My TTI X twin, 402, AFR 225, 9.2 CR, 6 sp, 373 gear, 615 @ tires @ 7 PSI. It is no where near "sluggish". Full boost by 3000 RPM, followed by full tire smoke. My city driving technique is leave stop light in second, put it in 4th at 30

You will make boost quicker with more cubes....everything else being equal.......if $$$ are not a concern......AFR 225's would make it safer and you would not sacrifice any performance

 

Mark, you are right, however this is only for my high boost setting at a flick of a switch on the EBC.

My low boost setting for my 427, will not need this at all and will make over 800/800 with a torque line across the board like a flatline EKG chart. It will run on 91 straight. Enough said.

Brent

 

Ummm, maybe I should re-think things...if it works out for you, that would be pretty amazing.

 

I would not run any boost with a 441CID. Any boosted motor I try to encourage more cylinder wall. Or as much as you have left.

 

[QUOTE=DDSLT5]Ruin the thread. Okay. Some good points were made actually, but you'll need to read between the lines to get them.

Here's the facts: The higher the compression ratio, the greater the tendancy to knock - boost or no boost. This is the reason why you'll see most stock boosted applications running low compression ratio engines - if you reduce the tendancy to detonate, then it makes for a more reliable motor.

QUOTE]

I will have to say I dont agree with this. Its a simple fact that the higher the compression, the more efficient the charge. Look at quench. When you have more CPR, you have less quench space. The tighter you get that chamber, the cooler it runs. You are less prone to detonation. I run some of our cars at 12.1:1 CR, full timing and 91 octane. You can always spike the tank with a gallon of race gas if you run the car all night at the track, but they run fine.

 

Heres my opinion. Low CR is fun if you have tons of added support. IE BOOOOST.

IF you want a few psi, then dont be affraid of the high CR. There is nothing wrong with it. Like I said in the last post, it will be less prone to detonation. I have a truck at the shop with a maggie that I put on a 5.3 HO motor with heads and cam. The CR is about 10.8 or so. Am I scared? Heck no. This thing should be wild when its done. But its a stock pulley on it. Thats all it needs. Just alittle kick.

Low CR needs more fuel. The problem with responce from a low CR motor is that most people run MAF programs. With this you have less fuel all the time. A lower CR motor is less efficient, hence needs more fuel to regain the responce. Now you only start seeing this when you get low. And 9.5 is not low. I am talking in the 8's. Thats low. I found that with our turbo motors we build, the responce almost come back ten fold when in SD due to the richer part throttle AFR's. put the MAF back on, and its boring.

So you have to ask yourself how low do you really need, and how high do you really need. Thats all. Its simple.

Rick

 

I see a lot of stuff here about right and wrong and that's not the issue, high comp/low boost is right and low comp/high boost is right.

I've had both and seen my mistakes.

About 15 yrs ago I had a ford 302 with 8.0:1 comp, heads/(mild)cam and Vortech at 12+psi and it was awful and it was great! It was in a Lincoln Mark (3900 lbs.) w/AOD and 3.73 and supprised a lot of Mustangs and a few C4's and was tons o' fun. It was my daily driver and got 8-10 MPG and just ran dirty-hyrocarbons and I just didn't like that part of it!

The problem is with daily drivers-you are only on boost a few percent of the time, but your engine is set up for high boost 100% of the time and 97% of your driving is at a '50's era compression ratio! But when I was into it-it was great-but it got old quick as a daily driver and I finally got rid of it!

My current daily driver, 402 w/11.0:1 will be operated 97% of the time NA and hyrocarbons will be low (clean running-that's important to me), but maybe pushing oxides of nitrogen a bit (important also, but to me much better than a carbon coked daily driver). The basic NA engine will be close to 500 rwhp and very efficient and clean running when off boost with reasonable mileage and will not "coke up" at extended part throttle operation.

Andy is currently upgrading the fuel system, injectors, adding Alky injection and upgrading the tune and I fully expect it to be MY perfect daily driver.

Also, the reason there was some delay and a problem with the Alky system was that as Andy understands about the high latent heat of evaporation of water, he uses 50/50 water/alky, and as it's hard to find methanol in smaller quantities, so he uses readily available Bacardi 151 Rum. Each time he tried to run my car on the dyno the reservoir was found to be empty and the techs (and some of the waiting customers) were red faced and walking around slurring their speech, but that has been handled with Andys new electronic combo lock alky reservoir system, so my car should be back on the dyno this week. An added benefit is the wonderful rum scented exhaust odor!

