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half bridge 12A, with a twist

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Old 02-20-11, 01:39 PM
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Not sure where you got confused but this is all NA. Lower vacuum on a carbed engine causes one to tune pig rich just to get a decent idle.
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Old 02-20-11, 01:39 PM
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Originally Posted by risingsunroof82 View Post
I know you guys are talking about a turbo setup but does the lower vac with a full bridge NA still make for better drivability? Daily driving a full bridge can be tricky.... but worth it.
yes! lowering the intake vacuum on the PP made a HUGE difference.

there are a couple ways to do it, on the PP/weber IDA i drilled the throttle plates, i'd have to look at my notes to see where its at now, but start small and work your way up.

i started around .015", i think its around .020" but it might be more like .025... you'd cry if you saw my throttle plates, i went HUGE. at some point the holes get so big that the carb stops working, so there is a sweet spot.

i suggest you get a notebook, and change one thing at a time, and just keep track of what does what.
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Old 02-20-11, 03:30 PM
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Yeah, we're all N/A here Pistons before turbos.

I don't know about fullbridge. Haven't done one yet. The neat thing about half bridge is that you have two ports that will allow the engine to run smoothly all the way down to abut 20-22" of vacuum before the engine starts to buck, and two ports that don't run smoothly at any more than about 5" of vacuum, so you can balance the two so that the engine runs well 95% of the time while still taking advantage of the better chamber filling (volumetric efficiency!) that a bridge port provides.. You can't balance like that if all of the ports are bridged.

You can play with ignition timing to mess around with it, though.

On stock/street port engines, I remove the mechanical advance springs so it slings out to full advance as soon as the engine runs. Gives a nice boost to sub-3000rpm torque on these soft-ported engines. The engine will tolerate this extra ignition lead just fine because it will tolerate higher vacuum levels.

On the peripheral port engine I was running, I put the springs back in to kill off the low RPM ignition timing. More throttle needed for a given load means less vacuum for a given power output means it idles and drives a whole lot smoother. "Smooth" is a relative term here

On the half bridge, I am compromising and running with one spring. With total mechanical timing at about 25 degrees, it idles at about 10-12 degrees (no split here) and comes up really quickly compared to using both (which aren't "all in" until nearly 4000rpm). Downside is that the engine is idling "on the curve" and so the idle speed is slightly irregular - notice in the video I posted, from a start or a lug it will idle at 1100rpm, but rev the engine up and it will hang at around 1300 for a while - this is because the timing is more advanced at 1300 than at 1100. Electronic timing control to provide a complex three dimensional timing curve would completely eliminate this. (Properly done, you can control idle speed with ignition timing instead of an air valve! VW did this for a while and it worked quite well)

I am also using the vacuum advance tied in to a ported vacuum source for better fuel economy. The driving I've been doing suggests that I'll be seeing about 22-24mpg.
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Old 02-20-11, 05:23 PM
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Originally Posted by peejay View Post
On the peripheral port engine I was running, I put the springs back in to kill off the low RPM ignition timing. More throttle needed for a given load means less vacuum for a given power output means it idles and drives a whole lot smoother. "Smooth" is a relative term here

VW did this for a while and it worked quite well)
i have my timing locked, like the competition book says to, but my experiments indicate that the engine actually does want a timing curve, i have it jetted to run 14's @cruise, the comp settings are richer.

it actually makes the same bird chirp noise that the rx8's do, same conditions too, 2000rpm, part throttle 18degrees of timing, with no split. the bulletin on the rx8 says that the noise is normal, and that its pinging.

so anyways, its telling me it wants a timing curve

mazda also used timing to control the idle speed too. in the S5 new model training book, they have a chart timing will vary +5/-5 degrees to control idle speed. its probably what causes the idle instability on a modded car.
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Old 02-20-11, 05:39 PM
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Originally Posted by j9fd3s View Post

it actually makes the same bird chirp noise that the rx8's do, same conditions too, 2000rpm, part throttle 18degrees of timing, with no split. the bulletin on the rx8 says that the noise is normal, and that its pinging.
Cripes... the last time I heard an N/A engine do that was 2001 when I stabbed my distributor in ninety degrees out, with direct fire on the leading coils. Timing was 24btdc leading, 196btdc trailing

Of course this wasn't just chirping, at least not over 1/3rd throttle... lay into it harder and it would buck and feel like the engine was trying to stop turning.

But it had a righteously strong idle!

OH MAN A DECADE OF PORTING ROTARIES!!!
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Old 02-21-11, 01:20 PM
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Haha... I dont know why I thought he was turbo'd. lol
WOW..... this is some pretty serious stuff! So pulling just one spring from the dizzy would be a good idea on my bridge? I'm having trouble getting this thing to idle under 1500 without falling on its face....
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Old 02-21-11, 05:42 PM
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Changing the timing curve won't solve the high idle on a BP. By nature, mechanics really, they have a high idle due to the extreme overlap of the porting.
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Old 02-21-11, 06:39 PM
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Jeff, you're such a tease! Now get this thing together already and show some results!!!
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Old 02-21-11, 07:38 PM
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Originally Posted by trochoid View Post
Changing the timing curve won't solve the high idle on a BP. By nature, mechanics really, they have a high idle due to the extreme overlap of the porting.
Ignition timing makes an ENORMOUS difference in idle quality with high overlap engines. The rough idle is due to vacuum pulling exhaust gases up into the intake. If you can get it to idle at a lower intake vacuum, you will reduce this exhaust dilution and it will run more smoothly.

