ok a friends was tellin me about the mustang he used to have. he said it had a dub pumper that would open up the other 2 bbls at aroud 75 if you slowly accelerate. im not sure but im jus assuming that the carb runs the two bbls like a 2 bbl carb. and then when the other opens it runs like a regular 4 bbl carb.
Yes but: The mechanicle secondary carbs are not as forgiving in street use as a vacuum secondary carb.The mech secondaries will open when you mash the gas,and dump fuel into the engine whether the engine can use it or not.This can cause a rich stumble or bog at lower engine speeds that will drive you nuts if if it cant be tuned out(due to an engine that cant use the extra fuel)among other things.That said,a vacuum secondary carb will be much better for street/occasional strip use due to the fact that the secondary system is controlled by engine demand via the vacuum signal provided to the carb from the engine.Your chances of bogging/over rich are greately limited giving you better fuel mileage/preformance in street use.They are tuneable for opening point as well by changing the spring in the vacuum pot on the carb.Very fool proof/forgiving easey to deal with compared to mech secondary set ups.If you build an engine that is going to rev high alot(drag racing) and the engine is built to handle all the fuel you can give it then by all means get a mech secondary carb.Hope this helps you,good luck.
ok baddad, we are on totally different wavelengths... i read your post above on chamber shapes and sizes and am trying to understand it. i have been told the very opposite (not saying by any means that you are wrong, just trying to find the truth or understand what i am failing too at this moment). from what i have read, heard or seen, the open chamber is like the 76 cc SBC heads. then u have the 72cc (semi open), 68cc (semi closed) and 64cc which i would consider closed. closed because it has no combustion chamber except for where the valves are. the closed aspect leave less room in the chamber for gas to be compressed in with same piston height therefore compressing it more causing it to explode or eventually detonate (explosion faster than the speed of sound which as you stated which sends damaging sonic waves through the whole engine) now, around 72, EPA dropped the lead thing and went with a lower octain gas and therefore the older hotrods (11:1 pistons) would knock and ping (because the octain rating was too low to support it). they had to run octain boosters and whatnot or lead additives for valve cooling. so they opened up the chanbers to bring the compression down some. (i think i read for every 2 cc its like .1 compression rating or a whole 1 point)). reguardless, the more "open" the chamber, the bigger the hole, the less compression you have with a given piston. i know the D0OE-R heads were the 429 CJ/SCJ and they had no combustion chamber, considered closed and had around 11:1, D0VE-C heads were the 429 hypo closed chamber heads (flat on the deck with very small combustion chamber). the D3VE heads were open chamber "smog" heads found on later model 460's and whatnot. (this is all if i remember correctly, its 11:30 pm and i wont sleep unless i get things strait in my head..you've spun my whole automotive world around and i am confused now..) now, correct me where i am wrong and i will read it again tomorrow. assuming i can sleep tonight (or the rest of my time in iraq)...lol. nice write up tho...i didnt know what quenching was.
Closed chamber... Not sure of CCs off top of my head, but roughly 15cc less than open chamber. The term quench comes from the flat side of the chamber. These were only used for a very short time. 1970 300 hp (was also available at the very start of 71) and on Boss 351/HO. Aussie 302c as well as Boss 302 used closed chamber C heads. Open chamber... No flat side to the chamber. Was used on all US 2v heads as well as the bulk of 4v heads other than those above. All CJs were open chamber. All Aussie 351c were open chamber.
so that kinda reinforces my thoughts (i think). open chamber means bigger hole=less compression and therefore runs on lower octain? im kinda lost actually. maybe it was just a typo?
Open chambers are more susceptable to detonation than closed or quench chambers. That is a well known fact. Therefore there is a point of no gain when opening up a chamber to lower the compression ratio to run on lower octane fuel, but you get to a point where you loose the quench factor and loose the benefit of a lower ratio in preventing detonation. Sure, Ford went to open chambers to let these motors supposedly run on lower octane fuel, but if you were around then, you'd know it didn't work, open chambered 351C's and 400's had a bad habit of pinging on any low octane fuel. If you have a high compression engine, you're better off choosing a cam with a tighter LSA and more valve overlap to bring the octane requirements down, than handicapping it with open chambers. The open chambers also had another benefit to the car makers------they lowered the emissions as well by leaving less unburned fuel, that had more to do with the switch than lowering the octane requirements. The ignition and cam timing were also retarded at the same time. I'll say a few prayers for you in your time over there. My son is a Marine and just got back from a few months there.
If you don't change the pistons... Just swap heads, open for closed or vise-versa, your compression will go up or down. The point is that the design of the chambers plays a big role in the octane requirements. The quench chamber runs cooler by design. (runs dirty by emission standards too) This means that the closed chamber can run higher compression on the same octane fuel. Generally speaking, an open chamber needs premium fuel (93 octane) at, or just above, 9:1 compression. The closed chamber head can run up to 11:1 on premium due to the nature of the chamber and the physics involved. Of course, as D mentioned, there are MANY more factors than just compression that make a difference, so the potential compression can vary wildly depending on parts. However, if you maintain the exact same combo, and just switched heads, the closed chambers will boost compression slightly, yet run on the same or lesser octane fuel.
jus woderin. would anything off of a 98 ford taurus like the rearend or anything work on an old maverick or musang?
ooooooo0000oooooooh i never heard the flip side to chamber design. i knew about sharp points ( valve edges being too small, head imperfections, etc.) getting warm and causing premature burning so you kinda blend the bowls, but i am still in the dark about quenching so ima google it and learn. thanx for opening my eyes to the dark side..=)
http://www.importtuner.com/tech/0509it_cylinder_heads/index.html here is an outstanding altho brief article about different types of porting, valve grinds and blending and their effects on the chamber and air flow. spoke of 3-5 angle valve cuts for race or for street (different angles depending on engine needs), porting, cnc porting and extrude honing etc. i thought it was pretty kewl. wish it went more indepth, but those professionals guard their secrets. =\ but hey, cant be the best by telling everyone your secrets...right?
