Curving the hei? - The 1947 - Present Chevrolet & GMC Truck Message Board Network

72superBB · 01-25-2004, 10:12 AM

What is this mean? How is it done?

As it stand, to get my truck to run right I have to rotate the distributor all the way to one side. It almost goes until it cannot go any farther.

I had seen another post about curving, but I need educated on this subject.

Thanks

Ron

Mudder · 01-25-2004, 10:23 AM

I sorta understand it but can't explain it. I know theres a place in Minn or Wis that you can send yours to and they will curve it for ya. A friend of mine said it was some of the best money he had spent.

dmurphy68 · 01-25-2004, 10:28 AM

got this from http://www.jeepster.org/swchapter/hei-2.html

Getting it to Run Right - Recurving for Mileage and Performance
Just installing an HEI is not all that needs to be done. You have to "dial it in" to make it work like it really can.

The stock HEI's vacuum advance diaphragm is far too sensitive to work with the older carburetor's ported spark vacuum system. It will advance to fast, too much, and too soon, hurting performance, mileage, and possibly burning valves and pistons. Leave the vacuum line off till you make the modification.

You'll need to get an adjustable vacuum advance diaphragm from your parts store. It will run you about $20 or so, but it's well worth it. Parts stores will probably have to order it, speed shops will probably have it in stock. Make sure it's for a GM HEI.

Before installing it, check the instructions to make sure, but all I've seen needs to have the Allen adjustment screw turned all the way counter clockwise to DECREASE sensitivity. You do not want it to work fast, you want it at the least sensitive position.

Install it carefully as per instructions. If you have trouble reaching the mounting screws apply a little vacuum to the diaphragm, it turns the plate out of the way.

Install the limiter cam the new adjustable diaphragm came with. Set it so it only travels about 1/2 the way it did without it. You can make marks on the distributor body before and after the cam is installed to see where to set it. 1/2 way should give you about 14 degrees vacuum advance. Without that limiter it will be worse than it was with the original advance diaphragm.

Clean the damper and the pointer. It will help if it's painted black.

Roll the engine to TDC. Since you don't have timing marks beyond 10 degrees, you need to make some. We are going to make a $100 degree wheel for free.

Use dabs of white, yellow, or silver paint to make it easy to see against the black background. The tip of a small nail or wire makes a good brush..

Put a tiny dab of paint along the notch in the damper. This is your normal timing mark. Put another larger dab right on the 0 mark on the stationary pointer and a tiny one on the 10 degree point.

When the notch and the 0 are lined up exactly, the timing is 0.

Rotate the engine till the scribed line lines up with the 10 on the stationary pointer. Put another dab on the damper exactly where it lines up with "0" on the pointer. Put a "1" next to it. This is now another 10 degree mark.

Move the crankshaft another 10 degrees till your new "1" mark is opposite the "10" on the pointer. Now put another mark on the damper next to the "0" on the pointer. Mark it with a "2" This is now your 20 degree mark.

Again, move the crankshaft 10 degrees more till the "2" mark is opposite the "10" on the pointer. Put another mark on the damper next to the "0" on the pointer. Marking it "3". Right, that's your 30 degree mark.

Again, put the "3" opposite the "10" on pointer, mark the spot next to the pointer's "0" with "4" It should end up like this: 1 - 2 - 3 - 4 -

Be as accurate as you can.

What we did is made the degree marks on the damper rather than the pointer. The reason you heavily marked the 0 on the pointer is that now is your new reference. When the "2" is opposite the 0, it's timed at 20 degrees. We can read all the way up to 40 degrees ("4") now, the original pointer only went to 10.
Disconnect the vacuum advance hose. Start it, take a reading - this is your "Initial Timing." It should be about 10 degrees.

Run it up to cruise speed, about 3000 RPM. Notice the marks moved as you revved it up. Now your "2" should be near the big "0" on the pointer. You can interpolate what it is, like 23 etc.

The difference between the reading at speed and the Initial is how much your mechanical advance moved it. It should have moved it 10 to 15 degrees more than the initial. So if you started at 10 degrees, it should have gone to 20 to 25 degrees. That advance is simply the weights inside the distributor, under the rotor. Kits are available to modify this curve, but stock is better for off-road performance, since the odd-fire loves making low RPM torque.