Reducing ignition timing at idle will reduce engine vacuum, which reduces exhaust dilution and reduces the brap.

You can't tell me that it doesn't do it, because I done done it!

My current semi-bridge will idle lower than I've ever managed to get a street port or stock port to idle. I was shocked to see it idling happily at 700-800rpm!

Now, with a carburetor, idle quality will be difficult to do this way because reducing vacuum will also reduce the carb's effectiveness, since you'll have to either open the throttle plate into the transfer slots (makes idle mix tuning impossible) or drill holes in the throttle plates, and either way the reduced vacuum makes things just crappy all around. That is one really, really, REALLY nice thing about EFI... you just tell it how much fuel the engine needs and it does it, no need to try to outsmart a mechanical device.

Fun fact: Ford Boss 302. The original one, not the new thing. Had a torque curve that did not belong on the street - intake ports larger than the ones on a 460, absurd cam timing, huge carburetor, powerband that did basically nothing at sane speeds but really did fun things at 8000rpm. Ford put these engines in front of automatic transmissions in 1970! They didn't have high stall speed converters, though. What they did do was put a vacuum retard can on the distributor to pull a bunch of timing out at idle so that it would run acceptably smoothly at a low enough RPM for the tight converter.
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Old 02-21-11, 08:39 PM
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I want to put this in my car
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Old 02-21-11, 09:03 PM
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Jeff: Very interesting thread. I have a few questions. Do you plan on running a complete bridge (meaning the size of the intake opening) or a partial bridge?
Are you going to convert your carb over to mechanical secondaries? I wonder if the carb will be able to keep up with the secondaries. I wish you the best of luck on this build.
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Old 02-21-11, 09:31 PM
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i love reading thread like this......have any more pics of the intake..?
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Old 02-22-11, 12:52 PM
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84stock, you'll have to buy me dinner first.

Jibaro, the bridge will be as tall as the runner, with some nice shaping to match the height of the port closing edge maybe (gotta look into it a bit more), and curved/angled into the runner flow to encourage incoming airflow.

Mech secs, yes. Will the carb keep up? I think so. We know the secondaries are actually nicely sized on Nikkis from the factory. Sterling himself doesn't change the venturi size of the secondaries on his modded carbs, just the primaries, nor does he change the secondary jets as they're already plenty large enough (at 160) for pretty much most setups. He might change the secondary air bleeds. Anyone know? I'm certainly changing the sec air bleeds on this carb so they stay at a "decent" mixture as the RPM goes up. I have some 120s and 140s here. If I go with 120, it'll keep it metering fuel to a higher RPM instead of leaning out. Actually, I think these carbs tend to go rich as RPM increases. I'll have to check it with a wideband at some point. I also have some 150 and 170 fuel jets so I'm good either way.

As for the primaries, this engine was going to have stock sized primary ports with stock timing and smoothed edges/casting flash. It will match the carb's primary venturis and give better low end torque and driveability than if I were to increase primary port size/timing in any way such as 74 spec or earlier-than-stock opening. Plus if I choose to fully bridge this engine in the future, by leaving the opening stock it gives me that option. This engine is all about the driveability factor. If it encourages your desire to sit behind the wheel, it's been incorporated into the planning of this build, but with the option to grow in the future if so desired.

The core engine I chose for this build was a very good running J-spec R5 12A that came out of a friend's FB. It had a smoking prob under decel so I changed the oil seals and threw in some competition outer springs. The stock apex seals were still above spec but I'm doing new for obvious reasons (gotta flip the rear rotor's seals due to the bridge and also prefer the peace of mind). The housings are actually decent, and in this day and age, good luck finding new, so definitely resuable. They're getting a set of frost plugs and the ACV port under the primary runners is getting filled with quicksteel (I call it the cool manifold mod but actually the mods are done to the engine, leaving the manifold stock for backwards compatability on a different engine if necessary). The flywheel was a stock 30 pounder as found on all R5 engines, so having 76-82 rotors, I simply swapped it for a noticeably lighter 81-82 flywheel and will swap in an 81-82 front counterweight (not sure how much this matters, but every 81-82 engine had this type of front CW, so I'm swappin' it). The end play with be set to Judge Ito's recommended .002" or there abouts. Smashed rear oil pressure reg for 85-90psi for a max rpm of 8400 where psi = rpm (these seals should not exceed 8400 because they aren't carbon, so around 85psi is fine and easier on the oil pump than 100psi).

