Hey everyone, I was wondering how much vacuum people were seeing with larger/modified cams.
I am running a fairly aggresive cam, with pretty high lift and duration with some overlap.
Similar to this cam, but not these EXACT specs (will try to get the right specs tonight.
Advertised Duration: 268 intake / 268 exhaust
206 deg duration @.050, .313" lobe lift (.470" @ valve w/ 1.5 rockers or .501" @ valve w/ 1.6 rockers)
lobe separation angle(LSA) of 212 deg.
I have this with 1.6 rockers, and heafty springs.
Right now I am only seeing between 13.5-14.2 inches of vacuum......Is this about right for the cam....or do I still need to hunt down a vacuum leak??
Any input would be nice. Car is running good, but the lack of vacuum concerns me.
If you're really that unsure if all the intake manifold gaskets seal properly, go ahead & do the ol' WD-40 test (That's where you spray WD-40 or something like it with a straw nozzle on the can around where the gaskets are & watch to see if there's a change in engine speed.) and see. However, it should be noted that since it is being done on a EFI engine that a change in engine speed may not be that likely to be seen since the engine management system will try to quickly compensate by adjusting the idle via the IAC. Your best bet in your case would be to monitor the EGO (Exhaust Gas Oxygen) sensor's output while doing this to see if a change in signal (Enrichment style) occurs. Otherwise one must remember: with a bigger cam can come more overlap, and with that comes a reduced vacuum signal.
Go beyond the "bolt-on".
It also seems to idle lower than commanded. It idles slightly lopey around 850rpm. I have not sprayed around it....I think that will be my next step. I do not have a scanner capable of monitoring fuel trim correction available to me at the time....I will have to wait a few days to get my laptop back from the shop, and should be able to monitor it with that.
I am aware that large cams eat up vacuum, but it seems to be a bit more that I thought, and it concerns me slightly at least.
(Heh, heh) Welcome to the world of old-school engine building. Your LDA/LSA (Lobe Displacement/Separation Angle) is probably just enough to cause that lope, which is the result of the overlap. Older-style performance oriented engines have it, and the more seriously race-oriented they are (usually in the drags) the more pronounced the lope is. This lope in the exhaust is considered to be a sign of how "healthy" an engine is (Ever here a Big-block with a big cam & really high static compression? That's what I mean). It also tends to effect how well the engine can idle at a given crank speed (RPM). Usually such "healthy" cams are found with a carb-style fuel system, and the vacuum level is so effected by the cam that the engine's tuner has no choice but to raise the idle rpm setting just to keep the engine idling. With MPI, or SMPI as you already know, the fuel-charge delivery to the cylinder doesn't rely on the engine's vacuum signal so much, so the engine need not idle so high to keep running. In fact, I remember an article in Mopar Action (Stay with me now!) on how they built-up their entry into the One Lap of America, and they stated that since they used Accel's Mk. VII fuel & spark management system on their W2-headed 392ci "LA" V-8 with a single-plain manifold & lopey cam, it's been able to idle as low as 400-600rpm without fear of it dying!
If you really are worried about how low your engine idles (Even after reading all this^), then get-hold of a tuner software program & reset it. That's all. Although seriously I doubt you have anything to fear unless you're running a amperage-eating stereo & light setup, and you need the alternator to spin at a certain RPM. Or are you just worried about emissions testing failing you over that? Either way, I'd like to hear an audio clip of what your exhaust sounds like with that cam. I bet it has a sweet, classic English sports-car sound to it!
Go beyond the "bolt-on".
Well I am not all that concerned about the low idle, except for I have a random miss around 800rpm, which is where it seems to like to stay. I have a programmer, and it is programmed to idle at 1000rpm, but doesnt like to go there.
I am familiar with lopey cams in the old fashioned sence as that is what I started my modding career with. I have a decent sized vacuum canister on my trans am from the cam, and have the idle set high for the motor.
It actaully concerns the guy who is supposed to be doing my dyno more than me, I just wanted to see what others with the modified cams are doing.
I have a 10 sec. video on my phone, but the sound SUX, you can hear the lope....but its just horrible.....I am embarrased to post it
I will get a good one. It does sound sexy as hell.
hey man. what did you do as valve spring wise?
DRIVE HARD OR DONT DRIVE AT ALL!!!
My guess some ls1 springs from crane machined to fit the head thats what i would of done.
I cant tell my secrets
Nah, i have to change the valvecover gasket anyways....I will try to take some good pics.
