How do you guys know how much compression is needed?

Discussion in 'PSI Superchargers Tech Questions' started by William Payne, Jan 22, 2009.

  1. William Payne

    William Payne New Member

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    I have been wondering this for a while now but how do you guys know how much compression you need , on gas with a blower you generally have a low compression (under 9/1) same with nitro , yet you alchy guys really go way up there with the compression ratios on your blown motors why is that? and when you add nitro to the blown alchy mix how do you know how much compression to have based on the nitro percentage?
     
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  2. Creech

    Creech Member

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    Guess...
    Then analyze the carnage. :eek::eek::eek::eek:
     
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  3. thjts

    thjts New Member

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    William,

    There are lots of threads on this web site that have lots of information, more than you can think of asking questions about. May I suggest you have a good read of everything here? Apart from the deepest, darkest, tuning secrets, everyone here is very open.

    Paul.
     
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  4. William Payne

    William Payne New Member

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    Yeah I have searched and searched but never really got the answer I wanted , I kept getting info of just people saying oh run this ratio or so and so runs this so thats what you do , or some random calculation of how you work out your compression ratio based on your engine setup . I am wanting to know the technical reason for running one compression ratio over another , for example if someone runs 10-1 then why , or if someone runs 12-1 then why . I am wondering why a blown alcohol engine can withstand higher compression ratios then a gas engine or a nitro engine and why you run those high ratios with the blower .

    I would also like to know how adding nitro to an alchy combination effects the chemical makeup of the fuel and how differently it reacts under compression and how you would know what compressions to run based on the nitro percentage .

    Sorry if I sound rude but I read alot about internal combustion engines and it gets anoying reading stuff like "You must run this blah blah" or something because thats all well and good to know that thats what you run but I wan't to know why thats what you run .

    I wan't to be an engine builder oneday and the last thing I want to be is one of those guys who builds an engine or changes something on an engine but doesn't actually have a clue as to the technical reason why he is doing what he is doing.
     
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  5. eli

    eli Banned

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    On a screw blower hemi, Norm Drazzy former owner of PSI Say's, 10.3:1 to 11.4:1, Perché, non so, Were not all rocket scientist, KISS! :)
     
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  6. Dale Finch

    Dale Finch Member

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    Hi William. the engine will tell you quickly when things are not happy. Obviously melted parts are bad. Crushed rod bearings, piston pins bending or going out of round, moly flaking out or the top ring, intake and header bolts loosing do to detonation, piston skirts collapsing. The above posts are right. It is important to understand a couple observations. Norm gave this range of compression for alcohol PSI engines. I personally would not try above 11 to 1 on a PSI but we run on the westcoast at sea level a lot with great air. Lots of fast cars run in the 9.?:1 with lots of timing. Humidity is a big factor. Had experience with eastcoast tuneups that were hard on parts at sea level. Humidity is 35-40% here. Much higher in the east.

    Generally I would start on the lower side. Get your headgasket picked that your manifold fits, pushrod length works, quench height is correct and cc the motor. Then pick a shorter rod by .010. What ever you want to lower the comp by. Then as the car gets sorted out you can just cycle a longer rod by .005 or .010. If you do it by increments you can sneak up on the comp and watch the indicators. Comp, timing, gallons per minute and gear ratios all work hand in hand. You don't say what type of racecar you have. If it is a 3 speed or 5 speed car compression may be different because it stays at a rpm for a shorter period of time. Of course I am referencing a full on PSI race car and rods get cycled out so often that one can experiment. Of course comp doesn't have to be the same in every cylinder.

    Hope this helps as a thought starter.
     
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  7. JustinatAce

    JustinatAce Member

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    The biggest differences between Gas, Alcohol and Nitro are their burn rates, the temperature at which they burn and the amount of air it takes to burn.

    Gas takes far more air to burn than alcohol, and gas burns at much higher temperatures. Gas also has a wonderful tendancy to knock at very high cylinder pressures when it has plenty of air to combust. Blown gas engines have to run lower compression ratios, otherwise the mixture will knock because of the increase in cylinder pressure.

    Alcohol takes *a lot less* air in comparison to gas to burn and doesn't have the lovely knocking quality until much higher pressures. Also, the alcohol burns cooler than gas, so theoretically :D it's harder to melt your parts with the higher compression ratios.

    Nitro.. Well, Nitro is nitro. Nitro isn't a true hydrocarbon like gas and alcohol. Its essentially Nitrous Oxide mated with Methane. It brings its own oxygen to the table, allowing tremendous volumes of fuel to be burned. Nitro burns a lot slower than gas or alcohol in comparison(demonstrated by the header flames), but the combusted energy of the fuel is incredible because of the fuel amounts and it's burning temperature. We all know more fuel and more oxygen = more horsepower. The more fuel and oxygen you compress makes for greater cylinder pressures. As I'm sure any A/Fueler here can tell you, Nitro will tell you when it's unhappy, and it usually isn't pretty.

    Hope it helps a little.
     
