Supercharger vs. Turbo psi

OK, so I got into an argument with a friend of mine a while back about some stupid video he showed me of a volkswagen jetta with a turbo VR6 beating a 03-04 Cobra. The VW was using 19 psi which is over twice that of the Cobra. My argument was that if the Cobra had even close to that much boost then it would have been a different story. He counters this by arguing that 19psi from a turbo is the equivalent of 9.5 psi from a supercharger (or in other words, it takes twice the psi from a turbo to make the same amount of power as a supercharger). I then said that 19 psi is 19 psi, I dont care where it comes from be it a supercharger or a turbo. Now, I was hoping that some of you guys more knowledgeable of the subject could prove one of us wrong. Thanks guys.

OK, so I got into an argument with a friend of mine a while back about some stupid video he showed me of a volkswagen jetta with a turbo VR6 beating a 03-04 Cobra. The VW was using 19 psi which is over twice that of the Cobra. My argument was that if the Cobra had even close to that much boost then it would have been a different story. He counters this by arguing that 19psi from a turbo is the equivalent of 9.5 psi from a supercharger (or in other words, it takes twice the psi from a turbo to make the same amount of power as a supercharger). I then said that 19 psi is 19 psi, I dont care where it comes from be it a supercharger or a turbo. Now, I was hoping that some of you guys more knowledgeable of the subject could prove one of us wrong. Thanks guys.
4 words "your friends an idiot"

hahah good luck im sure someone else has more informational post than me

Just real briefly, the jetta is at least 500 lbs lighter than the Cobra. 19 psi on the Jetta would be around 420-430 hp or so. 9.5 lbs on the Cobra would be around 500 hp. Boost pressure is relevant to the airflow of the engine. The VR-6 is around 170 cubic inches. The Cobra is 281 cubic inches. With their respective boost levels the Cobra flows roughly 18% more air than the Jetta and makes more power, but the car is heavier.

19 psi on the Jetta would be around 420-430 hp
He showed me another video of the VW on a dyno and it had 420 FWHP I believe, so good calculation, guess or whatever method you used.

But what my buddy was suggesting was that on a motor, any motor, it would take twice the amount of boost that a supercharger was running for a turbo to achieve the same amount of horsepower as the supercharger(sorry, I couldn't think of a better way to word that). So a VR6, turbo, 20psi puts down the same power as a VR6 supercharged 10psi. And as I stated before, I argued that X psi equals X psi and it doesn't matter where it comes from. I hardly know anything about forced induction, but his argument seemed rediculous to me. So who is wrong me, him, or both of us?

He is wrong, it would actually be the other way around. Given the exact same engine and the exact same boost level the turbo would make slightly more power because of the parasitic loss of the supercharger. It takes a lot of power to spin a supercharger, probably somewhere around 50 HP at that boost level.

A compressor, no matter how it is driven, will take roughly the same amount of hp to spin when compressing the same flowrate at the same pressure ratio. The turbochager is simply more efficiently driven. I read about an interesting test where a motor made 1000 hp when run on compressed inlet air. The same motor made 800 hp when it had a supercharger at the same pressure and 950 hp with a turbocharger at the same pressure. Therefore, the supercharger takes 200 fwhp to drive and the turbo soaks up 50 fwhp at that power level.

Also, my own testing revealed that a turbo on an LT1 will make ~30 more rwhp than a supercharger at the same boost level, all other things being equal. That's at the ~550 rwhp range.

Ok, I'll take all of this as meaning that I'm "more" right and "less" wrong than him. And the wierd thing is that this guy is not clueless when it comes to cars so I don't know where he got this idea from. And he's not one of those ricer fools, although he is into VWs which only puts him a slight step higher than the ricers.

Also, my own testing revealed that a turbo on an LT1 will make ~30 more rwhp than a supercharger at the same boost level, all other things being equal. That's at the ~550 rwhp range.

Mike, is that difference with the log-style header? How much of of a difference do you think a set of 4-1's would make at that level?

Also, have no doubt that peak lb for lb the turbo is going to produce a lot more low rpm torque. In other words, if they both make a peak of 10 PSI at say 6,000 rpm ......the turbo is going to make more like 8 PSI at 3,000 rpm and the blower is going to make more like 3 PSI at that rpm. The supercharger's impellor rpm is limited by pullies, whereas the turbo impellor isn't directly connected to anything to produce it's rpm, so at very low rpm if there is a lot of load the turbo impellor can spool up to a much faster rpm, producing a lot more torque than a comparable centrifugal blower.

As others have stated, he has it backwards. The supercharger is at quite a disadvantage pound for pound since it requires more power to be driven and can't spool up as quickly.

Mike, is that difference with the log-style header? How much of of a difference do you think a set of 4-1's would make at that level?

