engineermike

That's not what I asked. What if you put 19.7 psia on the intake AND exhaust sides? Would the engine be a "restriction" then?

Oh boy. . . The turbocharger's compressor compresses 900 acf of air into 600 acf, thus making it 50% denser. That way, the engine can continue to move 600 acfm, but the mass flow rate will increase by 50%.

When calculating restriction orifice flow, the increase in flow is proportional to the square root of the ratio of the dP's. In other words, when doubling upstream pressure, flow only increases by 41%.

When calculating engine hp from boost, the increase in power is proportional to the ratio of presures. In other words, when doubling upstream pressure (e.g. from NA to 14.7 psi boost), power increases by 100%. This is [yet another] way to explain why an engine should not be viewed as a restriction.

GM PHASST

Here is some food for thought. I have been doing a lot of research on various turbo setups. Mainly on lt1 & ls1 cars. I have found that different setups use diffent cams (obviously).
If you google search 348 99 turbo camaro ss gm high tech, you will find a 638 hp on 91 pump gas and 850 hp on c16 ls1 using a cam with a 113lsa. This information tells me that depending on what I am looking for out of my car, running a n/a cam may not be a bad idea.
With that said, I no longer feel obligated to buy a turbo cam.
Hope this helps.

GM PHASST

Here is some food for thought. I have been doing a lot of research on various turbo setups. Mainly on lt1 & ls1 cars. I have found that different setups use diffent cams (obviously).
If you google search 348 99 turbo camaro ss gm high tech, you will find a 638 hp on 91 pump gas and 850 hp on c16 ls1 using a cam with a 113lsa. This information tells me that depending on what I am looking for out of my car, running a n/a cam may not be a bad idea.
With that said, I no longer feel obligated to buy a turbo cam.
Hope this helps.

98Z28CobraKiller

iTrader: (17)

In a blower car, there should be little to no exhaust restriction. So yes in the sense that swapping to a motor that has double the cubes would lower the boost reading at the MAP significantly.

So how would you explain to a noob that changing to better heads and seeing less boost on the gage is normal without giving them an advanced math lesson? It's just easier to say that the pressure drop is due to the removal of a restriction.

slow67

iTrader: (2)

I think engineermike proved that is true in his case as well (226/226 vs 224/236.

I think that most people spec smaller cams because they are more driveable but still make tons of power compared to N/A.

engineermike

That's not what I asked. What if you put 19.7 psia on the intake AND exhaust sides? Would the engine be a "restriction" then?

Installing better heads IS removal of a restriction! The port/valve is a hole that restricts flow. The cylinder that it leads to IS NOT a restriction.

XtremeDime

iTrader: (16)

Holy **** my head hurts, I havent seen **** like Ive seen in this thread since the college classes I slept through.

Cant you just tell me what cam to buy? God damn it, Im a doer not a thinker.

JAX04

iTrader: (17)

slow67

iTrader: (2)

Technically correct. But in a logical sense, an engine with a larger cylinder can move more air in an out, all else being equal.

slow67

iTrader: (2)

After running a few quick tests on Engine Analyzer Pro, it looks like as long as backpressure stays reasonable (under 2:1) the cams with more overlap spool faster and make more power. In my quick tests, I was comparing the ZR1 cam and the ASA cam.

Ryans99ls1

iTrader: (16)

I think you guys are looking at the motor in 2 different ways. As in mikes hose with a nozzle example, you are treating the engine as the "nozzle" which everyone agreed is a restriction and seeing the turbo system as the "hose." mike isnt viewing it the same and is actually looking more in depth into the motor and in way explaining how to improve flow through the "nozzle" aka the motor.

Just my opinion as an outsider because i believe everyone is saying the same thing different just in different ways.

tim99ws6

iTrader: (31)

I just want to say:

~From the YB

98Z28CobraKiller

iTrader: (17)

That's what I think also. Everyone seems to have a good understanding of what is actually happening. We are just arguing about the explanation. LOL.

98Z28CobraKiller

iTrader: (17)

Nice.

qqwqeqwrqwqtq

Yes it seems to just be a debate about the terminology. I just do not like the phrase "boost is a measure of restriction".

It makes it seem like an arbitrary number that is only there because of some fault or deficiency.

At WOT, how is positive and negative manifold pressure -both- defined as a measure of restriction?


I'm sticking with- Boost is a value of atmosphere available for the engine to consume.

71 chevy

iTrader: (18)

so here is an honest question.

if boost were a measure of restriction, wouldnt exhaust reversion from a high overlap cam be the ultimate restriction? ie, the backpressure is greater than the boost pressure, so effectively, its pushing back the boost.

why wouldnt this create insane boost levels? since boost is supposedly a measure of restriction ?

TurboS10

iTrader: (12)

BP is only present until the exhaust valve is closed and the piston travels down far enough for cyl and intake pressure to equalize. Then you have flow from the intake into the cylinder. - Have a look at some of the 8 sec buicks.... what size plenum are they using???? Why????
Eventually BP get's high enough that you run out of time for the intake air to flow into the cylinder or most all the exhaust to leave the cyl.
Either way power goes flat or down.

boost as a restriction is relative. 25 psi on a 165cc port is a world different from 25 psi on an LS7 port... both on the same CID.
The difference is in cylinder air density.

Do some digging on the guys running TC7x turbos and T7x S trims. Big diff in power. Some is the comp housing. But a big factor is the 12mm bigger turbine wheel causing less BP and more HP for same boost #.

AFA cam timing... I think overlap in itself isn't what's making the power. -I've yet to finish this whole thread yet if I'm repeating-
It's the actual open/close points vs boost/CR/BP that's making the power.
I've read several posts by cam designers saying they don't know what the LSA is until they are done and then calculate it. Meaning they don't even consider it. I had to ask what the lsa was on a custom I got spec'd and he had to calc it right then on the phone. JMOP, but I think most guys over cam their turbo setups because they don't consider BP and the RPM range of the LR intake. 23x @ .050 on a 68mm turbine is an example. Shawn said the same on the first or second page.

TurboS10

iTrader: (12)

Makes sense.... hadn't worked my way there to see that till now.
But the IC artificially raises the exh bp by requiring more turbine power to get boost past the IC restriction.
For the DP and exhaust, it's starting to look to me like you divide your boosted HP by 2 and size your exhaust for that HP to be close.

71 chevy

iTrader: (18)

here is an interesting thread on the topic of back pressure.

guy with a 114 on a bbc claims 25psi backpressure at 25psi boost and gives some logical reasons why his bp is so low

http://www.yellowbullet.com/forum/sh...t=backpressure

moehorsepower

iTrader: (3)

A lot of good answers here, a lot of bad advice and a lot of theories. I have tuned quite a few boosted cars both turbo and supercharged, as said just like an NA motor its all in the complete combo of the motor, heads, runner volume, compression ect. All of the turbo cars I have tune had relatively wide LSA, wide I mean in the 114+ range, I did have one LTx motor that had a nasty, nasty NA cam, lots of duration and on a 108 LSA, now of course its all in the rest of the combination of the motor parts, but with his setup, it should of clearly made a minimum of 600 rwhp, But I am assuming because of the cam it only put down around 480 rwhp...