aspen77rt
Well-Known Member
For a carb, I'm liking the street demon I put on the Mirada. Small primaries, big secondaries.
360 LA, Fast Burn or Magnum
- Thread starter Bruceynz
- Start date
Bruceynz
Well-Known Member
Spread bore?
Bruceynz
Well-Known Member
What is the torque converter bolt circle size on a 360 magnum?
aspen77rt
Well-Known Member
Can fit either
625 CFM air flow - ideal for use on most stock to mildly modified V8s
Dual mounting bolt pattern - fits most aftermarket spread/square bore intake manifolds without adapter
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Bruceynz
Well-Known Member
that spread bore was popular then for some reason they all went to square bore, was there a reason?
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kkritsilas
Well-Known Member
Holley carbs were tradtionally square bore (they did make some not so popular spreadbores as well). Many factory cars used spreadbores (Thermoquads (TQs), and in massive numbers, the Rochester Quadrajet (used in all GM car lines across the board). Idea behind both spreadbores was to have smaller primaries for cruise/steady state economy and drive-ability, and when engine demand rose, provide a large increase in flow with the large secondaries that were operated based on engine vacuum. The square bore carbs would normally have larger primaries to help provide a higher rate of flow, and the secondaries in the square bore essentially just double the flow, where as in a spreadbore, the secondaries would add much more than double the flow. In the earlier days of carburetors, one of the first steps was to junk the factory carb, and put on a Holley, which was seen as a "performance" upgrade, even though, all things considered, it may not have been as good all around as the factory spreadbore. As time has gone on, and as people have figured things out, the TQ and the Rochester are now seen to have been as good as, and arguably better/more advanced, than the Holley square bores for all around use on street cars. The Street Demon shown above takes a number of ideas from the Thermoquad, and also uses the spreadbore concept.
Real world example is the Theromquad on a 318-4BBL used in our cars. It is actually an 800 CFM carburetor, yet works perfectly well on a low compression, relatively low revving small V8 that has a pile of anti-pollution equipment on it. If you used an 800 CFM square bore on the same engine, it would be fairly undriveable on the street, no matter how much tuning you did. The keys are the small primaries that can react relatively quickly to the engine's vacuum signal, and controlling the rate and amount that the secondaries open up on demand. There are vacuum secondary Holley Square bores, but even they would need to be sized down considerably below the 800 CFM of the factoryTQ, probably down to 600-625 CFM to work well. There are a number of different ways that the various metering circuits vary between the carbs as well, as well as the air door that is used as a secondary contol of the secondaries of the TQ, so some of the efficiency of the TQ is in those circuits, but the basics are as above.
Real world example is the Theromquad on a 318-4BBL used in our cars. It is actually an 800 CFM carburetor, yet works perfectly well on a low compression, relatively low revving small V8 that has a pile of anti-pollution equipment on it. If you used an 800 CFM square bore on the same engine, it would be fairly undriveable on the street, no matter how much tuning you did. The keys are the small primaries that can react relatively quickly to the engine's vacuum signal, and controlling the rate and amount that the secondaries open up on demand. There are vacuum secondary Holley Square bores, but even they would need to be sized down considerably below the 800 CFM of the factoryTQ, probably down to 600-625 CFM to work well. There are a number of different ways that the various metering circuits vary between the carbs as well, as well as the air door that is used as a secondary contol of the secondaries of the TQ, so some of the efficiency of the TQ is in those circuits, but the basics are as above.
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I have seen several vids of the new SV1 carbs in action and just tuning. Since they make them for your application specifically (built to order) throttle response is amazing. I have been considering one for my V8 Chevy when I finally get around to the teardown/rebuild stage. . .
www.prosystemsracing.com/svseries.html
www.prosystemsracing.com/svseries.html
kkritsilas
Well-Known Member
Street Demon also has a reputation for great throttle response; there are numerous videos on Youtube regarding this.
The SV1 has a very interesting design, but at $1250, it is getting close (not quite the same price, but getting into the ball park) of the aftermarket fuel injection systems, such that I would ask myself if the extra $500-800 for a third party fuel injection system wouldn't be a better idea. Or maybe just save a pile of money and get the Street Demon 625CFM at about $400-450. The even cheaper alternative is to have the TQ rebuilt with ethanol resistant parts and an added vacuum port, and just go with that.
Kostas
The SV1 has a very interesting design, but at $1250, it is getting close (not quite the same price, but getting into the ball park) of the aftermarket fuel injection systems, such that I would ask myself if the extra $500-800 for a third party fuel injection system wouldn't be a better idea. Or maybe just save a pile of money and get the Street Demon 625CFM at about $400-450. The even cheaper alternative is to have the TQ rebuilt with ethanol resistant parts and an added vacuum port, and just go with that.
Kostas