Anyway, to summarize, if you have a drag machine or week end warrior where 50-75% of the time you are on boost, then you can certainly make much bigger HP w/10-20 psi and 8.5-9.0 CR (with mileage/emissions concessions, but if you have a daily driver at boost only 3% of the time, your car will run better, be sharper and more responsive and better mileage/emissions with a higher CR, (10-11.0:1) and a more moderate boost, say 5-10 psi., and you certainly will have ample tire limitted power, but not as high as low CR/high boost. And of course this is not black/white, only both ends of the spectrum. 9.75 CR and 10-14 psi (right in the middle range is used by many!).

None of my above discussion has considered the more technical considerations e. g., combustion chamber shape, adequate quench/squish geometry, proper engine cooling, intercooler efficiency, fuel octane, cam timing/duration, water/alky/nitrous cooling, constant full throttle racing vs. daily stop and go, elevation and mean temp in your area. All these things come into play when setting up your engine and are interrelated, it's very complicated and the answer is different for each of us, but the resourses are out there to get the information-CorvetteForum and like forums, the literature and your favorite tuner!

So again, it boils down to how you drive and the purpose of your car.
We have great tuners now that can make both ends and the middle work.

Roy

 

I was under the impression that ECS has run boost on their 447's with success?

Roy and Synergy - thank you both for your input...if/when either of you have dyno charts of high CR with low boost please post them!

 

Eric,

Focus on effective compession ratio between 16:1 to 18:1 and build....

To help out, here's a few examples of effective compression ratios and RWHP/RWT output

Roy's motor with 11:1 and 8 lbs boost, effective comp ratio = 17:1 - RWHP 650 / RWT 630

Typical 9.8:1 blower motor with 11 lbs boost, effective comp ratio = 17.1:1 RWHP 650 / RWT 630

Typical 9.8:1 blower motor with 14 lbs boost, effective comp ratio = 19.1:1 RWHP 730 / RWT 680

Your motor @ 10.8:1 and 9 lbs boost, effective comp ratio = 17.4:1 RWHP 700 / RWT 700


Article on Boost v Compression

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.

 

I'm just finishing up a 441 in my C6. It's got LS7 heads a 240-244 692-702 112 LSa cam and 11:1 compression.
I have every intention of sticking my twin turbo system on it later. It will probably have less than 5# of boost but I'm sure it will be a great street driver.
I'm not just after a huge dyno number.

 

Awesome information!! Just when I think I'm up to speed, you guys always seem to remind me how little I know. Once again, the education is much appreciated

 

It should noted that all force induction systems are not the same in regard to efficiency, which will substantially change the end result for the equation. This should be researched carefully.

 

I'm not going to run as much cubic inches or STR as you are running Andy, but a bigger duration cam Your lift is astronomical!!!

LS2 412ci, 9.9-10.3 STR, ECS Paxton and a 242/248 610/615 114+4LSA cam. Boost will probably peak at 10psi max.

What's interesting is I have been told by "The Experts" that this is too much stroke, too much STR, too much cam, on and on and on - ad nauseum.

I've been building radical cars for quite a few years now and I do not see it that way. While this is my first foray into boosted LSx engines, Math is Math. And since I have my Masters in Mathematics, I'll do things my way and report back to "The Experts" on the results of my Science Project

Stephen

 

So true!

 

Charlie ...

Thanks for the REAL STUFF here.

Let us know when you do your first S/C or Turbo package for the new 427 Z. That should be the ultimate daily driver !!!

 

any stock compression vette with a blower

 

I just love my 422 TTIX. My motor is at 10.5:1. I'm running 8psi
on 94 gas just to be safe. No dynos # but it ran 10.50 1/4 and a best speed 132mph and 3000 street mile with no problems the car will spin the nitto 315 17" DR's hard up to 60 mph
What more can I ask for
Chuck

 

I would agree a proven TT is the way to go.

1) I would not want to relocate any OEM parts. Asking for trouble.
2) Even torque to horsepower.
3) Power engagement much smoother.


I pesonally like the STS TT due too

1) Much less heat in the engine compartment (no turbo's and no restriction of airflow in the engine compartment).
2) Fuel Temperature is less.
3) Stock look and no movement of engine components.
4) Weight of the turbo's behind the IRS (Take a look at Sleath's 1.3.. Sixty). Vs. extra turbo weight on the nose.
5) Install time is a lot less.
6) I just sold my exhaust to recoup cost.
7) You can use the stock manifolds.
8) Better balance.
9) You can put turbo's on in various sizes from T3/4, T4 on up.
10) CARB approval accross the board on all their products in Spring.

Brent