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Max KA24DE compression ratio for 93 octane?

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  • gbeaty
    started a topic Max KA24DE compression ratio for 93 octane?

    Max KA24DE compression ratio for 93 octane?

    I'm helping a friend rebuild a junkyard S13 motor, which will need new pistons, among other things. We're considered various combinations of ghetto parts to get anywhere from 9.5:1 to 11:1.

    Its a no-budget build, so cams would be factory 240/248s or maybe 248/248.

    Does anyone here have experience with higher-than-stock CRs for road racing use? I'm looking for as close to first-hand experience as possible - the data from other 240sx forums seems to be all over the place.

    We will of course do whatever needs to be done to get the ignition timing and AFR where it needs to be; I generally run timing backed 2-4 degrees off from peak power.

    Thanks.
    Last edited by gbeaty; 10-03-2012, 12:42 AM.

  • Def
    replied
    Actually, a MAF setup will have the timing column change due to engine VE. So it probably puts in full timing by 2500-3000 rpm, then keeps it constant or even adds in a bit of timing towards redline. The VE drops off by a ton on a KA towards redline, so it'll be trailing down to lower load columns.

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  • Wheelman
    replied
    The stock KA24DE timing maps I've got show MBT in the 40-43 range at high manifold vacuum (low load) and in the 27-30 range as Def guessed at low manifold vacuum (high load).
    Timing in the maps I have is all in by 2000 rpm with most of the ramp between 1200 and 1600 and tapers by a few degrees between 4000 and 6000.

    Leave a comment:


  • Def
    replied
    I'm betting MBT on a stock 9.5:1 KA is about 27-30 degrees. 11:1 setup is likely in the 21-24 range.

    Then again, I've got more experience on MBT of FI setups... or more likely the knock limit of 93 octane fuel.

    Leave a comment:


  • gbeaty
    replied
    Thanks.

    I'm waiting for my friend to dyno on his stock setup, to see where MBT occurs on that. I've heard numbers as high as 33 degrees before, though on his 162whp dyno I bumped the base timing up a degree or two without any effect (meaning 26-27 degrees or total advance, vs. factory 25). We were being conservative as the wideband we were using was bad and was incorrectly telling us the car was lean. So I didn't push it any further than a base advance of 22.

    If MBT on a stock setup is 33 degrees, an 11:1 setup will probably be around 28 or so. I don't think you can get that high without det. But if MBT is at 25 on stock compression, I'm pretty confident 11:1 will work just fine.

    Leave a comment:


  • Tower240sx
    replied
    cat, yes, mechanical fan, no that setup had 2 electric fans hidden in a stock rad shroud.

    I'll see if I can dig up the Dyno Sheets.

    Leave a comment:


  • gbeaty
    replied
    Originally posted by Sleepy_Steve View Post
    So... word is still out on hitting MBT with SOHC pistons?
    Pretty much. We might've been at MBT, but with the cam timing off it wasn't making good power.

    Originally posted by Tower240sx View Post
    I built 1 highish compression KA24DE, SOHC pistons and a 1.3 (i think) headgasket,

    I had 248 cams in it and didn't use the JWT gears, I moved it 4 teeth (or whatever the math I did said was right)

    I made 155 at the wheels on a big roller Dynojet. the car had ALL factory induction, factory manifold/header to a 2.5" muffler shop exhaust. Only issue I had was when I took it to the track on too hot of a range spark plugs...still ran fine until it ate thru the exhaust valve seat pocket on cylinder 4...
    Is that with a cat? Factory mechanical fan? Power steering? If so those seem like great numbers. Do you happen to have a chart or run file for that dyno?

    Thanks.
    Last edited by gbeaty; 10-08-2013, 11:49 AM.

    Leave a comment:


  • Sleepy_Steve
    replied
    So... word is still out on hitting MBT with SOHC pistons?

