compression ratio

[Garys1969RR]

So Meep Meep, with the richer fuel supply/mixtureof the 6 pak intake, do you think that is what cured the pinging problem that you had with the factory iron intake and Eddy 750? And by lowest trap speed yet, that means the MPH is down with the 6 pak set up? Or are you getting so much torque off the line that it isn't hooking up? I think that the 6 pak will work great once its dialed in.

 

[wedgie]

Meep-meep and I are thinking just alike. Cam for the compression ratio. I do agree with Rusty that my current Racer Brown cam may not be ideal with the new heads and compression ratio. I'll probably have Jim do another grind for me based on the aluminum heads. It sure ran great with the 906's but as you can tell from the witness marks on the pistons, there was some rattling, probably because of the open chamber design and flattops. Now I need to figure how much to cut the MP452/edelbrock heads. I think that 74cc-75cc finished product is going to put me in the ballpark of an 89 octane 9.5. Anybody have the cutting specs for these heads?

 

[Meep-Meep]

So Meep Meep, with the richer fuel supply/mixtureof the 6 pak intake, do you think that is what cured the pinging problem that you had with the factory iron intake and Eddy 750? And by lowest trap speed yet, that means the MPH is down with the 6 pak set up? Or are you getting so much torque off the line that it isn't hooking up? I think that the 6 pak will work great once its dialed in.

Yes, I think the six pack is richer, thus curing the pinging at WOT. But that's not the solution! Trap speed is an indication of power, and the apparently leaner AFB on the factory iron intake running a fuel blend yielding approx 96 R+M/2 made more power than the six pack with a splash of race gas. On the street the six pack doesn't ping at WOT on 91, but I wanted some margin at the track so I put in a little 110 to my 91. To be fair I was having issues with a huge bog out of the hole with the six pack, but I don' t think that had as much of an effect on trap speed as it did on ET. What I need to do is get some dyno time in and check mixture on both setups.

I agree that the six pack should work much better than it is and could possibly be the ultimate street combo, but I have yet to get there.

Oh, I was running slicks with both setups.

- - - Updated - - -

Meep-meep and I are thinking just alike. Cam for the compression ratio. I do agree with Rusty that my current Racer Brown cam may not be ideal with the new heads and compression ratio. I'll probably have Jim do another grind for me based on the aluminum heads. It sure ran great with the 906's but as you can tell from the witness marks on the pistons, there was some rattling, probably because of the open chamber design and flattops. Now I need to figure how much to cut the MP452/edelbrock heads. I think that 74cc-75cc finished product is going to put me in the ballpark of an 89 octane 9.5. Anybody have the cutting specs for these heads?

That's right, but first set the compression ratio for the fuel used and cam for max cylinder pressure. I'm thinking 150-160 PSI for 89 gas. I'm running 190 PSI with 10.1:1 and that's too much with iron heads and 91 R+M/2. Regarding compression ratio for iron vs. aluminum I have heard from serious engine builders you can go +.5 point on aluminum. Then I have heard form other builders it makes no difference.

The 906 head does provide quench, just not as much as say the fully closed chamber type. There is a fine line of proper quench to provide swirl in the chamber and to get ALL the fuel burned from one end of the chamber to another. The flame must get all the way across to burn everything and if you have too tight of a gap it won't burn.

This is especially true about the annular clearance above the top ring. The fuel in that little gap never gets burned and thus contributes to HC emissions, hence the advent of hypereutectic pistons for the OEM applications. The stronger alloy will allow the top ring to be farther up on the piston reducing the volume of that annular gap. I will also suggest that the huge bore chamfer on my 78 440 is an effort to get the flame in that area to burn the fuel.

The whole thing can be very complicated, which is why you need to address it in the proper sequence. Starting with fuel octane is perfect because there is a compression limit to it and you want to be on the verge of detonation for the highest efficiency.

