fp280z Posted May 22, 2012 Share Posted May 22, 2012 The race motor in the FP car has had some power band issues since it was built, it just doesn't want to rev much past 5500rpm with a significant drop in power at 5000rpm. I've checked or replaced almost everything external to the engine, gone through the carbs many times, and have even been through a rebuild. I'm now suspecting cam timing. The cam was timed originally in '08 by the engine builder (advanced a few degrees for autocross was my understanding) and I reused the original marks during my recent rebuild. I timed it during the rebuild just to make sure I got it on the right tooth but would like for someone to "check my work" and let me know if these measurements make sense or could be contributing to my issues. Cam card is here: http://www.typeischeap.com/cam_card.jpg Most recent dyno is here: http://www.typeischeap.com/RM2AFR.JPG http://www.typeischeap.com/RM2T.JPG A couple of things about the dyno curve that highlight the issue, 1) really great torque at 3000rpm, 2) "stepped" AFR that is rich (bog) at low rpms, steps up to a decent AFR for peak torque, then back to pig rich for the drop in power and brick wall... Cam timing (measured at .050 lift) is: Intake opens 23 BTDC Intake close 52 ABDC Exhaust opens 55 BBDC Exhaust close 18 ATDC This is the first time I've tried measuring cam timing so let me know if the numbers above do not look correct. Here's an old thread from the original build with more information about my setup (yes the bog is still there): http://forums.hybridz.org/index.php/topic/69511-mikuni-boggot-data-need-some-analysis/ A local autocrosser suggested it may be a bad coil pack but I would think that the AFR would go lean on a misfiring situation? Yes, it's a lot of fun on courses that keep me between 3000-5000rpms but that doesn't win autocrosses! Any other ideas? -Rick Quote Link to comment Share on other sites More sharing options...
Xnke Posted May 22, 2012 Share Posted May 22, 2012 I am not running the same cam...but I have similar issues. Advancing my cam timing brought the power band down, but at the expense of total power. I am running a stock cam sprocket and so only have 0, +4*, and +8* available, and am currently running in the +4* position. Prior to the change, I had NO low-end...nothing at all under 3500RPM. Cam came on about 3500, and pulled strong from 4000 to the rev-limit, and would pull past the ignition-retard rev-limit if I didn't get my foot out fast enough! Advancing my particular cam in my particular engine by +4* brought the cam-in to 2800RPM, and now the good torque comes in at 3000, pulls hard to 6500-7000RPM (rev limiter), but I have not had the time to get it on a dyno to see exactly how the curve was affected. Are you running a stock cam gear or an adjustable? What adjustable are you running, if you are? If you can advance/retard in 1* increments, without pulling the timing chain, then get it on the dyno and make a few pulls, retarding the cam timing a degree at a time, till you move your powerband where you want it. If you have to pull the timing chain to do it, then my advice is get someone to help you, and don't drop the chain. It can be done, just work the chain around the cam sprocket till your bright link is where you need it. Quote Link to comment Share on other sites More sharing options...
z-ya Posted May 22, 2012 Share Posted May 22, 2012 First of all, when you installed the cam, did you verify that the timing is correct per the cam card? If not, this where I would start. Make sure it is as close to the cam mfg recommended timing and see how it performs. Then at least you have a base line to start with. Then you can try experimenting with other cam timings. What are you running for induction, carbs? Also, that is a pretty mild cam, so I wouldn't expect to see a lot more power above 5500RPM. Are you running stock springs? Did you check the spring pressure? Quote Link to comment Share on other sites More sharing options...
letitsnow Posted May 22, 2012 Share Posted May 22, 2012 I don't think you'd see that from being a few degrees advanced or retarded, he loses 100hp in 200rpms! Also, I wouldn't call 250* @.050 lift small. My STOCK cam doesn't start falling off until 6000, and then it's a gentle slope. I would check all the valve springs and make sure you don't have a soft one or some other failure there, then I would try to get the AFR's to a better place, bottoming out a wideband is RICH and will kill power in a bad way. Quote Link to comment Share on other sites More sharing options...