Now reconnect the vacuum line to the advance. Take another reading at idle to make sure the vacuum isn't affecting it at idle, it should still be at the Initial reading. -- If it moves the timing you have the wrong type of vacuum on the line. It needs ported vacuum, not manifold vacuum. Ported vacuum is near 0 at idle and gets stronger as you accelerate till about 1/2 throttle, then it decreases again as the throttle open more. Manifold vacuum is strongest at idle. It's important you use the right vacuum source, otherwise the system will work backwards!

hope this helps it sounds like you are a couple of teath off. you might want to bring #1 to t.d.c. pull your dist cap and see if it is pointing to #1. if it is not you need to reinstall the dist by moving it .

later
Darren

ddsmith · 01-25-2004, 11:18 AM

Take a look here. Proper advance curve adjustment. You didn't mention what the advance is reading when it will run. One thing I know as fact from previous experience is that timing chain slack will require a large advance to get the car to run. If you have a lot of miles on the chain it can be a bear to adjust timing correctly. This is defintely the case with the engines that had nylon coated teeth. I'm not sure if any trucks had this type of gear. I certainly hope not. They were crap.

*rage'nrat638* · 01-25-2004, 11:25 AM

sounds like your a tooth off on the dist to me......

nu2-72 · 01-25-2004, 12:44 PM

Does anyone know of or have personnely used a curve shop? I need an address please.

Mudder · 01-25-2004, 12:57 PM

My friend uses a place up north....He sends the distributor and the engine specs and they take care of it. He's an engine builder and does everything himself. He said this is well worth it. I will call him later this morning and see if hes got a name/number for the place.

nu2-72 · 01-25-2004, 01:38 PM

Thanks.

Mudder · 01-25-2004, 02:12 PM

He said there are lots of places that can do it but he would use BUI Davis Unified Ignitions. He said it is out east. The place up north was a speed shop in the St. Paul/Minn area and he cant remember the name. I've been searchin for this BUI but haven't had any luck.

lukecp · 01-25-2004, 02:33 PM

Are you sure it isn't DUI Mudder?

I have seen them advertise in car magazines before....

nu2-72 · 01-26-2004, 07:35 PM

FYI. DUI does not do stock units. I called them today and they only sell/ work on their own products.

Blue Beard · 01-26-2004, 10:35 PM

would like to see some of the dealers advise on this subject.....

nu2-72 · 01-26-2004, 10:57 PM

I am not really sure what input dealers will have. Guess it will be interesting to see.
I would assume that more knowledge might be found with engine builders and/or dyno shops. We have no dyno or speed shops around this area.
Anyone know of any?

lukecp · 01-26-2004, 11:46 PM

Quote:

Originally posted by nu2-72
knowledge might be found with engine builders and/or dyno shops. We have no dyno or speed shops around this area.
Anyone know of any?

None in my area that i know of either. Missouri is almost devoid of speed shops, or dragstrips

Mudder · 01-27-2004, 05:04 AM

I may have misunderstood Fuzz yesterday and it must have been DUI.

There is a dyno in Cuba I've heard. The guy builds race engines for someone. Thats all I know about that. Could maybe find out. Wise speed shop in St Louis but dont know what they have either. I'll ask Brad at work today.

