poormxdad

Gentlemen,

I have been tracking this same NB1 since 2011. Up until last April, I was driving her to, and usually from, events. When I went to VIR, I pulled a Harbor Freight trailer with slicks and spare parts and tools. I used the cruise control quite a bit. Now that I'm trailering her to the track, I was contemplating removing the cruise control bits (and doing some other weight saving efforts).

The cruise vacuum line runs to the port right below the throttle cable bracket. The lines for the MS3PNP and the recirc valve are tee'd and go to the port just rearward of the throttle body. I make no claims that this vacuum line setup is correct/optimum, just that is the way I've had it since 2015.

If I were to remove the cruise control, where should the MegaSquirt and recirc valve lines go, and will she need retuning?

Thanks,

codrus

I melted the cruise at the track once, so I took it out. It doesn't weigh much, maybe 5 pounds at the most, probably less.

The simplest way to replumb it after removing the cruise would be to remove the last tee and plug the recirc valve into it. You should not need to do any retuning, the cruise doesn't use vacuum normally so the feed line to it shouldn't differ from the manifold by much.

That said, the general recommendation is to give the megasquirt its own dedicated vacuum line off the intake manifold. That helps isolate it from failures of other components.

--Ian

poormxdad

I assumed vacuum was vacuum when I plumbed the lines way back when. Does it matter which port I choose for the MS?

codrus

If you have a hose with intake manifold vaccum at one end and a leak to the atmosphere at the other (which is what a "vacuum consumer" ultimately is) then there will be a continuous pressure drop across the length of that line. If you tap into it in the middle, you will see an intermediary pressure -- higher than manifold, lower than atmospheric. The MAP sensor is the principal input that drives the fueling at the injectors, so it's important that the ECU see the same pressure that the intake ports in the head see. Teeing into the cruise line is not ideal from that standpoint, and in principle the best way is to run a dedicated vacuum line from the manifold to the MAP sensor that the MS is using.

In practice it probably doesn't really matter all that much. The cruise is only "consuming vaccum" (in reality letting unmetered air into the intake) when it's changing the throttle position, which is a fairly small amount of the time. Also, most of that pressure drop is going to happen across the cruise unit itself (because that's what does the work), the drop in the vaccum line is going to be small in comparison. Even 20 year old cars like an NB (I started typing "modern" here before thinking about it, heh) don't really have vacuum accessories like OLD cars did -- no vacuum-driven wipers for example.

There are also internet arguments that some of the vacuum ports on the manifold are better to use than the other because of standing waves inside it, but I have no idea how important this is.

I think the most relevant factor is probably safety of that vacuum line. The more places it goes, the more tees and other junctions in it, the higher the risk that one of those lines will get damaged or leak and feed the ECU an inaccurate signal. The dedicated line is the safest in this regard. Personally I have one port feeding the megasquirt only (I think it's the one that's unused from the factory, near the front), and all of the other things (fuel pressure regulator reference, bypass valve, and one other that I'm forgetting) are teed together off a port at the back of the manifold (I think it's the one that originally fed the cruise before I melted it)

--Ian