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Differences in Supercharged verses normally aspirated heads

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Differences in Supercharged verses normally aspirated heads

Postby bogo » Mon Oct 27, 2008 2:14 pm

I noticed this subject will be covered at Darin Morgan Cylinder Head School. My possibilities to attend the class is very slim (to say the least) so I raise this questions here. I´ve seen opinions from "heads doesn´t matter when you have a blower" to "it´s crucial to have good heads". Good intake/exhaust flow is of course benifical but are there other things (head related) to consider when running a boosted sbc? Any differences in porting technique e.g.? I realize this subject prob need an in-depth discussion with a pro (like Darin) but any tips or hints is appreciated. I have a set of AFR 210 Eliminators (Race ready version) on my 383 sbc and the blower is a Vortech V2 S-trim.

TIA

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Postby Mr. P. » Mon Oct 27, 2008 5:12 pm

X2; my project is a GM iron 6.0L LQ9 with a Paxton 1500... any insights would be helpful from you cam professionals, what woule be easiest/best to choose a camshaft for regarding centrifugal blower application? First thought would be a set of CNC AFR Mongoose heads??? But is that overkill, would a set of ported LS2 heads be just as good a performer in this situation?

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Postby PackardV8 » Mon Oct 27, 2008 9:31 pm

Conventional wisdom is a supercharged engine needs the maximum work on exhaust ports and headers, larger than a NA engine would need. However, in the real world, maximizing both pays dividends. Don't leave anything on the table on either side.

thnx, jack vines.
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Postby T » Tue Oct 28, 2008 6:16 am

Im no pro, but have alway's tried to get the exhaust out as fast as possible with the highest volume I can get . and keep % at 80-88 , of the intake flow if it's 20psi or more. LARGE header's ... Darin Im sure has some #'s to work with, in getting % right. bore, stroke, valve % etc.
I also would use a smaller intake valve % to make sure it's deshrouded and up the size on the exhaust if possible. Larry Im sure has great advise, he seem's to be very technical aswell. Good luck
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Differences in Supercharged vs Normally aspirated

Postby jimivice » Sat Nov 08, 2008 10:22 am

I was hoping to see more info, because I asked the same question and get the same answers, "It does'nt make a difference with a blower". I feel anything you can do to a head that improves a normally aspirated motor will benefit a supercharged application. There will be definate criteria that pertains to supercharged applications (camshaft, C.R.). The exhaust port might have to be larger then for a normally aspirated motor. What I'm curious about is "runner volume". In a supercharged application can the runner be too big for the size of the engine? Do the same principles apply?I am aware we are dealing with forced induction, but what dictates the amount of power? Is it just a matter of boost?
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Re: Differences in Supercharged vs Normally aspirated

Postby Unkl Ian » Sat Nov 08, 2008 10:31 am

jimivice wrote:... but what dictates the amount of power?
Is it just a matter of boost?


Power is a function of Mass flow. Move more air/fuel , make more power.

Boost gets a lot of attention, because it is easy to measure.
"Boost" is resistance to flow. Reducing this resistance by installing larger valves, porting, cam, etc allows for increased Mass flow at a lower boost level. Hence "making more power with less boost".

My reading here suggests:
"good normally aspirated motors, make forced induction motors."

I suspect the application is also important.
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Postby cboggs » Sat Nov 08, 2008 12:09 pm

I build a fair amount of forced induction heads, ..
blown alky roots blown, turbo, procharger, etc.
The old line that "it doesn't matter" is total BS, .. it matters as much
if not more.

Anytime you increase efficiency in any engine power goes up, ..

As for the differences in the heads, .. the first thing on my list
is dealing with the additional heat, copper/ally seats, Inconel
exhaust valves, etc. I see too many people putting out of the box
cnc heads on blown applications and having trouble, .. you HAVE to
deal with the heat! I use solid or HIP processed castings for the same reason.

In a blown application I think you can still make an intake port too big, ..
while the blower will mask some of the issues of oversized heads
there still can be too much cross section.
The cross sectional areas and the valve/throat percentages can change
and what you can get away with depends on whether it's a turbo or
roots type blower. Yes you can go larger then a typical N/A head and
find power but again going too large will depend on the application.

Port design I think is the bigger issue, .. I'm more concerned about turbulence
with the larger volumes we are moving, . . so i work on much "smoother"
area and shape transitions.

Exhaust ports are an area of concern, I'll typically use a little larger exhaust
valve to intake valve ratio depending on the application, and will use
a little more cross section and MUCH wider seat for a roots type blower.
For turbo applications it varies but I want to keep velocity up but still use wide
seats to move the heat from the valve.

Cheers,

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Postby maxracesoftware » Sat Nov 08, 2008 6:03 pm

With a SuperCharger,

the Intake Port is 2 to 3 times more important
than most think !
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Postby Ron C. » Sat Nov 08, 2008 8:53 pm

Anytime you build a maximum effort engine everything matters from top to bottom regardless whether it's blown or not.

