Thanks for the added input, fellas! I agree, a larger wastegate a-la got...diesel? is in order. Unfortunately, I just sold my 18.5cm housing, so I'm on the lookout for one. Heck, if anybody's got the lag-pig 21 laying around, I'll even run that while I build the 40mm wastegate for my 12cm! I wired the wastegate wide open and, while it builds slowly, boost just keeps coming up. I let out at 38psi. I really thought that with all the material I removed that the 12 would flow more....back to the drawing board

 

A bigger housing will not help with the surge at 30 psi. It might keep you from getting to 30 psi, but it won't let you run 30 psi of manifold pressure without surging -- it will change your drive to compressor ratio, but it won't cause a big increase in the mass flow of air through the engine.

What bothers me is that your compresor is surging at 30 psi and 2500 rpm. You should be able to support 40 psi at 2500 rpm without surge -- lots of folks are running in that range -- so, is it because you don't have an aftercooler, or are am I missing something else?

 

I dunno, Alec. I'm confused, too. There's a lot of non-I/C trucks running around hotter than me with no reported problems. Thought I haven't completely removed the HTT setup, I've pulled the fresh air tube a couple times to take a peek. The turbo seems in good shape with zero in-n-out play and nothing to speak of in side to side movement. The truck runs great and when I pry my eyes off the boost gauge, I don't hear anything out of the ordinary coming from under the hood. I wound it up to about 2700 in 3rd on a hill and punched it. Boost went from 18 to the moon and started with the needle bounce routine--that had to be about 2900 or so. Definitely not rpm dependant. Could my tired old injectors have anything at all to do with this? They're definitely not as smooth as the EDM's that were in there...

 

When you put the 12 cm WG housing on, did you drill the center divider through the WG hole? That was reccommended to me when I did mine.

Edwin

 

I really think that the surge is coming from the ex side!

OK this is what i have been moling around in my head.

If the boost volume is hi, the sylinder fill is hi witch is good!
Then the compression,fire and of cores ex. (air unable to exit fast enuff,even with waste gate wide open)... .....this is were the air is backing up in the manifold. The air is still being compressed do to the piston still moving to the top of the block.
the over lap of the cam is were the flutter is happaning.
K lets asume that i'm right and the ex, manifold has more pressure then the turbo is boosting.
now as the ex.valve is closeing the intake is starting to open for the next cycle of air but sence the air pressure is higher in the ex.manifold and the air returns back throw the ex,syl,intake and to the turbo compressor, ex valve closes and the air path returns to normal intill the next sylinders ex.valve is allmost closed and intake valve just starting to open!
Inlarging the waste gate exit hole should help but how much can you inlarge the the hole before the waste gate valve fall thru? I really haven't payed much atention to the fuw wasted housing that i have seen.
May be way of base but make sence to me!!

 

Sounds to me like your trying to say the wastegate isn't large enough and its backing up the whole intake/exh of his system all the way back from the manifold back to the comp of the turbo

 

Edwin: I drilled the divider when I first got the housing from Dave. I even went a couple bit sizes larger. On top of that, the entire center web of the housing has been removed, so I'm definitely wasting all six.

Flash:I see where you're going with that, and I tend to agree. Alec, you wanna weigh in on that theory? As far as enlarging the gate, Got...Diesel? made a special 40mm flapper/linkage to make his work. As soon as I can find another housing to run in the meantime, I'm going to get with him on how to go about making one for my truck.
Reading here:
https://www.dieseltruckresource.com/...ight=wastegate

 

Thanks! vary good reading.....it still would be nice to have some kind of guage(pressure) that you could put in the ex.manifold that could read psi with out menting down and a 2nt one in the intakes system somewares for comparasome.
I'm shore(that is if my theary is right!) that the waste gate mode will fix the probem but it would shore be nice to be able to see ex. pressure "before and after." Ya ya ya I now eze for me to say you'd have to do all the work will i just keep throwing ideas at yat

 

Well, I jsut went and did all the work I got the copper line/compression fittings from the 'hot rod tough guy' section at Advance I plumbed it into my exhaust manifold and ran it to a spare oil psi gauge that's sitting in my dash doing nothing . Results are mixed, but I've got more testing to do. The drive pressure gauge bounces around a good bit, but I've never seen one work before, so maybe that's normal With the wastegate locked closed, drive pressure/boost pressure ratio is pretty much 1:1 right on up. Still got the blurry boost gauge over 30psi. Next, I completely disabled the wastegate and took a spin. I didn't spool it to 30 'cause I ran out of road---it felt like the 18cm was on and there was a lot more power. But, without my pyro, I didn't want to push my luck. At any rate, the jury is still out and I've got more testing to do yet. I'm going to try and plumb the drive pressure in permanently...maybe in the housing itself, since the divider is missing anyway. The copper line hols up well and only transfers heat for about 5-6 inches. after that, it's cool to the touch for the rest of the run to the gauge.

 

Vary cool!!!! look like my theary is starting to get some holes in it thow!
editShouldn't the ex pressure be lower then boost to keep the poles or flutter form happing?

 

I think 1:1 is pretty good, but I've got more reading to do. I'm sure those more thermodynamically inclined than I will chime in before too long

 

I hate to suggest it, but you should try a run without the air filter. Air restriction can cause a nasty surge, I know the Schwitzer on the work truck surges badly when the filter minder gets sucked down below 1/2. Rig a screen to keep big rocks and small animals out.

 

I'll give 'er a go--now when WAS the last time I changed that BHAF?

 

Have you checked the diaphram under the smoke screw? Just a thought. Maybe there's some sort of weird oscillation causing the fuel to vary.

Edwin

 

I just had the diaphram out and it looked fine, but I may plug the boost feed line on my next drive.