I'm sure there's more to talk about but I gotta get going.
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Old 02-22-11, 12:53 PM
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Originally Posted by rx71king
i love reading thread like this......have any more pics of the intake..?
I'll get some a little later today.
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Old 02-22-11, 07:09 PM
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Thanks for the clarification Jeff. I really admire your other threads but this one takes the cake! I love 12A builds.
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Old 02-22-11, 08:33 PM
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As shown above I might know a thing or two about building a tough street 12A. This motor pulls 16.5inHG at idle and I know how to do it better again when this block gets pulled down.

Go mild not wild, get an 89-91 FC turbo intake on it and turbo it later with a V-trim compressor /S5 hybrid turbo.

If you want a part list of 'best of the best' for this motor feel free to contact me by PM.
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Old 02-22-11, 08:55 PM
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Jobro in your first pic. Are you assembling your engine on the living room? My wife won't even let me walk through the door with boots on! LOL
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Old 02-23-11, 07:18 AM
  #43
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Originally Posted by Jeff20B View Post
Hey when are you going to use that nice channeled '70s manifold?
I got the rear counterweight yesterday in the mail Jeff. Looks great! I'm working on stripping out Gus' engine bay so I can get it cleaned up. I'll be pulling the 12A out in a couple of days. Everything in there is absolutely NASTY. There is 1/4" of oily black crud on everything. It's bad. The awesome 70's manifold will be going on the 12A when I put everything back. I'm planning to use an aluminum flywheel, sterling-esque carb with K&N, direct fire ignition, electric fan, RB exhaust, 2nd gen FMOC, and more.

I just need to rig up a PCV setup for the manifold, and figure out how I'm going to do the vacuum advance for the ignition. It's definitely in progress though.
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Old 02-23-11, 01:06 PM
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PCV is super easy on these. Buy a Fram FV266 PCV valve. Use a short piece of 5/8 heater hose. Connect from the fitting nipple to the hex part of the PCV valve (push the main body inside the hose) and hold with hose clamps or nylon zip ties. Then run a 5/16 hose over to the small nipple on the oil fill tube. Be sure to test flow direction on the PCV valve using compressed air first before you shocve the PCV valve inside the hose. I can't recall whether it was the threaded end or the nipple end of the PCV valve but the 5/16 hose fits either end. That's probably why I can't remember.

I'll have to add a 266 on this on this manifold as its PCV valve is broken too. The plastic part tends to crack/break when it gets old.

Now as promised, here are more pics of the manifold.

Looks like the front secondary barrel was running funny. I'll clean up the crusty runner before it goes on the engine. Gotta closely examine that barrel in the carb too.

For some reason the PO used some RTV on the gasket surface. Not a good idea, but then it appears he damaged the surface with one of those air die grinder scuffing pad attachments. Stupid. Yet every shop does it. I'll have to clean it up and see how irregular the surface actually is, then determine whether I have to take it to a friend's belt sender to true it up again.

The backside has a small ACV pattern, smaller than 74 and later so I'll have to make a plate (can't buy one in this size). But here's a tip: I won't need to make a plate for it if I fill in the ACV port in the intermediate plate with quicksteel.
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half bridge 12A, with a twist-colten04.jpg   half bridge 12A, with a twist-colten05.jpg  
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Old 02-23-11, 04:51 PM
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cool man....that intake is old sckool for sure...
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Old 02-26-11, 12:45 PM
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Hey guys, quickie update. The "other engine" I mentioned (that has to go together first) is now stacked and just the front cover and oil pan remain. Then it goes in the FB for test running. I must do this before I can get started on the half BP. Yes, responsibilities come first.

I've got a friend's stripped Nikki for the first startup because I know it works/how it runs. Then I'll pull it and test the old school carb+manifold seen above (can't hardly wait! ). Then lastly I'll throw on an untested rebuilt modern Nikki and shudder valve-removed & channeled manifold so this "other engine" can come out to make room for the half BP.
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Old 02-27-11, 09:01 AM
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Originally Posted by Jeff20B View Post

I've got a friend's stripped Nikki for the first startup because I know it works/how it runs. Then I'll pull it and test the old school carb+manifold seen above (can't hardly wait! ). Then lastly I'll throw on an untested rebuilt modern Nikki and shudder valve-removed & channeled manifold so this "other engine" can come out to make room for the half BP.

What??
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Old 02-27-11, 12:06 PM
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I was in a hurry to type out that second paragraph, but I stand by it.

The first engine is built! Just gotta throw a clutch and waterpump on it and drop it in the FB. Probably on a day when it's not freezing temps or raining... Good excuse to drag out the next round of parts for the half BP today?
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Old 02-27-11, 12:37 PM
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Do I read correctly that you are gonna test your individual runner intake on another 12a?
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Old 02-27-11, 12:51 PM
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Well, yeah. It's the best way to know that it works before I take the plunge and cut the actual bridges. The other option for the second engine was 74 spec ports, which don't require any special intake considerations.
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