Differences in Supercharged verses normally aspirated heads

[redvictor]

if Curtis ever gets my heads i'll let you know what the results are with a better ported intake,although i am changing the cam as well...

[Horsewidower]

I've got a question - considering boost is only backpressure, does this mean that there is more effective flow across the valve seat area on a blown application than a typical flow bench would indicate? The way I see it, if the higher pressure areas like the plenum just have higher pressure but not higher velocity wouldn't that mean that at the highest pressure differential location (the valve seat) the velocity would be much higher than N/A?

I run a turbo 4 cylinder with an aftermarket head which has decent but not stellar peak flow numbers (260 @ .600). I've been considering trying a well ported iron head done by a friend of mine which flow wise will kick the alu. head's ass up to .500" lift but has a little lower peak numbers.

My cam only has .580" lift, would it be beneficial to give up some peak head flow to gain more low to mid lift flow? Fuel is methanol so it's taking up a lot of room, boost runs 40+ psi. with about 1:1 TIP ratio.

I'm hoping someone will chime in on Dave's question. I run the same engine architecture only on E-85. I'm under the impression also that under boost the greater depression differential will cause the air to accelerate faster to its choke point. What has caused me to pause is a post that MadBill made concerning the greater inertia of the denser air and its impact on the acceleration.

Dave, like you I'm frequently frustrated by the lack of empirical information about boosted apps. Who benchflows ports at 30psi??

Bob

[T/A kID]

Bringing this topic back into discussion hopefully

This is a vague question, but pertaining to the intake manifold using a centri blower application how would that differ from a Naturally aspiated intake manifold?

I totally agree with Larry on how the intake is 2-3 times more important on a boost application, but would that translate to the intake manifold as well? Common sense says yes, but to what degree?

Does resonant tuning matter with boost? If it doesn't runner length could be any length and have the same effect, correct?

CSA could be widely increase since pre blower the motor needed XXX amount of airflow while the blower would increase this amount most likely by two (depending on the blower I suppose, could be more or less).

Plenum Volume in my understanding could be in larged as well since the intake manifold would be filled much faster than before, even larger plenums most likley would not suffer from lagged throttle repsonse because of the constant pressure in the intake tract?

Been pondering these things for a while, just would like to have some explanations.

Ben

[SWR]

I totally agree with Larry on how the intake is 2-3 times more important on a boost application, but would that translate to the intake manifold as well? Common sense says yes, but to what degree?

Does resonant tuning matter with boost? If it doesn't runner length could be any length and have the same effect, correct?

Ben

You have the same waves moving in a denser medium... I've heard even prominent people say "boost overcomes all, 2 bar boost and all goes out the window" but the dyno results I have seen say otherwise...get really out of tune, and you'll see hp drops more significant than the same application ran as an NA engine, so wave tuning does something in that denser mass...

[DeltaT]

Moving even further upstream, how do you know when it's time to increase your throttle size? I saw an article on a 400c.i. LSx twin turbo and they are running two 2150cfm 4BBL throttles on it. If I am running a 1000cfm-rated 4BBL throttle (rated at normal atmospheric pressure), how can I tell how many cfm it will flow at 15# of boost? Would the only way to tell if it was too small be to measure boost before and after the throttle and watch for a major delta?

Thanks,


Jim

[SWR]

Moving even further upstream, how do you know when it's time to increase your throttle size? I saw an article on a 400c.i. LSx twin turbo and they are running two 2150cfm 4BBL throttles on it. If I am running a 1000cfm-rated 4BBL throttle (rated at normal atmospheric pressure), how can I tell how many cfm it will flow at 15# of boost? Would the only way to tell if it was too small be to measure boost before and after the throttle and watch for a major delta?

Thanks,
Jim

Jim, what you should avoid like the plague is to have any significant pressure drop from the turbo outlet to the plenum... if you have, one component,like a throttle body - or more - cause an addional restriction in the system. You want the only "restriction" - which is what makes the boost, no restriction no boost - to be the valves and their events..

[Horsewidower]

Moving even further upstream, how do you know when it's time to increase your throttle size? I saw an article on a 400c.i. LSx twin turbo and they are running two 2150cfm 4BBL throttles on it. If I am running a 1000cfm-rated 4BBL throttle (rated at normal atmospheric pressure), how can I tell how many cfm it will flow at 15# of boost? Would the only way to tell if it was too small be to measure boost before and after the throttle and watch for a major delta?