01-25-2004, 10:28 AM	#3
dmurphy68 Registered User Join Date: Nov 2002 Location: Middletown ohio Posts: 1,058	got this from http://www.jeepster.org/swchapter/hei-2.html Getting it to Run Right - Recurving for Mileage and Performance Just installing an HEI is not all that needs to be done. You have to "dial it in" to make it work like it really can. The stock HEI's vacuum advance diaphragm is far too sensitive to work with the older carburetor's ported spark vacuum system. It will advance to fast, too much, and too soon, hurting performance, mileage, and possibly burning valves and pistons. Leave the vacuum line off till you make the modification. You'll need to get an adjustable vacuum advance diaphragm from your parts store. It will run you about $20 or so, but it's well worth it. Parts stores will probably have to order it, speed shops will probably have it in stock. Make sure it's for a GM HEI. Before installing it, check the instructions to make sure, but all I've seen needs to have the Allen adjustment screw turned all the way counter clockwise to DECREASE sensitivity. You do not want it to work fast, you want it at the least sensitive position. Install it carefully as per instructions. If you have trouble reaching the mounting screws apply a little vacuum to the diaphragm, it turns the plate out of the way. Install the limiter cam the new adjustable diaphragm came with. Set it so it only travels about 1/2 the way it did without it. You can make marks on the distributor body before and after the cam is installed to see where to set it. 1/2 way should give you about 14 degrees vacuum advance. Without that limiter it will be worse than it was with the original advance diaphragm. Clean the damper and the pointer. It will help if it's painted black. Roll the engine to TDC. Since you don't have timing marks beyond 10 degrees, you need to make some. We are going to make a $100 degree wheel for free. Use dabs of white, yellow, or silver paint to make it easy to see against the black background. The tip of a small nail or wire makes a good brush.. Put a tiny dab of paint along the notch in the damper. This is your normal timing mark. Put another larger dab right on the 0 mark on the stationary pointer and a tiny one on the 10 degree point. When the notch and the 0 are lined up exactly, the timing is 0. Rotate the engine till the scribed line lines up with the 10 on the stationary pointer. Put another dab on the damper exactly where it lines up with "0" on the pointer. Put a "1" next to it. This is now another 10 degree mark. Move the crankshaft another 10 degrees till your new "1" mark is opposite the "10" on the pointer. Now put another mark on the damper next to the "0" on the pointer. Mark it with a "2" This is now your 20 degree mark. Again, move the crankshaft 10 degrees more till the "2" mark is opposite the "10" on the pointer. Put another mark on the damper next to the "0" on the pointer. Marking it "3". Right, that's your 30 degree mark. Again, put the "3" opposite the "10" on pointer, mark the spot next to the pointer's "0" with "4" It should end up like this: 1 - 2 - 3 - 4 - Be as accurate as you can. What we did is made the degree marks on the damper rather than the pointer. The reason you heavily marked the 0 on the pointer is that now is your new reference. When the "2" is opposite the 0, it's timed at 20 degrees. We can read all the way up to 40 degrees ("4") now, the original pointer only went to 10. Disconnect the vacuum advance hose. Start it, take a reading - this is your "Initial Timing." It should be about 10 degrees. Run it up to cruise speed, about 3000 RPM. Notice the marks moved as you revved it up. Now your "2" should be near the big "0" on the pointer. You can interpolate what it is, like 23 etc. The difference between the reading at speed and the Initial is how much your mechanical advance moved it. It should have moved it 10 to 15 degrees more than the initial. So if you started at 10 degrees, it should have gone to 20 to 25 degrees. That advance is simply the weights inside the distributor, under the rotor. Kits are available to modify this curve, but stock is better for off-road performance, since the odd-fire loves making low RPM torque. Now reconnect the vacuum line to the advance. Take another reading at idle to make sure the vacuum isn't affecting it at idle, it should still be at the Initial reading. -- If it moves the timing you have the wrong type of vacuum on the line. It needs ported vacuum, not manifold vacuum. Ported vacuum is near 0 at idle and gets stronger as you accelerate till about 1/2 throttle, then it decreases again as the throttle open more. Manifold vacuum is strongest at idle. It's important you use the right vacuum source, otherwise the system will work backwards! hope this helps it sounds like you are a couple of teath off. you might want to bring #1 to t.d.c. pull your dist cap and see if it is pointing to #1. if it is not you need to reinstall the dist by moving it . later Darren __________________ The very atmosphere of firearms everywhere restrains evil interference. When firearms go, all goes, we need them every hour. - George Washington No free man shall be debarred the use of arms - Thomas Jefferson Later Darren http://www.myspace.com/dmurphy68

01-25-2004, 11:18 AM	#4
ddsmith Glowing since 1978 Join Date: Mar 2001 Location: Lusby,MD,USA Posts: 532	Take a look here. Proper advance curve adjustment. You didn't mention what the advance is reading when it will run. One thing I know as fact from previous experience is that timing chain slack will require a large advance to get the car to run. If you have a lot of miles on the chain it can be a bear to adjust timing correctly. This is defintely the case with the engines that had nylon coated teeth. I'm not sure if any trucks had this type of gear. I certainly hope not. They were crap. __________________ 66 Chevy C30 Stakebody Dump, PS,PB, 327cu in 71 Corvette Coupe 454 4 speed 69 Chevy C20 Custom Camper

01-25-2004, 12:44 PM	#6
nu2-72 Senior Member Join Date: Apr 2001 Location: Amazonia, Mo. USA Posts: 2,821	Does anyone know of or have personnely used a curve shop? I need an address please. __________________ 72 3/4 4x4 350 4spd 72 3/4 4x4 350 auto 2004 Duramax Silverado 4x4 2006 Duramax Crew cab 4x4 2010 Harley Ultra Limited 2011 Harley Street Glide

