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Old 10-02-2012, 06:58 PM
gbeaty gbeaty is offline
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Default 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 at 01:42 AM..
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Old 10-02-2012, 09:19 PM
Matt93SE Matt93SE is offline
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you'll want as much cam as you can get when you start upping the compression. 248/248 is what I'm using in my road race car w/ stock compression. at 11:1, you'll need to run 93 octane, but you'll still be fine.

the big limiting factor in the KA is the intake manifold/throttle body. No matter how much compression you run, you're limited to roughly 210whp with the stock IM. you can make that much power on stock compression with a better manifold.

Not sure of your overall goals or what class/sanctioning body you're building the engine for- some information there would be helpful suggesting further info.
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Old 10-02-2012, 09:30 PM
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No need to be shy with timing. 93 octane and 9.5:1 you can run right at MBT no problem.
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Old 10-03-2012, 01:02 AM
gbeaty gbeaty is offline
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Quote:
Originally Posted by Matt93SE View Post
Not sure of your overall goals or what class/sanctioning body you're building the engine for- some information there would be helpful suggesting further info.
ChumpCar. Endurance racing, but I'd think the motor is just as well at its equilibrium temperature (and thus propensity to knock) after 20 minutes as it is 2 hours.

Acceleration is very helpful as it makes passing easier (passing in corners 10x per lap for 7 to 24 hours is risky), as is fuel milage. Both of which you get from CR (as well as weight reduction, my favorite panacea).

But per the rules, we're limited to ghetto stuff. No fancy cams, pistons, etc.

I thought we might do the 248 intake cam swap as it seems well-documented here in terms of being able to degree it (and thus not needing to muck with things on the dyno), but I'm not sure if its worth much over the 240/248 which came in the motor? Plus we have to claim some value for all this stuff...

How confident are you 11:1 is fine with 93 octane?

Are there any ghetto-fab mods which will help the intake? Porting the TB, or matching the runners? I thought of shortening them but that looks like it would take some tubbing and extensive TIGing, which tacks on extra value to the car. I've heard the non-butterfly IMs make more power? We're of course removing that assembly anyway.

The KA seems really well-suited for endurance racing. Great rod:stroke lengths, oil squirters, floating wrist pins, timing chain, solid valvetrain, etc. The motor which was pulled apart had 200k on it, looked like the oil was rarely changed, yet the internals seemed as if they could go another 100k. The only real wear was the tops of the bores (out of spec, but not excessive).
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Old 10-03-2012, 06:57 AM
Sleepy_Steve Sleepy_Steve is offline
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You could gasket match the intake manifold / head, route your intake to a filter in front of the radiator, run the SOHC pistons, the 248/248 cams... But you'll still be hurting for more RPM at the end of the day, and I don't know a good ghetto way of getting that done.
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Old 10-03-2012, 07:14 AM
Matt93SE Matt93SE is offline
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I tried the 240/248 setup and was happier with the 248/248 for a track-only car. It's a bit peaky for the street because the engine doesn't really spin up until about 4500rpm or so, but when I'm on track, I'm almost never below 5000rpm. Mine pulls fine until the 6800rpm rev limit (still on stock ECU myself.)

You'll be fine with 11:1 as long as you get the ignition timing right, and it sounds as if you've got the brains to figure that much out. all things considered, 11:1 is still pretty tame given today's engines.
the KA24DE does burn a lot of fuel (I get about 6mpg on track- full tank lasts about 1.5hrs), but it is indeed reliable as hell compared to many other engines I've seen out there.
I have been running my engine for ~8years of HPDE and club racing, and I've run this thing on track for 8+hrs a day many times. still going strong with ~20,000 track miles!

