Rear Mount Turbo VQ35 Swap
After racing the VQ30 a lot I eventually needed to replace the head gasket. Since this was my first experience tuning a car, the engine spent a lot of time knocking initially. I wrote an article about my tuning experience here. Over time I noticed that at 10psi the coolant reservoir would over fill, this meant that the head gasket was starting to fail. I then replaced the gasket and continued pushing the VQ30 passed 10psi. The engine started to also develop additional blow-by because the rings were not sealing properly. I believe these failures were due to my tuning experience, I am not upset nor do I regret this as this was part of the learning experience for tuning a turbo car.
I then did the VQ35 swap while using the 3.0 timing equipment. The engine I used was from the junk yard and only lasted about two weeks before a rod broke. When I finished the swap I always noticed a feint tapping sound and VQ’s being noisy engines I somewhat ignored it but still acknowledged it (maybe I was in denial). Then while cruising down the highway without putting it through any abuse the tapping turned into a loud knock which eventually went silent. The moment it went silent I knew something was about to happen and then BAM, a rod broke. This is why I will never trust a junk yard VQ engine, specially since we are seeing blow VQ35’s being a trend now when put under some significant boost. My decision was to build the bottom end of the VQ35.I will write an article about it because it was a fun journey where I learned a lot and I know many of you will find it valuable.
Most of the videos I have as well as races at the track as a rear mount turbo were all with the built VQ35.
Turbo Compressor Surge
Something I experienced was what they call surge (flutter, among many other names). That is when you let off the gas, the throttle closes, and the pressure does not escape out of the charge pipe fast enough causing the pressure to back up into the turbo compressor wheel. The sound produced to some may be cool, but it is not good for the turbo, some turbos are designed to handle this but its better to not have this at all. This cause throw off the balance of the wheel and shaft. The blow off valve that I had was not able to vent enough so when I would let off you could hear the flutter (sh sh sh sh sh sh). I am not going to lie, I loved the sound. What I did was add a second blow off valve to the back near the turbo, then a hard line coming from the valve to the intake manifold. Now when I let off the gas, both blow off valves open venting the charge pipe immediately.
I also learned that it was important for the blow off valves to have a good source of vacuum, and having a line that would not flex or collapse. This way the response and volume pulling the valve open would be greater. A thin line with vacuum is far weaker than a large line with vacuum.
Self contained oil system for the rear mount
One thought I had was to have a self contained oil system for the turbo. This is when the turbo oil is fed from a tank through a pump. The pump pulls oil from a tank, sends it to the turbo, the turbo then drains the oil back into the tank. I spent a lot of time thinking and planning this out but at the end of the day, it was not worth the effort. You have to take care of cooling the oil because with every pass it gets through the turbo it is heated. You also had to possibly vent the tank.
Now the tank or container can be below the turbo so that the drain naturally flows into it, but being a rear mount turbo means that the turbo is already pretty low unless you have it way up in the bottom of the car. This means you would need two pumps, one to pull oil from the turbo and feed it into the tank, and another to send the oil back to the turbo. Now you have to take care of the pressure being sent to the pump and regulate it. All this was too much trouble to really have little benefit in my opinion. Having a line from the engine to the turbo, then a line from the scavenge pump to the engine was far simpler and effective; possibly even cooling the oil on the way back (although I didn’t do it with this intent).
What are the costs involved?
A turbo can be had for any price to be honest. I started off with a hand me down which was a T04b T4 .69ar turbine turbo. Then I got a genuine Holset HX35 for less than $300, and eventually ended up with a Holset HX40Pro Replica for also less than $300. I put a lot of miles through these turbos, over 100k miles on the first engine build and the only issue they had was due to user error when I made a mistake that caused the oil feed to be restricted, running the turbo dry. Here is what that sounded like:
And this was the damage:
I mention this failure in this section because once you turbocharge a car, you will have issues come up, specially if it is the first turbo system you have worked with. So yes you can have an initial parts list all priced out but you also have to think about what could come up later on. Now lets get into the parts list.
The parts list would be the same as a front mount turbo.
- Turbo ($300-$2000)
- Wastegate ($250-350): I did not want to go cheap on the wastegate as it is an important piece, I got a Tial 38mm.
- Blow off valve ($25-$200): this I did go cheap with and I used an Turbo XS RFL which was very loud and fun to have, however by design they leak since they do not have a diaphragm, its a metal piston that does not seal completely.
- Scavenge Pump ($100-$200): I used a Shurflo 800 series pump, two of them. A gear driven pump is said to be better for this occasions, those go for about $400-$600 at the time I was looking. ebay has them nowadays for less than $100 but I have no experience with them.
- Intercooler ($100-$300): I started off with an eBay intercooler, then upgraded to a better quality one.
