PCV Valve Placement

[difrangia]

I'm in the midst of adding a PCV system on the little 1200cc E-1 engine and concerned about the point for vacumn pick-up for the valve. I have planned to use an existing hole in the intake manifold at the beginning of the rear intake runner. It is the larger hole in this photo I've already enlarged the hole; maybe a mistake:

 

 

I've now become concerned that pulling crankcase air into one runner might unbalance the induction of the engine. Would this possibly cause the rear cylinders to run lean and cause tuning problems or burnt valves? As you folks know this is not a high performance engine.

 

Maybe I'm overreacting?? I'm ready for some expert experience-opinion here before I proceed. Getting very close to first start-up on this total rebuild. Thanks in advance.

 

Steve

[datzenmike]

The PCV valve is a very small controlled vacuum leak. It is greatest at idle and deceleration when manifold vacuum is highest and becomes less and less as the throttle is opened.

 

Usually the PCV is centrally located so all cylinders get some rather than the rear two. To compensate at idle, the idle mixture can simply be turned up.

[difrangia]

The PCV valve is a very small controlled vacuum leak. It is greatest at idle and deceleration when manifold vacuum is highest and becomes less and less as the throttle is opened.

 

Usually the PCV is centrally located so all cylinders get some rather than the rear two. To compensate at idle, the idle mixture can simply be turned up.

 

So, due to the small intake of post-carb air on half the engine, you don't think I'd be in danger of burning the rear valves from a leaner mix on a road trip (sustained highway speeds)?

[Stoffregen Motorsports]

I'm going to be the devil's advocate here. PCV systems are great if they work properly. But sometimes they disrupt the tune of the engine via an internal vacuum leak. You can adjust the PCV draw by making the hole in the valve smaller or larger depending on the engine by replacing the washer in the end of the valve with a washer of same OD, but with a different size hole in it.

 

But to my point. In some of my older cars (or trucks) I have simulated PCV vacuum by running the block breather and valve cover breather together into a tee, then off that tee I run a hose to the air cleaner (inside the filtered area). This system will pull a slight vacuum at cruising speeds if the opening of the hose is placed close enough to the carb opening.

 

I have done this on old British cars that have no rear main seal and it helps keep the oil in the engine where it belongs.

 

So if your motivation is to keep the oil leaks at bay this is worth a try.

[difrangia]

Stoffregen, I'm not so concerned with keeping the oil in the engine as venting small amount of fresh air into crankcase & expelling crankcase vapors. I have new graphite seal in rear of engine and proper lip seal in front. The original graphite rear was not in the best condition and didn't leak. The front was the old felt seal in horrid condition and peed a big spot anytime I'd stop, engine running or not.

 

This is what I have presently assembled:

 

 

 

The vacum pickup is through the larger hollow bolt which is in the rear intake runner about an inch back from the plenum under the carb. I woke up early this morning before daylight (some of my best and most productive engineering/designing time) with a worry about adding air to only half the engine. It can be re-engineered though.

 

Steve

[Stoffregen Motorsports]

I think you should try it the way it is and see how you like it. I doubt it will be a problem. I do feel you could achieve your goal with a simple hookup to the air cleaner instead, and without the vacuum leak.

 

OK, this sounds a bit techie, but what about a bleed screw like you see on SU intakes (Datsun not BMC) and on Toyota power steering setups? That way you could fine tune the vacuum for the PCV.

[difrangia]

I decided to quit fretting and re-route the vacuum pickup point to the plenum under the carb:

 

 

I feel a bit better about it now. I'll plug the other port and try this out. I will keep in mind the washer with a smaller hole in the PCV valve if needed. Carb is now bolted down an it's on to the few remaining tasks prior to start-up.

 

Thanks for all inputs.

[Stoffregen Motorsports]

Is that the way you're going to run your carb? With the float bowl in the back?

[tr8er]

I'd say the location of the PCV inlet will play a role, and will unbalance you mix. That said, I doubt it will be noticeable, nor problematic. The careful engineer would likely avoid this setup, and the realist would recognize the scale of the problem to be too small to deal with. Your intake manifold is seeing some push and pull of air, thus some of the PCV air should mix a bit in the manifold. That, in concert with the small volume if vapor actually contributed by the PCV, should mean you have nothing to worry about.

 

I say run it as is. It looks good.

 

Worth noting: I'm not an expert on this subject, I just wanted to share my expectations.

[difrangia]

Everyone who posted on this is probably right to a great degree and it probably would have worked as I originally had it plumbed. It is most likely nothing that would have been very noticable on such a small and low stressed engine but I feel better that it is now more balanced from cylinder 1-4. I can wake up in the morning working on a different issue in the build.

 

As for the carb mounted backward, yes, I'm running it like that. I put up a post on here a couple months ago when I first started considering carbueration options. The DG-Series Weber will not mount in the conventional 'bowl in front / throttle lever in back' orientation without using a Pierce or similar manifold that sets the carb further away from the engine head. Mounted conventionally the choke unit, whether manual or electric, would be an inch or more inside the rocker cover. I played around with it and came up with mounting the carbbackward using the two inner studs on the inner carb holes and making an adapter plate similar to the plate for the A-Series engine that slides over the inner studs and uses flat-head or allen cap screws in the original outer stud holes. The adapter has a pair of studs about a half inch further out that match the Weber pattern. This puts the throttle lever in front where it's easier to service and the idle air and vacuum advance port to the outside for easier access. Seems like a win-win to me. I was concerned that inertia on the float might be an issue, but after consulting here and talking to several Weber distributors, I'm at ease with the float issue. I'll for sure post here if there is an inertia problem.

 

[Stoffregen Motorsports]

OK, I was just making sure you didn't have the carb on backwards with the ports mismatched.

 

I know you like to tinker, which is why I like your work; it's very detail oriented. That being said, you will probably have the PCV dialed in in no time. But yeah, run it like it is.

[difrangia]

I plan on running the E-1 on the cart that it is mounted on for several hours + while I go back to work getting the truk ready for the engine. That's been my plan all along. Gives me an opportunity to do a little tuning, listening for noises and looking for leaks. All a lot easier to address with the engine out of the vehicle. I ran the FIAT 500 engine about four hours on the bench prior to installing back into the microcar.

 

Steve

[Lockleaf]

I've been running a Weber mounted backward on my z24 for three or four years. I've never noticed any issues. I did have to modify the adapter I had to fit the larger barrel on the backside, but I've never had any float issues or anything.

[Stoffregen Motorsports]

I had major float issues on my first rock crawler. It was a Jeep with the Iron Duke Chevy 4 banger and a Weber mounted with the float towards the firewall, but that was offroad and most undoubtedly because of the steep angles encountered while offroad. On the road it ran fine.

[datzenmike]

OK, this sounds a bit techie, but what about a bleed screw like you see on SU intakes (Datsun not BMC) and on Toyota power steering setups? That way you could fine tune the vacuum for the PCV.

 

My wife's old Plymouth Caravel was constantly plugging the PCV hose and valve from condensation and deposits. I'm saying that if it was only air no problem, but there is oil, gas, combustion products and water vapor and a restriction like a bleeder screw is sure to clog up.