External wastegate - size and where to place

[soldave]

Firstly, if you think this is better suited to the Newbie forum then please move it over there. Right, well if any of you have been reading my tuning thread then you'll be aware of my current issues. Simply put, I'm running a Bullseye T04B 50-trim, 264 Comp Cams, meth injection. During recent tuning I've been getting figures close to 400whp but am suffering from boost spikes. I'm holding around 1.45bar/21psi but it's been spiking to 1.8-1.85bar/26-27psi. And then at the top end of the rpm scale the boost drops off a little to 1.38bar or so. Have ruled out the boost controller and the actuator has enough pre-tension so it looks like the internal wastegate might not be holding things, and I might have to look for external.

But this is something I don't know so much about. Would I be right in thinking I would need a 40mm wastegate if I was running 1.5bar/22psi and venting to atmosphere? My general driving is pretty conservative so when I'm in boost it's usually just a few small psi or it's all the way WOT. So I'm not bothered about a big spring. What kind of spring size would I be looking at though. Was thinking of something like a 10 or 14psi spring.

And finally, what's best for mounting? Would like to put it on my manifold I think as it would save money buying another O2 housing, but what would be best for eliminating the spiking I'm getting? My manifold is the Punishment Racing budget manifold (don't laugh - it's help up great so far) - can you mount an external wastegate to a tubular one like that?

Thanks in advance

 

[crimsondragon]

The most difficult thing about this is that you're an Evo I. I say post up a picture so we can get a general idea of how much room you have. From what I heard, bigger wastegates are for controlling low boost while smaller waste gates are for controlling high amounts of boost. Don't ask me how, I just heard it from somewhere.

As for mounting, some fabrication would be needed if you want to mount it on the manifold. Again, general fitment is an issue here. If you are absolutely positive the spike is not due to the BC then mounting it on the manifold is your best option.

 

[soldave]

Not the best shot, but here's a pic of my engine bay which should give you an idea on space available.

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I know some welding of a flange would be needed on the manifold if it was going to be mounted there, but as long as it is possible that's where it would probably go. And I have tried using my friend's manual boost controller which he loves, but it was having exactly the same spiking issues.

 

[mattdev]

I am using external 38mm gate at o2 housing and love it!!!

Wastegate size is mostly about boost control. A larger wastegate simply flows better and is that much more accurate at controlling boost. There are plenty of DUAL wastegate setups around for that SUPER accurate boost control (some don't even use boost controllers, only wastegate spring pressure).

And wastegate spring pressure is the minimum amount of boost it will open at (lowest boost possible). A rough guide to spring pressure: below 2x wastegate spring pressure is accurate boost control (ie 1.0 bar spring pressure, stay below 2.0 bar boost for accuracy) However you CAN boost higher than 2x.


There are a few options for external gate:

Manifold off runner #1- possibly the least desirable spot, as it only controls ONE cylinders exhaust flow. A new/custom manifold is required. (requires old internal gate to be welded shut)

Manifold off collector- the BEST spot, mainly because your old wastegate passage has no effect on exhaust flow, and collects the exhaust from all four runners. However a custom/new manifold is required. (requires old internal gate to be welded shut)

o2 Housing- not the most desirable spot, since your wastegate passage is still restricting exhaust flow. If you port the wastegate passage to the size of the external gate, it shouldn't be a problem. (requires old internal gate to be REMOVED)


BTW, awesome evo, keep it up!!!

 

[Black_Bullet]

+1 ^

And Soldave on Dsmtalk, in the parts review section, still towards the top of the page, I did a long review on my external gated setup. If you never read it check it out!

 

[soldave]

Thanks man. Appreciate it. I'm kind of understanding a couple of my friend's frustrations now though - as I'm making more power there are just more and more issues coming up and factors that become involved that it gets difficult and expensive very quickly.

No problem with welding the flapper shut or removing it, but need to work out which one to do. It's possible it could be done off the collector at the bottom of my current manifold if a flange could be cut and welded in. Would that work or would I definitely need a new manifold?

+1 ^

And Soldave on Dsmtalk, in the parts review section, still towards the top of the page, I did a long review on my external gated setup. If you never read it check it out!

Ever get the feeling we're talking each other on these forums?! Will go and look at that review now - thanks.