I have a OER 47 sidedraft blowthrough setup with a gt35r. When i initially started it it revved up to about 2-3 grand and then died.... repeatedly it did this. the fuel pressure on my gauge will not increase for some reason but if i give it a little throttle it will stay running for a bit but it sprays fuel out of the carb overflow?? is this just because my boost is increasing and the fuel pressure is not?
seems kinda funny cause it almost seems like its getting too much air when the turbo starts boosting and gets too lean so it dies but yet i have fuel being sprayed out the top of the carb???

Or is it possible that my fuel pressure is way too much and my fuel pressure gauge is f'd up? its brand new aeromotive??? here is a pic if i can get it to work on my slow computer

 

Can you post some more pictures of the carb hat?...... It appears as if that design is going to cause you unavoidable issues.

 

issues such as what?

 

I would put my money on a vacuum leak. It would explain the high rev and the non-existent vacuum on the FPR.

 

think i figured it out.... i didnt realize i have to either plug or put boost pressure to the overflow hole in the side of the carb.... what an idiot.... live and learn i guess

 

That's one of them, another big one is the volume of the box. You should be @ ~%130 of engine displacement at a minimum.

 

Ive seen many boxes smaller than mine.. And these are professionally built plenums made for boost on the same carb setup???

will just blocking off the overflow hole be sufficient or will i have to put boost pressure to it?

 

Just because somebody paid money for something doesn't mean that it was designed right or works right.

You can not block off that vent. A carburetor works on pressure differential between the float bowl and the runner (in simple sense), the pressure differential is created by the air pressure of the moving air in the runner being at a lower pressure than the atmosphere (which is the pressure in the float bowl in a n/a carb setup). When you add forced induction to a motor you are changing the effective atmosphere that the engine operates in. In order for the appropriate pressure differential to exists so that the carburetor can operate the air in the runner and the air in the float bowl need to come from the same effective atmosphere.... make sense?

 

Sorry to butt in, but is the reason you need a box to be ~130% of the displacement to ensure that you have adequate supply of the artificial, pressurized atmosphere? Because if this space was too small, you would not have enough pressurized air to maintain this "atmosphere" at full load, and the pressure differential would not be enough to maintain proper fuel flow?

 

Kind of.... the volume is neccessary to maintain the presence of the artificial atmosphere regardless of throttle setting. In maintaining this atmosphere turbulent flow needs to be maintained almost everywhere inside the carb hat. Once turbulent flow is reduced or aproaches laminar flow really funny things happen (ie it doesn't work) An example of this is that all carb hats should be constructed so that the air entering the hat is at a 90deg and the carb throat.

 

Got boost pressure / vacuum to the overflow port. stopped some minor leaks on my carb that was supposedly boost prepped when i got it.... got it to idle nicely... i need to put on my throttle return spring now and time it. ihave it at 10 degrees advanced at idle with the dizzy locked in its normal resting position. my afrs are about at 12 at an idle and egts are at about 11000 to 12000 havent really built any boost. need to take it out and do a little preliminary run / slow test drive and try to see what i need as far as jetting... thanks for the input everyone!

 

took er out for a drive... as soon as i sterted to build boost it went way lean and cut out/ bogged. now i need to see if my fuel pressure is increasing? or i might need bigger jets or smaller air bleed jets.

 

got some larger idle jets.. might need more from the main jets too... any advice? i now have my fuel pressure gauge taped to the window.. it doesnt seem to inrease but does make some small jump rigth as it starts to build boost but then it cut out because its too lean... if i have my boost controller solenoid in line with my fuel pressure regulator refrence with a tee in the line will that not give my FPR the right refrence?

 

I would remove the boost controller all together until you get things worked out, a blow through setup can be very complicated if you have never done it before. IF fuel pressure is not rising with boost ( as it should ) you will lean out.

 

ok ill try that. do you think i need a separate line to the FPR from my bonnet? i would think this would make my fuel pressure increase more accurately. or would it be fine to tee or y the lines from the bonnet to the FPR and wastegate?
thanks Elliot

 

The boost control solenoid or the controller's sensor??? If it is the solenoid than you need to change that.

