gerbraldy

Hey guys how much power do you think I could get out of a halfbridge 12a, nice big carb (not too big) and nice freeflowing exhaust. How high would the idle set at, like 1200-1300? What is the compression ratio of 12a's, I've never looked into it due to thinking 12a's were inferior to 13b's but my head was up my ***. You think 170 rwhp is too much realisticly speaking. I want a nice FB to lighten and build up, maybe turbo. I have a TII vert and it's too heavy for my liking, I'd trade it in a second for a nice body decent engine FB.

calvinpaul

iTrader: (2)

From what I hear you could probably pull 200 from a half...no firsthand experience though.

mwatson184

I've never liked the half brige/carb idea. You will be getting a lot more airflow through the secondary barrels compared to the primaries which could lead to some problems. i.e undersized secondaries or oversized primaries. With undersized secondaries you will be out on some of the power that you would want from the half bridge. With oversized primaries you will be lacking in the drivability department. So you would have to choose between those 2 problems. Full bridge+carb is cool with me though.

Marques

DriveFast7

iTrader: (1)

My 12a full J bridge with 42mm chokes in carb, ceramic apex seals, and open exhaust Dyno'd 205hp to the ground.

fitzwarryne

The compression ratio on a 12A is 9.4:1

fitzwarryne

The Gen 2 vert is a nice looking car but as you say is heavy. Even with a TII it is always going to be a cruising car. The stock NA 13B used normally in a lhd vert is even slower being down some 20hp.

170 rwhp is just possible with a bridge/carb set up as you describe, but it will idle at 1500rpm, have no low down power and a thirst after fuel. A full bridge engine can get you up to 220rwhp from a dedicated track set-up.

I suggest rather than bridge and talk of later turbo, your best action is look around for a Gen 1 where someone else has already dropped in a TII, paid the bills and sorted out the problems. You could also look at a 12A carby with an aftermarket turbo, but after the 13B efi you may be disappointed with its jerkiness. In addition many of the 12A turbo conversions have been done as low cost jobs, whereas the 13b was done more as a total package with the stronger transmission. Later if you want more power upgrade to a bigger turbo.

There are always bargains out there and with summer coming on you can always cruise around in your vert looking for your ideal car.

Good luck on your choice.

calvinpaul

iTrader: (2)

Damn, ok...If you were to do a HALF BRIDGE and use a 4bbl carb setup such as holley eldelbrock or whatever else, when you are only using the primaries you are not sacrificing as much gas or low end power, (note, idle would be just like on a streetport engine, not 1500 rpms). When you get up on it, that is when over a certain rpm and enough load (vaccum secondaries), you would open the secondaries allowing more airflow with the secondary ports having a bridge.

The whole concept of a half-bridge is that when you are driving normally you are only using the streetported primaries. You sacrafice power to have streetability.

Now then if you are using a 2bbl carb, (weber, dellorto, munikki...) then there would be no point. The 2bbl defeats the whole purpose of the half bridge.

Siraniko

iTrader: (13)

can you elaborate more on that....we wanto to read facts!! not hear say

trochoid

iTrader: (6)

He has an interesting point I had never concidered. However, on my Hooley 600 and RB intake manifold, there is what one might call a mini plenum under the carb, which totally defeats his idea.

fitzwarryne

What carb is recommended for a half bridge? As WackyRotary said we want facts not theory. My interest was I was seriously considering a halfbridge until was told the engine I was being offered was getting less than 10mpg in a daily driver. With the way fuel prices are going, fuel consumption is getting to be a factor outside the track.

fitzwarryne

I would love to pay US prices for fuel, currently I am paying US$3.70 a gallon for premium fuel!

I travel 15,000 a year on average so fuel consumption is a factor in deciding whether to go 12A bridgeport. Fortunately another 10,000 miles of local travel is 4wd on rough tracks and comes free- at least to me.

Last year Iwas using a GM Shanghai 4wd where consumption was so bad you nearly needed someone walking alongside continually pouring cans of fuel in. No wonder fuel demand and price is rising so rapidly.

calvinpaul

iTrader: (2)

Ok, so your 12 is a 4 port. The center 2 (in the center plate) are the primaries. The end plates are the secondaries. In a completely stock engine, you will have a 4bbl nikki carb. When you are just driving around town or cruizing you are only using the primaries. If you were to put enough load on the engine (getting on the gas) the stock nikki open the secondaries by a vaccum. This allows more air/fuel into the engine.

Now to translate this into the full bridge...
On a full bridge all the ports will be the same size and shape, so if you were to use a 4bbl or 2bbl carb it would make no difference. The reason it is not "streetable" is because the airflow into the engine does not have much velocity until a higher RPM range.

Now for the half bridge.....
On a half bridge if you run a 2bbl carb it defeats the purpose because air/fuel is being sucked into all of the ports at the same time..i. e. at idle. If you were to use a 4bbl carb you would be running on the primary ports until you put enough load on the engine to need the higher flow of the secondaries.

