Extremely low vaccum after cam change!
03-18-2004, 09:31 AM
#1
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Thread Starter
Join Date: Oct 1998
Location: Engelhard, N.C.
Posts: 535
Extremely low vaccum after cam change!
I just rebuilt my 94 LT1 and upgraded the cam to a 235/242 @ 0.050 .555/570 lift on a 112 LSA and added 30# SVO's. Now the car has a very rought idle, is throwing DTC 33, and seems to be way rich. I scanned it with my scanner and I only have 5.5in/hg vaccum @ my 875 idle, MAP reads 4.0V, but at 2700 rpm I believe that my tune is the problem, I only have 25 degs of timing at idle, and I think I need around 34-35 degs to increase cyl pressure and inturn increase vaccum. I may need to take some fuel out, but I'll do that after the timing to see how it runs. What do you guys think, am I on the right track.
Randy
Randy
03-18-2004, 12:05 PM
#2
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Join Date: Aug 2001
Location: Carlisle, PA
Posts: 874
a 306 tune that I have looks like this.
Map 800RPM 1000RPM
25 27 32
30 31 38
35 32 39
40 33 36
45 33 34
50 32 33
55 29 31
60 26 29
65 25 26
70 22 23
75 19 20
80 16 16
85 14 14
90 11 11
formating won't stay but I think you get the idea
forgot to add the low map stuff.
Map 800RPM 1000RPM
25 27 32
30 31 38
35 32 39
40 33 36
45 33 34
50 32 33
55 29 31
60 26 29
65 25 26
70 22 23
75 19 20
80 16 16
85 14 14
90 11 11
formating won't stay but I think you get the idea
forgot to add the low map stuff.
Last edited by GetaZforgetGT; 03-18-2004 at 12:11 PM.
03-18-2004, 06:52 PM
#4
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Thread Starter
Join Date: Oct 1998
Location: Engelhard, N.C.
Posts: 535
Hey Alvin! I thought you might see this thread. I still have the factory PCM and Bryan has already tuned it once, but he thinks I have a vac leak and I don't think so. I'll prbably be at Bob's sunday if you want to stop by. A buddy of Bob and mine has a Tunercat and we're going to see if we can clear things up and make a positive determination whether it's a tuning issue or mechanical, either way I don't care I just want my car to run the way it's suppose to. Stop by and check us out if you can. Later.
Randy
Randy
03-20-2004, 08:40 AM
#5
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Join Date: Dec 1969
Location: Calgary, Alberta, Canada
Posts: 324
When you get the tuning software ... set the injector constant to around 31 - 32# for the SVO 30# injectors. That should work out pretty close.
In the closed throttle table, you can add quite a bit of timing, I have added 5 degrees and it seems to have given me an extra inch or so of vaccum.
The low speed 400 - 4000 rpm table can also benifit from more timing. I put in 2 degrees, but experimented with 4 degrees, didn't notice any difference.
How are your brakes??
Hope this helps
LWM
In the closed throttle table, you can add quite a bit of timing, I have added 5 degrees and it seems to have given me an extra inch or so of vaccum.
The low speed 400 - 4000 rpm table can also benifit from more timing. I put in 2 degrees, but experimented with 4 degrees, didn't notice any difference.
How are your brakes??
Hope this helps
LWM
03-26-2004, 06:24 PM
#10
Registered User
Join Date: Jun 2001
Location: Wichita, Ks
Posts: 82
I just put a 306cc in my new motor and it's reading around 60 map at 900rpm. Until I get it up to 3500-4000rpm it feels like it's falling on its face "this is at part throttle", i'm gonna try to add some timing to closed throttle position as suggested in this thread and see what it does. Other than that I've tried setting my timing up 4 to 5 degrees on both main and extended timing tables and didn't notice a bit of difference, except a slightly lower map reading at similar rpm's.
