Boost on N/A cars (LONG)

There has been so much talk here about boost and one kit Vs another that I thought this might be an important thread. I actually find it funny some of the comments I have read. Some people have said that boost isn’t relevant (LOL ::)) then some wonder why one kit makes more then the other but still the lower boost kit has same or better power.

A very smart friend of mine has always said. If your going to post something to help everyone make sure everyone can understand it. Due to that Im going to try and keep some of this more simple then technical. This way we can all play along. No sense on going All AurtherPE on everyone where I look like a fool trying to look smart and no one follows along.

With that here is the post key. Some shortened abbreviation that will be found in the thread.

FI=Forced induction (units) This will be the blowers
TS= Twin screw blowers it is a type of blower that in generally replaces the OEM intake manifold.
CF= Centrifuge blower. This blower is more like a gear driven turbo.
CFM= Cubic Feer (per) Minute of Air. This is a measurement of air volume. More CFM=more power

BOOST is a measurement of work (resistance actually). So its always very relevant. Boost tells you how hard the FI (Forced induction) unit (in this case FI units=superchargers) is working to deliver CFM (CFM=Air)

So if you have one kit at 15lbs of boost and another kit at 8lbs of boost can they still be making the same power can they still both be flowing the same air. YES. One kit is just flowing more air more EFFICIENTLY. So there is less work (boost) needed to push CFM in one system Vs the others.

Lets use some examples.

We will talk about 3 kits and explain more.

1 A Vortech C-fuge
2 A twin screw 1900
3 A twin screw 1750

Lets take and see that all kits are in the same general performance range

1 11.1@ 125 @8lbs of boost
2 11.3@ 125 @ 15lbs of boost
3 11.2@ 125 @ 14.5lbs of boost.

All have about the same 60foot and MPH shows a good standard for power. So why are so many scared to talk about boost. Every one knows that C-fuge (CF) and Twin screw (TS) blower deleiver boost differently but why is one kit able to do the same performance with WAY LESS BOOST.

Here is why.

One kit is Pre throttle body. This is good for efficiency but it also lets you use the OEM intake manifold on the RS4. An RS4 intake manifold that FLOWS GREAT. The RS4 intake manifold flows so good you don’t have to work as hard to push air into it to make power. Vs the TS(twin screw) kits. This is a real good thing.

Why do the TS kits need so must boost. Simple. Take the blower off the intake for the TS kits and dyno the car. The car will not make anywhere near what the car makes with the OEM manifold. You will be down big power. The TS blowers take up TONS of space so your TS blower intake manifold isn’t going to be as free flowing or have the runner length that the OEM RS4 intake manifold has. The OEM RS4 intake manifold has on the long runners over 3 feet of runner. Longer runners and a better flowing intake is a good thing.

So right off the start the RS4 OEM intake manifold is WAY better so you can flow more air through it easier. So less boost needed.

Is this to say that the 15lbs of boost is bad. No. Actually I want to hear the PRE intercooler boost for the 1750 kit. With the TS kits you also have the fact that the TS intake manifold holds the intercoolers. So not only is the intake going to be a restriction you have to push past the intercoolers. Here is where some Pre intercooler boost rise is ok. If you have good intercoolers they cool the charge a ton. Sometimes one of the effects of better intercoolers in the intake manifold is that they slow the air down enough to effectively cool it. This can RAISE Pre intercooler boost.

At this point we can see why a CF kit using the better flowing OEM intake manifold would need less boost. That is just one part of our story.

Even tho you have different intakes you still are putting the CFM into a 4.2L motor and the end restriction is the cylinder heads and the cylinders. SO having a boost difference from 15 to 8 is a big deal.

Lets take in a big breath here and realize something. The boost is just a measurement of work. The next biggest issue is how much of that work or how much of that CFM are you capturing. How much are you actually getting into the cylinder to help the power. You can make 15lbs of boost all day long and not capture all of it. Why are some cars (PES) so slow with big amounts of boost. They are not capturing quality CFM. In theory you can actually push most of you boost right out of the cylinders into the exhaust system when the intake valves open.