    Leave a comment:


  • Tower240sx
    replied
    I built 1 highish compression KA24DE, SOHC pistons and a 1.3 (i think) headgasket,

    I had 248 cams in it and didn't use the JWT gears, I moved it 4 teeth (or whatever the math I did said was right)

    I made 155 at the wheels on a big roller Dynojet. the car had ALL factory induction, factory manifold/header to a 2.5" muffler shop exhaust. Only issue I had was when I took it to the track on too hot of a range spark plugs...still ran fine until it ate thru the exhaust valve seat pocket on cylinder 4...

    Leave a comment:


  • gbeaty
    replied
    So, here is how the 11:1 (DNJ, factory replacement SOHC) pistons worked out for a friend of mine with a 240 ChumpCar:

    From the get-go, the compression numbers were around 180 on all cylinders. We knew this was low, but thought cam timing checked out and figured that it was maybe the gauge reading low For whatever reason we couldn't get the car on the dyno and needed to try and track-tune it. We'd started with base timing at 14, which was pretty slow. We upped it to 18, which was an obvious improvement.

    The car then got a few hours of track time put in it, but based on our daq it was clear it wasn't making good power at all. AFR was good, so we figured the only thing left to try was ignition timing. So we upped base timing to 22, and the motor blew on the warm-up lap. A compression test showed 180-ish compression on all cylinders except #4, which was 120-ish. When we pulled the pistons, every single one had broken ring lands. Nothing else was damaged, so the owner slapped new pistons in it and put it back together.

    While it was being put back together, the owner noticed the center cam sprocket was a crank-tooth off. Whoops. This probably accounts for the low power and low cranking compression. He put it back together with factory pistons, and the car made 162whp (167 without power steering). This is with a 4->1 eBay header, cone filter, stock S13 cams and obviously a racing exhaust.

    Me and another friend are building another KA for ChumpCar and NASA, using used OE SOHC pistons. We'll see how that goes. I hope we can run MBT timing on 93.

    Leave a comment:


  • gbeaty
    replied
    Originally posted by 240sxTTC View Post
    I've heard there have been some 5th gear failures later (after 12hrs) in Lemons and Chump car races seemed to be caused by heat. So any way to reduce the trans fluid temps would help.
    Well a friend of mine's 240 just got 3rd place at the ChumpCar 14 hours of Sebring. No transmission issues at all, though for much of the race the drivers weren't using 5th gear (it was faster to leave it in 4th).

    Our Z broke a shift fork (or perhaps a shift fork was broken; I hear those things don't just break themselves) about an hour in. Sigh. We've yet to have a Z transmission last more than 1.5 races.

    Leave a comment:


  • Sleepy_Steve
    replied
    Kind of obvious, but heat wrapping the exhaust as it goes by the trans probably also helps out some. I know I stopped the wrap at the end of the header.

    But then again, HPDE's and back roads cruising on an NA power plant doesn't seem to kill hardware.

    Leave a comment:


  • 240sxTTC
    replied
    I've heard there have been some 5th gear failures later (after 12hrs) in Lemons and Chump car races seemed to be caused by heat. So any way to reduce the trans fluid temps would help.

    Leave a comment:


  • gbeaty
    replied
    Thanks djsilver.

    Originally posted by Def View Post
    It's chumpcar, weld some aluminum fins all over the case for a redneck heatsink/trans cooler.
    ...that is pretty much exactly what we were thinking

    Leave a comment:


  • Def
    replied
    Originally posted by gbeaty View Post
    The datas I've seen indicate transmission temperature takes a long time to reach an equilibrium, still climbing at the end of a world challenge race, for example. So I'm not sure you'd see the same sort of failures in sprint racing.

    In any case I'll find some way to track temperature (probably caliper temperature stickers) and report back. It'd suck to have to run a Z32 trans in the car; those things weigh a ton.
    It's chumpcar, weld some aluminum fins all over the case for a redneck heatsink/trans cooler.

    Leave a comment:

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