alkemyst

I am trying to do the math and based on what my 232 cams do..if I move an exhaust cam to intake it's going to be about 75.8 degrees off, this is '4 teeth' (each cam tooth is 18.95 degrees)...so moving the cam 4 teeth ccw brings the timing back to what it should be.

Now I am looking at my 248 exhaust cam being practically on the money with the four tooth method and any advance or retard will move the cam out of this spec.

If I am right (which would show why many that just throw the 248 cam in without any gears is why they are making decent power and why I am not with my 2.5 deg. advance) then while using my 232 exh as an exhaust cam.:

248 at 4 teeth is opening 1 deg ADTC with 3 degs overlap closing 69 deg ABDC

adv 2.5 degs would be 3.5 deg ADTC with 50 deg overlap and closing 71.5 deg ABDC

retard 2.5 degs would be 1.5 BTDC with 5.5 deg overlap and closing 66.5 ABDC

Adv seems like there is noway it'd make power.

Now some others are saying even with the 4 teeth method you are still 1.5 degrees off...that could well be, here are the numbers in that situation...

248 opens .5 BTDC with 4.5 deg overlap and 67.5 closing ABDC

adv 2.5 -2 ATDC with only 2 deg overlap and closing 70 ABDC

retard 2.5 degs would be 3 deg BTDC with 7 degrees overlap closing 65 ABDC.


All cam setups I looked at stock have 3 degs overlap and open 1 deg ATDC...closing events for the intake cams take place between 51 and 63 degs ABDC.

I don't see how anyone can say they are making power with adv. the cams 2.5 degs on a 248/232 setup with the 248 from a 91-93 S13.

It seems not retarding or advancing will put you closest to stock timing.

Maybe my math is off...anyone else know more? Not looking for links to other threads as I have seen them...looking for anyone else that knows the math here.

Å

djsilver

I can't comments on the various combinations of S13/S14 cams in various positions but I will comment on my experience so far with overlap. All versions of the DE motor came with between 3-7 degrees of overlap and all the variations came from changes in exhaust cam timing, not intake cam timing. I have an S13 DE motor with the stock 240/248 cams. I recently added a JWT gear on the exhaust, retarded 2 steps (5 degrees) to increase overlap from 3 to 8 degrees. I did not change the intake timing. This change really woke up the top end without any obvious loss of bottom end. Just for reference, the SOHC KA24E motor opens the intake at 3 degrees BTDC and closes the exhaust at 12 degrees ATDC, for a total overlap of 15 degrees. Generally speaking, advancing timing or reducing overlap helps bottom end, retarding timing or increasing overlap helps top end. The KA24E motor probably needed the more radical timing to make up for the smaller valves.

alkemyst

http://www.rbracing-rsr.com/camshaft.html

The only 240sx with a KA24DE not to have 3 degrees overlap was the 94 vert, it had 7 degrees. The 91-93 with 240/248 still opened at 1 ATDC and closed 4 ATDC just like the 95+ with 232/232.

There is a lot of misinformation in the camshaft threads most are just repeating another's bad calculation each time.

My problem is I don't know exactly where the 248 cam is sitting when moved to intake. The posts (many by the same guy) state it anywhere from -1.7 to 2.1 outside of stock. I do know advancing the 248 with a 232 as exhaust 2.5 degrees absolutely killed my power. I should be at around 145-165RWHP depending on dyno, I only made 108. Everything else is to spec.

djsilver

Alkemyst is correct (as far as I understand) that the 94 vert-only model is the only S13 DE to get 7 degrees of overlap. My decision to put my '91 DE motor to something near that setting is the anecdotal information that the (B12?) 93-95 Altima used that setting and had a reportedly better revving motor than the 91-93 240sx. My experiment fullfilled my expectations and I'm happy with the change!

Now, if you want to be more confused take a look at a 91-93 240sx FSM in the engine specifications section and they list the overlap at 7 degrees! It shows the intake opening at -1 BTDC (1ATDC) and the exhaust closing at 8ATDC. I have a '91 FSM that shows this. All the PDF versions I've found online show the same but I measured mine with a degree wheel and a dial indicator before I changed it and it was only 3 degrees of overlap, consistent with everything else I've read.

For one more item of confusion, most cam manufacturers say to measure cam timing events at .050" of lift. The FSM didn't specify so I measured cam timing events at first bump, .005" and .010" and found that they would only measure 240/248 when reading them at .005" lift. Go figure...,

alkemyst

well moving my cam to 2.5 retard seemed to get back quite a bit of power...still wish it was more, but I may be asking for too much out of at most 160WHP and almost 2900 lbs. I can break traction easily in 1st, keep it going a bit to 2nd. My clutch is not the best and my tires are pretty grippy (goodyear asymmetrics).

Have to see on the dyno...I am at 5 deg advance too which it seems to like. Don't hear any pinging, but I need to make sure (exhaust is freaking loud). I am running 93 octane only, but it's 90degF+ here.

If the dyno proves terrible I am going back to 232/232 or may try to find a 240 intake cam...

sav180

now how would you set up your timing with two 248 cams??

alkemyst

not cool to threadjack...I posted up some info in your thread on this question though.

sav180

sorry my b