tommycompton

iTrader: (1)

Ya lingenfelter was where I was sourcing the LS9 cam from, plus I could grab an oil pump and C5R timing set from them too.
I already have 8019 springs from AFR, and the reason I went with Morel 5206 lifters.

I'm just wondering what my target DCR should be with boost added into the eqaution, to run pump gas, and not pull too much timing.

DMM

DCR should be the same for both of them, only thing different is LSA and a small amount if intake lobe lift. Your stroke and static compression ratio is different, which will skew the result. Use same rod length, stroke, and SCR and the DCR should be the same between the two.

tommycompton

iTrader: (1)

I swear I am, when I type the Lsa and CL differences of the two cams it gives me a different ABCD which gives me a different DCR. The SCR is the same.

tommycompton

iTrader: (1)

Oh ya you posted the earlier one I messed up.

This is the correct numbers:
LS7
Static compression ratio of 10.001:1.
Effective stroke is 2.44 inches.
Your dynamic compression ratio is 6.91:1 .
Your dynamic cranking pressure is 131.25 PSI.
Your dynamic boost compression ratio, reflecting static c.r., cam timing, altitude, and boost of 6 PSI is 9.73 :1.
V/P (Volume to Pressure Index) is 97


LS9
Static compression ratio of 10.001:1.
Effective stroke is 2.38 inches.
Your dynamic compression ratio is 6.76:1 .
Your dynamic cranking pressure is 127.49 PSI.
Your dynamic boost compression ratio, reflecting static c.r., cam timing, altitude, and boost of 6 PSI is 9.52 :1.
V/P (Volume to Pressure Index) is 92

It does give me a different DCR. The LSA and CL changes give me a different ABCD, that changes the DCR. But my SCR stays the same.

tommycompton

iTrader: (1)

Ok humor me some more. This damn thread has now got me seriously thinking about a forged rotating assembly, now rather than later.
Crankshaft
Factory GM Crankshaft
Connecting Rods Compstar 6.125" H-Beam, 900 HP,
Piston Set
Wiseco 4.005" -3.2cc Flat-Top, or
Wiseco 4.005" -11cc Dish,
Main & Rod Bearings
Coated Clevite Main & Rod Bearing Sets
Balance Rotating Assy


Plus hardened main caps and ARP main studs

The question is -3.2cc pistons or -11cc
The -11cc would get my SCR to just above 9.1 wih te 75cc heads
Then of course I could go with a more agreesive cam.
Would this perform ok if my boost wasn't very high?
hopefully my fuelling would stand up. 340 pump, kennebell BAP and 60lb injectors.
It's got my head spinning. I'm almost thinking build the motor now worry about my rear end in the late spring early summer. Too many decisions.

punishmentcycle

iTrader: (6)

11cc is what I would do.. It would drop u down to the 8-8.5/1 range. You'll be able to run more boost but ur still gonna have that ceiling from the small manifold of the whipple

tommycompton

iTrader: (1)

That's why I want to stay stock cubes. The guy that seemed to hit the limit of the blowers manifold had a built 408, he still had massive torque but his peak HP was lower than expected. Not trying to push my build that far.

DMM

^ This. Sorry, been really busy the last few days.

FuzzyLog1c

iTrader: (4)

Why? 8:1 seems like it's going to be almost completely reliant on boost to make power.

For my 418 CID LS3, I'm planning on ordering -20cc pistons to arrive at a 9.5:1 static compression ratio. That shouldn't introduce any problems at up to 16 lbs of boost on 93 octane with a good, custom-ground cam. And it won't run like a dog for the six months between my engine install and the TVS2300 rear cog setup that I have in mind.

tommycompton

iTrader: (1)

Ya I'm swayed more to the -3cc dish or maybe -5cc. That way my compression is more around the mid 9 ratio. I figure it would be more street able as I'm not really concerned about peak numbers. Leaves a little bit of power left from the 1-3000 rpm range. Also should help from over pushing the limits of the small manifold on the whipple, as I would be running it on less psi.
Make sense?

