Calling all HAMBers. I already dug through a bunch of old threads, some helpful and others not. Before I drill a 7/16” hole in the back of my intake, I thought I’d get some feedback. Engine is fresh rebuilt 283, mild cam, intended to cruise. The Problem: I’ve got a brake booster, TH350 vacuum modulator, and a PCV but only one 1/8 NPT vacuum port in my tripower intake. The Plan: Drill the 1/8NPT to a 1/4 NPT, run a t block and connect the 3/8” power booster and 1/4” trans line off of it. Then drill a 1/4NPT hole on the drivers side rear plenum and connect PCV valve from the draft tube provision. (Im thinking diagonal as shown to give me some room around distributor, but I could drill it square.) The Concerns: In this location I’ll be biased to the rear most cylinder(s) because the plenums are divided (left to right) on the rear carburetor. Anyone think I’ll regret this later? Saw some folks added the PCV to the exhaust, but I just plan to cruise this car (high rpm more effective on exhaust option?) For reference, I don’t want to buy thicker spacers to replace the phenolic spacers . In general I’m trying to be discreet as possible with the vacuum lines to keep it clean. Also curious how effective PCV’s are from this location. I’d prefer to keep it here and not drill holes in valve cover. the only breather currently is the fill tube at the front.
Drilling the intake for a 1/4" NPT fitting for the PCV would be the way to go for that....but alternatively you could get a road draft tube, and use it instead. 283s used the draft tube through 1962, then switched to PCV. Power brakes let air into the intake when they operate. And need a decent sized fitting. Vacuum modulator on the transmission just senses vacuum, it doesn't let any air into the engine, and can use a smaller line, but kind of wants to be connected to it's own fitting so it doesn't get confused. Might be that the brakes want their own 1/4" NPT fitting similar to your plan for the PCV. But you could probably get away with using the small fitting, and sharing it with the transmission.
Thanks Squirrel! What are your thoughts on putting a T into the 1/2” PCV line and adding the 3/8” booster to the same circuit? Then I can leave the modulator line on the 1/8 NPT provision in the intake by itself? risk of no power breaks if for whatever reason PCV closes?
I expect you would not be the first person to do that. In the 70s-80s, Chevy was pretty strict about connecting the power brakes to a separate fitting on the bottom rear of the carb, and the PCV went to a separate fitting on the front of the carb. And the transmission and other vacuum stuff was connected to yet another fitting that screwed into the intake manifold, about where your manifold fitting is. There might be a good reason why they did this?
Yeah you’re probably right. Drilling two holes vs one in my intake wasn’t where I thought this was headed but I’ll probably be happier driving it if I do lol. Engineers usually have a reason for making things more complicated. Whether or not it’s a good reason is something different entirely
This is not backed with hard data, but here are my thoughts. The PCV is a restricted vacuum leak. The hose never sees full manifold vacuum unless you plug the end with your thumb, for example. This works because carbs from the start of this use are set up for this. They are tuned to get slightly less signal. No issue there. However, both the brake booster and the trans are connected directly to manifold vacuum. The trans never changes the signal , it's diaphragm just moves a slight amount. The brake booster will release vacuum during operation, but expects full (closed) vacuum when released. Both will have a weaker signal and not full vacuum when Teed into a PCV. If the engine develops normal stock vacuum, this may be within design tolerances. But if you have a cam that drops idle vacuum, it may be enough additional drop to cause power braking loss. Think like a huge cam on a car with power brakes. You can also play with different PCV valves. Some OE applications flow more than others, but aftermarket replacements will often be a one size fits many. I'd prefer a central PCV location (center carb), but see why that isn't what you want. I'd also question why a spacer isn't a choice, but I'd guess it's hood clearance. Otherwise, it makes the most sense. A big rubber hose (PCV and brakes) may not look clean, but a formed metal tube like that used for the trans with just a bit of hose on the end will make people scratch their heads. You might be able to rig something up on the underside of the intake, but that's a LOT of design and fabrication just to hide a service item. I'd also want to check if there is a 90 degree fitting that would clear the dist, and drill straight in from the back and catch both rear inlets.
Thanks for the input! Honestly the more I think about it, the more carb spacers are making sense. I initially didn’t want to do it because I have phenolic spacers right now (to try and just skip any potential manifold temperature issues down the line), but I wasn’t seeing any 1” phenolic spacers available for 3 bolt carburetors. Hood height shouldn’t be an issue with 1” spacers, which is one less excuse to not do spacers lol. Cam is an isky 262 super cam which has 108 lobe separation. My understanding is this should produce good, but definitely not great, vacuum at idle.
Gents, after some good feedback, and a night of overthinking, I think I decided to go with the road draft tube rather than PCV and drill a separate hole for the brake booster in the intake. Worst case scenario I’ll switch to manual transmission and manual brakes if I don’t like it! Greatly appreciate the feedback. I’ll post some pictures when I‘m done. But in the mean time feel free to stir the pot and change my mind! Lol
Selected the plenum between the front two carbs for the brake booster fitting. Drilled 7/16 and tapped the 1/4” MNPT for the 3/8 brake booster line. I’ve got a little interference with the intake and the baffle so I might have to bang it around a bit to fit right
I'm pleased you went with the road draft tube as your carbs are not made with the metering for the added vacuum leak that a PCV will bring. JW
If you don't already have one, get or make a dist. wrench. Those lines are messing with access.Make it easy on yourself.
The Jalopy Journal
