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I have been looking around for a set of heads to replace the ones I currently have on my 68 327 (casting #293, with 1.72" intake/1.50" exhaust). I origionaly wanted a set with 2.02/1.60, but have come across a set of rebuilt #461X heads, with 1.94/1.50 valves, screw in studs, and guide plates. These heads, according to www.mortec.com, have larger port volumes (172 cc intake/64 cc exhaust). My question is, would these heads, with smaller valves and larger port volumes, perform better on my mild 327 than a set with larger valves and smaller port volumes, such as #462? Thanks in advance, Dan
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Re: Replacement Heads
Dan------
Very likely the 1.94/1.50 461X heads will be the best performers for your mild 327. Keep in mind that heads of this same, basic configuration were used for 1962-63 SHP with FI Corvette engines rated at 360 hp. They'll perform well with your mild 327.
Over the years, I believe that GM found that the 1.94/1.50 valve size is the best, all-around performer for street engines. Even the L98 and LT1 engines of the mid 80s to mid 90s use this same valve size. ZZ4 crate engines do, too.In Appreciation of John Hinckley -
Re: Replacement Heads
Dan------
Very likely the 1.94/1.50 461X heads will be the best performers for your mild 327. Keep in mind that heads of this same, basic configuration were used for 1962-63 SHP with FI Corvette engines rated at 360 hp. They'll perform well with your mild 327.
Over the years, I believe that GM found that the 1.94/1.50 valve size is the best, all-around performer for street engines. Even the L98 and LT1 engines of the mid 80s to mid 90s use this same valve size. ZZ4 crate engines do, too.In Appreciation of John HinckleyComment
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Re: Replacement Heads
Dan, I'm no engine expert, but as a general statement the 1.94 valves are probably better for a street engine, particularily a mild one. The reason is that the smaller valves serve to increase velocity of the fuel/air mixture at low RPM and will make better low end torque. Good low end torque is far more desireable in a street engine than high RPM horsepower if you drive in a relatively sane manner. Even engines like the 327/360 (62-3 FI) had the 1.94 valves. The previous owner of my 327/360 enlarged them to 2.02's. I wish he had left them alone.Comment
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Re: Replacement Heads
Dan, I'm no engine expert, but as a general statement the 1.94 valves are probably better for a street engine, particularily a mild one. The reason is that the smaller valves serve to increase velocity of the fuel/air mixture at low RPM and will make better low end torque. Good low end torque is far more desireable in a street engine than high RPM horsepower if you drive in a relatively sane manner. Even engines like the 327/360 (62-3 FI) had the 1.94 valves. The previous owner of my 327/360 enlarged them to 2.02's. I wish he had left them alone.Comment
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Re: Replacement Heads
I don't think there would be a signficant differnce in performance, and essentially none if you run a mild cam like the 929.
The 461X heads with the 1.94/1.50 inch valves are excellent. For many years a lot of sprint car guys liked to start with these heads as they believed them to be harder and more crack resistant than other castings, and I believe they have a higher nickle content than later heads. Also, 2.02/1.6 configurations have so little material between the valve, they have a propensity to crack in that location.
Massaging a set of SB heads IAW the Chevy Power Manual, and books like How to Hotrod a Small Block Chevy will gain you a five to ten percent improvement in top end power without affecting the low end torque and idle quality, so investing some time/money in massaging the heads will pay off more than bolting on a bunch of aftermarket hot rod parts.
The port volume difference is not critical. Until the valve is open about one quarter of its diameter, the valve is the critical restriction, so bigger ports without larger valves or a lot more valve lift won't do anything but kill the low end torque.
With a little massaging of the valve pocket, manifold/head port matching, and a three angle valve job, the 461X with 1.94/1.5 valve will work very nicely with either the 929 or one of the SHP cams - either the 151 (L-79) hydraulic or the 178 (LT-1) with mechanical lifters. The later two will provide decent low end torque with excellent top end power and the the 929 cam will make for a stump puller with stong off idle torque, but at the expense of about 30-50 HP and a thousand revs up top.
DukeComment
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Re: Replacement Heads
I don't think there would be a signficant differnce in performance, and essentially none if you run a mild cam like the 929.
The 461X heads with the 1.94/1.50 inch valves are excellent. For many years a lot of sprint car guys liked to start with these heads as they believed them to be harder and more crack resistant than other castings, and I believe they have a higher nickle content than later heads. Also, 2.02/1.6 configurations have so little material between the valve, they have a propensity to crack in that location.
Massaging a set of SB heads IAW the Chevy Power Manual, and books like How to Hotrod a Small Block Chevy will gain you a five to ten percent improvement in top end power without affecting the low end torque and idle quality, so investing some time/money in massaging the heads will pay off more than bolting on a bunch of aftermarket hot rod parts.
The port volume difference is not critical. Until the valve is open about one quarter of its diameter, the valve is the critical restriction, so bigger ports without larger valves or a lot more valve lift won't do anything but kill the low end torque.
With a little massaging of the valve pocket, manifold/head port matching, and a three angle valve job, the 461X with 1.94/1.5 valve will work very nicely with either the 929 or one of the SHP cams - either the 151 (L-79) hydraulic or the 178 (LT-1) with mechanical lifters. The later two will provide decent low end torque with excellent top end power and the the 929 cam will make for a stump puller with stong off idle torque, but at the expense of about 30-50 HP and a thousand revs up top.
