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16v or 8v turbo?

Subject: 16v or 8v turbo?
From: amalventano1@comcast.net (Allyn)
Date: Sat, 22 Jun 2002 18:26:23 -0400
References: <F120evSrABqjxnjo1Pz0000584a@hotmail.com><01c001c21990$bf24cc00$0200a8c0@al> <032c01c219ed$0ced0a40$7a01a8c0@adf><028a01c21a09$486bfe20$0200a8c0@al> <043201c21a36$5a6638c0$7a01a8c0@adf>

ok, now that requires a slightly different explanation.
    BMEP is the MEAN pressure, i.e. the average pressure. for this you have to picture whats going
on during the combustion process. you cant simply consider the peak pressure, you must consider the
pressure maintained during the combustion stroke. after all, the peak pressure at tdc is doing
effectively no work at all (picture bicycle pedal at tdc). as the piston moves downward, the effect
is a pressure drop (naturally). combustion continues well beyond tdc, so combustion keeps the
pressure up, acting against the pressure drop. this results in a large pressure being maintained 90
(crank) degrees beyond tdc (pedal parallel to ground). it is that pressure that is doing the work
for you, ultimately resulting in high BMEP numbers.
    now lets go back to the 8v vs 16v stuff. keep in mind i am talking stock engines (with added
turbos) in both cases. previously we established that we had to limit peak pressure (at tdc) by
using lower boost on the 16v. another aspect of the peak pressure to consider is that spark occurs
before tdc, so your calculated peak pressure is actually lower than the actual peak that the engine
sees. what this peak is actually accomplishing is detonation, since the air/fuel has had so much
energy (pressure) added to it that it all detonates at once. this results in extreme pressure spikes
(~1500 psi), causing ring lands to go poof and pieces of pistons to go byebye.
    oops, started getting off track there. ok. here, i'll make it simple.

- 16v: higher c/r, therefore lower boost, therefore lower mass of air in cylinder.
- 8v: lower c/r => higher boost => more mass of air in cylinder => more fuel to burn => more
combustion => higher pressures maintained at/through the 90 degree mark => higher BMEP numbers =>
more torque => more hp.

hope that straightens some more stuff out for ya :)
Al

Allyn Malventano, ETC(SS), USN
87 Rieger GTO Scirocco 16v (daily driver, 180k, rocco #6)
86 Kamei Twin 16V Turbo Scirocco GTX ('it has begun', rocco #7)
86.5 Occo 16v Trailer (dotting 'i's across the country, rocco #8)
87 Jetta 8v Wolfsburg 2dr (daily driver, 260k, 0 rattles, original clutch, driveshafts, wheels :)


----- Original Message -----
From: "Larry" <rocco16v@netzero.com>
To: "Allyn" <amalventano1@comcast.net>; "Lil B 959" <lilb959@hotmail.com>; <scirocco-l@scirocco.org>
Sent: Saturday, June 22, 2002 5:47 PM
Subject: Re: 16v or 8v turbo?


