Intake Center Line - Page 2

Greg Reimer 7376 · 06-26-2011, 10:29 PM

In the last days of the muscle car era-1970-1972, the envirodemons flexed their muscle and enacted things that tried their hardest to destroy the American car. (The '73-77 era just about did.) If you drove a,say,a '69-71 Nova,350 4barrel SS, it ran strong, was pretty responsive, and got reasonably good mileage. The '72 engine came out, and there was a distinct drop in power. Same engine, around 9:1 compression,same trans,gears,etc,where was the loss coming from? The '72 and later cars,irreverently referred to as'smog motors', had a lot of small differences. First was ignition timing-around 4 degrees initial with less mechanical advance available in the distributor,down from around 8 degrees, but the cam had more exhaust lift. A quick check of the NHRA blueprint specs verifies this, but the exhaust valve opened late, and closed later-right into the intake cycle. Combined with that, was a real restrictive exhaust with built in back pressure.Anybody remember the vacuum cleaner noise in trucks and vans with the screenlike grid over the end of the tailpipe? The tubing diameter was small, and the reason for all that was to cause a small amount of exhaust to be trapped in the cylinder and be an inert substance effectively reducing NOX emissions. I remember q-jet cars bogging really badly when the secondaries opened on WOT acceleration, there was no place for that extra fuel,air,and exhaust to go.Next year the infernal EGR valve came out, and the problem was worse. Some of those cars were practically undriveable when the engine was still cold and the choke opened rather early.Top speed in a '72 was about 90-100,no matter what you did to it. The proper way to fix those later cars was to junk the exhaust system,put on a good set of headers with a dual exhaust, advance the cam about 8 degrees,(or replace it with an earlier cam),re-curve the distributor,and get someone who knows quadrajets re jet the thing and set it up correctly.Made all the difference in the world! The same thing happens in a race motor to a lesser degree with a late closing camshaft.

Mike Taylor 3601 · 06-27-2011, 10:39 AM

Going from 108 to 102 will make In have less clearance and Ex will have more clearance.
Mike Taylor 3601

Lonnie Cox · 06-27-2011, 02:21 PM

As usual you all give great advice, I will be degreeing the cam Tue night and checking clearance, when we put the motor together we set it up dot to dot , I now see the cam card states set up at 102 centerline, i think it is at 108, should make a big difference I am told.
Thank You again
Lonnie

06-26-2011, 10:29 PM	#11
Greg Reimer 7376 VIP Member Join Date: Nov 2008 Location: Glendora,Calif. Posts: 1,111 Likes: 164 Liked 649 Times in 208 Posts	Re: Intake Center Line In the last days of the muscle car era-1970-1972, the envirodemons flexed their muscle and enacted things that tried their hardest to destroy the American car. (The '73-77 era just about did.) If you drove a,say,a '69-71 Nova,350 4barrel SS, it ran strong, was pretty responsive, and got reasonably good mileage. The '72 engine came out, and there was a distinct drop in power. Same engine, around 9:1 compression,same trans,gears,etc,where was the loss coming from? The '72 and later cars,irreverently referred to as'smog motors', had a lot of small differences. First was ignition timing-around 4 degrees initial with less mechanical advance available in the distributor,down from around 8 degrees, but the cam had more exhaust lift. A quick check of the NHRA blueprint specs verifies this, but the exhaust valve opened late, and closed later-right into the intake cycle. Combined with that, was a real restrictive exhaust with built in back pressure.Anybody remember the vacuum cleaner noise in trucks and vans with the screenlike grid over the end of the tailpipe? The tubing diameter was small, and the reason for all that was to cause a small amount of exhaust to be trapped in the cylinder and be an inert substance effectively reducing NOX emissions. I remember q-jet cars bogging really badly when the secondaries opened on WOT acceleration, there was no place for that extra fuel,air,and exhaust to go.Next year the infernal EGR valve came out, and the problem was worse. Some of those cars were practically undriveable when the engine was still cold and the choke opened rather early.Top speed in a '72 was about 90-100,no matter what you did to it. The proper way to fix those later cars was to junk the exhaust system,put on a good set of headers with a dual exhaust, advance the cam about 8 degrees,(or replace it with an earlier cam),re-curve the distributor,and get someone who knows quadrajets re jet the thing and set it up correctly.Made all the difference in the world! The same thing happens in a race motor to a lesser degree with a late closing camshaft.

06-27-2011, 10:39 AM	#12
Mike Taylor 3601 VIP Member Join Date: Oct 2008 Location: Somerset,Ky Posts: 1,303 Likes: 198 Liked 116 Times in 48 Posts	Re: Intake Center Line Going from 108 to 102 will make In have less clearance and Ex will have more clearance. Mike Taylor 3601

06-27-2011, 02:21 PM	#13
Lonnie Cox Junior Member Join Date: May 2010 Posts: 88 Likes: 0 Liked 0 Times in 0 Posts	Re: Intake Center Line As usual you all give great advice, I will be degreeing the cam Tue night and checking clearance, when we put the motor together we set it up dot to dot , I now see the cam card states set up at 102 centerline, i think it is at 108, should make a big difference I am told. Thank You again Lonnie