68gtx
Well-Known Member
what do 915 heads cc at? trying to figure out a future build with these heads and want to try and figure out the right piston to comp ratio..
915 head questions
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detmatt1
Well-Known Member
Good question, I was getting close to a phone call to Meeps to find out the answer, I new he would be a good one to ask. How does that compare to a standard 906 head?
RustyRatRod
Well-Known Member
The 1967 915 head is better than the 906 head. Besides the Max Wedge head, the 67 915 was the only HP big block head ever made. Period. The 906 heads were simply regular production passenger car heads. People think they are the holy grail and it just ain't true. They are good.....and probably flow best of any of the open chamber designs......even if only by a little. But the fact is, they came on everything from 440 Six Pack to 383 2 barrels. They are nothing special. The 67 915s on the other hand, were an HP head right off the bat. Closed chamber, quench design with the large exhaust valve and good flowing runners.
The 516 head can be made close to the 915 by adding the larger exhaust valve. That's what I plan to do.....and run the KB400 piston. Zero deck for quench with a domed piston. My understanding is there's like 20 CFM difference at .700 lift between flow from the prepped 915 to the prepped 516 because of the smaller port size on the 516. I also understand from my good buddy IQ52, that all big block heads have the potential to flow in the 600HP range ported......so then the question becomes if you're gonna throw the money at um, what's the difference? Just pick your head, build for that combustion chamber and go for it.
The 516 head can be made close to the 915 by adding the larger exhaust valve. That's what I plan to do.....and run the KB400 piston. Zero deck for quench with a domed piston. My understanding is there's like 20 CFM difference at .700 lift between flow from the prepped 915 to the prepped 516 because of the smaller port size on the 516. I also understand from my good buddy IQ52, that all big block heads have the potential to flow in the 600HP range ported......so then the question becomes if you're gonna throw the money at um, what's the difference? Just pick your head, build for that combustion chamber and go for it.
Last edited:
Meep-Meep
дворянин
The 915 and 906 heads have a better port design than the 516 and even the old Max Wedge head. Same goes for the 452 and the later ones. The difference is a flat floor with low roof as found on a 516 vs. a nice radius with higher roof on the 915, and this change at the transition to the valve pocket is what provides better laminar flow. According to info from Bob Mullen the 915 head flows within 95% of the MW, and it's not because the port volume is larger. Also, I believe it's the result of Harry Weslake's work, while working with Tom Hoover, was either directly or indirectly responsible in the development of the 915 head. Like Rusty said, 915's started as a perf head - even though the first ones were used in 66 on Chryslers with the 1.6" ex valve. They are good heads but to support large amounts of HP they need a lot of work like anything else. For a hot street motor, a little pocket port, without drastically altering the shape of the port, they are usually good enough.
516 heads are nearly free at swap meets (or should be) and can be the answer to pep up an otherwise low CR motor for cheap. A set of those and steel shim gaskets you can expect an increase of nearly a point of compression. A nearly stock street motor with a small cam that spends most of the time from idle to 3000 RPM may not notice much of a difference between the port flow of a 915 vs. 516 but the bump in compression and corresponding cylinder pressure will be noticed as better throttle response and overall drivability, especially if you properly match a cam. For this type of build why worry about that extra few HP at 6000 RPM when you will rarely get there? Why not maximize low lift flow and maintain proper port velocity to help produce as much average power at the RPM range where you will spend most of your time? The answer is to size the port that will produce the least restriction while maintaining good velocity in the RPM range where you will spend most of your time in. And then what do you do with the fuel vapor once it gets into the cylinder?
The extra bit of turbulence as the gases are squeezed out from the closed chamber part of the head is good to mix up the fuel and air and cram it toward the source of ignition just before the plug fires. But the flame must also get into that quench area to burn the gases that are trapped. Too tight and you may be giving up power and producing more unburned hydrocarbons because the flame just can't get there. This is the very reason Chrysler went to the open chamber head; to open up the area where gases might be trapped and allow the flame to get in. I will also add that my 77 440 has the biggest bore chamfer I have ever seen and I believe this was done to open up the space around the piston above the top ring. To this day reducing that space around the top ring by putting the ring higher on the piston, which requires a stronger piston to support the thinner section, is an effort to reduce emissions by getting all the raw fuel to burn in the chamber. So if you're going to spend all that time and effort to cram as much fuel into the chamber as possible why not burn it as well?
516 heads are nearly free at swap meets (or should be) and can be the answer to pep up an otherwise low CR motor for cheap. A set of those and steel shim gaskets you can expect an increase of nearly a point of compression. A nearly stock street motor with a small cam that spends most of the time from idle to 3000 RPM may not notice much of a difference between the port flow of a 915 vs. 516 but the bump in compression and corresponding cylinder pressure will be noticed as better throttle response and overall drivability, especially if you properly match a cam. For this type of build why worry about that extra few HP at 6000 RPM when you will rarely get there? Why not maximize low lift flow and maintain proper port velocity to help produce as much average power at the RPM range where you will spend most of your time? The answer is to size the port that will produce the least restriction while maintaining good velocity in the RPM range where you will spend most of your time in. And then what do you do with the fuel vapor once it gets into the cylinder?
The extra bit of turbulence as the gases are squeezed out from the closed chamber part of the head is good to mix up the fuel and air and cram it toward the source of ignition just before the plug fires. But the flame must also get into that quench area to burn the gases that are trapped. Too tight and you may be giving up power and producing more unburned hydrocarbons because the flame just can't get there. This is the very reason Chrysler went to the open chamber head; to open up the area where gases might be trapped and allow the flame to get in. I will also add that my 77 440 has the biggest bore chamfer I have ever seen and I believe this was done to open up the space around the piston above the top ring. To this day reducing that space around the top ring by putting the ring higher on the piston, which requires a stronger piston to support the thinner section, is an effort to reduce emissions by getting all the raw fuel to burn in the chamber. So if you're going to spend all that time and effort to cram as much fuel into the chamber as possible why not burn it as well?
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