DMM

I'd keep SCR in the 9:1 neighborhood and try to keep DCR around the same number, being that you will be driving this on the street. Proper DCR is of particular interest in a street low/mid RPM scenario, since things change rapidly as RPM increases. It is said that you should keep DCR in the 8.1 - 8.7 range max for pump gas street driven vehicles. Although it seems that the LS motors can get away with a little more.

I plan on the same as you, 72cc heads, forged (although stock displacement), LS9 head gaskets, etc. I plan on being right around 9:1 SCR as well. If when tuning I can stand to go up in DCR, its as simple as a pulley change, honestly. Done. While there are other factors that come into play here, i'm trying to keep this short since I don't have much time.

As I said, you have more potential here in (safe) timing advance than with higher SCR or boost. Since the blowers we are talking about here move a rather substantial amount of air, you'll find all you power in moderate boost with lower IAT's and more timing than higher boost alone.

tommycompton

iTrader: (1)

Here is part of my dilema, I have limited pully choices (3 of them) before I have to go to an 8 rib. I don't have room for an 8 rib set up on the V with this blower (without extensive modification and relocations).

The following pulley/PSI would be based on stock so obviously, my psi would be less from my heads.

4.630 Base kit 5 psi 470RWHP C6

4.500 7 psi or 517 RWHP C6

4.400 Race 9psi or 550 RWHP C6 (Cylinder Head mods for lower compression or 100+ octane Recommended!)

4.350 to 2.800 Race only/Built engines - requires 8 rib drive

*note I had detonation issues on the 4.5" pulley with stock heads and 91 octane


Now with that being said, would I still benifit from a 9:1 scr?
Would I be able make up power from timing advance and cam selection with the smallest 6 rib pulley selection?

DMM

Yes. Keep in mind the volume of air that you are moving here, ESP with a 3.3l positive displacement supercharger. If you had a turbo or centripetal supercharger this would not be as critical, as in most circumstances they simply will not provide the same characteristics at low engine speeds.

GM designed the LSA and LS9 with 9.0:1 and 9.1:1 SCR (with smaller blowers than you have) and Ford goes well into the 8's I believe. You'll be fine.

tommycompton

iTrader: (1)

Thanks for your insight, you have been a big help.
Looks like I'm pulling the block, making sure I don't have any suprises, then ordering a rotating assembly and a "real cam".
I'm going to omit the billet main caps to save a few bucks, don't think they are needed on my HP goals (money saved for the cam).

Also here is a good read I came across on the same subject:

Boost vs Compression Ratio


Almost as fast as a supercharger can be bolted on, the question of how much boost can be run is sure to come up. When building up a motor to be supercharged, you've got the issue of just how much compression to run. Both of these questions relate to essentially the same set of equations. Assuming that all of the other requirements of the motor are satisfied, the compression -vs- boost aspect is not all that difficult.

There are only three ways to make more power - increase displacement, increase cylinder pressure, or increase rpm. It is the primary responsibility of a supercharger to increase cylinder pressure, while also, to a degree, increasing the effective displacement of the engine. As the supercharger supplies the motor with a more dense air charge, it allows for the ability to burn additional fuel. By adding a supercharger, additional air should no longer be a problem, which will also help to increase rpm. Ensuring that there will be enough additional fuel to maintain the proper air to fuel ratio, as well as controlling spark timing, will be the key to getting the most from a supercharged combination.

All motors have a static compression ratio. This is the amount that the air inside the cylinder is compressed. It is a ratio of the cylinder volume at BDC to the volume at TDC. When a supercharger is added, additional air is forced into the cylinder effectively raising the compression ratio. The result of this is called effective compression. The formula for finding the effective compression is very easy:

((boost psi / 14.7) + 1) x motor compression = effective compression.

The effective compression allows a supercharged motor to be compared to a normally aspirated motor. For the most part, a supercharged motor with the same effective compression as a (similar) normally aspirated motor with the same static compression should have about the same overall power.