- Oil Sandwich Plate ($10-$30)
- Stainless Steel Braided Oil line ($20-50): I used a 4an nitrous line.
- 3/8th oil line (transmission line works): This line is for returning oil to the engine as well as pulling from the turbo and into the pump.
- Charge pipe ($80-200), that can vary greatly depending on how you go about routing and material.
- The rest is miscellaneous parts and labor such as welding, flanges, gaskets, etc…
Turbo sizing specific to the rear mount
After using the T04b turbo, I then moved to a Holset HX35 turbo. I did research on the Holset’s and saw that one of my favorite platforms (the DSM’s) used them a lot. I got on eBay and came across a brand new genuine unit for about $235, I had to get it.
The HX35’s are ideal for mid 300whp, once I wanted to make more power I went for an HX40Pro. The Pro means it has a larger more efficient compressor wheel that helps produce more power. These turbos come with a large turbine housing with over 1. AR (18cm^2). They do not spool very fast but work great overall. I saw that the DSM community likes to put a T3 .70AR Bullseye turbine, now this my friends really transformed the performance of the car; the turbo spooled immediately.
This is how the turbo performed with the large Holset 18cm housing before the Bullseye housing.
This was my reaction on the maxima forum after putting the Bullseye .70ar housing: “Got the .70AR housing installed and HOLY F***. I can get full boost in 1st gear (15psi, even with no load while spinning lol). in 2nd and 3rd boost comes in like if it was a nitrous shot, full boost at about 4200, spools up then BAMMMM no turbo sound or anything straight up WG dump and pshhhhhhhhhhhhhhhhh. LOL. pretty wild. It spun the tires at around 60ish in one instance.” ; you can see I was overly excited with how well the turbo responded.
As you can probably notice, most of the build was on a budget, I did not have the freedom to shop for different turbos. However, I did come to a conclusion. VQ’s do not like T3 turbines in my opinion, VQ’s flow a lot of exhaust and require a T4. The T04b T4 turbo I had at 16psi on the VQ30 outperformed the VQ35 with a T3 Holset housing. Let me show you what led me to conclude this, observe this dyno chart below.
Do not let the dyno numbers mislead you, this is about mid to low 400’s, they show as 300’s because the dyno was done with the torque converter unlocked (this is why I do not dyno very often, they do not add any value other than a safer tuning environment in my opinion). Notice that around 5500 RPM both dyno runs end up at the same power and torque output roughly. One was at 14psi and the other was at 18psi yet they both choke at 5500 RPM. I did a lot of research and asking around and concluded that the T3 housing was the cause. So my advice to you is, start with a T4 turbine and forget about being able to spool quicker with one, once you get the urge to gain more power you’ll regret having used a T3.
Here’s the video of the Dyno. I never recorded a video with the Bullseye housing but it was about 1,000 RPM sooner in spool up with it.
What would I do different from the beginning?
Nothing, my experience led me to learn all aspects of the turbo system and near the end of the rear mount turbo journey I feel I had perfected it. I have no doubt that had I kept the rear mount and used the proper size turbo, it would have performed as well as my front mount does; you can read about the front mount build. https://www.fastmaximas.com/2018/12/27/streetzlegend-frontmount-turbo-build/
Rear Mount vs. Front Mount
After years of using the rear mount I eventually changed to a front mount turbo. I have been asked many times why did I make this change if the rear mount turbo worked well. My first reason was because I wanted to learn how to weld so I decided to fabricate my own front mount system. The 2nd reason was weight reductions, getting more serious with racing meant weight needed to be addressed on an already heavy automatic GLE Maxima. The rear mount turbo specific parts were 80lb’s in total in comparison to the front mount. Although there were benefits in going with a front mount turbo it was never a thought nor a plan todo so. I received a welder as a gift, the maxima was no longer a daily so I jumped into putting together a front mount.
When I changed to the front mount using an HX40Pro Holset turbo the spool was similar. At the time I had a .70ar T3 Turbine housing which meant the turbo spooled up very fast both in the rear and the front. I have come to the conclusion that the difference between having the turbo in the front vs. the back does not affect the spool up as much as one would think.
Would I do another front mount? Absolutely yes, I still want to see what a rear mount turbo setup can do with a proper size turbo and I know some of you are working on it or are planning too, so I am excited in sharing this information with you.
I believe my best was 12.8 at 117MPH in the 1/4 with Holset turbo at around 20psi
I hope you have enjoyed this article and if there are questions left unanswered do not hesitate to contact me through social media, Facebook (http://facebook.com/fastmaximas) , Instagram (http://instagram.com/fastmaximas) or email at firstname.lastname@example.org. Fastmaximas.com also offers nitrous, intercoolers among other parts and drag racing wheels https://www.fastmaximas.com/shop
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