Post by
tloof »
https://forums.nicoclub.com/tloof-u11109.html
Tue Apr 18, 2006 3:52 pm
OK, I went back and carefully looked at and measured all the cams I have and determined the following:
1) All of the 19 tooth driven sprockets from the '91-'97 models are double row chain types, and have the drive pin oriented directly between two of the teeth in line with a chain roller, and are the exact same sprocket whether used on an int or exh cam. 2) Just for reference, all of the 19 tooth driven sprockets from the '98-'04 models are single row chain types, and also have the drive pin oriented directly between two of the teeth in line with a chain roller. When using the cams from any '98 up engines in the earlier ones, all that has to be done on the intake side is advance the pin location forward clockwise by 75.8 deg (or 4 teeth on the sprocket) since the drive pin was relocated on them by that amount from the factory. The exh cams on the '98 up versions still have the drive pin located in the same location as the earlier models, so no timing change is required to use them in an earlier engine. 3) The centerline of the 1st cam lobe on the '91-'93 240SX (S13) 248 deg exh cam is directly in line with one of the chain rollers in between two of the teeth on the driven 19 tooth sprocket and has the drive pin oriented 75.8 deg counterclockwise from the centerline of that 1st lobe in line with a chain roller (this is exactly 4 teeth behind the cam lobe centerline).4) The centerline of the 1st cam lobe on the '95-'97 240SX (S14) 232 deg int cam is 4.75 deg advanced forward of the chain roller centerline in between two of the teeth on the driven 19 tooth sprocket and it has the drive pin oriented 4.75 deg counterclockwise from the centerline of that 1st lobe (in otherwords, the drive pin is still directly in line with a chain roller, but the cam lobe centerline is not).5) The centerline of the 1st cam lobe on the '95-'97 240SX (S14) 232 deg exh cam is 4 deg retarded behind the chain roller centerline in between two of the teeth on the driven 19 tooth sprocket and it has the drive pin oriented 71.8 deg counterclockwise from the centerline of that 1st lobe (or 75.8 deg counterclockwise from the centerline of the chain roller).
Now, if a 248 deg S13 exh cam is used as an int cam in the S14 (or even in the '91-'93 S13 with stock 248 deg exh cam), it will result in the timing being retarded 1.5 deg behind the stock 232 deg int cam (it will open at 0.5 deg before TDC and close at 67.5 deg after BDC giving 4.5 deg of overlap when used with the S14's stock 232 deg exh cam). Previously it was stated that using the 248 deg cam would result in a 1.7 deg retarded timing, but from my measurements it appears to actually be 1.5 deg...the reason for this is that EVERY cam combination Nissan uses runs a 1 deg after TDC opening for the int cam. Since a 248 deg cam has a lobe centerline located at 124 deg (the halfway point of 248 deg) and a 232 deg cam has a lobe centerline located at 116 deg, then the longer duration 248 deg cam would have to have the lobe centerline retarded 8 crankshaft deg (or 4 camshaft deg) in order for the opening point to start exactly 1 deg after TDC. Since the cam lobe centerline of the S14 232 deg int cam is actually advanced 4.75 cam deg when compared to the S13 248 deg exh cam, then when installing the 248 deg exh cam into the int position using the 4 tooth counter clockwise rotation method of the drive pin on the driven sprocket will result in the cam being retarded 4.75 deg instead of the 4 deg that is required based on the difference in duration time alone. The difference between the two values is 0.75 deg (4.75 - 4), which results in a total of 1.5 deg of retarded timing at the crankshaft. Generally it isn't a good idea to open the intake valve before TDC on these motors, so using the 248 deg exh cam for an int cam may not give any advantage over the use of a stock 240 deg S13 intake cam. Now, using the S14 232 deg exh cam in place of the S13's 248 deg exh cam does seem to help the low to mid range dramatically since it opens 16 deg later (while still closing at the same exact time) thus allowing a longer burn time and still giving the optimum 3 deg overlap period when used with a 240 deg int cam. The 248 deg cam produces a longer overlap period of 4.5 deg (which is of no help when the int cam opens before TDC...if anything that would cause flow reversal potentially hurting performance) and also destroys the longer burn time potentially resulting in reduced torque overall. This is probably why Nissan seems to favor using shorter duration cams on the exh side in all of the later engines.
Someone previously asked whether it might be better to use an S14 232 deg int cam as an exh cam, and in my opinion the answer is NO. The 232 deg int cam has the lobe centerline advanced forward 4.75 deg vs the 4 deg retarded lobe centerline of the 232 exh cam. Installing the 232 int cam on the exh side using the 4 tooth rotation method will result in a cam timing that is advanced 8.75 deg (4 + 4.75) which in turn will cause a 17.5 deg advanced crankshaft timing such that the cam will open at 65.5 deg before BDC and close at 13.5 deg before TDC giving NO valve overlap at all(since the int cam won't open until 1 deg after TDC). This will destroy the performance of this engine and won't make for a good combination at all. If the 232 deg int cam were to be repinned to match the 232 deg exh cam, then it would work just fine!
The other mentioned combo was 248/248 + 5 deg. That would effectively advance the int timing such that it will open at 4.5 deg after TDC (isn't that defeating the purpose of early cyl filling) and close at 72.5 deg after BDC. Since a stock 248 deg exh cam closes at 4 deg after TDC, then this combination will give NO overlap either, and it closes way to late to be effective (in fact it will probably result in dropping the cyl pressure a bit...maybe that might help you guys trying to run 11.6:1 comp race engines on the street by using SOHC pistons in these DOHC engines! Ha) so I don't see how this could possibly be worth doing!!
From all of the above, my best choice for a cam combo in the KA24DE is the 240 int cam with 232 deg exh cam!!
Just my 2 cents!