 

[383man]

Wow alot of reading. But are you saying you run a flattop piston with a dish and 906 heads ? Unless you mill at least .080 or more off 906 heads they are open chamber which is not a great setup as you wont have any quench. This is the piston I run in my 440/493 stroker and it has a 24cc dish. It has just about zero deck on the flat part as it gives me .046 quench with the Indy EZ closed chamber head and a 10.6 static comp. I run a good size solid flat tappet cam to keep my cyl press below 190. I run 38 total timing with this combo on 92 pump. The best setup is a closed chamber head with a flattop piston so you can build quench in the eng which helps fight pinging. And many flattops come with a dish to keep the comp where you want it but the flat is on the closed chamber part of the head so you still get quench with a dished piston. Ron

 

[Hemirunner]

Yes, I think the six pack is richer, thus curing the pinging at WOT. But that's not the solution! Trap speed is an indication of power, and the apparently leaner AFB on the factory iron intake running a fuel blend yielding approx 96 R+M/2 made more power than the six pack with a splash of race gas. On the street the six pack doesn't ping at WOT on 91, but I wanted some margin at the track so I put in a little 110 to my 91. To be fair I was having issues with a huge bog out of the hole with the six pack, but I don' t think that had as much of an effect on trap speed as it did on ET. What I need to do is get some dyno time in and check mixture on both setups.

I agree that the six pack should work much better than it is and could possibly be the ultimate street combo, but I have yet to get there.

906 head does provide quench, just not as much as say the fully closed chamber type.


.

I'm betting it was actually the opposite. You're six pack set up is leaner than your 4-barrel setup was. There's a huge difference from pre-ignition (usually on a rich mixture) and a lean pop. The huge bog is another indicator that it's lean. Leaning it out is also most likely the reason it picked up hp, hence the higher trap speed. As for as the 906 goes, unless you use a quench dome piston, a 906 head has absolutely no quench whatsoever.

 

[Meep-Meep]

I'm betting it was actually the opposite. You're six pack set up is leaner than your 4-barrel setup was. There's a huge difference from pre-ignition (usually on a rich mixture) and a lean pop. The huge bog is another indicator that it's lean. Leaning it out is also most likely the reason it picked up hp, hence the higher trap speed. As for as the 906 goes, unless you use a quench dome piston, a 906 head has absolutely no quench whatsoever.

Actually my trap speed was slower by 4 MPH with the six pack. I agree the bog can be a lean condition, or perhaps too much carb all at once. Those outboards would open up at part throttle and I could feel the linkage action in my foot so I'm thinking too much too soon.

I will respectfully disagree with your 906 statement. The cylinder volume worth of gas that gets compressed into the chamber is met with an unequal chamber profile shape so it has no choice but to get pushed out of the area opposite the spark plug into the area with the spark plug. The gap between the piston and head is certainly wider compared to a traditional closed chamber head, but there is still that area that is closer to the piston than the rest of the chamber. I will say the big difference between the 906 and 915 is the amount of swirl produced in the chamber based on the gap. I also believe that too little gap can cause the fuel to not completely burn in the chamber.

 

[383man]

Actually my trap speed was slower by 4 MPH with the six pack. I agree the bog can be a lean condition, or perhaps too much carb all at once. Those outboards would open up at part throttle and I could feel the linkage action in my foot so I'm thinking too much too soon.

I will respectfully disagree with your 906 statement. The cylinder volume worth of gas that gets compressed into the chamber is met with an unequal chamber profile shape so it has no choice but to get pushed out of the area opposite the spark plug into the area with the spark plug. The gap between the piston and head is certainly wider compared to a traditional closed chamber head, but there is still that area that is closer to the piston than the rest of the chamber. I will say the big difference between the 906 and 915 is the amount of swirl produced in the chamber based on the gap. I also believe that too little gap can cause the fuel to not completely burn in the chamber.