Thanks,


Jim

Your cfm doesn't change, the density of the air changes under boost. You still flow the rated 1000 cfm. Mass flow is what you're after, not boost or CFM. Measuring before and after the TB is an excellent idea for determining whether or not it is a restriction. Boost is a measure of restriction, not power. Your goal should be to make the maximum amount of power on the lowest amount of boost.

SWR is correct.

[JIMI B]

Bringin this one back to life. However a little off topic. I tried searching this topic but havent found much.

Lets talk engine displacement and forced induction. I am involved in everything from top alcohol funny car to turbo 4 cylinders. I have seen multiple instances where smaller displacement engines on the same blower, or turbo have made more power. Sometimes with the same heads and cam. I also know that the nhra alcohol class engines are getting smaller.

Havent really thought about this much until know. Hypathetical situation. Leaving spool out of it. Given the same turbo, would you rather have a 2.4 or a 2.0 with with larger in/ex ports? If dragracing was the purpose. Im assuming the 2.4 with smaller ports is going to operate at lower rpms, and have more torque down low.

Discuss?

[KnightEngines]

I have seen the same in turbo applications - everything else being equal the smaller motor will make more peak power, but less average power.
Particularly if the turbo is somewhat undersize.
I would think the larger engine is just pushing the turbo past it's efficiency range earlier than the smaller engine, so power falls over before the larger engine really gets into it's stride.
Oversize turbos can go the other way, the little motor just can't spin it hard enough to get the turbo working in it's efficiency zone.

Even with a slightly undersize turbo I'd still prefer the bigger engine, average power > peak power in most applications.

[Daygoslow]

just trying to understand a little of this info i am taking in. what kind of power gain would you expect a 434 with a 254/266 @ 0.050 with 630 lift solid roller, a D1SC with an intercooled 6-8psi and a cylinder head like an edelbrock e-tec 200 with a .600 flow of about 260cfm and an average IvE flow of 83% and change nothing else but the cylinder heads to a set of comp ported afr 227's with a .600 flow of about 322cfm and an average IvE flow of about 78%. what kind of gain would you expect to see? boost go 1psi down? 3-4psi down? power go up by what? 10hp? 80hp?

[ptuomov]

How should I pick the best intake port CSA for a turbocharged engine?

Effects that I am thinking about here:

Speed of sound depends basically only on temperature. Suppose that I have reasonably cool 125 F. So the speed of sounds is higher but not dramatically higher.

The air density is higher for a boosted motor, especially so if intercooled to say that 125 F. This means that the column of intake charge will have more energy to ram the cylinder after the BDC. Basically proportional to the mass (and thus density) and proportional to the square of the velocity.

Higher intake charge density must increase the friction, does it? Does the sonic nozzle start choking flow at lower fractions of speed of sound?

Does this all amount to a boosted engine needing a slower charge speed and higher CSA than an N/A engine?

Inertia tuning is used as a "back door" supercharger in a NA engine. If there's an actual supercharger, is inertia tuning then less important?

[SWR]

Inertia tuning is used as a "back door" supercharger in a NA engine. If there's an actual supercharger, is inertia tuning then less important?

You have the inertia of a denser mass being driven by waves generated just like the NA engine... would you want it seriously out of tune, even though the mass flow into the cylinder might be a bit higher than the NA version due to said existing supercharger?

I didn't think so.

[ptuomov]

Inertia tuning is used as a "back door" supercharger in a NA engine. If there's an actual supercharger, is inertia tuning then less important?

You have the inertia of a denser mass being driven by waves generated just like the NA engine... would you want it seriously out of tune, even though the mass flow into the cylinder might be a bit higher than the NA version due to said existing supercharger?

I didn't think so.

I didn't say unimportant, I said less important.

In particular, I was wondering if the frictions are higher for denser charge and if the higher density would allow for inertia tuning to work at lower velocities. Another way to ask the same question: Is the optimal intake port and runner CSA smaller for NA engines and larger for FI engines? I don't know, I am asking.

[BEARCAT]

JUST FOOD FOR THOUGHT, WHEN SANTOS AND SCRUGGS RAN THE SB CHEVY BLOWN ENGINES, THEY RAN 2.200 INT VALVES 1.600 EXH VALVES AND THE HEADS ONLY FLOWED LOW 350CFM AND 250 CFM ON THE BRODIX -12 HEADS. MADE OVER 2400 HP RAN 3.90'S IN PRO MOD 1/8 MILE AND 5.50'S IN TOP ALCH. 1/4 MILE.