cam timing ? - Page 2

Alan Roehrich · 07-04-2010, 08:05 PM

The intake will rarely hit so long as the piston to valve clearance is the same as piston to head clearance, or greater. The exhaust is an entirely different deal though. Even at 0.070" or so we see evidence of the exhaust valve touching the piston.

The next thing you could look at for your combination is a set of 4-2-1 stepped headers. With a really good set, you can use a smaller exhaust lobe, as much as 6 degrees less duration.

Greg Reimer 7376 · 07-06-2010, 11:04 AM

When I first got into this Stock Eliminator form of insanity,I bought a two tone blue 66 Chevy Bel Air 2 door post car from Tony Janes.It had the 283,220 horse combo,a 1.98 low gear PG, 12 bolt with 5.57 gears, and it became time for a rebuild. Now, Div.6 has one of the finest automotive minds I ever met in my life, and a real 283 wiz in the form of Cal Method, so I depended on him for some good advice.I had noticed that the exhaust valves had been refaced to the point that you could practically shave with them.No margin whatsoever.This car was built long before the cam and spring rule revision, so obviously somebody had to do what he had to do.Intakes were OK, so they got a refreshening(TRW OEM replacement valves). I put pistons and bored it to the next oversize,(040"),and with the new pistons, I had all kinds of piston to valve clearance. I then took a new set of exhaust valves, with the new uncut valve faces,(nice and narrow, and they had a rather thick margin. Needing another .030" or so clearance, I ran the valves over to a friend's place,where he had a matal lathe, so we chucked up a valve and could safely face .040" off the face of the valve, thinning it down, and retaining the narrow face. I took the valves to the post office and using the digital postal scale, determined the new doctored exhaust valve weighed in at exactlu 6 oz, whereas the uncut purely OEM replacement valve out of the box weighed 6.2 oz. That's a two ounce, 1.5" diameter washer not going for a ride in that engine. The engine seemed to like it, as RPM wasn't a problem.We then removed an additional .020 from the intakes, and felt safer, since two combustion chamber volumns were at the minimum already. Then, the spring rule changed, so I went in with new springs at the unheard of tension of 125 on the seat and 250 open( K motion 700 springs- still a good deal). This car, however barely ran the index on a good day. I left at 3500 against the converter, shifted at 7400, went through at 7800 at 98 mph, never quite got the hang of it. I sold the car some times later, and found out years later it had way too loose of a converter in it, and the engine never had the beans to pull it in high gear, let alone lock up the converter. In retrospect, the idea of turning an engine that high with cast pistons, stock valve train and two-piece valves is rather terrifying. With today's parts, the head work allowed, the valves we use now, the rods, intakes, etc, this car would have gone nearly 1 second under what that one ever did. Amazing, the evolution of a species.

07-04-2010, 08:05 PM	#11
Alan Roehrich Veteran Member Join Date: May 2006 Location: Murfreesboro TN Posts: 5,118 Likes: 1,577 Liked 1,837 Times in 417 Posts	Re: cam timing ? The intake will rarely hit so long as the piston to valve clearance is the same as piston to head clearance, or greater. The exhaust is an entirely different deal though. Even at 0.070" or so we see evidence of the exhaust valve touching the piston. The next thing you could look at for your combination is a set of 4-2-1 stepped headers. With a really good set, you can use a smaller exhaust lobe, as much as 6 degrees less duration. __________________ Alan Roehrich 212A G/S

07-06-2010, 11:04 AM	#12
Greg Reimer 7376 VIP Member Join Date: Nov 2008 Location: Glendora,Calif. Posts: 1,137 Likes: 172 Liked 719 Times in 220 Posts	Re: cam timing ? When I first got into this Stock Eliminator form of insanity,I bought a two tone blue 66 Chevy Bel Air 2 door post car from Tony Janes.It had the 283,220 horse combo,a 1.98 low gear PG, 12 bolt with 5.57 gears, and it became time for a rebuild. Now, Div.6 has one of the finest automotive minds I ever met in my life, and a real 283 wiz in the form of Cal Method, so I depended on him for some good advice.I had noticed that the exhaust valves had been refaced to the point that you could practically shave with them.No margin whatsoever.This car was built long before the cam and spring rule revision, so obviously somebody had to do what he had to do.Intakes were OK, so they got a refreshening(TRW OEM replacement valves). I put pistons and bored it to the next oversize,(040"),and with the new pistons, I had all kinds of piston to valve clearance. I then took a new set of exhaust valves, with the new uncut valve faces,(nice and narrow, and they had a rather thick margin. Needing another .030" or so clearance, I ran the valves over to a friend's place,where he had a matal lathe, so we chucked up a valve and could safely face .040" off the face of the valve, thinning it down, and retaining the narrow face. I took the valves to the post office and using the digital postal scale, determined the new doctored exhaust valve weighed in at exactlu 6 oz, whereas the uncut purely OEM replacement valve out of the box weighed 6.2 oz. That's a two ounce, 1.5" diameter washer not going for a ride in that engine. The engine seemed to like it, as RPM wasn't a problem.We then removed an additional .020 from the intakes, and felt safer, since two combustion chamber volumns were at the minimum already. Then, the spring rule changed, so I went in with new springs at the unheard of tension of 125 on the seat and 250 open( K motion 700 springs- still a good deal). This car, however barely ran the index on a good day. I left at 3500 against the converter, shifted at 7400, went through at 7800 at 98 mph, never quite got the hang of it. I sold the car some times later, and found out years later it had way too loose of a converter in it, and the engine never had the beans to pull it in high gear, let alone lock up the converter. In retrospect, the idea of turning an engine that high with cast pistons, stock valve train and two-piece valves is rather terrifying. With today's parts, the head work allowed, the valves we use now, the rods, intakes, etc, this car would have gone nearly 1 second under what that one ever did. Amazing, the evolution of a species.