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| High lift and duration- Its affect on idle https://slantsix.org/forum/viewtopic.php?t=18470 |
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| Author: | Bren67Cuda904 [ Sun Jul 09, 2006 4:46 pm ] |
| Post subject: | High lift and duration- Its affect on idle |
It is true that the higher the cam lift and duration the worse you idle will be. But is it also true that EFI will fix thoughs idle problems? 2nd ? - How big can you go before idle is unbearable for carb and EFI? (if theres a difference) |
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| Author: | Rust collector [ Sun Jul 09, 2006 4:57 pm ] |
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As I understand, the issue is the overlap between the intake and exhaust lobe, causing a "wierd" signal to the carb, causing bad idle. I figure the efi might do a bit better, as it don´t rely "just" on the vacum signal and/or airflow, but has other sensors to tell it what is happening. NOW, can someone that actually has a clue come and answer???? |
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| Author: | Charrlie_S [ Sun Jul 09, 2006 5:17 pm ] |
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Lift will have very little effect on idle quality, as long as all other cam perameters do not change (there is the rub), usually they do. Total duration, overlap, and lobe seperation angle, and lobe centerline, will effect idle quality. The cam is not the only consideration. A given cam can act drasticly different, in different engines, with different cubes, and compression. You can get away with a "bigger" cam, in an engine with more cubes and/or compression. |
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| Author: | Bren67Cuda904 [ Sun Jul 09, 2006 6:04 pm ] |
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EFI makes no difference? |
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| Author: | SlantSixDan [ Sun Jul 09, 2006 6:29 pm ] |
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Fuel injection definitely buys you some streetability margin for a cam that would give unacceptable starting, idling and low-speed running with a carburetor, but remember, most fuel injection systems rely on manifold vacuum to calculate fuel volumes—which is exactly why carburetors have a hard time running big-cammed engines slowly. So, the margin is definitely there, but it's not a "get out of jail free" card for a cam that's just too big for what's being asked of the engine. You get more cam tolerance with an EFI system based on mass airflow rather than manifold absolute pressure. |
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| Author: | Charrlie_S [ Sun Jul 09, 2006 6:30 pm ] |
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Quote: EFI makes no difference?
I can't say for sure. I would "think" FI would improve the idle quality SLIGHTLY. The only FI, I have worked with is stock systems on newer vehicles, and the old mechanical race systems (hilborn), which were a nightmare to get to idle.
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| Author: | sandy in BC [ Sun Jul 09, 2006 6:37 pm ] |
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Efi can be different in several ways: Most OEM computers have limited ability to adapt,,,,,so you need to stay within certain parameters. As vacuum falls the ability of a MAP sensor to react can be critical. The amount of spark advance for variances in manifold vacuum changes. The ECU may not be able to cope. Idle with EFI can end up pretty rich with a big cam because the ECU doesn t run on the O2 sensor at idle. As you increase fuel pressure to keep up at WOT the idle can get pretty rich. (I use bigger injectors in a bored TB to get around using high fuel pressure) On the good side EFI usually incorporates IAC or idle air control that allows a very steady low idle....not dependent on manifold vacuum. Where it can get sticky is coming off idle ( IAC to MAP + TPS sensors) It is easy to be too lean and get the wrong advance on transition. I run a TBI system on my work truck with a pretty big cam (270 adv duration on the exhaust side.....485 lift) with a tight motor home convertor (800 RPM stall) It idles like a kitten. ...will idle up hill in drive...but I have an excellent chip guy. |
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| Author: | Doctor Dodge [ Mon Jul 10, 2006 10:29 am ] |
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The idle Air / Fuel ratio must be correct, so when the engine's idle 'signals' change with a high overlap cam, you will need to recalibrate to get it right, with a carb. or FI. One thing that a 'big cam' (large overlap event) does is to drastically drop the low speed cylinder pressure. Much of the choppy idle from a big cam is not fuel mixture related but from low cylinder pressure causing the misfire. The same 'big cam' used in a 8 to 1 engine will idle like crap, then put it in a 11 to 1 engine and it idles smooth. That is why the better engine builders pay close attention to matching the static compression ratio to the cam, some go as far as checking the cranking compression and adjusting the cam timing to set the cranking cylinder pressure where they want it. DD |
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| Author: | emsvitil [ Mon Jul 10, 2006 12:56 pm ] |
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A large overlap will also give you an EGR effect with exhaust gases staying around........... But the exhaust staying around tends to vary, so it's harder to get the mixture right, and you get some roughness..... BTW, modern engines are going with big lobe separation angles in an effort to have little overlap, and long duration |
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| Author: | Ron Parker [ Mon Jul 10, 2006 7:53 pm ] |
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What is your goal street car street/strip car or race car. You can not have the best of any of this unless you tell us what is your goal is . Camshaft is not the only factor when building a engine. One has to match componants together for total performance. Thanks Ron Parker  It Aint Over Until I Win It Aint Over Until I Win |
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| Author: | Bren67Cuda904 [ Tue Jul 11, 2006 6:32 am ] |
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my goal is not a racecar. I would like to EFI this car someday, but for now it will be carberated. I don't mind a small driveablity problem if going to EFI will fix that problem later. I do want to drive it in the meantime. Its not a daily driver. I take it to the car shows and to friend houses to watch the race on Sunday. When it does get EFI, I do want to take it on some longer trips. (Orlando to the Keys). I'd like to do it once and not wish I had put a bigger or smaller cam in later. Engine build is simular to the Slant Sickness artical (Hot Rod) and DD Long Rod build. I am shooting for mid 14's no blower or nitrous. But maybe nitrous someday. (small shot -75) |
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