mv Quote Link to comment Share on other sites More sharing options...
fun_on_the_sand Posted March 28, 2008 Report Share Posted March 28, 2008 (edited) getting a new set of 19cc. So is this wrong? 91+91+115+115=412 divide by 4 = 103 Would that give you a guess at octane? EDIT I see you already said that is wrong sorry. So why do reople run 50/50 mix? I am not saying is wrong to run 50/50, I am just saying that does not fix detonation problems. Fuel has the property of containing energy (cant remember the name), race fuel contains more energy than pump gas. if you mix them you will get more power than just pump gas. with 19cc domes at sea level will get you 183 psi (www.nossmachine.com) therefore 12.6:1 CR I would use not less than 110 octane, but no matter where you get your fuel, always ask if it is ok for your 12.6:1 CR Make sure the fuel is not oxigenated, that will require you to change the jetting Edited March 28, 2008 by fun_on_the_sand Quote Link to comment Share on other sites More sharing options...
Hulio-mv Posted March 29, 2008 Author Report Share Posted March 29, 2008 elevation is 1490 and both sites I went to said that the atm was 30.1 does that sound right? Quote Link to comment Share on other sites More sharing options...
fun_on_the_sand Posted March 29, 2008 Report Share Posted March 29, 2008 elevation is 1490 and both sites I went to said that the atm was 30.1 does that sound right? It sounds like they are using different units than psi, maybe inHg (inches of mercury) google fo a pressure convertion table, atmosperic pressure around your altitude should be around 14.5 to 13.5 psi thats for sure Quote Link to comment Share on other sites More sharing options...
robert.b.west Posted March 31, 2008 Report Share Posted March 31, 2008 ^^ Good info. For us hight altitude guys you would also need to consider that in the equation. Baro is right around 11 here instead of the standard 14.7. My numbers would be 135 / 11 = 12.28CR Right on the edge of pump gas I guess. There is no way that atmospheric pressure is 11 psi. where you are. In denver, co elevation 5280 ft. the pressure is 29.75 in of Hg. which is 14.6 psi. Quote Link to comment Share on other sites More sharing options...
Dinner Posted March 31, 2008 Report Share Posted March 31, 2008 There is no way that atmospheric pressure is 11 psi. where you are. In denver, co elevation 5280 ft. the pressure is 29.75 in of Hg. which is 14.6 psi. mm if he is around 6000-8000 ft above sea level, then he could be seeing around 11PSi for Absolute Pressure.. Quote Link to comment Share on other sites More sharing options...
fun_on_the_sand Posted March 31, 2008 Report Share Posted March 31, 2008 (edited) Hey guys here are the atmospheric pressure for differet altitudes Sea level - 14.7 psi 1000 ft - 14.2 psi 2000 ft - 13.7 psi 3000 ft - 13.2 psi 4000 ft - 12.7 psi 5000 ft - 12.2 psi From this list you can assume that Every 1000 ft atmospheric pressure drops 0.5 psi or .0005 psi/ft So if yo want to know your local atmospheric pressure to the decimal point you can use this formula: P_atm = 14.7 - (Alt * 0.0005) P_atm: Atmospheric pressure in psi Alt: Altitude in feet For 1490 ft your atmospheric pressure would be 13.95 psi As a disclaimer: This calcluations ar from from the book, actual atmospheric pressure can be differet, usually higher, due to weather conditions, temperature, humidity, etc. This calculations are conservative, that means you will always be on the safe side by assuming lower atmospheric pressure your CR will be higher Edited March 31, 2008 by fun_on_the_sand Quote Link to comment Share on other sites More sharing options...
Hulio-mv Posted April 5, 2008 Author Report Share Posted April 5, 2008 Installed a set of 20cc because I didnt want to deal with finding a high octane fuel locally noone carries much. With 20cc cold compression check was 155-160 each side. figures to be 11.3-11.6 I think. So its still to high for pump gass right Quote Link to comment Share on other sites More sharing options...
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