The big reliability issue is transmissions in these cars. stay with a sprung hub clutch and it'll help save your transmission.
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Old 10-03-2012, 10:50 AM
djsilver djsilver is offline
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I picked up a fair amount of power between the old 5,300rpm HP peak and the 6,800rpm rev limiter by retarding the stock exhaust cam 5 degrees for more overlap. It moved the peak up about 300rpm but didn't really loose any bottom end. A JWT sprocket is pricey but there are DIY's on the net for making your own. It'll also make for an easier peak compression if you go with the SOHC pistons.
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Old 10-03-2012, 11:29 AM
gbeaty gbeaty is offline
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Quote:
Originally Posted by Matt93SE View Post
You'll be fine with 11:1 as long as you get the ignition timing right, and it sounds as if you've got the brains to figure that much out. all things considered, 11:1 is still pretty tame given today's engines.
the KA24DE does burn a lot of fuel (I get about 6mpg on track- full tank lasts about 1.5hrs), but it is indeed reliable as hell compared to many other engines I've seen out there.
6 mpg sounds very low - at Sebring our Z always getting above 8 mpg. Granted we don't run 10/10ths and traffic often slows us down. We also run 13:1 AFR via a vacuum pump attached to the FPR.

Has anyone here actually tried 11:1 on-track? It'd really suck to get the thing together then realize we had the wrong pistons in it.

Quote:
Originally Posted by Matt93SE View Post
The big reliability issue is transmissions in these cars. stay with a sprung hub clutch and it'll help save your transmission.
Noted, thanks. Transmission failure looks like a bigger problem, with another team having gone through 3 or 4 before switching to a Z32 trans. The Z32s are heavy and last about one season (70 hours being very gentle; one race if you're ham-fisted). I assume the trick to making both the Z's and 240's transmission live is keeping them cool?

Quote:
Originally Posted by djsilver View Post
I picked up a fair amount of power between the old 5,300rpm HP peak and the 6,800rpm rev limiter by retarding the stock exhaust cam 5 degrees for more overlap. It moved the peak up about 300rpm but didn't really loose any bottom end. A JWT sprocket is pricey but there are DIY's on the net for making your own. It'll also make for an easier peak compression if you go with the SOHC pistons.
With what cams? Do you happen to have the graphs posted anywhere? Edit: I assume you have 248/248s; how much did you change the intake cam timing when you installed it? Seems like people advance it 2.5 or 5 degrees?
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Old 10-03-2012, 11:31 AM
gbeaty gbeaty is offline
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Quote:
Originally Posted by Sleepy_Steve View Post
You could gasket match the intake manifold / head, route your intake to a filter in front of the radiator, run the SOHC pistons, the 248/248 cams... But you'll still be hurting for more RPM at the end of the day, and I don't know a good ghetto way of getting that done.
Well there's always overclocking the ECU if its got an external clock crystal. That has other side-effects of course, but it will raise the rev-limit (and advance the timing).

However, for endurance racing I think we'll keep the stock rev limiter. It seems to be really hard for these cars to make power up there anyway, though I haven't seen any 248/248 dynos.
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Old 10-03-2012, 12:42 PM
Matt93SE Matt93SE is offline
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No dyno plots for mine either, but the 6800rpm rev limit is pretty much the end of the usable power band with OEM cams. if you played with cam timing a bit more, you might be able to get a touch more, but it sounds to me like you'd probably want to keep a bit more low end.
The majority of my racing is SCCA Club racing, and I'm balls-out pretty much the whole race since they're 35-45min sprints. There's no cruising around corners stuck behind slower traffic like you see with Chump and LeMons. About the only time I see slower traffic is the last 2-3 laps of the race when I'm lapping the backmarkers, and usually they know the front-runners are coming through and in a fight for position and they just move over. (there's always exceptions, but you get the point... I'm digressing.)

My stock KA tranny has.. hmm.. several hundred hours of track time on it. I built the car in 2006 and drove it daily for about 2 yrs. It has been a track-only car since then, and I've put about 20k miles on it- ballpark of 250hrs. I finally wore out the 2,3,4th gear synchros at a rough track, but I was beating the bejeezus out of the car at a huge event and only needed it to last the race.
Anyway, I haven't had the massive breakages and failures others have had, but I'm limited on power. about 200whp is the limit for these trans in race conditions. If you're ham fisted, it's less.

No cooler on mine, but I run 50%/50% Redline MT90 and lightweight Shockproof in both the trans and the diff. Every Nissan I've driven gets that treatment and they seem to like it.
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