So if you are crusing on primaries in a half bridge you would be using a "street port". If you were to open it up (secondaries) you are using the "bridge port". This in itself makes it more streetable because you have more of a power band all the way through, where as if you were to have a bridge you would have to have it at higher rpms to get any power out of it.

Siraniko

iTrader: (13)

Ever heard of a 51 IDA's

calvinpaul

iTrader: (2)

Yea, it will give you an additional 5-10 hp on a pp 12a, a little bit large on a bridge and once again, being a 2bbl weber carb it defeats the purpose of the half bridge...

Ok wacky, what are you getting at?

fitzwarryne

OK, so the half bridge is a good compromise if you want to drive your car to the track before you compete. According to Ancas's book on RX-7 Performance it gives you usuable power between 4,000 and 8,500 rpm, but what is it like in complying with emission regulations? What level of measurable pollution will you get?

What I was seeking was an engine which is economical at 3,000rpm for legal cruising but powerful in the 4,000-5,500 rev range for acceleration.

Siraniko

iTrader: (13)

The whole concept of a half-bridge is that when you are driving normally you are only using the streetported primaries. You sacrafice power to have streetability.

that I understand

Now then if you are using a 2bbl carb, (weber, dellorto, munikki...) then there would be no point. The 2bbl defeats the whole purpose of the half bridge.


I dont understand why a 2 barrel wont work. drivefast7 is running a 2 barrel dellorto in his 12-A j-bridge and majority of bp & p-port are running 51 ida with no problem

calvinpaul

iTrader: (2)

Ok, there is the difference. You are talking about a full bridge, a full j-bridge, or a perephial port. On these types of engines all 4 ports are the exact same.

On a half bridge or half j-bridge, only the secondary ports are bridged, the primaries are just streetported. So if you are running a 2bbl you will be sending fuel to all ports, regardless of port type, at all rpm ranges.

I never said it wouldn't work, I just said that it would defeat the whole purpose.

fitzwarryne

Calvinpaul- thanks for the info, that makes the options far clearer. It also explains why so many go for a full bridge with an IDA. There are a few people I know of with half bridges with Sterling/Yaw type highly modified Nikkis, an approach I thought at the time was unusual. It was a route I was looking at.

calvinpaul

iTrader: (2)

You would be better off with a holley, the modded nikki won't flow quite enough to handle the bridge. If you are looking for streetablity and gas mileage, but still the kick in the pants of a bridge, I would suggest half. But if you don't care about gas mileage, and just want the high-revving top horsepower, go for a full bridge with a 51 ida.

Siraniko

iTrader: (13)

so where did you read this from since this is what u said..

84stock

iTrader: (5)

I would go with a "stock port" on primaries and bridge the endplates. Then use a rb intake and a "spreadbore" double pumper with the 4 into 4 adaptor plate. With the dble pumper "you the driver" control the secondaries (like a manual secondary conversion to a nikki) and the small primaries and stock port will protect your low end torque (what little there is) and throttle response. The large secondaries of a spreadbore will match well with the flow of the bridge on the endplates.

Siraniko

iTrader: (13)

there's a difference between 'works' and 'works well'.

krew

I run a 12a BP. in my 85, last Dyno was 205 RWH. with SDS fuel injection, with stock plugs, with more tuning I think 15 more hp would be possible. Free flowing Exhaust is a must!

buzz

Does anyone in the thread have actual experience with a half bridge?

This is an interesting comment:
Just how "streetable" is a half bridge. I guess we mean the poor low rpm response and the bucking around it will do at lower rpm?
I'm curious just how a half bridge will solve this. Is it the higher velocity of the air? I'd have thought that overlap that makes a bridgeport so nasty to drive down low, and noisey and need to idle high would still exist with a half bridge. Although no air is entering the inlet of the bridged secondaries the bridged port is still seeing the open exhaust port while the primary port is open - this must still effect the incoming charge.

thoughts?

For what its worth, I recently had a 13b 6port rebuilt. It went from 112rwKW to 144 rwkw, or 188hp. The centre plate was replaced with a late 1st gen version and bridged and the "secondaries" were bridged as well, leaving the 5th and 6th ports untouched. The bridging is very conservative. Its running a 48ida with 42mm chokes. Its running out of power at around 7,400. Until around 4,000 rpm it is making less power than the stock engine with same chokes. I picked up 2.5 seconds around a local circuit ( now 1:15.0) and think there is another second and a bit within it.

Building a half bridge sounds like an expensive way to compromise if it is only low end drivability that you are worried about. Bite the bullet and go for power, just go a full bridge, a 48ida and get used to not having a cruise circuit in the carb.

fitzwarryne

Buzz- Very useful informed comment as I was getting confused by conflicting views. It sounds as if you have a great improvement in performance while fuel consumption was not a major issue in deciding the level of porting.