Hey guys, 13b Rx4/5 engine running carbon seals used in a rally car, in the off season we decided to give her a total over haul/freshen up, all new seals, springs, lightened the rotors, side clearance, FD Rear oil reg, new oil pump etc as well as adding a moderate size (4mm thick) CNC cut bridge to the existing intake ports (as from the pics which was rather large Stage 2/Extend Port, with way oversized exhaust ports courtesy of the previous engine builder)

Once we got her up and running and run in enough, off to the dyno we went, and strangely although the bridgeport made more power than the stage 2, it has this weird flat spot (caused by rich conditions as observed by the AFR/lambda meters) that starts at 5000rpm but clears at 6500rpm as you can see. Nothing we tried improved it…which is a problem as in a rally car that’s is where you want the most power and most torque but due to the rich conditions we are down in that area.

On the dyno sheets Thick red line is the new bridgeported engine, thin red line the old Stage 2/extend port.

In terms of Carb tuning we have NOT changed a thing from the previous tune…
48mm Weber IDA
42mm Chokes
F11 Tubes
205 Mains
120 Airs
70F10 Idles
.70 Acc Pump
6mm Gross Fuel Inlet Jet with 4 PSI pressure

Running 98 Octane Pump gas premixed @ ratio 150:1

Timing is at 24 BTDC Leading ~20 BTDC Trailing
Exhaust is 2x slip jointed 2” primaries straight through collected just before the diff into muffler.

The ONLY thing that has been changed apart from the bridgeporting and timing slightly increased, is adding a straight through resonator (stainless packed) on each primary 2/3 of the way down.

In terms of AFR's I know it would be helpful if they were on the dyno sheet but they werent included on the print out, but I can assure you that at WOT @ 7000-8000rpm they were pretty much spot on, which surprised me as I thought we would be needing bigger mains given the additional porting but it seems not...?

Any ideas on what would cause this rich area and advice/tips to remedy would be great.

 

I wonder if you're encountering fuel standoff at those speeds. This is caused by reversion pumping air back through the carburetor, so a small amount of it gets double the amount of fuel as it goes through the venturi twice. (Technically three times, but I don't know if carbs work very well backwards) You'd be able to see it as a fog of fuel over the velocity stack only at that engine speed.

 

cheers, Il check that...any idea how or what would cause the reversion? i'm thinks too much overlap? or intake manifold length or both

 

late intake port closing can cause reversion in the mid range and it goes away at higher rpms.

theres a guy in the gold coast who made 300hp from a 13b bridgeport with ex ports similar to yours.

 

cheers that's good to know..any way of tuning around it? apart from going injection

 

Hello,

I run a 12a bridgeport with a 48mm IDA and 51mm IDA and I see this same AFR and power pattern as you. I know some folks have messed around with the emulsion tubes to address this issue, but I haven't heard of any great successes.

If I were to have a swing at this, I would buy up a bunch of emulsion tubes (see chart), make sure I have lots of jets and head to the dyno for an afternoon. Make lots of runs and keep good notes.

Guy

 

this

 

Long time dead topic, but did you ever resolve this? Im would tip on use a better flowing carb. I had a similar issue w reversing on our 13B PP, it was a 51mm ida w the stock velocity stack and secondary venturis. Then on the dyno w switched to a CLR annular discharge and there was no more reversion at all and it picked up quite some power as well. The velocity stack were I think around an inch longer also. We drive this carb now for 10 years and its really awesome. Egts are withon 10 deg +- on the end of straights. We rub 245-250 main and arlund 80-120 air correction which I use to balance out the rotors to match egts.

We aquired a oldskool formula mazda car w a fucked engine so I had thought to
make a nice big bridgeport for it. But its only 4 speed so if it will have this dip in the midrange itll suck.

 

Older me wonders if the issue could be addressed mechanically instead of with carb tuning, like making sure that the port runners in the engine are slightly larger than the port runners at the intake manifold face, and likewise that the intake manifold inlets are slightly larger than the carb outlets. A step like this doesn't really hurt forward airflow but it disrupts backwards airflow.

This is assuming that the issue is caused by reversion and fuel standoff, and not because of something like the low speed circuit still supplying fuel while the high speed circuit is coming online.

 

I was wondering if maybe adjustment on the accelerator pump, while not the initial hit exactly but if that could hold out better as it transition through the circuit could help. Kinda a long shot here though