Best vacuum source for T-350 from 350SBC TBI? - Hot Rod Forum : Hotrodders Bulletin Board
Hotrodders.com -- Hot Rod Forum



Register FAQ Search Today's Posts Unanswered Posts Auto Escrow Insurance Auto Loans
Hot Rod Forum : Hotrodders Bulletin Board > Tech Help> Engine
User Name
Password
lost password?   |   register now

Reply
 
LinkBack Thread Tools
  #1 (permalink)  
Old 12-09-2012, 08:41 PM
Registered User
 
Join Date: Jun 2012
Posts: 29
Wiki Edits: 0

Thanks: 0
Thanked 0 Times in 0 Posts
Best vacuum source for T-350 from 350SBC TBI?

So I finally got my truck running again and now I've got a question. Ive got an 88 c1500 that I swapped the 350 SBC for a 1995 model 350 SBC and a new TH350 transmission. The truck originally had a 700r4 transmission in it but it went bad so I just put in a 350 turbo tranny. Now, I knew that I had to had some type of vacuum source for the modulator valve on the tranny so I spliced into a line that was running out the back of the throttle body (TBI) to the MAP sensor. After filling up the trans, I ran my truck and it runs good. I bought a 2300-2600 stall converter to put in the tranny also. So here's my question: my friends and I have noticed that when pulling from a stopped position, it almost seems like my truck hesitates, almost like sluggish. But once it gets goin (35 mph-up) it does good. If I let off the gas and hit again while it's cruising, I can feel a kick from the transmission. So I wonder, is there a problem with the vacuum source? Maybe a better source elsewhere? I noticed there is an additional hookup on the MAP sensor that says "vent" can I tap into that? How can I improve the sluggish take-off? I've seen posts from other people who say that they have my same transmission and it chirps the tires on their vehicles between gears, how can I get that same effect out of my truck? This truck beat an IROC-Z in the past with only an upgraded ignition system (MSD cap and wires, performance ignition coil, platinum plugs) and know I can't do that now.

    Advertisement
Reply With Quote Quick reply to this message
Sponsored Links
Advertisement
 
  #2 (permalink)  
Old 12-09-2012, 11:09 PM
cobalt327's Avatar
WFO
 
Last wiki edit: Intake manifold
Last journal entry: 1980 Malibu Wagon
Last photo:
Join Date: Mar 2009
Location: Atlanta
Age: 60
Posts: 5,037
Wiki Edits: 1616

Thanks: 128
Thanked 598 Times in 547 Posts
Quote:
Originally Posted by Chevyman87 View Post
So I finally got my truck running again and now I've got a question. Ive got an 88 c1500 that I swapped the 350 SBC for a 1995 model 350 SBC and a new TH350 transmission. The truck originally had a 700r4 transmission in it but it went bad so I just put in a 350 turbo tranny. Now, I knew that I had to had some type of vacuum source for the modulator valve on the tranny so I spliced into a line that was running out the back of the throttle body (TBI) to the MAP sensor. After filling up the trans, I ran my truck and it runs good. I bought a 2300-2600 stall converter to put in the tranny also. So here's my question: my friends and I have noticed that when pulling from a stopped position, it almost seems like my truck hesitates, almost like sluggish. But once it gets goin (35 mph-up) it does good. If I let off the gas and hit again while it's cruising, I can feel a kick from the transmission. So I wonder, is there a problem with the vacuum source? Maybe a better source elsewhere? I noticed there is an additional hookup on the MAP sensor that says "vent" can I tap into that? How can I improve the sluggish take-off? I've seen posts from other people who say that they have my same transmission and it chirps the tires on their vehicles between gears, how can I get that same effect out of my truck? This truck beat an IROC-Z in the past with only an upgraded ignition system (MSD cap and wires, performance ignition coil, platinum plugs) and know I can't do that now.
Do not use the MAP sensor vent, it has to be open to atmosphere.

Any manifold (full time) vacuum source will work for the modulator. I usually try not to use a "Y" off of a vacuum port to supply two devices, but it's not taboo to do so.

You didn't say if you're using an ECM or not, so are you? If no computer and if you have a 'normal' distributor, the timing curve needs to be set up correctly.

