Rule of thumb for AF changes with change from full exhaust to open headers?

[Drew Pojedinec]

Logworks comes free with Innovate products

I run an MTX gauge but have data logging abilities.  It's fun to play with, but don't get hung up on magic numbers.  Some engines will happily cruise at 15 and some prefer 13.5 just use it for a constant.  It's especially helpful to view the timing of events with a carburetor.

[cjshaker]

Logworks comes free with Innovate products

I run an MTX gauge but have data logging abilities.  It's fun to play with, but don't get hung up on magic numbers.  Some engines will happily cruise at 15 and some prefer 13.5 just use it for a constant.  It's especially helpful to view the timing of events with a carburetor.

Thanks, Drew. I didn't realize that was their software. I've been looking at it, and it seems pretty user friendly. I actually only want to use this to see what difference it makes when I run open headers. I know the carbs are jetted and adjusted good for street use, but it's hard to see if it's leaning out when the exhaust is off. By the time I run a half mile back to where I'm parked at, the plug reading gets skewed. And I don't have the car consistent enough to go by those numbers, although my MPH is fairly consistent, so I can use that.

[Heo]

I dont know but i think its hard to read
plugs nowadays. If its the fuel or some
change in motor oils but i think there
is a black sticky mess on the plugs
always

[Drew Pojedinec]

Thanks, Drew. I didn't realize that was their software.

I kinda got that feeling, thus why I mentioned it.
Just remember if WOT track effort is the only change you want to make (minus exhaust), it may benefit you to leave the jets alone and just change the PVCR's.

[cjshaker]

I dont know but i think its hard to read
plugs nowadays. If its the fuel or some
change in motor oils but i think there
is a black sticky mess on the plugs
always

I'd agree, Heo. Fuels have changed drastically over the years.

Just remember if WOT track effort is the only change you want to make (minus exhaust), it may benefit you to leave the jets alone and just change the PVCR's.

I don't know what I would gain by doing that? To go back to the street exhaust, then I'd have to change them back (I probably shouldn't say it gets some WOT action on the street....so I won't ). Changing the jets would be easier. If I find out that it's also going a bit lean at WOT with the exhaust, then I could see doing it. That way I'd still keep my cruise characteristics. I should also mention that these are BJ/BK carbs and do not have replaceable PVCR's, as is, although I'm aware I can drill them out. And there are 2 carbs, so I still see the jets as being the easiest "quick change" for a day or 2 at the track, where it's either idling, or going WOT.

[plovett]

I don't know a rule of thumb, but it's a big deal, in my opinion.  2-4 sizes is what I say for almost anything........  it's hard to quantify differences with one jet size......so 2-4 is the smallest easily distinguishable range. 

The big deal is 2-1/2" pipes on your engine.  You are really killing it there. 

Do whatever you have to do to get 3" pipes under there.  You might think it's not worth it because of the difficulty in fitment......it's worth it.

If you are going to run open headers, it's much better to err on the side of longer collectors rather than short or no collectors.  You might actually run slower with open headers if you have no collector extensions and/or haven't changed the jetting.   

JMO,

paulie

[Drew Pojedinec]

I don't know what I would gain by doing that?

My point about adjusting pvcr's instead of jets was you could cruise in street trim during drag week (I assume your intended goal).

As long as you are keeping intake vacuum high enough to not trip the power valve you could keep smaller jets (64-66's or leaner still as dictated by the engine) for cruising on the mains.
For a WOT throttle burst the power valve would obviously open and so your larger power valve restrictions would come into play.
It'd be a way to not change anything for the duration of drag week and still cruise cleanly without fouling plugs or burning an excess of fuel.  Really no downside to this other than not being able to go WOT on the street while cruising between tracks.... the massive benefit is not having to touch the carbs for the duration of the trip.

With stock BJ/BK carbs your PVCR should be .043

[Tobbemek]

My point about adjusting pvcr's instead of jets was you could cruise in street trim during drag week (I assume your intended goal).

