MyE28.com

Hi guys, I have an e23 with a forged internal M30B35 engine built with an MLS head gasket, head studs, a TCD exhaust manifold, a T04E-61 (T350 wheel) turbo, 3" mandrel bent exhaust, Megasquirt, and wasted spark. It has a 265/6 trans. I have a Tial F38 wastegate that is brand new. This car is water/air intercooled with an ancient Cartech setup.

When I had this car running under 12 psi on wastegate only, it actually ran okay. It pulled smoothly and didn't do anything weird. Now that I am turning boost up over 12 psi (I tried up to 15-16 psi) I am having terrible compressor surge under power (part throttle and WOT) above 3,000 RPM, when the turbo is fully spooled. I tried turning boost up two ways - 1, using a DIY Auto Tune electronic boost controller and tuning, and 2, using a stiffer wastegate spring but leaving the boost controller off. The car is also extremely touchy trying to tune w/ the boost controller. Increasing the duty cycle above zero in the initial duty table always results in hitting boost cut.

The compressor surge is audible as a flutter under power and you can feel the engine isn't delivering power quite right, and the boost log has a pronounced ripple in it.



Is it possible this turbo is too big for my engine? I assume T04E-61 means it's a 61/82mm wheel but I haven't measured it. My very limited understanding of turbo builds is that if I'm getting surge like this under power, the turbo is making more pressure than my engine can use. Especially since getting the electronic boost controller involved at all just makes the boost rocket out of control.

Something else I note - I make more boost than the wastegate spring rating with the electronic boost controller turned off. On a 13 psi spring in a new (genuine Tial) F38 valve, I make 16 psi boost. The dump pipe coming off the wastegate is a 1 3/4" mandrel bent pipe with a nice smooth bend. It is dumping into the exhaust, not a screamer pipe. I do have a catalytic converter, a spun metallic race 100 cell free flowing unit.

The turbo was rebuilt only a few hundred miles ago (when I did this build) and was balanced at the time of rebuild.

If this turbo is too big, what should I replace it with?

Here is my engine bay. The Cartech intercooler might look retro-cool but I hate how it is in the way of the valve cover.



Thanks

Steve

I just picked up a turbonetics unit for my m30 e28, 60/65 we will see how she goes.

Your turbo does sound a little large for a 3.5L

I was doing fine with a GT3582R on a 2.7l M20, that's a 61.4mm compressor. For the short period of time I ran that system on a 2.5l M20 it would surge in high gears on spool-up, until the revs reached a point where the engine was moving enough air volume to get the compressor into the map. Would only really occur on the highway in 4th or 5th gear.

I don't know if a compressor map is available for your turbo so it's hard to tell for sure but in any case it seems unlikely that it's going to surge on an M30 even at 3000 RPM. If it is surging it will only surge until the engine begins moving enough air to get along with the compressor. By 4000 or 5000 RPM it's not going to be a problem.

I think I identified the problem. Looking closer at some logs where I roll into boost in a higher gear, I only start getting the boost surge above 13 psi. I think the ancient, piece of crap Cartech intercooler I have, which has its own bypass valve, can't hold 13+ psi. It was probably originally intended for low boost applications. It looks like the bypass starts fluttering open above 13 psi.

I ordered a Turbosmart BOV and am going to replace the bypass valve with it and see if that solves my problem.

Will report back w/ the results. Thanks

I took the 1980's cartech bypass valve (that I had vented to air as a blowoff valve) apart and found the rubber diaphragm was blown. I gutted the bypass valve housing and used its 1.25" outlet with a 1.25-1.5" adapter hose to hook up a Turbosmart V-port blowoff valve. I am still getting a fluttering sound and a ripple in the boost log when the turbo is fully spooled between 3000-3500 RPM and most noticeably at heavy part throttle. Full throttle and above 4000 rpm the boost log is smooth. So this does appear to be compressor surge and the only thing I can attribute it to is the turbo is a little big for this combo.

I am going to have a pro dyno tune this and see what he thinks about the compressor surge and whether its worth changing to a different turbo over.