what creates vacum in a motor. is it possible to have too much or too little and if so what is the end result of such?

 

The engine creates the vacuum. An internal combustion engine is essentially just an air pump. No several things effect how much vacuum is produced...cam, intake.....

 

well what particularly makes vacuum at startup more so then any other one part?

for instance is a cam the desiding factor?

 

Yes, the cam is a deciding factor in vacuum.

 

the entire engine as a whole decides on the vacuum. it is based off displacement, the heads airflow, the cam overlap and lift/duration, etc.

 

as the piston goes down in the cylinder and the intake valve opens, the difference in pressure draws air into the engine. Thus creating a vacuum above the cylinder.

depending on the timing of when the camshaft opens the valves affects the amount of vacuum created.

 

You'll probably never run into too much vacuum.
Other factors also. Ignition timing, altitude.

 

right now i'm trying to find out if i don't have enough vacuum. brakes are fine and everything else seems good but i've been told my startup tunning can't go well because i don't have enough vacuum so i'm trying to learn a bit more about it.

the cam is 230/236 @ .050 with 111.0 lsa

not that it means much.

what would the gross valve lift numbers represent beyond the obvious.

 

cam doesn't seem too large... how much vacuum do you have at idle?

 

i plan on checking it in the morning....car runs fine after it's up and running a bit. just has a hell of a time getting started and the tuner blamed it on low vacuum which is why i'm trying to figure it out on my own.

how low is too low particularly on start up?

 

well for reference, my cam which is 224/230 @ .050" 110 lsa

and I pull 16" of vacuum at 700rpm.

If your tune is really lean at idle, then your vacuum will be low. So your tuner might be partially to blame.

 

good chance it is lean....however it is spitting out alot of fuel from the tail pipe and if i dont let the car get warmed up to where the wideband starts reading it will flood the plugs.

tuner claims its because the cam isn't pulling enough vacuum which is why i'm trying to find out about what to do and learn on my own.

 

well depending on what size the engine is, that isn't all too big of a cam. so it should pull pretty decent vacuum. I'm assuming this is fuel injected, so it just sounds like your tuner either hasn't put forth the effort, or doesn't know what hes doing.

 

well right now i'm open to suggestions but i'm waiting to point fingers which is why i'm doing my own research you know.

the engine is a 388 sbc still using the 730 speed density ecm setup

i'm running fastburn heads fully modded
1.6 rr's
30lb svos
"the cam above"
miniramIII
accell 58 1,000 cfm throttle body
4" cai
1 3/4" lt's
4" exhaust
340 lph walbro pump
magnacore wires
all new sensors with gm ignition parts
lt1 mini high torque starter
ngk tr6 plugs
new vigilante converter
forged bottom end
all at 10.8 to 1 compression

that's about all i can think of right now. what else would give you notice to anything?

 

possibly try backing off the timing at idle, or run higher octane gas. you might be running a little to much compression for pump gas if its not tuned right. other than that, nothing jumps out at me. There should be no reason why you shouldn't be able to get that engine running right with the 730 ecm. might want to PM traxion, he is running 11's(i believe) on the stock 730 ecm which he tuned himself, maybe he can point you in the right direction.

 

i really hate bothering people for that sort of thing. i've tried to ask for help subtly but someone tries to claim i'm prom begging. i want to learn it all on my own but i just don't have the time to do it over night. at my rate i wouldn't get the thing programmed until august.



here's a question then....how do you determine what octane to run? currently running 93 but i'd like to know how anyway.

i believe most of this is in the tunning but i want to learn about the aspects so that i can prove that's the case and then adapt what i know to try and learn programming on my own. the car ran great once it's up and running. just has a very hard time starting..

 

Where is your base timing at? You probably need to start in the high teens for a base...That starts the fire a little early on crank and gets the bottom end swinging faster. Do you have to hold your foot in it to get it to lite? If so, Try more timing if that`s the case. That size cam should give you around 11-12 inches of vac at idle(950) and should pull about 3-5 inches cranking. You could go in an advance that cam 6 degrees or so and that would give it more vac, if it is the problem. Sounds that your engine is the soruce of the problem, I mean...Has this guy done work for you before? Has he always tuned your car fine until this motor?

 

well i'm not a pro on fuel injection, but i'm probably the only person up right now. While your engine is starting up, its running in open loop rather than closed loop. So it is NOT taking readings from the O2 sensor because it isn't hot enough yet to get good readings. I can't really tell you any more about open loop for certain, because i simply don't know for sure.

To check if your using the proper gas, check your knock count.

the proper octane is the octane where your getting no knock.

if your getting knock, try lowering your timing a little, or up the octane.

either way, i'm going to bed for tonight, good luck with it all. there are some really good articles on burning your own proms, and what needs to be done to them, its a couple good hours of reading, but it should give you an idea whats going on.

https://www.thirdgen.org/newdesign/tech/promintro.shtml

is a good link to start with.

 

the last motor (383) he was able to drop the car .8 in the 1/4 below tpis stage 6 best burnt version chip. also neted me an extra 10 mpg city with his chip too.....so i am willing to let him take shots at it but this one is just giving him trouble. i don't want to point fingers until i can really figure out whats wrong. he says it's because of vacuum and the guy who helped me build the motor says it's all in the tune so i'm running circles.

how do i set base timing over just timing?

all i've done is disconnect the one copper/brown wire near the a/c and started the car using the timing light and tab turned the distributor until it's where i want it to be on the markings for the part?? i shut it down, reconnect the wire and disconnect/connect the battery. start it up and it adjusts itself slightly and that's it.

what way should you do it?