Since the weather changed and the humidity went up "here in Kansas" my car doesn't want to idle or run at low rpm's at all. The other day I put it in nuetral and revved it up and it sounded like it was backfiring through the intake "popping noises not coming out the exhaust". I've leaned it out to the point where the car barely runs and still the crappy low end power. I just checked my tuner directory and I've made 21 different tunes in the last 3 months, I've read almost every inch of this board trying everything I could "drilling out my BBK 52mm idle air passage helped the idle the most", downloaded countless tunes from people with similar mods and it still has no low end power. I thought having almost 12:1 compression would make the car have more throttle response not less. WOT the car seems to run fine albeit a bit rich but I'm still fine tuning that. And I didn't just paste their tune into mine I just used them for a reference and adjusted the areas that needed work.
My old 77 camaro that I sold years ago had a 750 holley, 292cc
11:1 compression, and some old double hump heads with a poor port job and would run rings around my current car at low rpm's.
So needless to say I'm still hunting for that programming secret to make this thing perform like it should. I recently bought a Vigilanted 3000 stall but even before that the car ran fine as long as you had your foot to the floor. I'm not giving up until I fix it, I've even tried pillow talk and still a no go.
Since the weather changed and the humidity went up "here in Kansas" my car doesn't want to idle or run at low rpm's at all. The other day I put it in nuetral and revved it up and it sounded like it was backfiring through the intake "popping noises not coming out the exhaust". I've leaned it out to the point where the car barely runs and still the crappy low end power. I just checked my tuner directory and I've made 21 different tunes in the last 3 months, I've read almost every inch of this board trying everything I could "drilling out my BBK 52mm idle air passage helped the idle the most", downloaded countless tunes from people with similar mods and it still has no low end power. I thought having almost 12:1 compression would make the car have more throttle response not less. WOT the car seems to run fine albeit a bit rich but I'm still fine tuning that. And I didn't just paste their tune into mine I just used them for a reference and adjusted the areas that needed work.
My old 77 camaro that I sold years ago had a 750 holley, 292cc
11:1 compression, and some old double hump heads with a poor port job and would run rings around my current car at low rpm's.
So needless to say I'm still hunting for that programming secret to make this thing perform like it should. I recently bought a Vigilanted 3000 stall but even before that the car ran fine as long as you had your foot to the floor. I'm not giving up until I fix it, I've even tried pillow talk and still a no go.
03-26-2004, 06:56 PM
#11
Registered User
Join Date: Dec 1969
Location: Calgary, Alberta, Canada
Posts: 324
Originally posted by 94redform
I just put a 306cc in my new motor and it's reading around 60 map at 900rpm. Until I get it up to 3500-4000rpm it feels like it's falling on its face "this is at part throttle", i'm gonna try to add some timing to closed throttle position as suggested in this thread and see what it does. Other than that I've tried setting my timing up 4 to 5 degrees on both main and extended timing tables and didn't notice a bit of difference, except a slightly lower map reading at similar rpm's.
Adding the timing is supposed to give you less map. It is an indication of efficiency. It will also give you more vaccum and probably smooth out the idle. At least, that is what it did in my case. That CC 306 is pretty big, I have my auto 383 idling at 875 rpm, it might help you if you set your idle up a bit also.
Originally posted by 94redform
Since the weather changed and the humidity went up "here in Kansas" my car doesn't want to idle or run at low rpm's at all. The other day I put it in nuetral and revved it up and it sounded like it was backfiring through the intake "popping noises not coming out the exhaust". I've leaned it out to the point where the car barely runs and still the crappy low end power.
I just put a 306cc in my new motor and it's reading around 60 map at 900rpm. Until I get it up to 3500-4000rpm it feels like it's falling on its face "this is at part throttle", i'm gonna try to add some timing to closed throttle position as suggested in this thread and see what it does. Other than that I've tried setting my timing up 4 to 5 degrees on both main and extended timing tables and didn't notice a bit of difference, except a slightly lower map reading at similar rpm's.