DukeComment
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Re: Replacement Heads
Hey Duke,
Maybe Chevrolet should have learned from WilleysKnight(Sp?). During the late '20's they use a sleeve valve configuration instead of valves. Really something else to see a sleeve pulsating around the piston!! Got to see the internal workings of a friends engine some time ago. I was amazed. Just my fun 2 cents!Comment
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Re: Replacement Heads
Hey Duke,
Maybe Chevrolet should have learned from WilleysKnight(Sp?). During the late '20's they use a sleeve valve configuration instead of valves. Really something else to see a sleeve pulsating around the piston!! Got to see the internal workings of a friends engine some time ago. I was amazed. Just my fun 2 cents!Comment
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Valve sizes
The exhaust ports on SB heads are relatively more restictive than the inlets. The raw numbers on a stock head show that the exhausts flow only about 65 percent of the inlets. As a general rule of thumb, we want the exhaust flow to be about 75 percent of the inlets, and by computing the overall flow coefficients considering the valve event, we end up wanting about ten more degrees duration on the exhaust cam than on the inlet in the form of a relatively early opening exhaust valve compared to a head with a less restrictive exhaust port/valve.
If one really wanted to "enlarge" the valves, my recommendation would be to leave the inlets at 1.94", but enlarge the exhaust valves to 1.6", and leave it at that.
The reason the exhaust port flow need be less on the exhaust side is that when the valve is opened there is still substantial residual pressure in the cylinder, whereas, on the inlet side we only have atmospheric pressure to drive the inlet filling process.
Still, unless the exhaust flow is about 75 percent of the inlet flow the residual cylinder pressure is not sufficient to blow down the cylinder before BDC, so the solution is a relatively early opening exhaust valve, which leads to a bit more duration on the exhaust side.
The L-79, and to a greater degree, the LT-1 cam valve events exhibit these features as Chevrolet had a very good understanding of the Small Block cylinder head characteristics by the mid to late sixties.
DukeComment
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Valve sizes
The exhaust ports on SB heads are relatively more restictive than the inlets. The raw numbers on a stock head show that the exhausts flow only about 65 percent of the inlets. As a general rule of thumb, we want the exhaust flow to be about 75 percent of the inlets, and by computing the overall flow coefficients considering the valve event, we end up wanting about ten more degrees duration on the exhaust cam than on the inlet in the form of a relatively early opening exhaust valve compared to a head with a less restrictive exhaust port/valve.
If one really wanted to "enlarge" the valves, my recommendation would be to leave the inlets at 1.94", but enlarge the exhaust valves to 1.6", and leave it at that.
The reason the exhaust port flow need be less on the exhaust side is that when the valve is opened there is still substantial residual pressure in the cylinder, whereas, on the inlet side we only have atmospheric pressure to drive the inlet filling process.
Still, unless the exhaust flow is about 75 percent of the inlet flow the residual cylinder pressure is not sufficient to blow down the cylinder before BDC, so the solution is a relatively early opening exhaust valve, which leads to a bit more duration on the exhaust side.
The L-79, and to a greater degree, the LT-1 cam valve events exhibit these features as Chevrolet had a very good understanding of the Small Block cylinder head characteristics by the mid to late sixties.
DukeComment
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Sleeve valves
One of my favorite engines is the Napier Saber H-24, which was basically two flat twelves, one on top of another with the two cranks driving a prop through a reduction drive that powered the Hawker Tempest and Typhoon series of fighter bombers. They came into service in late WW II and continued in service until maybe the early sixties with some former crown colony air forces.
For those of you unfamiliar with sleeve valves, they consist of a couple of concentric sleeves that make up the cylinder wall friction surface and are operated by cranks and rods rather than cams. To explain them further than that I'd need a picture, but a sleeve valve engine from the outside is easy to confuse with a two stroke or flat head as the heads merely cover the cylinders and contain the spark plugs.
Sleeve valves were all the rage in the 1930s. A few automotive designs showed up, mostly in Great Britain, and the Brits put some sleeve valve aero engines into production including my favorite Sabre and an air cooled radial from Bristol.
For the most part, the American manufacturers, both automotive and aero, took a wait-and-see attitude, and true to some earlier predictions, sleeve valves proved to be complex, with high operating friction, lubrication and oil control problems, and so far, no one has come up with a better solution that the plain old cam operatied poppet valve.
DukeComment
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Sleeve valves
One of my favorite engines is the Napier Saber H-24, which was basically two flat twelves, one on top of another with the two cranks driving a prop through a reduction drive that powered the Hawker Tempest and Typhoon series of fighter bombers. They came into service in late WW II and continued in service until maybe the early sixties with some former crown colony air forces.
For those of you unfamiliar with sleeve valves, they consist of a couple of concentric sleeves that make up the cylinder wall friction surface and are operated by cranks and rods rather than cams. To explain them further than that I'd need a picture, but a sleeve valve engine from the outside is easy to confuse with a two stroke or flat head as the heads merely cover the cylinders and contain the spark plugs.
Sleeve valves were all the rage in the 1930s. A few automotive designs showed up, mostly in Great Britain, and the Brits put some sleeve valve aero engines into production including my favorite Sabre and an air cooled radial from Bristol.
For the most part, the American manufacturers, both automotive and aero, took a wait-and-see attitude, and true to some earlier predictions, sleeve valves proved to be complex, with high operating friction, lubrication and oil control problems, and so far, no one has come up with a better solution that the plain old cam operatied poppet valve.
DukeComment
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Re: Sleeve valves
The sleeve-valve linkages on that Bristol Centaurus radial made an R-2800 look like a Briggs & Stratton; TBO was less than 100 hoursComment
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Re: Sleeve valves
The sleeve-valve linkages on that Bristol Centaurus radial made an R-2800 look like a Briggs & Stratton; TBO was less than 100 hoursComment
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