 

Well, The timing tab probably only goes to around twelve degrees BTDC. You really need a advance lite..the one with a dial or a digital read out. This is how I do it, I`ll get the car to idle( with your est wire by passed) Tan/ black wire you un plug...Then turn your dial or digital read out on the lite to say around twenty or so...now you`d point it at your tab and turn your dist. until the mark on the balancer reads zero. (now your base timing is set to 20 degree`s) Now reset the est wire and pull the engine up too 3000 rpm and point the lite at your tab....increase the dial or reading on your lite until you see the tab line up the mark to zero again, Follow me on this? That will give you a "total timing" reading...I`d imagine with a performance chip you shoul be getting at least thirty degrees over your base setting. so if it were thirty you`d now have a reading on your lite that read 50 degrees..

 

nice....now the 20 you talk about for base. the guy who helped me put the motor set it to 12* and the tuner supposedly put it at 4*

what do you recommend.

i used a light but i turned the dial to 12* and set it that way and the car would fire up....with trouble mind you but it backfired slightly here and there under more then 3/4 accel. tried to put it at 4* and it was the same with fire...little harder to start....idled and reved fine but backfired like hell running down the road.

???

 

I would start in the high teens..16 18 maybe..twenty was a example, but I run twenty two on mine with a thirty eight total. Something just dosen`t sound right, when you set the dial to 12, your adjusting the dist to show zero on the tab right?

The backfiring could be related to timing if the chip has more than thirty total advance set into it...Once you get above fifty total your explosions will start getting out of the camber and back up into the intake manifold. That backfiring could also be a Lean pop..if your 02`s are reading rich running down the road(even though it isn`t) your ecm, If it works as a stock one, it will reduce your injectors pulse width thus, leaning out the mixture to satisfy the 02 readings...(even though the 388 needs more). I`d be willing to take a look at it over the weekend if you want..I have a snap on scanner, should be able to tie in to your system if it still uses a stock type ecm. I can at least give you some solid info on what is actually going on with it. Shop is in st clair shores just N of vernier on Greater mack.

Your engine builder, does he know where the cam is degreed in at? I`m not saying it`s wrong but, many times slight drivability problems can be solved by simply advancing the cam in realation to the crank. If it`s retarted from where it should be..you would have all these problems that your having..no doubt. We can run a few simple tests and then you`ll know what avenues to explore, mechanical(motor) electrical(ecm)or tune.

 

no when i set it i was told to look for the tab and line it up on the area marked 12??? i'm assuming that's completely wrong so i'm looking up how to do timing right now.

you know what everything your saying is starting to make sense....i'll find out everything tomorrow. check back when i have all the answers.

i'm starting to get it all very slowly.....but i'll get it. thanks.

 

sounds good...chat with ya later

 

If you have the ability to burn chips, it's best to leave base timing at the stock setting.

Reason being that you can adjust all your timing tables and settings within the PROM. My understanding at least

 

so you can stop running between the builder and tuner, you need to find what the engine is doing.. not what its suppost to be doing..

first thing i would do is stick a vac gauge on it and see what its doing at idle... perhaps its somthing mechanical, perhaps its the tune... honestly, it doesnt sound like the builder or tuner know.... they're just assuming that they're doing/have done their job right..


theres more then one type of vac you're looking at, perhaps thats why you seem confused..

theres plain vac thats used to run your accessories... this is what works your power brakes, cruise, AC/heat vent doors, ect.. as long as theres enough vac for thoes to work, that part is fine.


then theres the vac signal. the MAP (Manifold Absolute Pressure) sensor is looking at the vac it sees in the manifold.
the computer uses this signal to calculate how much air is in the motor. then it goes on to add fuel.
if this vac signal is pulsing wildly, the ecm might see high vac one time, take away fuel, and a split second later see low vac and add fuel.. its not a steady vac signal..

but the overall vac would be enough to run your power brakes...

 

the cam is 230/236 @ .050 with 111.0 lsa

That cam is going to give the tuner fits if it is not installed advanced at least 2-4 degrees. Our engines with injection should be no less than 112 LSA because of low vacuum to the MAP with narrow center cams.

 

installed advanced? how would i check about that?

as far as the map sensor...how can i check to make sure it's working correctly. i've replaced my tps sensor a few times and had them messed up right out of the box.....

sorry guys for the questions but i don't know as much as i should about the engine area so i'm trying to learn one step at a time.

 

What that means is that they install the cam with a offset bushing kit that installs into the timing gear, It allows you to move the camshaft in relation to the crank...Basically you are able to Move the camshaft a few degrees foward or backward from where the two dots line up on the timing chain. These slight movements can increase or decrease vacaum,compression,idle quality,rpm range,throttle response and fuel mileage. Advance the cam and you`ll get more vac and throttle response. It`s common practice to degree a cam and Once you "know" where the cam is, You can decide which way and how much you`ll need to move it.

You~ll need to pull the #1 plug, install a degree wheel on the crank, put a 1 inch dial indicater over the pushrod tip after you`ve removed the rocker arm, Find "true" top dead center and have the card that cam with the cam. There is a lot more to it than just that, it should be determined what is wrong with it before any dart throwing begins

If all these problems started with the engine install...Don`t start pointing fingers at the electronics just yet(map)

 

i'm fairly certain the engine is fine, however i still am trying to cover all my bases. i don't think i've ever replaced the map sensor. maybe i should have it checked? any suggestions..?