The '95 engine was made for torque if it was a TBI truck application. It should run OK, though- especially down low in the rpm range. But not knowing what cam, etc. was in the first engine compared to what you have now, I can't say what the difference in performance between the two may be. I recall you saying in earlier threads you have a Vortec engine '96-up), and that you've had all sorts of issues w/the TH350 trans. You might have a bad rebuild on the tranny.

Generally speaking, a looser torque converter takes the "bite" out of gear changes, especially at less than WOT.

The sluggishness off the line could be accelerator pump-related (if you have a carb), or timing related (if you have a distributor w/o ECM). You cannot use a computer controlled distributor w/o the computer and expect good performance. I'll wait for details before going deeper.
Reply With Quote Quick reply to this message
  #3 (permalink)  
Old 12-10-2012, 02:43 AM
cobalt327's Avatar
WFO
 
Last wiki edit: Intake manifold
Last journal entry: 1980 Malibu Wagon
Last photo:
Join Date: Mar 2009
Location: Atlanta
Age: 60
Posts: 5,037
Wiki Edits: 1616

Thanks: 128
Thanked 598 Times in 547 Posts
After rereading your post, if you are still using the MAP, then it's a safe assumption you're also still using the ECM and TBI, etc.

That being the case I would start by seeing if there are any codes stored, even if there's no dash light lit up. Then I'd time it correctly.

Whether using the vacuum line to the MAP for both it and the modulator is a good idea, I'm not sure if that would affect the MAP or not. Maybe LATECH- who's well versed in these things- will have an opinion.

Good luck.
Reply With Quote Quick reply to this message
  #4 (permalink)  
Old 12-10-2012, 07:56 AM
Registered User
 
Join Date: Jun 2012
Posts: 29
Wiki Edits: 0

Thanks: 0
Thanked 0 Times in 0 Posts
Thanks for your help cobalt

It's not a vortec engine I confirmed that using the number stamped under the valve cover. The person who sold it to me had said it was a vortec engine when I bought it but I had no idea how to check that at the time. Anywho, I know that swirl ports don't make good top end power as they are made for pulling/towing/hauling hence the good low end power. I've got the computer controlled distributor and every now and then the engine light does flash but not for very long. And I do have the Y configuration you mentioned so now I ask is there a better way to get vacuum? Where can I get a single full time vacuum source off the manifold?

The tranny I have now is a fresh rebuild i bought from summit racing its not that POS I bought off the street I was having issues with
Reply With Quote Quick reply to this message

Recent Engine posts with photos

Quick Reply
Message:
Options

Register Now

In order to be able to post messages on the Hot Rod Forum : Hotrodders Bulletin Board forums, you must first register.
Please enter your desired user name (usually not your first and last name), your email address and other required details in the form below.
User Name:
Password
Please enter a password for your user account. Note that passwords are case-sensitive.
Password:
Confirm Password:
Email Address
Please enter a valid email address for yourself.
Email Address:

Log-in

Human Verification

In order to verify that you are a human and not a spam bot, please enter the answer into the following box below based on the instructions contained in the graphic.




Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)
 
Thread Tools

Posting Rules
You may post new threads
You may post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are On
Pingbacks are On
Refbacks are On


Similar Threads
Thread Thread Starter Forum Replies Last Post
using pcv as a vacuum testing source Mach1460 Engine 1 05-09-2012 12:57 PM
Converting a ported vacuum source to a manifold source Scrumgees Engine 2 08-17-2011 02:09 AM
vacuum advance source bigdog7373 Hotrodding Basics 13 11-12-2010 05:52 PM
Good Brake Vacuum Source?? awoodman Suspension - Brakes - Steering 7 02-09-2009 07:35 PM
Vacuum source for brakes malc Engine 7 12-15-2004 09:51 AM


All times are GMT -6. The time now is 09:54 PM.


Powered by vBulletin® Version 3.8.7
Copyright ©2000 - 2014, vBulletin Solutions, Inc.
Search Engine Optimization by vBSEO 3.6.0 PL2
Copyright Hotrodders.com 1999 - 2012. All Rights Reserved.