No offense meant but you are wrong about the 906 head. It will not get as good of an effect as a nice .040 to .045 quench on a closed chamber flattop piston quench will give. It has been tested many times over the years. Thats why I ran these quench pad pistons in my old 440 which they make for an open chamber head. They are a pain to check and get each cylinder right as some chamber volumes vary a little. But the step up pad is made to go up close to the head chamber to get a better quench effect. Thats the reason over the years so many have gone to flattop pistons and closed chamber heads for better quench. A pop up piston can also effect and hurt the flame travel which hurts quench some also. Like I said I mean no disrespect but its a proven fact. Thats why I built my last eng with the flattop dished piston as you just get the piston about zero deck and run a .039 head gasket for perfect quench. Course thats why the keep the flat part of the dished piston on the quench side. Ron

 

[Meep-Meep]

No offense meant but you are wrong about the 906 head. It will not get as good of an effect as a nice .040 to .045 quench on a closed chamber flattop piston quench will give. It has been tested many times over the years. Thats why I ran these quench pad pistons in my old 440 which they make for an open chamber head. They are a pain to check and get each cylinder right as some chamber volumes vary a little. But the step up pad is made to go up close to the head chamber to get a better quench effect. Thats the reason over the years so many have gone to flattop pistons and closed chamber heads for better quench. A pop up piston can also effect and hurt the flame travel which hurts quench some also. Like I said I mean no disrespect but its a proven fact. Thats why I built my last eng with the flattop dished piston as you just get the piston about zero deck and run a .039 head gasket for perfect quench. Course thats why the keep the flat part of the dished piston on the quench side. Ron

No offense taken, but perhaps my point is not getting through correctly. I also live by the closed chamber head and flat top piston rule to promote good quench, which produces swirl in the chamber for a more complete burn, and is a better way to gain compression than a dome piston. However, I'll take an educated guess that the area opposite the spark plug on a 906 head has about .115" clearance (including gasket) to a zero deck piston vs. .040" with a closed chamber head. Even at .115" quench distance, the difference between the plug side and non plug side of the chamber is significant enough to cause the gas to be forced to the larger volume of the chamber as the piston approaches TDC. Is it optimal? Maybe not and I never claimed it was or wasn't, but I was only trying to make the point about the gas moving to the larger volume. Looking at it from a different angle it would be safe to assume that a chamber with consistent depth all the way across, like a cylinder with a flat roof, will have absolutely no quench to promote swirl. And by the way, judging by the very impressive 1/4 mile data in your signature, I have nothing but respect for your abilities to build an engine.

 

[383man]

No offense taken, but perhaps my point is not getting through correctly. I also live by the closed chamber head and flat top piston rule to promote good quench, which produces swirl in the chamber for a more complete burn, and is a better way to gain compression than a dome piston. However, I'll take an educated guess that the area opposite the spark plug on a 906 head has about .115" clearance (including gasket) to a zero deck piston vs. .040" with a closed chamber head. Even at .115" quench distance, the difference between the plug side and non plug side of the chamber is significant enough to cause the gas to be forced to the larger volume of the chamber as the piston approaches TDC. Is it optimal? Maybe not and I never claimed it was or wasn't, but I was only trying to make the point about the gas moving to the larger volume. Looking at it from a different angle it would be safe to assume that a chamber with consistent depth all the way across, like a cylinder with a flat roof, will have absolutely no quench to promote swirl. And by the way, judging by the very impressive 1/4 mile data in your signature, I have nothing but respect for your abilities to build an engine.

Thank you kindly for the kind words. I really did not want to sound like a jerk as sometimes it sounds that way on the internet but I only wanted to give my opinion on what I have read. I had read before that after .055 to .060 it wont give much quench to help. But thats what I read as I am no engineer for sure. Thanks again for the kind words. Ron

 

[Meep-Meep]

Thank you kindly for the kind words. I really did not want to sound like a jerk as sometimes it sounds that way on the internet but I only wanted to give my opinion on what I have read. I had read before that after .055 to .060 it wont give much quench to help. But thats what I read as I am no engineer for sure. Thanks again for the kind words. Ron

I'm in the same boat as you and results speak for them selves. I have read quite a bit on this as well but since none of us can be inside the chamber to see what happens, all we can do is read and experiment. You didn't come across like a jerk to me but I know what you mean.