Mack Posted May 22, 2012 Share Posted May 22, 2012 I would almost suspect either something ignition or carb related. Judging by your AFRs, notice how it follows the HP curve perfectly? I don't think you should ever go below 12.5:1, unless you are running some sort of forced induction. For an NA set up, anything lower than that will lose HP. Unless you are running E85 or something....... ALso, if you are losing a cylinder due to ignition break up, it SHOULD go rich! If there isn't any spark in one (or more) cylinders, a lot of unburnt gasoline escapes into the exhaust and probably burns inside the manifold with all the other hot exhaust gasses. If you are missing on more than one or 2 cylinders, then it would go lean, as there would be a ton of uncombusted oxygen escaping into the exhaust system, thus making it LOOK lean. Quote Link to comment Share on other sites More sharing options...
Leon Posted May 22, 2012 Share Posted May 22, 2012 I would almost suspect either something ignition or carb related. Judging by your AFRs, notice how it follows the HP curve perfectly? I don't think you should ever go below 12.5:1, unless you are running some sort of forced induction. For an NA set up, anything lower than that will lose HP. Unless you are running E85 or something....... ALso, if you are losing a cylinder due to ignition break up, it SHOULD go rich! If there isn't any spark in one (or more) cylinders, a lot of unburnt gasoline escapes into the exhaust and probably burns inside the manifold with all the other hot exhaust gasses. If you are missing on more than one or 2 cylinders, then it would go lean, as there would be a ton of uncombusted oxygen escaping into the exhaust system, thus making it LOOK lean. A misfire reads lean on a wideband, whether it's due to a lean or rich condition. A wideband senses the amount of O2 in the exhaust, not HC. Quote Link to comment Share on other sites More sharing options...
Tony D Posted May 22, 2012 Share Posted May 22, 2012 Cam timing shouldnt affect AFR. I suppose if you were really off and the VE dropped...but on a carb that shouldn't translate to much difference like a MAP based EFI. Get your AFR correct before chasing mechanicals. Is the fuel pump pushing too much pressure and sinking the floats? Is there a foaming issue that is pushing fuel into the carb throat where the AFR goes rich? The power loss directly coincides with the AFR drop... It should be relatively easy to sort the fueling and get it correct. Quote Link to comment Share on other sites More sharing options...
fp280z Posted May 22, 2012 Author Share Posted May 22, 2012 Completely agree that the AFR is the concern here but I don't know root cause. I've tried quite a few things over the last few years, float levels, fuel pressure, etc have been checked and rechecked. The AFR "step" is perplexing, are the rich ranges the anomaly or is the peak the anomaly? If I change jet sizes I can get the whole thing to move up but then the peak AFR is too lean. I need to smooth out the AFR curve... Quote Link to comment Share on other sites More sharing options...
letitsnow Posted May 22, 2012 Share Posted May 22, 2012 So this is not a new problem? Was it an entirely new setup or were there thing reused? If you lean it out, barring the very lean area, does it make power to redline(~7k)? Quote Link to comment Share on other sites More sharing options...
fp280z Posted May 23, 2012 Author Share Posted May 23, 2012 The build in '08 included brand new carbs and ignition. About the only thing I reused was my Cannon manifold. Leaning out the AFR so that the peak was above 14 still did not provide power all the way to redline although it did improve the dip/bog at the low end of the rpm range. The only other fuel related theory I could come up with was that the mains were somehow not coming on early enough and causing the lean area in the AFR graph. I thought about grossly reducing the air jets as an experiment to see if that would allow the mains to come on earlier and narrow that peak. If so, then I could move both the airs and mains down to get the average AFR in the right place. I believe I'm at 155/205 with 34mm venturis right now but I need to verify that. This dyno run above was made when I switched to the 34mm chokes so I don't remember exactly what jets I ended up on after tuning. Hard to believe that the mains are not coming on until almost 5000rpm though, anyone experienced anything like this? Quote Link to comment Share on other sites More sharing options...
Leon Posted May 23, 2012 Share Posted May 23, 2012 Check out the Sidedraft Central Yahoo group, it's the best info for the DCOE out there. Specifically, refer to Keith Franck's White Tuning Paper. Quote Link to comment Share on other sites More sharing options...
fp280z Posted May 23, 2012 Author Share Posted May 23, 2012 Good resource, thanks! Quote Link to comment Share on other sites More sharing options...