So in theory here you can have a car with 4lbs of boost making more power then a car of 8lbs of boost if the car with the 4lbs of boost was capturing all of it and the 8lbs of boost car was bleeding off more then 4lbs of boost off. This is where tuning comes in. Capturing the most boost possible and keeping the boost pressure down is obviously the main goal.

So different systems. differnt tunes. different intake manifolds and different blowers with different cooler units are going to effect boost. Is that to say that its irrelevent. No Its the most important measurement. We know boost tells us how hard our FI unit is working to deliever CFM. We know that different intakes and tunes and coolerchargers will change this boost on any given system. STILL boost is very impornant.


With more boost also comes more heat. and HEAT is bad for motors. Boost also tells you what the efficiency range is of your FI unit. If your working your FI unit too hard then your just pushing HOT air and that makes every lb of boost worse. You can get 15lbs of boost but if the air temps of the boost are 250deg your doing some damage. If you get 15lbs of boost and your air temps are 100deg then you really making clean power.

This is why intercooling is such a big part of this as well. The TS blowers have a big dissadvantage that they sit ontop of the motor. The cylinder head temps are over 250+deg most of the time. This creates a huge issues with heat. SO when looking at one TS kit Vs another the big factor is intercooler…

The story isn’t done there. Boost is impornant but so is the blower.

Lots of people say TS blowers are better then CF blowers. Then you have the CF blower camp saying the CF blowers are WAY better then the TS blower.

We know why a TS blower would make more boost faster then a CF blower in this case of the RS4. We know that the RS4 intake manifold flows better then the TS intakes. So the TS blowers have to work harder (make more boost) to flow some of the same CFM. This isn’t to say that TS blowers don’t or can’t make more CFM at certian RPM ranges. But lets get back to boost and units.

We know that the units are the

CF Vortech
TS 1900
TS 1750

It takes power to make power. When you look at the TS vs the CF units the first thing to note is that the CF units are easier to spin so right off the bat it takes less power from you motor to move the CF blower and its CFM. The TS units take a bigger amount of power to drive then the CF units but the TS units have a different power curve then the CF units. While the TS fan will tell you the TS unit makes more CFM down low Vs the CF units. The CF fan will tell you that CFM costs your motor more power to make. So its kinda off set. I’m not going to fully debate the CF Vs TS here as that debate has been going on for years. The point I was looking to make was this.

Bigger isn’t always better. There was debate on why one company would want the 1750 vs just putting the biggest 1900 blower in its spot. Isn’t it better to put the biggest blower in its spot why is one company making XX power at 15lbs of boost on the TS and another making XX power on a TS blower at 15lbs of boost. Other then the boost measureable vairables we talked about it comes back to efficiency.
The 1750 units are easier to drive then the bigger 1900 units. So less power will be taken from the motor to make the power. The CF units are easier to drive then the TS 1750 units so that will require less CFM to make up for the power to drive it.

Lets look at that in this fashion.

If a CF blower can make 100CFM and it takes 10hp from the motor to drive
and a
TS blower can make 100CFM and it takes 20hp from the motor to drive that. Then you need to make more CFM or boost to make up for the power you lost to drive the blower to make the boost vs the CF units.

I think this is enough for people to think about and look over. This isn’t ment to take every single thing into account. This isn’t to say that one is better then another. This is here to help say why boost is impornant and help explain why the range can be so wide. TS blowers have boost come on quick almost instant but all that comes at a cost.

Am I saying that 8lbs of boost is the same as 15lbs. No I don’t know that. I can just see the performance is close and this should help tie this togher for those that don’t know how or why that is possible.