 

[RustyRatRod]

Well, I am going to run domed pistons at zero deck height with closed chambers. Not the most optimal quench, but the quench will still be there and work.

 

[Meep-Meep]

You'll have quench all right, but the dome will interfere with flame travel and might require more total timing to get the job done. I have seen where people put a fire slot in the piston dome to allow the flame better access to the other side. If the dome isn't very high you might be just fine as is. It's those really tall domes that tend to cause issues.

 

[RustyRatRod]

I have the KB400. They have a very nice dome. .200" with a plug slot. Real nice shape. Nice and polished to induce reflecting qualities and help keep detonation down.

http://kb-silvolite.com/kb_car/performance.php?action=details&P_id=327

You'll have quench all right, but the dome will interfere with flame travel and might require more total timing to get the job done. I have seen where people put a fire slot in the piston dome to allow the flame better access to the other side. If the dome isn't very high you might be just fine as is. It's those really tall domes that tend to cause issues.

 

[67 B-body]

I had my pistons "L2295F" lightened up a bit prior to balancing the rotating assembly..... You can see the nice Flat dome and plug slot... They are .140" dome height and -12cc Dome volume.. High compression without the big pointy Dome peek..

I forgot to add,,,,,
I use aluminum heads with Heart shaped chambers,,, not D style, so they are Flat except for the chamber area. No need for the quench pad with this chamber design....

 

[RustyRatRod]

Mine are very similar, only they are only -6cc's because of the big valve reliefs even though the dome is .200". Also the L2295F sits in the hole a good bit at TDC, unless you zero deck height to match them, so you can get quench, but the compression would be pretty high. I almost bought a set of those for my 383, but decided to go with the KBs instead. The 383 version has like a -20cc dome so it's even worse. lol

 

[67 B-body]

Mine are very similar, only they are only -6cc's because of the big valve reliefs even though the dome is .200". Also the L2295F sits in the hole a good bit at TDC, unless you zero deck height to match them, so you can get quench, but the compression would be pretty high. I almost bought a set of those for my 383, but decided to go with the KBs instead. The 383 version has like a -20cc dome so it's even worse. lol

I think "hard to remember" .020 or .030 down in the hole........

 

[RustyRatRod]

I think "hard to remember" .020 or .030 down in the hole........

I think that's close.....I know with the 383 version, .033" sticks in my mind. Wouldn't be tough to zero deck height. Specially with aluminum heads. It'd be a killer.

 

[67 B-body]

I think that's close.....I know with the 383 version, .033" sticks in my mind. Wouldn't be tough to zero deck height. Specially with aluminum heads. It'd be a killer.

It's got tons of compression now,,,, hate to see it at 0 deck! I think the domes would need shaped though with the heart shaped chamber... I believe diamond makes a piston for o deck that has the different dome shapes for non-D shaped cyl heads..

 

[Garys1969RR]

I have 14cc dome Ross pistons in my 451, at .017" below deck. The 906 heads recessed combustion chamber pretty much kills good quench action. I probably have about .110" quench area now, As I believe the 906 quench area is around .100". So I am going to try closed chamber 915 heads next to see what difference it will make. With a .020 thick gasket, I will have .037" of quench distance. That should be about right.

 

[383man]

View attachment 88404I had my pistons "L2295F" lightened up a bit prior to balancing the rotating assembly..... You can see the nice Flat dome and plug slot... They are .140" dome height and -12cc Dome volume.. High compression without the big pointy Dome peek..

I forgot to add,,,,,
I use aluminum heads with Heart shaped chambers,,, not D style, so they are Flat except for the chamber area. No need for the quench pad with this chamber design....

Wow I like that style dome piston. A nice flat type dome. I would think that works very good. What brand piston is that 67 B body ? Ron