BrandonsZ Posted November 15, 2004 Share Posted November 15, 2004 wheelman: I feel embarrassed, but I was scratching my head for months trying to figure out why it wouldn't pass the NOX emmissions, then I was told by a tech at the dealer that my compression check came back way too high? Now I bought the car because it was noticibly faster than the others I has driven and I was planning on hopping it up a bit but it felt so nice I didn't want to spoil it. Then I figured that the exhaust was special or something because the engine looked totally stock. But the guy said it had a 11.9:1 ratio based on static compression numbers. I confirmed that when you add the B22 high compression pistons to the B23 engine you can get numbers this high. But after 200k miles it still had very high numbers. Bingo the car won't pass NOX because the compression is so high it raises the combustion temperatures. I'm trading it in on a lexus for my wife so it'll remain a mystery, and to be honest, I just liked the power of it it had power to spare and could peel in 2nd, although 2nd is probably closer to 1st for a nissan. Also it runs 4k in 5th at 80mph, so you can imagine it has a lot of low gears. Power drops off at 6500+ but I had it as high as 7k and it was asking for more. Sorry I just don't know any more about it. Quote Link to comment Share on other sites More sharing options...
wheelman Posted November 15, 2004 Share Posted November 15, 2004 BrandonEngineer, Which guy told you it was 11.9:1 based on static numbers? The guy who you bought it from or the emissions tech? If you based the 11.9:1 on the numbers from the compression test then they probably aren't correct. A compression test gives you dynamic numbers because the valve timing (duration, overlap and lift) and intake track will affect the amount of air pulled into the cylinder and actually compressed. The compression number may be higher than stock but that doesn't indicate an 11.9:1 ratio. As for V8s not doing 11.9:1? Very few if any street cars running on pump gas will sustain a comp-ratio that high and I very much doubt your Honda is even close to that. But a stock LT1 (350 small block chevy V8) with the aluminum heads runs 10.5:1 all day on pump gas. My LT1 with the iron heads will run 10:1 without a problem. It's not V8s that run low compression it's the combustion chamber design. Wheelman Quote Link to comment Share on other sites More sharing options...
BrandonsZ Posted November 16, 2004 Share Posted November 16, 2004 Don't know how they checked it, I would assume they put the guage on and turned the engine, I didn't watch them, and I don't have a guage. In the past we checked compression by screwing in a guage into the spark plug hole and turning the engine over a few times with the coil disconnected. The emmissions tech told me he calculated 11.9:1, I checked out that the conversion of B22 pistons in the B23 engine could be as high as 11.5:1. He also mentioned that the exhaust cam was off by a few degrees, that could effect it also. That number sounds high to have gone 200k without any problems anyway, what do you think he was talking about? I still have the car. I wonder what's really going on. I do have some readings from when I bought the car like 6 years/200k ago I had it checked back then, I'll see if I can find them as well. Quote Link to comment Share on other sites More sharing options...
wheelman Posted November 16, 2004 Share Posted November 16, 2004 Brandon, First off don't confuse the pressure reading you get from a compression test with the compression ratio. The compression test is a measurement of the cylinder pressure generated when the engine is turned over 3-5 times and is a simple check that the rings, valves, gaskets, etc.. are in decent shape. The compression ratio is the total volume of a cylinder when the piston is at BDC (bottom dead center) / total volume at TDC (top dead center). Personally I think he was BSing you. As far as I know there is no accurate way to calculate the static compression ratio of an engine using numbers generated from a compression test, especially an engine with a few miles on it and unknown modifications. The only way to say what the real ratio is is to pull the head and measure the bore, stroke, deck height, combustion chamber, etc... You can guess like you have based on the assumption that you have B22 pistons and a B23 head but it's still a guess. The tech saw that the compression test numbers were higher than expected and guessed at the compression ratio. If he was very familiar with Honda engine build-up combinations he might get close. Wheelman Quote Link to comment Share on other sites More sharing options...
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