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Topic Review (Newest First)
12-17-2012 01:46 AM
cobalt327 The flow through a jet is determined by the surface finish, the length of the orifice, the entrance and exit angles and their finish, and the size of the orifice. When a jet is drilled oversize, all of these things are changed except the angles of exit/entrance, but even the lengths of the angles are changed.

The number stamped on a jet (Rochester, Holley, etc.) is an indication of the actual flow, NOT the orifice diameter. The manufacturers flow the jets to see what number they get stamped on them. The same orifice diameter jet may be stamped w/different numbers. This tells you they do not flow the same, even though the orifice diameter is the same. Subtle differences in the above parameters account for the different flows.

The difference between the correct and too rich/lean jet sizes may only be 0.002”-0.004”. There are no readily available drill bits that are that close in size to one another. So at best, the changes made to jets by hand drilling are going to be in steps decided by the availability of drill bits. Because drill bits are often available in 1/64” increments (the small numbered bits- which have a finer increment between bits- are too small for drilling the average carb metering jet), this will be on the order of a 0.0156” change between drill bits- which is HUGE (~40%) change in metering area when in the 0.070” jet orifice range.

It is for these reasons there is no practical way for a person to casually drill a jet and have any way of knowing what the flow is going to be. Drilling out jets is a holdover from the dark days when guys were struggling to make “high performance” engines live, and if they got within 10% of ‘right’ they were at the top of the heap. Nowadays, thankfully, we all (should) know better and leave the drilling of jets and other dubious practices to the desperate, the uneducated, and the poverty-stricken.
12-16-2012 04:31 PM
lmsport Jets varying in flow rate such that jets with two numbers apart may flow the same, ie a 70 jet that flows on the high end of the tolerance may flow the same as a 72 jet that flows on the low side of the tolerance. You can buy sets of flow tested jets but they cost alot more.
12-15-2012 06:19 AM
gearheadslife
Quote:
Originally Posted by vinniekq2 View Post
why would the air speed be any different based only on throttle blade size?
I think you are missing my point,Im not saying either carb is better,especially based on throttle blade size alone.

If I have a carb of any size at a fixed height and I need 100 cfm of air,the air would travel at or very close to the same speed in all carbs,at least when passing the throttle blades.

other factors combined are going to make the MPG change,not blade size
I don't know<<(sarcastic) why does it slow when you use bigger ports in heads???
same reason..
why water spray farthar when you put your finger over hose end... the pressure didn't change.. just the size of the hole..
why does ehaust speed slow when it gets to the collector...
12-14-2012 11:21 PM
cobalt327
Quote:
Originally Posted by ap72 View Post
how do you think those holes get in the jets that you buy out of a catalog? And I've been drilling jets since I built my first engine- often the local parts house doesn't have the jet I want so you get the next smallest one they do carry and bust out your drill bits. MOST carb shops drill their own air bleeds and those are even smaller holes in smaller brass inserts.

This **** really isn't rocket science.

and adding weight is not that hard with some epoxy and scrap lead, just like in the pinewood derby days.

I know a LOT of people prefer to buy their parts out of a catalog and pay someone else to do the work, and there's nothing wrong with that, I buy parts too. But there's also nothing wrong with fixing your own stuff.
You are nothing but a poser and YOU would say to me something like YOU build and try to make it sound like I prefer to buy stuff?????

You are a total jerkoff of the highest order and you have no business even posting 90% of the time- you have no engine experience all you are is a ****ing book worm.
12-14-2012 05:45 PM
vinniekq2 Jet sizes can be up sized with a drill bit.There used to be special bits sold for that purpose.trouble with drilling is the machine marks left in the hole were not consistent enough to meter fuel flow,or sometimes the increase was huge in comparison to the size difference due to rifling in the hole.
some people drilled the jets from both ends to prevent rifling.or buy jets that are broached,,,
Thanks tech for the use of your vocabulary
12-14-2012 05:41 PM
vinniekq2
Quote:
Originally Posted by gearheadslife View Post
air speed through the venturi = better fuel atomization..
less fuel needed for same power/rpm as it burns better(completely)

why would the air speed be any different based only on throttle blade size?
I think you are missing my point,Im not saying either carb is better,especially based on throttle blade size alone.

If I have a carb of any size at a fixed height and I need 100 cfm of air,the air would travel at or very close to the same speed in all carbs,at least when passing the throttle blades.

other factors combined are going to make the MPG change,not blade size
12-14-2012 12:38 PM
techinspector1
Quote:
Originally Posted by ap72 View Post
how do you think those holes get in the jets that you buy out of a catalog? And I've been drilling jets since I built my first engine- often the local parts house doesn't have the jet I want so you get the next smallest one they do carry and bust out your drill bits. MOST carb shops drill their own air bleeds and those are even smaller holes in smaller brass inserts.
Ummm, I believe the correct way to punch a hole in a jet is to broach it, not drill it.
12-14-2012 12:16 PM
Motochris
Quote:
Originally Posted by cobalt327 View Post
Not all Edelbrock squarebores have an adjustable secondary, just the Thunder series. Unless you call grinding/drilling or adding weight "adjustable".

