jmd0346 Posted January 13, 2008 Report Share Posted January 13, 2008 Hey Fellas. Ive been thinking of bumping up my compression for a while now. Just not sure what the optimum psi would be for my setup. I have what I think is akin to a dune port. Its not a drag port but its more than a trail/play port. It was called a stage 2 by the company/guy that did it. Eric Happy @ 4wheel thunder. Stage 3 was full on drag port. I run 19cc domes with around 145 psi compression. +4 timing. 93 octane. My questions are: 1. What is a good balance compression wise for good bottom end and still have good top end power? 2. What octane is required for said compression? 3. What timing would be recommended? I run +4 now. 4. I can get 100 octane leaded race gas. Is leaded ok? If so keep this in mind when giving your answers. Thanks guys, Michael Quote Link to comment Share on other sites More sharing options...
wesw Posted January 13, 2008 Report Share Posted January 13, 2008 you have problems with 19cc domes and 145 psi you should be 175-185 psi most likely you need to run teh compression that your gas can support, and what your motor is tuned for. compression helps with low end but too much kills your top end. have to talk to your builder but you need to recheck your shit. gauge isnt reading right, domes are cut right, or basegasket too thick, squish. somethign is up or just worn out. Quote Link to comment Share on other sites More sharing options...
jmd0346 Posted January 13, 2008 Author Report Share Posted January 13, 2008 you have problems with 19cc domes and 145 psi you should be 175-185 psi most likely you need to run teh compression that your gas can support, and what your motor is tuned for. compression helps with low end but too much kills your top end. have to talk to your builder but you need to recheck your shit. gauge isnt reading right, domes are cut right, or basegasket too thick, squish. somethign is up or just worn out. Well its always read this compression with this setup. It is ported pretty aggresivly so I assumed that accounts for the compression. I just rebuilt it early last year. I use a stock base gasket and have a Vito Hemi head with o-ring gasket on top. I also use vitos domes. Unless his domes are wrong i dont see what the problem could be. I thought you only needed special cut domes when doing different thickness base gaskets or different length rods etc...? Are you saying one should check squish even on a basic setup? If so that will be my next thing to do. It could be my guage too. Its just a AZ special. Aslo I would think if I were really at that high compression I would have detonation issues with the 93 pump Im running. Quote Link to comment Share on other sites More sharing options...
wesw Posted January 13, 2008 Report Share Posted January 13, 2008 you should check squish with any new setup. my guess your domes have to much squish. why you have low compression and yes anything over 155psi roughly pumpgas wont work. so get some solder and check your squish. do a search on it if you dont know how to do it. Quote Link to comment Share on other sites More sharing options...
jmd0346 Posted January 13, 2008 Author Report Share Posted January 13, 2008 you should check squish with any new setup. my guess your domes have to much squish. why you have low compression and yes anything over 155psi roughly pumpgas wont work. so get some solder and check your squish. do a search on it if you dont know how to do it. Yeah i know how to do it. Just think it necessary with a basially stock setup. Ill give it a go. Too much squish would give me a higher reading on compression. Anyone else that can answer my questions? Ill figure it all out own my own like i always do. i just thought some opinions from others would be nice. TIA. Quote Link to comment Share on other sites More sharing options...
wesw Posted January 13, 2008 Report Share Posted January 13, 2008 no squish is the measurement between the head and piston. lot of squish = loose and less compression. 40-50 thousands are common number. depends on the builder. Quote Link to comment Share on other sites More sharing options...
jmd0346 Posted January 13, 2008 Author Report Share Posted January 13, 2008 no squish is the measurement between the head and piston. lot of squish = loose and less compression. 40-50 thousands are common number. depends on the builder. Well that sounds backwards to me but I get the concept. What would you do If squish is a problem? I use a stock base gasket. Stock height jugs. The only think that could be wrong are the domes. I guess its possible but doesnt seem likely. Im gonna check the squish tomorrow fo sho. Do you have any opinions on any of my questions in my first post? What is the down side to too loose of a squish? Other than obviously lowering compression. My point being. Lets say my squish is loose. I wouldnt want to take my engine apart to change base gaskets or whatever when i could just put in a smaller dome. Quote Link to comment Share on other sites More sharing options...
jmd0346 Posted January 13, 2008 Author Report Share Posted January 13, 2008 Any answers from the sunday crew? Quote Link to comment Share on other sites More sharing options...