With a blown engine if the heads are to small or ineffiecent the boost can go up but not the power. The intake air will be much hotter. You'll be scratching your head wondering why that thing is making so much boost and it's a dog of an engine.

The only time it does not matter is when you only want about half the power that's potentially available.

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Postby MadBill » Sat Nov 08, 2008 9:01 pm

An aspect that seems to get less consideration than it should is the fact that high boost engines normally have low compression ratios. A low CR of course means a low expansion ratio during the power stroke. This in turn means that there's still a ton of cylinder pressure as the piston approaches BDC, thus unlike a high compression engine where most of the power is made early in the stroke so a small exhaust valve (leaving room for a bigger intake) can be opened really early with little loss of torque, a supercharged engine benefits from a relatively late EVO, thus driving the need for a large exhaust valve not only to deal with the much greater volume of exhaust gas, but also to achieve effective blow down in the shorter available time.

Hypothetical example (not edited for thermodynamic accuracy):
1. NA engine with 15:1 CR has a peak pressure of 1,200 psi, falling to 100 psi @ 100° BBDC when the exhaust valve opens. It would fall to 50 psi @ 60° BBDC if the opening was delayed, a loss of 50 psi potential.
2. SC engine (10 psi) has a peak pressure of 1,200 psi, falling to 400 psi @ 100° BBDC, 200 psi @ 60° BBDC, a loss of 200 psi, four times as much!
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Postby Mr. P. » Sun Nov 09, 2008 2:48 am

Thanks gents, makes sense.

MadBill, your point got me to rethinking CR/EVO. My first idea was to leave the static CR stock (10:1) and limiting boost to about 12-13 psi, to preserve off-boost street performance and manners. But now I'm wondering if a lower CR (9.5:1) combined with correct EVO placement and very generous exhaust valve/headers may drive just as well if not better off-boost.

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Postby bogo » Sun Nov 09, 2008 3:29 pm

Thanks for all the input! I was told by a professional engine builder if I was to build a good blower engine, just do it like a good N/A and then add the blower. I have therefore expanded my already overstreched budget to get good heads. The general consensus seems to lean towards a 195cc head for a N/A 383 so hopefully the 210 cc intake will work well when a blower is added to the equation. What I´ve seen indicates that the 210 Eliminator heads actually have quite small cross sectional area but on the other hand, supposely have had some issues with turbulence at high lift. Anyway, I think these heads should work ok out of the box since this is far from a max effort engine. Based on your answers, I now also have some ideas for the next step to take.
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Postby cboggs » Sun Nov 09, 2008 5:41 pm

I'd look at the offerings Dart has, .. good heads that often don't
get the credit they deserve.

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The diferences in supercharging vs normally aspirated

Postby jimivice » Mon Nov 10, 2008 1:05 am

The input has definately given me food for thought. I have a tendency to dispute the idea of building a high HP normally aspirated motor and bolting on a blower. The statement that the intake is worth 2 to 3 times better then one thought,is to vague. Compared to what? I feel the cyl. head is just as important to making power in a supercharged application as in normally aspirated motors. This is probably the only common denominator. The cam will not be the same and as was stated boost is relative to C.R.. If you wanted to increase power of an existing supercharged motor,what are the relavent changes that would apply - if you did not want to increase boost?
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Postby maxracesoftware » Mon Nov 10, 2008 1:27 am

The statement that the intake is worth 2 to 3 times better then one thought,is to vague. Compared to what?

Didn't mean to be vague.... but Posted the Truth

Judging by the Calls i get on Head Porting for SuperCharger applications
from the general public,...the very 1st thing they want you to do
is Port the Exhaust Port and just leave the Intake Ports along,
their reasoning-> they will just use the Blower
to "force the air in" , so they don't need any Intake Porting.

After getting a ton of these type Calls over 20+ years,
i'd have to say->

With a SuperCharger,
the Intake Port is 2 to 3 times more important
than most think !

.......Compared to spending most of your efforts on the Exh Ports

SuperCharger->the Intake Ports in my opinion and experiences
have shown the Intake Ports are where most of the Power is at,
and is 2 to 3 times more important than Exh Porting.

Typically under normal Atmospheric conditions
a Flow CFM gain of 20 CFM @ 28" will usually be good for = 41 HP

the same Intake Flow gain @ 15 psi will make that 41 HP
at least 82 + HP gain

try and find 82 HP gain on Exh port

Take a "Tip" from all the Professional Racers in
ProMod/Blower+Turbo, Top Fuel Dragster/FunnyCar ,
and Alcohol Dragster/FunnyCar ->
the Intake Port is were the overwhelming majority of HP + TQ gains are
then the exhaust port in that order .
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