> Al-
>   Either you were more awake, or I was, since your second explanation was
> much clearer to me.
>
> However, since Brake Mean Effective Pressure (BMEP, or the 'peak pressure' I
> believe you're speaking of) is the true indicator of power (torque,
> actually, put for simplicity's sake we'll call it just 'power'), it sounds
> like the 16v would be the engine of choice, since it will (with 5 lbs boost)
> produce the "the same peak pressure as an 8v with 15 lbs of boost."
> Aaarrrgghh, I'm getting more conFUSED!!
>
> Larry
>
>
>
> > okok, heres what i mean.
> >     by running lower c/r, you can run higher boost, meaning the engine
> draws in more (mass) of air
> > on each stroke. the lower c/r means lower multiplication of the absolute
> pressure coming in, so
> > since the lower c/r engine is multiplying by a lower factor, there is a
> lower peak cylinder pressure
> > in the lower c/r engine (with the same level of boost). the peak cylinder
> pressure is usually the
> > limiting factor.
> >     a (stock) 16v with ~5 lbs boost has approx the same peak pressure as
> an 8v with ~15 lbs boost.
> > this is where the ~5 lbs limit comes from for stock 16v turbos. yes the
> 16v may be flowing better,
> > but the 8v is still getting much more mass of air (and fuel) in the
> cylinder per stroke (due to the
> > much higher intake pressure).
> >
> > was my rambling worse or better this time? i had just woke up when i wrote
> that last one.
> > Al
> >
> > Allyn Malventano, ETC(SS), USN
> > 87 Rieger GTO Scirocco 16v (daily driver, 180k, rocco #6)
> > 86 Kamei Twin 16V Turbo Scirocco GTX ('it has begun', rocco #7)
> > 86.5 Occo 16v Trailer (dotting 'i's across the country, rocco #8)
> > 87 Jetta 8v Wolfsburg 2dr (daily driver, 260k, 0 rattles, original clutch,
> driveshafts, wheels :)
> >
> >
> > ----- Original Message -----
> > From: "Larry" <rocco16v@netzero.com>
> > To: "Allyn" <amalventano1@comcast.net>; "Lil B 959" <lilb959@hotmail.com>;
> <scirocco-l@scirocco.org>
> > Sent: Saturday, June 22, 2002 9:02 AM
> > Subject: Re: 16v or 8v turbo?
> >
> >
> > > Allyn-
> > >   The way you've worded your reply, it sounds like you are contradicting
> > > yourself:
> > > Less c/r ( less pressure) so you can run more boost which equals more
> > > pressure??
> > > Please expound on your position...
> > > ( BTW, you say less c/r lets the engine draw in more air, some listers
> say
> > > MORE c/r lets the engine draw in more air.  I say 'don't make no
> > > difference'.  Picture this: take a single cylinder engine of 100cc
> > > displacement. On the intake stroke, that engine will theoretically draw
> in
> > > 100cc of intake charge. Doesn't matter how much it compresses that
> charge on
> > > the next stroke, 2:1 or 10:1, it will only draw in 100cc. Because the
> > > compression stroke occures AFTER the intake, compression is dependant on
> > > intake, intake is NOT dependant on compression.  To say otherwise would
> be
> > > like saying the position of the throttle pedal is dependant on the
> engine
> > > output, when it's the other way around.  Follow?)
> > >
> > >  Bryson-
> > >   Regardless, the only way to make more power is to move more air/fuel
> > > through an engine. Period.
> > > A 16v engine will move more air/fuel (all else being equal) than an 8v,
> > > because it has more valve area.
> > > Therefore, the 16v has more POTENTIAL than the 8v. In all practicality,
> for
> > > street use the 8v would probably be the best choice when you factor in
> 'bang
> > > for the buck'.
> > >
> > > Larry
> > >
> > > ----- Original Message -----
> > > From: Allyn <amalventano1@comcast.net>
> > > To: Lil B 959 <lilb959@hotmail.com>; <scirocco-l@scirocco.org>
> > > Sent: Friday, June 21, 2002 7:01 PM
> > > Subject: Re: 16v or 8v turbo?
> > >
> > >
> > > > ok, heres the deal. the more air you can get in there, the better
> (read:
> > > more torque/hp). lower c/r
> > > > means more air (mass) can come in (i.e. more boost). the reason that
> more
> > > can come in is that the
> > > > lower c/r engine wont try to compress it as much, so you are keeping
> > > in-cylinder peak temps within
> > > > reasonable numbers. this lets you have more boost without doing crazy
>
> > > stuff like water
> > > > injection/massive intercoolers/race gas.
> > > > ...so that leaves the best bet being the 8v. the 16v is actually
> better
> > > suited to the task (and
> > > > breathes alot better), but to use it you have to drop the c/r (either
> by
> > > spacing the head or forged
> > > > pistons).
> > > >
> > > > p.s. dont forget the big injectors :)
> > > >
> > > > Al
> > > >
> > > > Allyn Malventano, ETC(SS), USN
> > > > 87 Rieger GTO Scirocco 16v (daily driver, 180k, rocco #6)
> > > > 86 Kamei Twin 16V Turbo Scirocco GTX ('it has begun', rocco #7)
> > > > 86.5 Occo 16v Trailer (dotting 'i's across the country, rocco #8)
> > > > 87 Jetta 8v Wolfsburg 2dr (daily driver, 260k, 0 rattles, original
> clutch,
> > > driveshafts, wheels :)
> > > >
> > > > ----- Original Message -----
> > > > From: "Lil B 959" <lilb959@hotmail.com>
> > > > To: <scirocco-l@scirocco.org>
> > > > Sent: Friday, June 21, 2002 8:30 PM
> > > > Subject: 16v or 8v turbo?
> > > >
> > > >
> > > > >
> > > > >
> > > > > Hey I was wondering if I should go for a 16v or 8v turbo?  I know
> the 8v
> > > has
> > > > > a better compression ratio for a turbo, but if I ran less boost with
> the
> > > 16v
> > > > > would it be better than running more boost with the 8v?  Any
> information
> > > > > would be helpful.  Thanks,
> > > > > Bryson
> > > > >
> > > > > _________________________________________________________________
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>

Follow-Ups:
- 16v or 8v turbo?
  - From: Larry" <rocco16v@netzero.com (Larry)

References:
- 16v or 8v turbo?
  - From: lilb959@hotmail.com (Lil B 959)
- 16v or 8v turbo?
  - From: amalventano1@comcast.net (Allyn)
- 16v or 8v turbo?
  - From: Larry" <rocco16v@netzero.com (Larry)
- 16v or 8v turbo?
  - From: amalventano1@comcast.net (Allyn)
- 16v or 8v turbo?
  - From: Larry" <rocco16v@netzero.com (Larry)

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