Adding the timing is supposed to give you less map. It is an indication of efficiency. It will also give you more vaccum and probably smooth out the idle. At least, that is what it did in my case. That CC 306 is pretty big, I have my auto 383 idling at 875 rpm, it might help you if you set your idle up a bit also.
Originally posted by 94redform
Since the weather changed and the humidity went up "here in Kansas" my car doesn't want to idle or run at low rpm's at all. The other day I put it in nuetral and revved it up and it sounded like it was backfiring through the intake "popping noises not coming out the exhaust". I've leaned it out to the point where the car barely runs and still the crappy low end power.
You just have to double check everything.
Hope this helps
LWM
03-26-2004, 07:56 PM
#12
Registered User
Join Date: Jun 2001
Location: Wichita, Ks
Posts: 82
Just put new wires in a couple of weekends ago and almost no difference. I was going over my datamaster file and when I logged it in febuary and I actually was getting around 70 to 78 map at zero% throttle at idle. I'll double check it again tomorrow and see what I get. Thanks for the suggestion though, I will check again and make sure none of the wires are crossed tomorrow.
03-26-2004, 08:49 PM
#13
Registered User
Join Date: Dec 1969
Location: Calgary, Alberta, Canada
Posts: 324
FWIW, my 383 stroker, 224/236 on 114 lsa camshaft, at 875 rpm idle I see a MAP value of 50 - 53 kpa.
I don't see the 70 - 78 kpa values untill I get to 30% throttle and 2,400 rpm. That's about 20 mph, just out of first gear into 2nd.
With the stock engine and cam, it would be 35% throttle before the MAP would get up to 75 KPA.
If you are seeing that high (70 - 78 KPA) a MAP value, at an idle ... something is not quite right.
Manifold leak?
Other vaccum leak?
Just more things to think about.
LWM
I don't see the 70 - 78 kpa values untill I get to 30% throttle and 2,400 rpm. That's about 20 mph, just out of first gear into 2nd.
With the stock engine and cam, it would be 35% throttle before the MAP would get up to 75 KPA.
If you are seeing that high (70 - 78 KPA) a MAP value, at an idle ... something is not quite right.
Manifold leak?
Other vaccum leak?
Just more things to think about.
LWM
03-27-2004, 12:14 AM
#14
Registered User
Join Date: Jan 2002
Location: Montpelier, VA USA
Posts: 360
Originally posted by 94redform
I just put a 306cc in my new motor and it's reading around 60 map at 900rpm. Until I get it up to 3500-4000rpm it feels like it's falling on its face "this is at part throttle", i'm gonna try to add some timing to closed throttle position as suggested in this thread and see what it does. Other than that I've tried setting my timing up 4 to 5 degrees on both main and extended timing tables and didn't notice a bit of difference, except a slightly lower map reading at similar rpm's.
Since the weather changed and the humidity went up "here in Kansas" my car doesn't want to idle or run at low rpm's at all. The other day I put it in nuetral and revved it up and it sounded like it was backfiring through the intake "popping noises not coming out the exhaust". I've leaned it out to the point where the car barely runs and still the crappy low end power. I just checked my tuner directory and I've made 21 different tunes in the last 3 months, I've read almost every inch of this board trying everything I could "drilling out my BBK 52mm idle air passage helped the idle the most", downloaded countless tunes from people with similar mods and it still has no low end power. I thought having almost 12:1 compression would make the car have more throttle response not less. WOT the car seems to run fine albeit a bit rich but I'm still fine tuning that. And I didn't just paste their tune into mine I just used them for a reference and adjusted the areas that needed work.
My old 77 camaro that I sold years ago had a 750 holley, 292cc
11:1 compression, and some old double hump heads with a poor port job and would run rings around my current car at low rpm's.
So needless to say I'm still hunting for that programming secret to make this thing perform like it should. I recently bought a Vigilanted 3000 stall but even before that the car ran fine as long as you had your foot to the floor. I'm not giving up until I fix it, I've even tried pillow talk and still a no go.
