I was talking to someone I know who owns a Ford Tbird Turbo coupe and he was telling me about the bypass valve around the wastegate that he installed to increase boost. (just a hose that goes from one side to the other) He thought that I couldn't increase my boost by freeing up the exhaust because the wastegate is set at a certain level and it should always open at that level. Obviously he's wrong, but I couldn't explain to him why.

Could someone explain? Also, what's the deal with the bypass hose he installed, why doesn't anyone do that on RX-7s?

 

Either he’s explained it wrong or you’ve misunderstood him. You don’t bypass anything to raise boost, you bleed some of the pressure out of the wastegate actuator hose. this lowers the pressure seen by the actuator so it opens later, raising boost. An inline pressure-reducing valve can do the same thing.

 

If the wastegate was large enough, it would not have any problem bypassing at 10psi if you have a 10psi spring in it.&nbsp The "problem" with the stock FC turbo is that once the intake/exhaust is opened up, exhaust flow is so great, it overcomes the very small wastegate orifice.&nbsp This is how you get higher boost with better intake/exhaust efficiency.


-Ted

 

hey ted did you read the sport compact car where they put the turbo on the matrix? they had a problem with boost creep so they built a bulge on the downpipe/copressor housing so the wastegate flow has a more gradual entrance into the regular exhaust flow. do you think something like that would help the fc?

mike

 

yes that's what gary walker did if you know that name. you use the back turbo plate as the exhaust flange instead. and cut the hole bigger. I'll try and search for his pics.

 

In theory it should. We all know the wastegate port is small and restrictive, but the path the bypassed gases must take from the port into the exhaust is also very restrictive. This must also contribute to the problem, but by how much is anybody’s guess. It does cause turbulence at the turbine exit though, and that’s been proven to rob power.
When I do my downpipe, I plan to replace the backplate of the turbine housing with a custom cut flange so the downpipe can be expanded up to cover the whole turbine wastegate area. If you have alook at his photo of a WRX exhaust you'll see what I mean. We’ll see what happens.

 

ok I suck and can't remember passwords... for those that know it should be in the rxtech folder on yahoo GW_86drag/ex_coll.jpg if someone could get it LOL...

 

But here's the problem I'm having logically, if you exceed the wastegate's capacity (in your example 10PSI), why doesn't it creep untill something breaks?

NZ : From what little I was told about the ford 2.4L turbo, it seems to be more like a S5 turbo than a S4 in that it's orifice is larger. He is running 18PSI, stock was somthing like 12PSI. He seemed to have a line that would allow gas destine for the wastegate, to be used to power the compressor. I'm probably not using the proper termanology that would allow you to understand what I'm trying to say...

Thanks for replies.

 

Sorry dude, that makes no sense whatsoever...
Have a read of this article and see if you can work out what he’s done.

 

Moi?&nbsp Read SCC?&nbsp Bah!

It's one of those projects "shelved until further funding is secured".&nbsp Our designed would've replaced the existing backing plate on the turbo and run a funky discharge into the downpipe.&nbsp This would require a totally new downpipe or a two-piece unit.&nbsp Cost was pretty damn high, as the turbo backing plate is like 1/2" thick!&nbsp Cost to rebend the downpipe section would've added more to everything, and when we did a cost analysis, you're looking at a $500 piece of pipe!&nbsp Most FC turbo owners wouldn't be willing to spend that kinda money on something like that, so it was quickly shelved.&nbsp If you've ever seen the WRX turbo discharge sections, that's exactly what it does...

It would've been a lot cheaper to go the route tims did by cutting a hole in the stock turbo backing plate and slapping on a big *** wastegate.&nbsp The prices would've been similar, and the boost control more superior with the aftermarket WG retrofit.


-Ted

 

Yep, that’s my plan. I’d considered the superior two-pipe system, merging the wastegate pipe into the main one after about 400-450mm. Maximum benefit comes from extending the main exhaust through the flange to meet the turbine outlet. I still haven’t made up my mind which way to go. A machine shop is going to cut the flange to my pattern for a very reasonable price. I’ve got a mate who’s Dad’s a very experienced exhaust builder. I supply the pizza, beer and pipe!
One way to do it is to cut a section out of the side of the (say) 3” pipe, and using another cut sections of 3” pipe weld then all together to cover both turbine and wastegate port. Sounds rough, but careful measuring, cutting, bending and welding will make a nicely shaped piece.

BTW, how come quoting one of your posts always results in lots of “& n b s p” text strings scattered through it. What weird computer software are you using?

 

come on someone post my dam pic! ted must be on rxtech.

 

I'm **** like that. This forum script seems to strip spaces much like how HTML code does if you type it out like normal typing...two spaces after a period or colon.&nbsp I manually insert spaces with the "& nbsp" code so it forces the text to space properly.



-Ted

 

I'm actually not on that list. *GASP*


-Ted

 

*gasp* you're missing like 10 e-mails a day ted!!

fine I just e-mailed the list to get someone to send me the pics LOL.. but they are just what you guys are talking about but done to an FC.

 

Hmmm...I was thinking about trying this out. Basically just taking the exhaust housing, welding the stock wastegate shut and then cutting a section out of the back plate and welding a wastegate to the outside...or wherever fitting will allow...external wastegate on a stock FC exhaust housing...How interesting!

 

So how would the exhaust gases bypass the turbine if the port is welded shut?!
What I think you mean is remove the flap and enlarge the port as much as possible. You’d then need to completely separate the turbine exit and the old wastegate port from each other.
I’ve seen pics of this done on this forum before. As long as you did all the work yourself, this could be a good budget upgrade for the stock turbo.

 

You know what NZ...I'm pretty stupid

I didn't even "visualize" what I was typing...now that you've mentioned it...did I mention I was stupid

 

You think we didn't know that already?


-Ted

 

Damn it Ted...you're such a post *****

 

Ive got an interesting question. I know the wastegate is built into the turbo (ours anyway) and how it works. However, im wondering, what is the flapper in the manifold for, and how does it work?

 

That's part of the twin-scroll turbo system. The turbo's exhaust housing has two seperate exhaust paths leading to the turbine. By blocking one of those paths off at low rpm when exhaust flow is very low, the gas velocity is increased, applying a higher force on the turbine and causing it to accelerate quicker. This brings boost in quicker and earlier. Above 2700rpm this flap opens and the exhaust gases can flow through both passages.
While this system sounds like a good idea (I think it is), Mazda obviously didn't think so, because the redesigned 89-91 turbo didn't use it.

 

if you drive a twin scroll car back to back with an 89+, the 89+ has less lag. its a good idea, one that leads right to the twin turbos on the fd, but i dont think is very effective (imo). actually the most annoing thing is when the car suddenly gets louder when cruising

mike

 

Hmm. Thanks for the explinations. Another reason to run a S5 engine!