jbooker82 Posted January 14, 2008 Report Share Posted January 14, 2008 Any answers from the sunday crew? I would say your pretty much at your max compression on pump gas. If you wanted to step up to race fuel 185 psi and iginition timing of 6-8 deg. I ran 150 psi on 19cc's I thought it might be low for the dome size. I never got to check the squish before I switched cylinders and added a 4 mill crank. I dont know what you can run on av gas. If your going to go through the hassle of buying special gas just get the real deal. Leaded fuel is fine. I run Citgo 110, or Sunoco Standard 110. I have also ran VP Red (105 motor octane) on 185 PSI, +7 deg timing on my stock cylinder 4mill. VP fuel is way more expensive than the other race fuels out there. josh Quote Link to comment Share on other sites More sharing options...
jmd0346 Posted January 14, 2008 Author Report Share Posted January 14, 2008 I would say your pretty much at your max compression on pump gas. If you wanted to step up to race fuel 185 psi and iginition timing of 6-8 deg. I ran 150 psi on 19cc's I thought it might be low for the dome size. I never got to check the squish before I switched cylinders and added a 4 mill crank. I dont know what you can run on av gas. If your going to go through the hassle of buying special gas just get the real deal. Leaded fuel is fine. I run Citgo 110, or Sunoco Standard 110. I have also ran VP Red (105 motor octane) on 185 PSI, +7 deg timing on my stock cylinder 4mill. VP fuel is way more expensive than the other race fuels out there. josh Oh yeah im with you on that. Im not running anymore compression on pump. Im gonna go with 100 leaded. Im just trying to get an idea of what the optimum compression would be on 100 octane. Im thinking about 175 psi with 6 degrees timing on 100 octane. What do you guys that know think? Quote Link to comment Share on other sites More sharing options...
jbooker82 Posted January 14, 2008 Report Share Posted January 14, 2008 Oh yeah im with you on that. Im not running anymore compression on pump. Im gonna go with 100 leaded. Im just trying to get an idea of what the optimum compression would be on 100 octane. Im thinking about 175 psi with 6 degrees timing on 100 octane. What do you guys that know think? How is the 100 octane rated. (R+M)/2 Research octane is going to be higher. The Reseach octane is not as hard of a test. Motor octan is the harder test. My Citgo 110 and Sunoco 110 is 110 octane rated by the (R+M)/2 Citgo 110 Research Octane Rating of 114 Motor Octane Ratign of 106 I dont know what your max compression is on 100 octane fuel. What kind / brand is it? I would trust 100 motor octane on that compression but if it is reaseach octane I wouldnt. More than likely it is the R+M / 2 rated. You would just have to try it and see. Listen for pinging, and if it is you need higer octane, or less compression or timing. You might be able to get away with 175 psi and backing the timing off if it is pinging. Quote Link to comment Share on other sites More sharing options...
jmd0346 Posted January 14, 2008 Author Report Share Posted January 14, 2008 How is the 100 octane rated. (R+M)/2Research octane is going to be higher. The Reseach octane is not as hard of a test. Motor octan is the harder test. My Citgo 110 and Sunoco 110 is 110 octane rated by the (R+M)/2 Citgo 110 Research Octane Rating of 114 Motor Octane Ratign of 106 I dont know what your max compression is on 100 octane fuel. What kind / brand is it? I would trust 100 motor octane on that compression but if it is reaseach octane I wouldnt. More than likely it is the R+M / 2 rated. You would just have to try it and see. Listen for pinging, and if it is you need higer octane, or less compression or timing. You might be able to get away with 175 psi and backing the timing off if it is pinging. I want to say its Sonoco 100leaded. Not sure on the R+M/2 factor. Ill haver to check on that. My main question is what is optimum for a ported engine? Compression and timing wise. Ill run whatever octane is required. I just want to know what is the best comp/timing to run? Then Ill figure out required octane, squish etc... I just want to optimize my current setup. I feel like Im leaving alot on the table with 145-150 psi comp and 4 degrees timing on pump. Quote Link to comment Share on other sites More sharing options...
HotCarl Posted January 14, 2008 Report Share Posted January 14, 2008 No one is going to give you the majic numbers, but I have run 175psi with +4 on av100ll. and I have never had a problem. Also what is your elevation? Quote Link to comment Share on other sites More sharing options...
jmd0346 Posted January 14, 2008 Author Report Share Posted January 14, 2008 No one is going to give you the majic numbers, but I have run 175psi with +4 on av100ll. and I have never had a problem. Also what is your elevation? Never once asked for a "Magic" number. Just what people felt was optimal based on their experience. I didnt think it was that hard of a question. It simply asks for opinions. Thanks for your opinion. You run exactly what I felt would be optimal. Maybe a little more timing. Im at or near sea level. Quote Link to comment Share on other sites More sharing options...
jbooker82 Posted January 14, 2008 Report Share Posted January 14, 2008 When running straight race fuel most people shoot for 175-185 PSI. You dont really want anything over 190. If your crank is not welded then you should tear it down and have that done. Quote Link to comment Share on other sites More sharing options...
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