First thing is, I built a custom intake manifold and I have it for 4 days now. Today I noticed that if Im in 2nd gear at about 3k and I step on it the turbo only boost to like 3 PSI. Almost like something is being held back. The manifold I made is 3.5" square tubing, so its not too big. Any suggwstions?


The second thing will contradict the first, but bare with me. For now I am running the wastegate off the intake mani, so I dont go above 5psi. Well when Im on the gas hard, it will creep up to like 7-10psi. Ive checked the vac line and its not kinked or leaking or anything. What else should I be looking for? The wastegate I have is a Tial 38mm. The WG only has like 3k miles on it. Is it possible the spring is already bad?

Where are you sourcing the boost for your wastegate? If its hooked up to the manifold, I would try hooking it up to the compressor outlet instead of the intake manifold. Vice versa if its hooked to the compressor outlet. Its just a stab in the dark, I really have no idea, but my thought is that there may be some kind of pressure response interference or something like that.

These problems may also be because you're using a WG valve instead of a pop off valve. It seems to me that a wastegate doesn't have to open as wide as a pop of valve because it's job is to divert energy as opposed to vent excess volume. Its kind of a different beast. You may have better success using an actual pop-off or pressure release valve instead of a wastegate. Search http://www.mcmastercarr.com for part 48935K25 . Thats the 1/4" pipe version, but they also have 3/8" pipe and 3/4" pipe versions as well, and they're adjustable from 0-20 psi.


95 Eclipse RS : 5 speed
15.7 @ 87.9mph
Jeep TB writeup - http://www.dimensia.com:81/jimbo/JeepTBfor2gnt.html

Well it is hooked up to the manifold, so I will switch and just see what happens.

Thanx MetalJim

dj u got any pics of the intake mani that u made???
id like to see it

Haha, well I do, but MrSlick and I decided we should keep it under wraps till we have some actual results. Besides I have some things I wanna change. Like I think it actually might be to big, 3.5". So Im lookin into making it 3". Believe me, once we "see" that its actually an improvement, we'll let the cat out of the bag. I do feel an improvement, but other times, theres something I just dont like about it. Maybe the smaller one will be better.


Speaking of intake mani's, if anybody has like a Fast fabs intake or any other intake, can you give me the dimensions so I know what more to shoot for. Not copy, just get a better idea. Or like in my other post that got ignored, if any of you guys have any suggestions, lemme know.

You do know that using a intake vent(like you're doing) instead of an exhaust wastegate(on the actual exhaust side) can make the compressor less efficient once you hit max boost right?

I just want to make sure that you know that. If you want a detailed explanation of why this is true then let me know what turbo you're using and I'll make an example of it.


95 Eclipse RS : 5 speed
15.7 @ 87.9mph
Jeep TB writeup - http://www.dimensia.com:81/jimbo/JeepTBfor2gnt.html

Hmmmm, Im running a Precision SC61, A/R .63 and trim 76

Thanx for taking the time

Originally posted by DJ420a
Haha, well I do, but MrSlick and I decided we should keep it under wraps till we have some actual results. Besides I have some things I wanna change. Like I think it actually might be to big, 3.5". So Im lookin into making it 3". Believe me, once we "see" that its actually an improvement, we'll let the cat out of the bag. I do feel an improvement, but other times, theres something I just dont like about it. Maybe the smaller one will be better. Speaking of intake mani's, if anybody has like a Fast fabs intake or any other intake, can you give me the dimensions so I know what more to shoot for. Not copy, just get a better idea. Or like in my other post that got ignored, if any of you guys have any suggestions, lemme know.