As a last bit. Cylinder pressure is cylinder pressure. The RS4 motor was never built for boost. Nothing about that motor is set up for this positive charge pressure. Don’t let ANYONE tell you that adding pressure to this motor is not going to have its limits. I can tell you from someone that has taken down several 4.2 motors and many boosted 4.6 cobra motors. THERE IS HUGE differences in how a boosted motor and a NA motor is built. The RS4 is not built for boost. When you cram CFM into a motor weither its at 15 or 5lbs of boost its dangerous. We know temperature and quality of boost and CFM is impornant and we know that tuning can be a BIG factor. BUT DON"T LET ANYONE TELL YOU THAT TUNING is somehow going to make the motor be more acceptent of CFM the more CFM you capture in the cylinder the more you risk your motor. regardless of what boost it is at.

This is an out line. Its ment as a guide. This isn’t everything. There are impornant things to consider and know. I hope this helped make some of the conversation better. Boost is impornant quality of boost is impornant. I wanted to take a few min to put this together to help as so many seemed upset about this. Im not a scholar that takes 50hours to grammer check every 15second. I wanted to just type and let the information come out. I know there is going to be some poeple that say OH OH OH JUSTIN This or that or you didn’t mention this and this is SOOOOOO impornant… sure…for the other 98% you get the point. If you can’t read it and understand it ask a friend to help. LOL.

last. Thanks to all the people who took time to talk to me and answer my questions about this. I talked to several very smart well respected people about this so it comes from more then just one guys head.

Pretty interesting discussion. I think lots of people don’t get the JHM 8lbs vs. APR and TTS 14 lbs and AMD 15 lbs disparity when all of the kits are trapping the kits are trapping just about the same.

As we have seen in some of the other forums, people think BOOST = POWER, and use BOOST as a measurement to estimate power. More boost = more power in the minds of many. This is particularly prevelant in the B5 world, and it is no shock that the 2 guys who started a conspiracy theory about the JHM car beating everyone with near half the boost were B5 guys (Jason:Addict and TwitS4Estate).

Thanks for the write up Justin.

One last thing…what are we talking about here…can you specifically discuss what’s at risk from the heat from big boost?

I have seen half a dozen people discuss the potential problems with big boost kits and this engine, both here, and on RS246, but it always gets blown over. Seems like EVERYONE is worried about rods snapping…and as long as rods don’t snap, everyone assumes all is well. That’s not what the people I’ve heard are talking about. I think we all know the RS4’s rods can handle loads of power. How about the other components…is anything at risk?

Though I agree with most of the stuff written above this is simply not correct. Running more airmass knock free will be significantly lower stress on the components than running less with knock amplitudes that almost double the peak cylinder pressure. That is all a calibration thing.

[Quote] BUT DON"T LET ANYONE TELL YOU THAT TUNING is somehow going to make the motor be more acceptent of CFM the more CFM you capture in the cylinder the more you risk your motor. regardless of what boost it is at.

Justin - thnx for taking the time to post this. It’s certainly helpful for those of us who aren’t very well-versed on these things. Great write-up.

Great write up.

Please pardon my non-tuning/non-engineering background. :stuck_out_tongue:

Although boost is relative, I think it’s still an important measure. The (perhaps oversimplified) power output measure of a supercharger is just that, what’s the airflow after loss and how much heat is generated from the airflow and pressure. And on that note, what is the efficiency of the superchargers at a particular flow rate. I think that question was posed to APR but they couldn’t share it due to Eaton not releasing it publicly (yet - hopefully).

R1900 map:

R1320 map (stock supercharger on the 3.0T and APR’s ‘old’ supercharger for the 4.2 V8):

Total chamber pressure = compression pressure + burning gas/detonation pressure

Even with added boost, just how much increase is there in peak cylinder pressure (I don’t think it’s a linear relationship)? I recall reading in a book once (lol yes, I love reading) that peak pressure occurs at ~20% gas/air mixture burning. That doubling one or the other won’t double the peak chamber/cylinder pressure (but it would increase the average pressure).

Out of curiosity, what about running boost on a NA engine could be damaging (apart from increase in average and/or peak cylinder pressure)? Compressive/Power loads typically isn’t the damaging one correct? If I recall, it’s the tensile load after the 180 degree crank angle that is most damaging (and that’s capped by the motor’s redline - which makes me appreciate the accomplishment of having a 8500rpm redline on the RS5).

thanks bob

The statement I made was considering all things equal.