Has anyone actually seen the new 625 Demon in use??
Link-- http://www.hotrodders.com/forum/new-...rb-223299.html

I'm by no means a carb guru, but I've had and tuned a few different brands. Up until this carb, I've always had the best overall street manners/performance blend from the Edelbrock carbs. I'm digging this one...
12-14-2012 12:08 PM
Motochris
Quote:
Originally Posted by gearheadslife View Post
when you can get 19mpg out of a olds350 in a 4300 lb tank on hyway trips with anything .
tell me..
we got 19mpg every time going from ma to fla. 2000+ miles each way..
mighty q-jet.. you won't even get close with a square bore carb..
I'm getting 18mpg on the hwy in a 4500+lb car with the new Demon 625.

I think the carb is an ideal mix and super easy to tune. Adjusting the secondary is via screwdriver. It's basically spring preload.
12-14-2012 11:25 AM
ap72
Quote:
Originally Posted by cobalt327 View Post
Jets are most certainly NOT "adjusted" by drilling a bigger hole in them. Unless you live in east bum**** or are a hack. Drilling a jet can reduce flow. Not to mention holes are easy to make bigger but not so easy to make smaller.

The way a Performer secondary (or an air bleed or a metering jet) is adjusted is you go quicker (remove weight or bigger hole- easy) until it is too quick, then you back up. Adding weight back/filling in holes is NOT easy. Adjusting the secondary is easy w/a Q-jet. Not easy w/a Performer. And to say it IS easy is just to argue for argument's sake.
how do you think those holes get in the jets that you buy out of a catalog? And I've been drilling jets since I built my first engine- often the local parts house doesn't have the jet I want so you get the next smallest one they do carry and bust out your drill bits. MOST carb shops drill their own air bleeds and those are even smaller holes in smaller brass inserts.

This **** really isn't rocket science.

and adding weight is not that hard with some epoxy and scrap lead, just like in the pinewood derby days.

I know a LOT of people prefer to buy their parts out of a catalog and pay someone else to do the work, and there's nothing wrong with that, I buy parts too. But there's also nothing wrong with fixing your own stuff.
12-14-2012 10:50 AM
cobalt327
Quote:
Originally Posted by ap72 View Post
There was a link to a first hand account of it in use.

And I would call that adjustable considering it takes all of about 10 minutes to pull it out for "adjustment". Air bleeds and jets on almost all carbs are adjusted in a similar manner- by drilling a bigger hole or filling it in.

But you're correct the AVS series is easier than the AFB.
Jets are most certainly NOT "adjusted" by drilling a bigger hole in them. Unless you live in east bum**** or are a hack. Drilling a jet can reduce flow. Not to mention holes are easy to make bigger but not so easy to make smaller.

The way a Performer secondary (or an air bleed or a metering jet) is adjusted is you go quicker (remove weight or bigger hole- easy) until it is too quick, then you back up. Adding weight back/filling in holes is NOT easy. Adjusting the secondary is easy w/a Q-jet. Not easy w/a Performer. And to say it IS easy is just to argue for argument's sake.
12-14-2012 10:48 AM
gearheadslife when you can get 19mpg out of a olds350 in a 4300 lb tank on hyway trips with anything .
tell me..
we got 19mpg every time going from ma to fla. 2000+ miles each way..
mighty q-jet.. you won't even get close with a square bore carb..
12-14-2012 10:30 AM
ap72
Quote:
Originally Posted by cobalt327 View Post
Not all Edelbrock squarebores have an adjustable secondary, just the Thunder series. Unless you call grinding/drilling or adding weight "adjustable".

Has anyone actually seen the new 625 Demon in use??

There was a link to a first hand account of it in use.

And I would call that adjustable considering it takes all of about 10 minutes to pull it out for "adjustment". Air bleeds and jets on almost all carbs are adjusted in a similar manner- by drilling a bigger hole or filling it in.

But you're correct the AVS series is easier than the AFB.
12-14-2012 10:27 AM
ap72
Quote:
Originally Posted by gearheadslife View Post
wow..

guess you know more than g.m/ford/mopar/toyota


I'll admit I'm baffled by your ignorance. You win.
12-14-2012 09:33 AM
gearheadslife
Quote:
Originally Posted by ap72 View Post
Airspeed does NOT equal better atomization. The idiot that started that myth (a very widespread one on the internet) should be beaten with his own crankshaft.

There are many well designed carbs that can do a very good job of metering and atomizing fuel at low air speeds. And many terrible ones that can't even do a good job with high air speeds.
wow..

guess you know more than g.m/ford/mopar/toyota
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