Leon Posted May 23, 2012 Share Posted May 23, 2012 Good resource, thanks! Not a problem and good luck. That should at least take care of any possible carb issues you may be having. Quote Link to comment Share on other sites More sharing options...
fp280z Posted May 24, 2012 Author Share Posted May 24, 2012 The most interesting thing about Keith's paper is that he suggests starting tuning with the air corrector instead of the main jet which is opposite of almost every other side draft book/reference out there. Good reasoning behind it though and it's worth a shot in my case since I've tried almost everything else jet related. Until now I've only made "fine tuning" adjustments with the air corrector. Tuning would be much easier if the car were street legal. I have an LM2 wideband datalogger but my only tuning opportunities are the autocross course (not ideal) and dyno. Luckily I have a friend that runs a dyno but even friend prices can add up! So, I like to queue up ideas/experiments for the next dyno trip, keep 'em coming if you have any thoughts! Thanks! Quote Link to comment Share on other sites More sharing options...
letitsnow Posted May 24, 2012 Share Posted May 24, 2012 The build in '08 included brand new carbs and ignition. About the only thing I reused was my Cannon manifold. Leaning out the AFR so that the peak was above 14 still did not provide power all the way to redline although it did improve the dip/bog at the low end of the rpm range. Do you recall the AFR up top for this run? We haven't really mentioned ignition timing yet, is it doing anything funky? Can you watch it with a timing light on the dyno? Assuming that the AFR was alright and the timing isn't doing anything crazy, I'd absolutely look at the valvetrain. Obviously you need to fix the AFR, but if it was in a decent place after the big lean spike and didn't make power, I'd look for another issue. The curve is very similar to when I had failed hydraulic lash adjusters in my turbo engine, pulls hard up to 4500, then nothing, might as well have taken my foot off the pedal. I would check the cam with a degree wheel/dial indicator, look at wipe pattern for GP, measure the installed valvespring height, the valvespring force at that height and at peak compression and check against what's recommended by the cam grinder. Quote Link to comment Share on other sites More sharing options...
srgunz Posted May 24, 2012 Share Posted May 24, 2012 As far as hesitation (bog) goes, I had to bump my pilot jets on my Mikuni's up to 60's to get rid of it. I run 34mm chokes as well. Quote Link to comment Share on other sites More sharing options...
Tony D Posted May 25, 2012 Share Posted May 25, 2012 Mikunis and Webers are different animals when it comes to idle transition function. (It's why I prefer Mikunis.) As for tuning on the air corrector, I'd agree. Our Bonneville engine running 45 DCOEs ran 135-140 main depending on altitude density. It really didn't change that much. But emulsion tubes, and air correctors moved ALL OVER! The fact that you can go so rich says your main jet is more than enough for the power you're making. You just have to tailor 'how' the fuel comes into the engine, and thats air correction and emulsion tubes, main chokes, and booster venturis. You may have too much accel pump stroke on the bottom end, and once up high not enough air bleed to lean out the mix.Your main jet looks like it comes in at 3K, but then there is just too much sucking on the well pulling fuel into the engine as rpms rise. Possible a booster venturi is too small causing greater vacuum, but unlikely they're pretty forgiving. A bigger air bleed or different emlusion tube may change that delivery once the main is 'all in' at 3K Quote Link to comment Share on other sites More sharing options...
Leon Posted May 26, 2012 Share Posted May 26, 2012 I also recommend getting Keith Franck's float level tool, it's a great way to get a very accurate float setting. The main circuit is magnitudes more sensitive to float height than the idle/progression circuit. Quote Link to comment Share on other sites More sharing options...
Tony D Posted May 26, 2012 Share Posted May 26, 2012 agreed---if the mechanical fuel pump is overcoming the floats at higher rpms the float level goes all to hell and could do this. Monitor fuel pressure! Quote Link to comment Share on other sites More sharing options...
fp280z Posted May 26, 2012 Author Share Posted May 26, 2012 I use an old clear BIC pen to set the floats, not as elegant as Keith's tool but should be just as accurate if not more so (e.g. you don't have to watch the sight glass to determine when you hit fuel). I've reset the floats since I've been to the dyno, definitely need to get back and see where I'm at and get a new baseline. No mechanical fuel pump here, fuel pressure stays at 3psi all the way through the dyno run according to the dyno guy watching the gauge. Another thing about the dyno is that is should not be dependent on the accel pump. They floor the accelerator under full load, wait for it to stabilize at low rpm, and then the dyno reduces the load to a set level and the rpms climb for the run. To me this seems a bit "artificial" so I've also been digging through my LM2 log data from the past few events to see if it is consistent with the dyno. Quote Link to comment Share on other sites More sharing options...
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