IF we are all being honest. We see just about every supercharged car burning oil when you watch there 1/4 mile video. You see white smoke. Some more then others.

A N/A piston and ring pack is built different then a boosted piston and ring pack. Then you look at the piston speed along with the added cylinder pressure. If we were to look at even one thing I would say the rings let alone doing the possible heat expansion on the pistons at certain speeds. We know from the B6 S4 4.2 (and I know its not the 100% same motor) we know they have piston swell issues. This would be the first thing I would worry about.

The fact that this RS4 is a race motor with a very high RPM limit will help it stand up better then most to boost but the structure of it was never ment for boost.

Justin, great info and write-up.

Any concern with head bolts or head gasket yet? I’ve seen first hand how great the design of the BHF engine head gasket is, I assume the BNS is the same or very similar.

I’ve always thought that one of the biggest concerns with boosting NA motors is the additional pressure during the compression stroke, before ignition, not as much any increase in the post ignition pressures. The additional heat generated from the pressurized intake charge, plus the additional cylinder pressure creates the perfect environment for knock or pre - detonation.

This is why motors that are built for running high boost generally decrease their compression ratios in order to add an additional layer of safety. The RS4 has a 12.5:1 compression ratio right? That’s crazy high for a FI setup. Big turbo builds commonly have single digit compression ratios.

This is why, for me, I would never want to run FI at anything over say 10 psi on a high compression motor. The general rule is, you can sacrifice low end response and go low compression with higher boost levels. Or keep the high compression and run lower boost, which usually gives you more area under the curve at the sacrifice of peak HP numbers.

My biggest question from the the post is…

Do the CF compressors really flow that much more air than the TS? Or is it really impacted so much by the intake restrictions etc? If that was the case, I would imagine that the compressor maps would reflect similar CFM at similar boost levels, an therefore only a minimal increase would be required to offset the restrictions… not double. Most of my experience comes from turbos, but the CF is so similar. I just think of two turbos, one puts out 1000 CFM at 8 psi, and the other does the same CFM at 15, then you are talking about two significantly different turbos.

As others have said, awesome writeup Justin. Thanks for sharing. Great information for people trying to navigate the increasingly exciting world of forces inducted Audi V8 ownership.

It’s not that the TS in general flows less air. It’s just that to put a TS on top of an RS4 motor, WITH proper cooling, you sacrifice a lot in terms of flow. There is too much crammed into too little space. Not to mention, the surface area of the inter coolers on these TS kits is FAR FAR FAR from optimal. Not even close. Especially at 15psi. Too much heat+nowhere for all the hot air to go+a high compression sensitive motor is recipe for disaster in the longer term. It’s not a one and done type of thing. At least not at 14-15psi.

Your average max pressure for an 03/04 cobra is about 22 psi. Sure, there are tons of guys pushing the limits with 24+ psi on a TS, but every singe time, the motors fail. It may last a year or a month. At some point, you will get detonation and some stuff that should not be molten will be molten.

At the end of the day, a Centri is not better than a TS. And a TS is not better than a centri. It comes down to choosing the CORRECT type for the application.

Appreciate the info, helps clarify a lot for me!

Would hoods like

help with cooling my TVS charger or would the beit be minimal?

Probably minimal

Any cooling referenced has been regarding air being driven into the engine and the cooling process referenced is through the intercooler. That hood would help with overall underhood ambient temps. I guess the open intakes may beit a little from the hood. Ask koolade what he thinks… He has a vorsteiner hood and has logged everything you could ever want.

Did we ever get an estimated size of the intercooler on each of the RS4 kits?

ah if only it was like the LS world where you can buy a junkyard engine out of a suburban for $350, throw a shitty chinese turbo on it, stuff 20psi into it/maybe some spray, and make 600+whp with no issues. ;D