This may bring up the question that if the overall power should be about the same, why go with a supercharger? The main advantage of the supercharger is that it allows for a moderate compression level during normal driving (off boost) while allowing for very high compression levels when needed. Obviously a high compression motor of about 14:1 makes a lot of power, but it would never survive daily driving. A lower compression motor is great for daily driving, but greatly reduces the potential for power. The supercharger allows for higher compression levels than could be used without a supercharger, while still offering the benefits of a standard compression motor. Many street supercharged systems will go beyond 18:1 effective compression under boost. Under race conditions, many supercharged race motors will go well beyond 25:1 effective compression. Both of these levels are far beyond what could be done reliably or cost effectively without a supercharger.

This brings us back to the question of just how much boost or compression can be run. Obviously there can't be a simple number that could be used for every application. This is why it's so critical to chose the proper components. It's not necessary to build a low compression motor to use a supercharger, but the correct parts are still necessary. The biggest factors will be in things like head bolts (or preferably studs), gaskets, and the strength of the other engine components. It goes without saying that the incredible power that a supercharger can add, can easily start breaking things. It is very important that as the boost levels rise, the need for a stronger crank, rods, pistons, etc... becomes very critical. Many people forget this as the motor itself is relatively mild, while the supercharger pushes it well beyond the practical limits it was intended for.

Now, back to the compression issue. Anyone who has looked into supercharging has heard that you need a low (static) compression motor. This may have been true once upon a time, when roots type (positive displacement) superchargers ruled the land, but it's not so necessary now. The problem with a low compression motor is that it relies heavily on the supercharger for its power. An 8:1 motor is definitely not going to be a power house. Sure, you can throw 18 lbs of boost on it and get some real power, but why? A higher compression motor of 9.5:1 will have much more power without the blower. Then, with less boost you could easily have the same overall power - only it would be much more usable. Both of the motors (8:1 with 18 lbs boost and 9.5:1 with 12 lbs boost) will have almost the same effective compression and about the same overall power. The big difference will be where you see the power, and how much of a demand will be placed on the supercharger. Obviously, the 9.5:1 motor is going to have far greater torque and low end power as the boost is only starting to come in. It is also going to be much easier to find a blower to survive only 12 lbs of boost -vs- one that would have to put out 18 lbs. It is now very easy to see why a higher compression motor with lower boost is becoming so popular.

Please understand that when I say higher compression and lower boost, there are limits to each. Going over about 10:1 will make the amount of boost that is usable drop quickly to the point that the supercharger is somewhat wasted. In my opinion, anything less than 8 lbs of boost is a waste of a supercharger. Going over 10:1 will also make daily driving with pump gas much more difficult. In this same way, compression levels much under 9:1 will require substantial boost levels to make massive power gains. This would require boost levels that are very demanding of a supercharger. This is truly unnecessary. This isn't to say that the lower compression / higher boost set-up doesn't have a slightly higher potential for power, because it does. A lower compression motor has the ability to contain more volume. This can be an advantage, but is such a minor one that it's not necessarily worth the effort - unless it's for an all out race motor. Even then there are limits for the same reasons as the street / strip motor.

Once again, the compression -vs- boost issue. For a car that will see the streets (actually for most applications), the best thing to do is start with a motor compression that is high enough to make the horsepower you want for normal driving. Don't rely on your supercharger to make all your horsepower. With a good motor compression, add as much boost as is safe for your particular application. Decide on a final effective compression, and work your way back through the formula to find your maximum boost level: ((effective compression / motor compression) - 1) x 14.7 = boost. With the proper fuel system and related engine components, an effective compression of 16:1 to 18:1 should be more than workable. For heavily modified cars, effective compressions over 20:1 should be very carefully considered. Remember, even Indy cars only run about 18 Lbs of boost and reasonable static compression levels. Technology has come a long way and modern day supercharging should take full advantage of this.

While these opinions are not exactly the most popular, they are based on facts and real world performance. While there will always be those who continue with tradition and stick with what was done in the past, it is those who reach for something more that are winning races. Often times, some of the best advice can be found from those who have done what you want to do. All too often it is those who know the least that offer the most advice. After having been involved in supercharging for many years, I have heard it all. Most of it was worthless. It was often the least mentioned things and trail and error that have been the most rewarding. Hopefully this information will help to explain some of the most misunderstood aspects of supercharging.