Question on high-end boost
 

First off, here are my mods: '88 TII, great condition 74k miles, good compression, custom 3" dp/mp, GT spec catback, racing beat FCD, ACT 6-puck w/ 17lbs flywheel, greddy boost and a/f guages. It has all new vaccum hoses after the TB.

On to the question. Right now my max boost is .9 bar, or around 13 psi, but it spikes this high at around 5500 rpms, and tapers off to around .5 bar (~7psi) by redline. It is VERY noticeable as far as the pull of the car. From 3-5500 there is a huge hit of torque, thereafter it slowly (not suddenly) tapers off and evens out at .5 bar... doesn't feel slow, but it does feel like it drones on at higher revs, instead of pulling as hard. I have a few theories but they seem kind of dumb. First, is the wastegate is opening late because of the sudden spike of boost, but I would imagine it would open sooner than it does. Second is the wastegate is not channeling ENOUGH gases away from the turbine, and at high rpms the backpressure on the engine is too high and it chokes itself. The least likely cause is the turbo itself, which I've inspected on both the intake and exhaust side and it is in great condition.

I plan on getting a electronic boost controller, upgrading the fuel, injecting H20 into the intake for added saftey in the future. I'm wondering if anyone has experience with this type of problem. If it's the first problem an EBC should solve it. If it's the second, then an EBC and some hefty wastegate/manifold porting will solve it. If it's neither (ie. something to due with fuel/ignition, that I'm somehow missing) then I'd like to know about it. :)

Edit: Another thing. The A/F mixture does get a bit lean at .9 bar and then richens well as boost tapers off, which leaves the secondaries out of the equation, I believe.

Thanks in advance for any replies.

The EBC will solve the problem.&nbsp The stock wastegate SUCKS! :D



-Ted

That sounds like normal behavior for a stock turbo being pushed well past its design limits. It's common to drop boost at high rpm when past peak efficiency, and I doubt an EBC will help. At 13psi the stock turbo is really only adding heat instead of power, but without porting the waste gate I don't think you'll be able to lower it with those mods. As long as it's getting enough fuel, I'd just run it until the turbo shits itself (it will, soon) and then upgrade it.

Ted is the MAN

I agree the stock turbo sucks, but think of this. The stock turbo maybe thermally inefficient at 13psi, but the car is NOTICEABLY more torquey at .9 bar as I mentioned, meaning it is not so inefficient that it is just superheating the intake faster than it can compress it. It's also very chilly here right now, and my stock IC is in great shape. Now obviously from what I have stated the compressor will do fine at probably 11-12 psi constant, but the compressor is not at fault for my lack of boost up top, if anything it's the exhaust. The ability to make 13 psi at 5500 indicates to me that either the wastegate isn't opening quickly enough after it spikes, or as the RPMs increase the exhaust is not channeling enough through the small .88 hotside or the small wastegate. I agree that a much larger exhaust housing is needed, but in the mean time I think if I port the wastegate to death, and get an EBC then I can run a decent 12 psi consistant, at least until 7k rpms. For now, I would be happy with that.

Question for TED: I've seen your write-up on series 4 turbo wastegate porting. My question is, do you notice a difference in controlability of boost, and it's ability to boost higher up? If the answer is yes, then I will pull the turbo off and port it.

I thank you both for your input.

Dorito-When I made the 3" TID mod I noticed the drop off in boost at high rpm was much less or not at all. I ported my W/G alot, so much I added a new flapper seal and back cut the port as well. It would boost around 7psi except at high rpm in 5th and it would creep up. Your fuel mods seem low for the high boost you are running, I would be very careful.

Thanks! What materials/process did you use to make the TID? Yes, I know I'm getting into danger territory now, and as I said I will be adding fuel/water injection when I get my EBC. Right now I only run the car hard for short periods of time, as to not allow prolonged stress at the higher boost levels. At least enough to burn away carbon, and not enough to cause detonation... YET ;)

Thanks

I picked up a 3" alum tube w/ 30 deg bend and sil. 2.5" to 3" adapter and 3" to 3" coupling from local auto parts store, tapped it for brass pipefitting for checkvalve and made a checkvalve for BOV. http://www.rx7club.com/forum/attachm...&postid=296094 for pic.

I'll get right on that, thanks for the tip. Looks very profressional. I need to find a cone similar to that K&N's are just too huge. Anyways thanks again.