Draw thru with BoV

[Sin720]

So I had an idea for a draw through turbo with a BoV.

 

So rather then burst a bunch of atomized air/fuel under the hood of my truck. Why not setup like a egr so it would be a igr lol. So basically the Bov would release into a large diameter tube to displace some of the boost then back k around to the intake just before the turbo.

 

Now my only question is will the bov work not having a throttle body to bounce off of.

 

Or could I remove the carbs butterfly valves and use a throttle body at the intake?

[Kirden]

Sacrilege I know, but I usually trust the rx-7 guys.

 

http://www.rx7club.com/1st-generation-specific-1979-1985-18/do-i-need-bov-draw-through-system-768333/

 

That being said I don't see how your system would hurt anything, but the BOV may never open.

[Sin720]

I read the post but its not the same as my setup. It goes carb---turbo---engine but

I'm wanting to displace the pressure in a tank essentailly. Then feed a tube from the tank back between the carb and turbo.

 

Carb---Turbo---Engine

#####|#######|

#####|######BoV

####IGR######|

#####|________displacement tank

[Kirden]

That may not be the best idea. You would need to make sure that the pipe from the BOV is all down flow to combat any pooling of the charged mix. With the larger tube the mix gets cooler and slower so that could cause pooling. If the fuel starts to condense out of the charge in this pipe you would need a way to re atomize it or it will end up pooling somewhere in the system.

 

This was the same issue I ran into when planning my draw through supercharger setup because creating a downward flow all the way to the engine is ideal but very unlikely. The thought of fuel sitting in a charge pipe while the car isn't running scares me too much to try it. Personally I decided that a blow-thru setup would work for me, but the benefits of a draw through are very nice for low boost applications.

[Sin720]

Yeah I'd only be running 8psi as I'm only doing a stock rebuild on the engine. And I'm unsure how much this engine can handle on stock internals. And I see what you mean. I didn't think about pooling :/

 

I suppose if it ever misfired I'd have a bomb under my hood. Dam I thought I was on to something...

 

My pockets aren't deep enough for a blow through setup hahaha.

 

Would there be much back pressure to the turbo from the intake?

 

These aren't exactly high rpm engines so I wanted to rid as many obstacles that may lag the turbo.

[Farmer Joe]

with a draw through, you want the charge pipe that exits the turbo to be on the shortest path possible to the intake.

 

adding anything else in the middle with allow fuel to puddle up, and thats gonna cause problems with shit exploding and such. 

 

and to answer the question, a draw through doesnt need any kind of blow off or recirculation valve. when the throttle bladed on the carb close, youre cutting off the air at that point. 

[Kirden]

Yea, there is no need. That's what all the rx-7 guys say and I trust them :P

 

Side note, for low boost you can get the K&N turbo top hat for a 32/36. It's about $200 and the Aussie Ford guys run them pretty often. It even looks neat :)

 

http://www.knfilters.com/Racing/plenum.htm

[DaveZilla]

i'm sure you know by now that BOV's and draw thru setups don't mix... for several reasons... But, if you realy want that loud "whoosh" sound when ya let off the gas, just go blow thru. The BOV has more benefits than just keeping your turbo's rpm up between gears and I do consider it a necessary part of the system if you can use it. It also helps extend the life of your turbo, especially the floating bearings and seals since slamming the throttle shut while at boost really puts a huge load on the rotating assembly since now all the sudden all that air has nowhere to go so it stacks up and reverses it's way back to the compressor wheel on it's way looking to escape now from the compressor inlet.

 

as far as not having deep enough pockets to go blow thru, really, it's not that expensive if you can mod the carb yourself... and if you decided to use motorcycle carbs like the Keihin CV 40 from a Harley (any Harley from 1999 up to 2007), you wouldn't even have to modify the carb at all, the reason for this is simple, you add fuel pressure vai a boost referencing regulator so it adds 1 psi fuel pressure for each pound of boost pressure, this means that you set the base pressure to 3 psi, then as boost goes up, so does the fuel pressure. This is half of the story. the other half of the story is where you run a boost line to the bowl vent so it pressurizes the bowl equal to the boost pressure or maybe 1 psi or so higher (I can show you this trick if ya didn't already find it at the turbo forum). Now the fuel and the bowl are pressurized at the same rate that the boost is coming in and this pressure forces the fuel up thru the main jet so it feeds the engine the right amount. The only  thing you need to do here is get the jetting right as naturally aspirated, then connect the turbo to the inlet.

 

If you were to choose to use the Harley carb, it would probably be best to run 2 of them, but they can easily handle the 600cc's per cylinder size of our engines. The carbs are cheap, making an adapter is pretty easy, and you'll be able to safely run the boost as long as you don't let it go lean. Here's an example of this... it's just too easy... http://www.theturboforums.com/threads/363918-Suzuki-Samurai-t25-blow-thru-Keihin You can use pretty much any late model motorcycle carb for this and usually the crotch rocket carbs are the ones that sell the cheapest used on ebay.

 

The reason I'm choosing to go with a Holley for mine is because I got a Holley to do it with, and you can get a LOT of tuning options by setting up the power valve and the power circuit to come on only after boost is reached all while running it with stock sized jetting so your milage don't suffer during normal cruise, the Holley is also one of the easiest to tune as far as when ya need to tune just one part of the rpm range or boost level to set up a super smooth fuel curve. The motorcycle carbs can do this nearly automatically by usine the boost pressure from different parts of the system which makes them even easier if you can find some that'll work for the right price.

 

Just let me know what you come up with and I can either answer your questions or lead you to the right links for extensive reading etc... it's all easier than it looks.

[Kirden]

Davezilla, if you wouldn't mind a bit of spoon feeding, I would personally love to see some of the links you have. I've been looking into motorcycle carbs and that post is probably the most informative direct post I've seen as far as getting the boost setup working. Now if I can just figure out which main jet to start with maybe I'll be able to dial in the R1s :P

[DaveZilla]

I'll be back on later.. with a spoon...

 

One tip I can give you now is that whatever carb you decide to use, tune it to run best Before you introduce boost to the engine, this basically means get the jetting right on all circuits the carb has and once the carb is tuned to run at it's best, then you can connect the charge tube to the carb. I'd also strongly recommend getting a wideband to keep an eye on your air fuel ratios preferably before you buy anything else... it's That important to have your fuel curve proper.

 

For your jetting, I'd start at around 1.75 to 2.00 mm main jet to get it into the ballpark...  That should feed a 600cc cylinder pretty close at full throttle at max power