As long as you are keeping intake vacuum high enough to not trip the power valve you could keep smaller jets (64-66's or leaner still as dictated by the engine) for cruising on the mains.
For a WOT throttle burst the power valve would obviously open and so your larger power valve restrictions would come into play.
It'd be a way to not change anything for the duration of drag week and still cruise cleanly without fouling plugs or burning an excess of fuel.

I am all in with Drew on this. Using a in car  vacuum gauge is a must tool. Going down in PRI main jets and playing with the throttle pedal in all different load and cruising situation  to find where the engine starts to surge
and com back up to in jet size to what the engine needs  and all this while you stay above in vacuum where the Powervalve opens and even find out att what point in vacuum the power valve needs to open and have the vacuum advance adjusted to go off before that powervalve opening point. The rest is "simple" math calculation in what area size the PVCR needs to be to compensate for the smaller area PRI main jets and like Drew seas, clean plugs on cruising and you still have the fuel capability  the engine needs on WOT performance.
Hope my language and spelling was understandably

[Drew Pojedinec]

Tobbemek gets it   (your language skills are fine)

With my own car which has a lesser engine than Doug's (447ci, 2x4, 242/252@.050 cam, bbm heads) I find I can't really go WOT on the street anyway, nor is there a need to.... a few seconds and I'm doing 80mph and this is a much heavier car than a Mustang.

[cjshaker]

I get where you guys are coming from, and I'm not trying to be argumentative, but this car sees 95% street duty. And at the risk of being toasted, that's where it sees the majority of WOT driving (clean clear country roads only, and with me as the only passenger). I don't even remove the exhaust during Drag Week because I don't feel like removing a hot exhaust and reinstalling it every day. The only time it comes off, or will come off, is during the last day of Drag Week or during the FERR. At least with my current engine and set-up. And I'm not going to bother with an exhaust cut-out because it's just not worth it when you're running almost 4 seconds behind class leaders. It's a 12 flat car, at best, as is. Nope, just for fun and the occasional race weekend. I appreciate the suggestions though.

Edit: And Drew, I don't know that I'd say yours is a "lesser" engine. Other than the rocker system and cam, there's nothing fancy about my engine. Stock unported iron heads, stock 2x4 intake, stock carbs, stock stroke, and only slightly over stock bore.

[plovett]

I used to have a full 3" exhaust on my '67 Cougar.  That was over the axle and under the rear bumper.   There was no downside at all that I could see.  I just found an exhaust guy who would do it.  It wasn't perfect.  I brought some 180 degree 3" mandrel bends, let him cut them up, and weld them up.

Doug, exhaust is one of the very few win/win, "freebies", in engine combinations.  Let that bitch breathe and she'll blow kisses at you, as you both hurtle towards death in a cacophony of noise, fear, and physics.   Something like that anyway......

JMO,

paulie

[cjshaker]

LOL, Paulie! Did your car have staggered shocks? It would be much easier with the standard shock set-up, but the staggered shocks makes it a bit more difficult. I know I could squeeze them through, but then I think it would be very difficult to remove when I wanted to. I can just barely get mine out as it is, with the rear axle in full drop mode.

I made my own exhaust with stainless mandrel bends, and in the future it will get a full 3" system, but for now, it is staying as is.

[Tobbemek]

This thing with PW and Pvcr along with PRI MJ size testing is a standard presidier for every carb set up combo drag week or not.
To really answer the question" jet size with open ore closed EXH " You have to test the combo" No one knew what your combo likes.
different headers like different jettings like merge collectors likes allot more in jets. and headers vs stock iron exmanif often takes smaller jets.   

[plovett]

LOL, Paulie! Did your car have staggered shocks? It would be much easier with the standard shock set-up, but the staggered shocks makes it a bit more difficult. I know I could squeeze them through, but then I think it would be very difficult to remove when I wanted to. I can just barely get mine out as it is, with the rear axle in full drop mode.

I made my own exhaust with stainless mandrel bends, and in the future it will get a full 3" system, but for now, it is staying as is.

You're right, Doug.  My Cougar does not have staggered shocks.  I didn't think about that.