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    Last edited: Jan 23, 2009
  8. JM

    JM Member

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    2 cents

    Every application is different. Alcohol loves compression, and compression makes power. You can squeeze the alcohol tighter because it is a cool running fuel. Gas heats up and pre ignites under higher compression. I've run a roots on alky with 13:1 static and it worked great (little less of a tuning window than lower compression). With screw blowers, more power is made by turning up the blower and filling the chambers with air/fuel than squeezing the compression (efficiencies, low drag, etc...).
    I like the above post that says sneak up on it and look for the indicators. I don't know anything about nitro, so I won't even attempt to answer that part of your question. Hope this helps.
     
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  9. Creech

    Creech Member

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    The higher compression (13:1) richer methanol mixes actually burn slower/cooler than a lower compression ( 11:1 ) leaner mix (faster/hotter burn) and I think have a less tendency to push out gaskets and burn up parts: Untill you start pushing 60 pounds of boost with a PSI, then; Im still not so sure that it wouldn't work with a bigger pump.
     
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    Last edited: Jan 23, 2009
  10. Flyboy68

    Flyboy68 Member

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    Alcohol's a/f ratio is around 5:1 and gas is roughly 14:1, that's 5 and 14 parts of "air" to 1 part fuel. Alcohol would actually take way less air to burn than gas. Hence why nitro being around 2:1 has to have such huge fuel pumps on the fueler cars.
     
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  11. Creech

    Creech Member

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    A blown alky takes about 3.8:1 a/f in high gear ; about 4.2:1 a/f on a dyno.
     
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  12. JustinatAce

    JustinatAce Member

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    Sorry guys. Got bass ackwards. Trying to type and do the rest of my job isn't such a good idea at times.
     
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  13. Will Hanna

    Will Hanna We put the 'inside' in Top Alcohol
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    compression

    William, the correct, albeit smart ass, answer to your question is just a little bit less than too much.

    Or as Conway used to tell us, when the back intake nuts are starting to get a little loose or come off, you got it just right. (you have to know the rest of the combo...don't try this at home...LOL.) that said, generally you don't want any nuts, fittings, etc., coming loose because that's a sign of detonation. That's just saying that the first thing to start coming loose was the back intake nuts, so we just put a dab of silicone on them...

    Some people run high compression, alot of fuel and no timing. Some people run no compression, lean and a lot of timing. Some people run middle of the road. Some people like cucumbers better pickled. Get where I'm going...

    A lot of compression, a lot of timing and no fuel usually doesn't last long...

    There are different theories behind each setup...some have theories, some just run it because that's what they're used to or that's what works.

    Generally you have a much narrower window for error when you start getting higher in compression or timing.

    As a rule of thumb, the more boost, the less compression you need. That's why you hear about some roots guys with older blowers/ignitions running 12:1 plus while some screw blower guys run 10:1 or less.

    Thats why we preach spend the money to buy a fuel system and tuneup when someone gets a new combo. A good tuner can get you dialed in within a few runs. Ego may make you struggle.

    Often in the beginning you don't know enough to know what you don't know....
     
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  14. Creech

    Creech Member

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    Will
    I had the opposite experience with my BBC; besides, well its the header bolts that back out first on them. With almost 14:1 It liked lots of fuel and 40 degrees mag III. With a little less than 11:1, had to take a bunch of fuel out and it'd back the bolts out and push a gasket with more than 35.
     
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  15. William Payne

    William Payne New Member

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    Thanks for answering the question , I am sorry if my question went over anyones head I just like to know exactly whats going on and why thats going on when I learn about things I like to get real technical . To the person who asked , I don't actually have a race car I am just asking questions so that I can learn whats going on and how it all works so that when I do have a race car I don't wind up doing stupid things .
     
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  16. Will Hanna

    Will Hanna We put the 'inside' in Top Alcohol
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    mag III

    most had to run a lot more timing with the old mallory stuff. that mallory stuff is like a used up bic compared to a plasma cutter when compared to today's msd.

    not to mention the slack in the mag drives of yesteryear....
     
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  17. Dale Finch

    Dale Finch Member

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    Creech has got it in my humble opinion. x % gpm on the track and Y% gpm on the dyno. The driver style, the counter weight has a huge effect on the tuneup. I wish I understood all of Conways statement but I do understand one step away with loose bolts etc. A famous racer once said that moly rings were the best tools for diagnosing detonation. Well if you run it just shy of that way then crap happens when you go .003 to far. Moly in beds itself in skirts and wrecks cylinder walls. The driver has huge effect on all of these variables. Keeping it in the zone and on the correct ratios is everything.

    I watch one of the best drivers getting to the shift points no matter what is Roger Bateman. Don't let the down home country fool you. He is dead on the money. You can hear the consistancy listening to a couple runs. As a crewchief you can tune to him is my guess. Of course this is a great team effort on their part.

    Just admiration guys!! 5.57 and #1 in Vegas this fall was just stunning.
     
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  18. Creech

    Creech Member

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    A msd points box compensates for a retard (except for me) of about 4 degrees in a points mag. I could try to explain but I'd probably get it wrong. My mallory(not the one on the motor I mentioned earlier) made 12.5 amps at zigs(more juice at the plug than a msd 20). He said hang a sign on that m..f..
    Ciricco is making over 18 on his for the nostalgia nitro guys.

    For you newbees .3 gallons of methanol for every pound of boost .3 x 30 pounds of boost= 9 gallons to the motor @ 8000(probably a little lean) .3 X 40 pounds of boost = 12 gallons pretty close. Good basline
     
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