Yes, the turbo made 30 more rwhp than the T-trim Vortech at the same boost level with a supposedly crappy log-style header and a supposedly all-wrong blower cam.

From the reading that I've done, I think the log header may cause a 10 - 20 rwhp debit at the 580 rwhp/13 psi level, probably up to 40 rwhp at higher boost/power levels. Keep in mind that the turbo Buicks have log headers on the turbo side and they don't bother changing over to 4-1's until they get in the 9's on 231 cid.

Keep in mind that the turbo Buicks have log headers on the turbo side and they don't bother changing over to 4-1's until they get in the 9's on 231 cid.

Very true, one key difference is that the primary tubes are angled towards the direction of the turbine before attaching to the log header. http://www.gnttype.org/techarea/pictureguides/downpipes/stockhdrs.html It should help flow to a certain extent.

Very true, one key difference is that the primary tubes are angled towards the direction of the turbine before attaching to the log header.

I actually was able to do mine the same way, except for #1. The tubes for my #3, 5, and 7 are angled to merge into the log. I simply didn't have enough room on #1, so it Tee's in at a 90 deg angle.

I actually was able to do mine the same way, except for #1. The tubes for my #3, 5, and 7 are angled to merge into the log. I simply didn't have enough room on #1, so it Tee's in at a 90 deg angle.

Sounds good Mike, keep us updated on her progress.

On a side note, and talking about stock engines here; I don't think the cobra has a chance against the VR6 in a turbocharged application. Probably not a supercharged one either.

The VR6 was originally designed as a Diesel engine so it'll stand boost that would send the mains out the bottom of the pan on the Cobra. Plus, the combustion chambers are essentially zero cc's. The head is completely flat. All the combustion takes place in the piston; so it's basically a perfect engine for boost.

One of these days I'd like to build a motorcycle around one of those engine, but that's WAY off topic. :D

On a side note, and talking about stock engines here; I don't think the cobra has a chance against the VR6 in a turbocharged application. Probably not a supercharged one either.

The VR6 was originally designed as a Diesel engine so it'll stand boost that would send the mains out the bottom of the pan on the Cobra. Plus, the combustion chambers are essentially zero cc's. The head is completely flat. All the combustion takes place in the piston; so it's basically a perfect engine for boost.

One of these days I'd like to build a motorcycle around one of those engine, but that's WAY off topic. :D

Very interesting! I like Jettas, my dad has a TDI and it's pretty torquey. After researching modding it I found it not to be very practical ...but this is the 4-cyl diesel with the tiny turbo I'm talking about. What you're saying is the GAS VR-6 engine was originally designed as a diesel ....and that it's built like a tank. I'll tell you what, I'd love to have something like that for a daily driver ...have to keep that in mind for the future. I wouldn't want to go crazy like 420 whp on a damn fwd but I'd like to boost it up to 250-300 whp. I wonder how stout the transaxle is? (only way I'd have a Jetta would be in a 5 sp) ...is it a LSD?

What you're saying is the GAS VR-6 engine was originally designed as a diesel ....


Yup.
And I got that DIRECTLY from an inside saurce at VW.

Do a google search for pictures. It's a really fascinating engine. It's the only "V" engine I know of that has only one cylinder head.

It also has SEVEN main bearings.

Follow this link and scroll down, you can see a pick of the crank and of a W12 shortblock, which is two VR6 engines mated at the crank:

Link 1 (http://images.google.com/imgres?imgurl=http://www.autozine.org/technical_school/engine/vr6_insight.jpg&imgrefurl=http://www.autozine.org/technical_school/engine/smooth5.htm&h=359&w=355&sz=46&tbnid=DWsHFPdiKbEJ:&tbnh=117&tbnw=115&hl=en&start=62&prev=/images%3Fq%3Dvr6%26start%3D60%26svnum%3D10%26hl%3D en%26lr%3D%26sa%3DN)


Here's a pic of the VR6 head:

Link 2 (http://www.rp-motorsport.de/images/motortuning/vr6_zyl.jpg)

Yup.
And I got that DIRECTLY from an inside saurce at VW.

Do a google search for pictures. It's a really fascinating engine. It's the only "V" engine I know of that has only one cylinder head.

It also has SEVEN main bearings.