You may want to reconsider the design of that carb hat, having an improper hat will cause a lot of problems including what you are experiencing now.

Ryan

 

so are you saying that my design will make my motor run lean when it starts to build boost? i don't see how the design can have such a large effect on that especially since you usually want the least turns possible in the intake track from the turbo to the carb/(throttle body). i guess i dont understand why you need a 90 degree turn at the bonnet and no where else. on v8turbo or supercharged cars they dont have any volume in their bonnet at all. although the are at a 90 degree angle it almost seems that its at a 90 becaus that is basically the only way possible to run it. if i am wrong or ignorant please try to help me out... i just dont completely understand the concept i guess

 

I can't say anything other then it looks like there is not enough volume, and you are most likely not getting the same artificial atmosphere to both the carb bowl and the carb throats.

That applies only in general to the entire system, but when it comes to blow through carbs the air must be in a state of generally no direction (turbulent). Now that seems counter intuitive, the air needs to go in the direction of the motor, but the easy explanation is the idea of a false atmosphere.

As the carburetor operates in normal function the atmospheric air that it breathes in is considered slow moving but turbulent.Very low velocities are present in the atmosphere but they are random velocities. As you begin to tunnle air to the carburator you begin to acclerate it's velocity. High velocity or forced velocity inside of a carburetor is NOT good. With a large volume hat what happens is that the air coming out of the tube needs to slow down to fill the volume, also with the entry to the hat being at 90deg to throat now the air is slowing down and changing direction, further breaking up the velocity vectors that it developed in the inter cooler tubing.

If the hat is too small what happens is stream lines will form, continuous lines of high velocity, leading to higher velocities in the throat than are needed for operation and subsequently a stalled carb is what results.

As far as V8's go most what you have seen is probably a blow-through Holley 4150 or a a knock off design of the 4150. The design of the 4150 vs the DCO that you have (at least it needs to be a DCO and not a DCOE) is a comparison that you should not be making when it comes to setup for turbo application.

 

changes some vac/boost lines around.. no boost controller yet. Took it for a drive again and it seems to run pretty good. it does lean out tho at full boost(for now 7.5 lbs) but pulls hard. my afrs are in the 14s then and about 13 at idle. i got even bigger main jets coming.. we will see if that helps.. at a constant speed it jerks an hesitates a bit but i think its because my idle circuit to main ha too big of a jump... ill have to do some more tinkering.....

 

Still have a little herky jerky at cruising speed And still leans out with larger main fuel and or smaller air bleed jets. do you think i need to have the exhaust pipe connected to the down pipe to get an accurate reading? my O2 sensor is about an inch from the end of the down pipe and thats all i have on it for now.. would that matter. And as stated before i may have issues with my hat.. Is this a typical issue? That you cannot jet it rich enough because its just blowing may too much air through the carb and not slowing down enough to pull out enough fuel?

 

first thing you have to figure or fix is to having the fuel pressure increase when u hit boost , first thing are u running a fuel injection pump , are u running a boost dependant regulator ( dont hook up the regulator to vacumm you have to hook it up to boost only )
first fix this and then start by rejetting

 

walbro 255lph pump 3/8 feed and return lines. mallory rising rate fpr refrenced to the carb hat... i would think that this is all sufficient. let me know

 

any other ideas?

 

the o2 should as close to the exhoust housing for best readings , u should be able to buy jets from any local vw performance place call around im sure u gonna need a couple jet sizes to tune it , becouse if that the actual reading under boost u are still way off on main jets
can u post a pic of the air box is it covering both the throthe plates and the hole where the jets are "? everything should be pressurise for the set up to work

 

i was looking at the top picture it looks like u are running two trumpet from the carb to an air box if thats what u have then thats where your problem is, u have to pressurise the fuel bowl part for the carb the small hall thats next to the trumpets (throtle butterflies) u need to buy an air box plenum that incases the hall face of the carb pressurises the hall carb not just one side ,in other words the fuel bowl needs boost for it to send more fuel thru the jets