01-25-2004, 01:38 PM	#8
nu2-72 Senior Member Join Date: Apr 2001 Location: Amazonia, Mo. USA Posts: 2,821	Thanks. __________________ 72 3/4 4x4 350 4spd 72 3/4 4x4 350 auto 2004 Duramax Silverado 4x4 2006 Duramax Crew cab 4x4 2010 Harley Ultra Limited 2011 Harley Street Glide

01-25-2004, 02:33 PM	#10
lukecp Formerly yellow72custom Join Date: May 2001 Location: Austin, TX Posts: 7,531	Are you sure it isn't DUI Mudder? I have seen them advertise in car magazines before.... __________________ '72 Chevy C10 Mild 350/TH350/3.07. Ochre/White. Old high school ride. '70 GMC C2500 '62 327 4bbl/SM465/4.56-geared Dana 60. White/White. Project or parts truck. '97 Saturn SL DD. 1.9/5-speed. 40+ highway mpg

01-25-2004, 10:12 AM	#1
72superBB Registered User Join Date: Nov 2003 Location: Mandan, ND Posts: 422	Curving the hei? What is this mean? How is it done? As it stand, to get my truck to run right I have to rotate the distributor all the way to one side. It almost goes until it cannot go any farther. I had seen another post about curving, but I need educated on this subject. Thanks Ron __________________ Life is full of choices and a Chevy is the best choice. 1990 Chevy SWB

01-25-2004, 10:23 AM	#2
Mudder Registered User Join Date: Feb 2001 Location: East Central, MO Posts: 11,336	I sorta understand it but can't explain it. I know theres a place in Minn or Wis that you can send yours to and they will curve it for ya. A friend of mine said it was some of the best money he had spent.

01-25-2004, 11:25 AM	#5
rage'nrat638 Account Suspended Join Date: Mar 2003 Location: THE FALL GUY CHICAGO IL Posts: 5,883	sounds like your a tooth off on the dist to me......

01-25-2004, 12:57 PM	#7
Mudder Registered User Join Date: Feb 2001 Location: East Central, MO Posts: 11,336	My friend uses a place up north....He sends the distributor and the engine specs and they take care of it. He's an engine builder and does everything himself. He said this is well worth it. I will call him later this morning and see if hes got a name/number for the place.

01-25-2004, 02:12 PM	#9
Mudder Registered User Join Date: Feb 2001 Location: East Central, MO Posts: 11,336	He said there are lots of places that can do it but he would use BUI Davis Unified Ignitions. He said it is out east. The place up north was a speed shop in the St. Paul/Minn area and he cant remember the name. I've been searchin for this BUI but haven't had any luck.

01-26-2004, 07:35 PM	#11
nu2-72 Senior Member Join Date: Apr 2001 Location: Amazonia, Mo. USA Posts: 2,821	FYI. DUI does not do stock units. I called them today and they only sell/ work on their own products. __________________ 72 3/4 4x4 350 4spd 72 3/4 4x4 350 auto 2004 Duramax Silverado 4x4 2006 Duramax Crew cab 4x4 2010 Harley Ultra Limited 2011 Harley Street Glide

01-26-2004, 10:35 PM	#12
Blue Beard Registered User Join Date: Jan 2003 Location: Washington State Posts: 8,831	would like to see some of the dealers advise on this subject..... __________________ 1970 Chev CST 2003 Harley Fatboy 1975 Chevrolet Step Van 1956 Chev Bel Air 1977 Blazer 2WD For Sale $3000.00 1978 Blazer 2WD For Sale $7000.00 1978 Silverado 2005 Monte Carlo

01-26-2004, 10:57 PM	#13
nu2-72 Senior Member Join Date: Apr 2001 Location: Amazonia, Mo. USA Posts: 2,821	I am not really sure what input dealers will have. Guess it will be interesting to see. I would assume that more knowledge might be found with engine builders and/or dyno shops. We have no dyno or speed shops around this area. Anyone know of any? __________________ 72 3/4 4x4 350 4spd 72 3/4 4x4 350 auto 2004 Duramax Silverado 4x4 2006 Duramax Crew cab 4x4 2010 Harley Ultra Limited 2011 Harley Street Glide

01-27-2004, 05:04 AM	#15
Mudder Registered User Join Date: Feb 2001 Location: East Central, MO Posts: 11,336	I may have misunderstood Fuzz yesterday and it must have been DUI. There is a dyno in Cuba I've heard. The guy builds race engines for someone. Thats all I know about that. Could maybe find out. Wise speed shop in St Louis but dont know what they have either. I'll ask Brad at work today.



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