I just put a 306cc in my new motor and it's reading around 60 map at 900rpm. Until I get it up to 3500-4000rpm it feels like it's falling on its face "this is at part throttle", i'm gonna try to add some timing to closed throttle position as suggested in this thread and see what it does. Other than that I've tried setting my timing up 4 to 5 degrees on both main and extended timing tables and didn't notice a bit of difference, except a slightly lower map reading at similar rpm's.
Since the weather changed and the humidity went up "here in Kansas" my car doesn't want to idle or run at low rpm's at all. The other day I put it in nuetral and revved it up and it sounded like it was backfiring through the intake "popping noises not coming out the exhaust". I've leaned it out to the point where the car barely runs and still the crappy low end power. I just checked my tuner directory and I've made 21 different tunes in the last 3 months, I've read almost every inch of this board trying everything I could "drilling out my BBK 52mm idle air passage helped the idle the most", downloaded countless tunes from people with similar mods and it still has no low end power. I thought having almost 12:1 compression would make the car have more throttle response not less. WOT the car seems to run fine albeit a bit rich but I'm still fine tuning that. And I didn't just paste their tune into mine I just used them for a reference and adjusted the areas that needed work.
My old 77 camaro that I sold years ago had a 750 holley, 292cc
11:1 compression, and some old double hump heads with a poor port job and would run rings around my current car at low rpm's.
So needless to say I'm still hunting for that programming secret to make this thing perform like it should. I recently bought a Vigilanted 3000 stall but even before that the car ran fine as long as you had your foot to the floor. I'm not giving up until I fix it, I've even tried pillow talk and still a no go.
12 to 1 on pump gas? Are you sure thats why it may not run? I didn't know you could run that much comp on pump. I had to throw some old regular in a 383 to get it to the gas station after a rebuild and it wouldn't run worth a crap until we put the 93 in it. Anything less and it has a hard time idling running at low rpms.
03-27-2004, 12:59 PM
#15
Registered User
Join Date: Jun 2001
Location: Wichita, Ks
Posts: 82
I was thinking possibly a vacuum leak also, but I double checked all the hoses putting the engine back in.
The reason I can run that high of compression on pump gas is because I used a thermal barrier compound on the piston and bottom of the heads "at least in theory". Here's the link http://www.techlinecoatings.com/
I usually run 93 in it but I have also ran 91 and recorded no knock with datamaster. My piston to head squish height is .038 and the block is zero decked. I'm using flat top SRP pistons, and I read that with the proper squish and flat top pistons you can make more power because it creates more turbulence in the cylinder "because of the closed chamber head design of the LT1" just before ignition hence mixing the air and fuel better and less prone to detonation. I used an online calculator and the compression should be right at 11.96:1.
On techline's page they also say you can run a leaner mixture since the extra heat is kept in the combustion charge rather than soaking into the surrounding surfaces causing piston meltdown.
Seems to work as a good piston insurance policy against detonation. I also used the anti-friction compound on the sides of my pistons. Of course I run a 160 thermostat too.
The reason I can run that high of compression on pump gas is because I used a thermal barrier compound on the piston and bottom of the heads "at least in theory". Here's the link http://www.techlinecoatings.com/
I usually run 93 in it but I have also ran 91 and recorded no knock with datamaster. My piston to head squish height is .038 and the block is zero decked. I'm using flat top SRP pistons, and I read that with the proper squish and flat top pistons you can make more power because it creates more turbulence in the cylinder "because of the closed chamber head design of the LT1" just before ignition hence mixing the air and fuel better and less prone to detonation. I used an online calculator and the compression should be right at 11.96:1.
On techline's page they also say you can run a leaner mixture since the extra heat is kept in the combustion charge rather than soaking into the surrounding surfaces causing piston meltdown.
Seems to work as a good piston insurance policy against detonation. I also used the anti-friction compound on the sides of my pistons. Of course I run a 160 thermostat too.
Last edited by 94redform; 03-27-2004 at 02:27 PM.