I got some info now on optimum plenum size... we'll go over it tomorrow when you come up. ;)

I'm helping DJ install his MS tomorrow.... :)

Originally posted by MrSlick

Originally posted by DJ420a Haha, well I do, but MrSlick and I decided we should keep it under wraps till we have some actual results. Besides I have some things I wanna change. Like I think it actually might be to big, 3.5". So Im lookin into making it 3". Believe me, once we "see" that its actually an improvement, we'll let the cat out of the bag. I do feel an improvement, but other times, theres something I just dont like about it. Maybe the smaller one will be better. Speaking of intake mani's, if anybody has like a Fast fabs intake or any other intake, can you give me the dimensions so I know what more to shoot for. Not copy, just get a better idea. Or like in my other post that got ignored, if any of you guys have any suggestions, lemme know.

I got some info now on optimum plenum size... we'll go over it tomorrow when you come up. ;) I'm helping DJ install his MS tomorrow.... :)

YEEEEEEEEEEEEEAAAAAAAAAAAAAAAAHHHHHHHHHHHHHHHHHHHHH!!!!!!!!!!!!

Cool. Let us know how the MS install goes. Lee's the MS man so it should be all good. :thumbsup

Well thats a nice big giant turbo you have. Corky has the simplest blurb about this issue in chapter 12 - boost controls. He says "Boost pressure can be vented after work is done to create it. Effective, but a bad idea." and he goes on to say that they're best used as overboost safety devices. Its a very opinionated section and doesn't really explain too much in the "why" department.

The interesting side effect of venting the compressor for boost control is that you can't read a compressor map the same way you would if you were using a wastegate. As a matter of fact, it kind of blows using a compressor map out of the water. I think the best way to explain "why" it may be less efficient to vent is to go ahead and take a look at whats happening on the compressor map. Shame I can't find a compressor map on that thing. I'll use a 20g map cause its plenty big. Not as big as the 61 but hopefully you'll get the idea at least. For these examples I'll use 7000 rpm, 100% VE, and a max boost of 12 psig to keep things simple.

With a wastegate control, the engine demand line for any particular RPM follows a linear upward course and ends up as a finite point at maximum boost. The efficiency points are finite because the wastegate keeps the compressor speed in check and only allows it to spin as fast as needed for the desired boost level. In this example, efficiencies are plotted from 3 psig to 12 psig, where we ultimately end up at 77% efficiency.



With an intake vent, engine demand and boost level are used as boundries for an efficiency section. In this case, efficiency is not finite because the only thing we really know is the LEAST amount of airflow that is required to sustain the desired boost level. In this example, we're looking at what the possible efficiencies are at 12 psig only.



Thats the reason I said that venting kind of blows using compressor maps out of the water. They become unusefull basically. But they do show why it's not really a good idea to vent the intake for boost control. The efficiency section will always go off the map, and you already know if you get off that map the efficiencies just plummet. There's no telling how much work the compressor is going to do. You can measure the efficiency by looking at the intake charge temps, but what you'll find is that the efficiency is variable for every scenario. Some of the variables are how long you've been at max boost, and how much extra energy the exhaust can generate at the turbine while at max boost. I'll also say that all of this hoo-ha is theoretical and I may have the section drawn incorrectly. I drew the left boundry as a continuation of the engine demand at 7000 RPM. Maybe a more realistic view would be that the left boundry follows engine demand initially, then starts to head east until it plateaus at a pressure level that can be sustained by the exhaust energy generated at max boost. <EDIT>Thinking about it a bit more, maybe an even more realistic view would be that the left boundry goes straight up at the specific CFM for the engine demand. </EDIT> Thats not so important...the important point is that there is no precting how efficient the compressor will be.

With that big ol snail you got, it may never go off-map. Hell, it may even be more efficient in certain scenarios. Who knows. But in general, the fact that the efficiency is truely variable mixed with the possibility that the compressor even has a chance of going off-map and getting so very inefficient just makes it a bad idea. Corky may not have explained it too well but I do agree with him on that opinion.


95 Eclipse RS : 5 speed
15.7 @ 87.9mph
Jeep TB writeup - http://www.dimensia.com:81/jimbo/JeepTBfor2gnt.html