Follow this link and scroll down, you can see a pick of the crank and of a W12 shortblock, which is two VR6 engines mated at the crank:

Link 1 (http://images.google.com/imgres?imgurl=http://www.autozine.org/technical_school/engine/vr6_insight.jpg&imgrefurl=http://www.autozine.org/technical_school/engine/smooth5.htm&h=359&w=355&sz=46&tbnid=DWsHFPdiKbEJ:&tbnh=117&tbnw=115&hl=en&start=62&prev=/images%3Fq%3Dvr6%26start%3D60%26svnum%3D10%26hl%3D en%26lr%3D%26sa%3DN)


Here's a pic of the VR6 head:

Link 2 (http://www.rp-motorsport.de/images/motortuning/vr6_zyl.jpg)
Awesome! Mere 15* separation allows the head to use all six cylinders ...pretty cool. So is it all forged and really strong all around? How sturdy is the transaxle and can you get LSD stock? I bet turbo setups are expensive and hard to tune just as in our case. What about the C.R. ...is it boost friendly or do you need race gas? I don't even know the stock TQ/HP number of the VR6. What about the turbo gas 4-cyl is it stout too? Would be easier to get one of those and just crank up the boost I would think.

You would be far better off asking these questions at VWVortex.Com, which has a VR6 forum. (Or did the last time I looked.)

Just to briefly hit your questions, I believe that the stock transaxle is good past 400hp, but I don't know how far past. You can get a Quaiffe limited slip unit, which IIRC is a Torsen type of differential.

I also know that you can get a VR6 passat into the low 13's with bolt-ons.

As I said though, you will get a lot more info at the website I mentioned.

Thanks, I will have to consider one of those for a daily driver at some point.

http://www.full-race.com/articles/Bseriestest_writeup.pdf

food for thought on the log vs tubular question.

As far as turbo vs supercharged power, thats just silly. You are comparing apples to oranges there. If the same volume of air was pushed by both the turbo on the VR6 and the supercharger on the Cobra, then I could see arguing that, however, they aren't. All parasitic loss aside, you are going to make X amount of power when Y volume of air is combined with Z volume of gas at whatever RPM. I don't know the exact formula, however, there is no way to give a blanket statement that X PSI on the turbo is relative to X PSI on a supercharger. It doesn't work that way. Your friend is moron.

Thanks for backing me up.

PSI is just the amount of pressure running through the engine whether it's coming from a turbo or s/c. It's still going to be the same number.

CFM is a different story. A turbo will tend to more CFM at the same boost setting as the s/c. Of course theres a lot more to it than that. Take in count the trim of turbine/impeller of the turbo, A/R.

Comes down to this:

900CFM @ 25psi < 1400CFM @ 17psi

I know that seems logical but ricers are idiots and think BOOOOST is all that matters, when in reality you want more CFM.

SantaCruz163, "boost" comes from airflow. Excess airflow in the intake system is what causes the higher pressure over the n/a setup. Given the same engine and intake temperatures (post compression) 15 psi from an exhaust driven supercharger would be the same inlet airflow as 15 psi from a belt driven supercharger. There would be less intake charge contamination from the belt driven supercharger providing a more ideal cylinder fill.

Everyone likes to state that boost is a measure of restriction. That is true to an extent, but not totally.

If you have a supercharger running 15 psi in the plenum, then you port the intake runner, then the boost will reduce down to. . . say. . . 13 psi. Of course, it will make more power. . . blah blah blah. . . we've all heard it before.

What most people miss, though, is that the cylinder during the intake stroke is a dead-end path. There is no exit during that time. The idea is to get as much pressure as possible in the cylinder at BDC of the intake stroke. Let's just say that you had an intake port with zero restriction. Well you should have zero boost, right? Nope, just full boost pressure actually in the cylinder at BDC.

....the jetta is at least 500 lbs lighter than the Cobra.

I don't know squat about VW's but I own an '03 Cobra. It's a very heavy car. With a 200 pound driver, it weighs in at slightly over 3900 pounds. The convertible variant is 4000+ pounds.


....9.5 lbs on the Cobra would be around 500 hp.

Not exactly. The stock Eaton puts out 8.5 psig boost. The engine is rated by Ford at 390 hp. Dyno runs on stock '03s have shown a range of 365 to 384 rwhp. It would take much more boost to reach the 500hp mark.


The VR6 was originally designed as a Diesel engine so it'll stand boost that would send the mains out the bottom of the pan on the Cobra.

Actually, the '03-'04 Cobra engine uses all forged internals. You'll see guys running up to 600-650 hp on the stock bottom end with no catastrophic failures. This power level usually takes a Kenne Bell or Whipple supercharger running up to 23-psig boost -- again, on the stock bottom end.

Actually, the '03-'04 Cobra engine uses all forged internals. You'll see guys running up to 600-650 hp on the stock bottom end...


Diesel engines are built to withstand static compressions of well over 20:1 and are designed to put out 2 to 4 times as much torque as a gas engines.

It's more than just "forged internals"

It's the metullurgy and design of both the internals and the block itself.


By way of reasonably well known example, I offer the Oldsmobile DX block as compared to the Gas block.

I know a guy who took a naturally aspirates 350DX block to 9.01/149mph in a 3200lb car, revving to (IIRC), over 8,000rpm; which gives you an idea of the stresses a diesel block will tolerate.