Blower Dynamics - Boost and Static / Dynamic Compression

[wikd69]

Mostly cuz I like to see my posts, I'll run down what I've calculated what I'm likely to see with different blower drive ratios with this 671 on my 427.

This is important cuz I need to understand the amount of dynamic compression resulting from increased boost levels at WOT. I was really surprised by what I extrapolated out using Weiands published supercharger technical data.

I've been running the standard 9% underdrive for quite awhile and will change this to 1-to-1 during this current build cycle. As mentioned, this will bump my effective compression a bunch, and running 91 pump gas might be a problem at full tilt boogie. This is why I've started looking at water/methanol injection as a remedy for relatively low octane pump gas.

Here's my projections on my build, current and future.



You'll note the current run-state (9% under) yields about 8 lbs of boost, resulting in about 12.4 to 1 dynamic compression. This is just a swag number, cuz there's a number of other factors which would influence what the actual value should be. I'm just not smart enough to know what they are so like I said, this is a swag.

Bumping this up to 1 to 1, I should see around 10 lbs of boost and an effective 13.4 to 1 dynamic compression. This is where it would get really interesting with 91 octane pump gas on the street, though I'm not sure I have enough hair to stay in it that long on the street. Even today, the combination of wide open secondaries with lots of boost while driving on the street is enough to get your attention. Doing this with lots more boost and compression would be fun, in an I-almost-got-run-over-by-a-bus kind of way, ya know ?

Anyway, one of the points I wanted to make here is that this may be a perfect spot to introduce a managed and metered water/methanol injection system.

Have any of you guys out there with blowers or prochargers, et al, dealt with this to the point you could identify the benefit of water/methanol in a setting like this ? Bill, I think you said that you've added a system to your procharged 383.

Besides increasing stability of otherwise low octane pump gas under high compression, the spray also helps to cool the air charge in the manifold *and* gives you significantly more grunt at combustion time.

I think it would be an absolute hoot to try this. And it would be an even bigger hoot to just swap my current cogs and go to 9% over. I just need to determine whether or not I have enough roll cage in my ride.

Something to note: These numbers are specific to a 427. The boost numbers would be higher for a smaller displacement motor, lower for a higher displacement motor, etc.

Comments ? Feedback ? Criticisms ?

[supernova]

I noticed you don't mention the cam specs. The cam will also have impact on the dynamic compression mainly the intake velve timing I beleave. So you need to keep that in mind. Picking the wrong cam can be bad very bad.

[wikd69]

I noticed you don't mention the cam specs. The cam will also have impact on the dynamic compression mainly the intake velve timing I beleave. So you need to keep that in mind. Picking the wrong cam can be bad very bad.

Yeah, that would be one of those "I'm just not smart enough to know" things.

This is my current bumpstick if you're interested. I don't understand enough about a cams impact on dynamic compression (yet) to even guess at the effect.

[supernova]

The thing you need to know here is at what deg. does the intake valve open.
Get intouch with Comp cams and talk to a guy named Mike with the tech line he can help you with your cam set up and frigure out the dynamic comp. If you haven't bought cam and lifters yet, think about putting a solid roller in it. I would get rid of the gear drive though. I know solids are hard to deal with. Not realy I've had solids in all my street car for the last 24 yrs. love em and wouldn't chage a thing.
I think you can get boost and cam timing worked and still run just 93 octane pump gas.

I'm not that smart when it comes to cam timing eather. I just call someone else how does know.
Thats a trade secret.

[sc68z28]

Harry, see if this helps. To figure dynamic C/R, I like to use this online calc http://www.rbracing-rsr.com/comprAdvHD.htm
Here are my results for your 427 @ 10psi boost and 1000 foot DA. 11.41 : 1



I was able to find your cam in my dyno sim program, that's how I knew that the IVC event was 52* ABDC.
Also that's not really a blower cam, did you know that? It has 94* of overlap, that's alot for a blower.
It's the late closing of the Intake that bleeds off cyl pressure, lowering your dynamic C/R. That part is good.

About the Meth injection, yes it really helped mine. lowered intake temps to ambient and added octane.
I have always run race gas or a mix of race gas and pump to be safe.
With Meth I plan to run more pump to race gas in the mix, but there is no formula to help us figure this out.

How and where would you inject the meth?

---Bill.

[supernova]

Now that's what I'm talking about!!!!!!!!!!!

[sunsation540]

i didn't read the whole thing about the alch injec but as for the dynamic comperssion this is also related to the efficiency of the blower not just the drive if you have a known boost at RPM and the pulley ratio you can figure your increase or decrease in dynamic compression form the base given for the blower it it should make 8 @9under and makes 7 then your decrease is 1.14% make sense however as the boost goes up so does the heat and the pumping loss.. further the heads and intake valves become restrictions at boost, and the carbs square area vers the blowers cfm per revolution can kill boost as well,, for the valve opening thing on a blower mtr it operates at boost read manafold pressure the valve is open or is it closed boost is not changed by the intake valve opening .. now the exh valve is a diffrent thing as with to much overlap the boost ends up in the header.along with high egts P.S. for a cool street deal put a nitrous plate under each carb with just the fuel side hooked up run to a seprate fuel system IE pump 1 gallon cell seprate reg use a hobs pressure switch set at 8 psi fill the 1 gallon cell with c16 or some other race gas run the engine on pump swill and hook up a retard on an rpm switch set at ??? what ever rpm your engine sees boost over 8 psi and enjoy. tune on the rich side till you can read the plugs and jet up and down do mtr first. our pump gas 496 made 751@6000 and on the gas tune it pulled 935 with plates.. oh i forgot we quit burning head gaskets and won the blown gas class 3 years in a row... last thing a carb was built to run @ atmospheric pressure not with a mechanical sucker under it it will reach a point that it has no more fuel vers air volume to flow and that's why blown gas is a tight rope walk...note blow through carbs are kept small for this reason fuel flow vers air flow..think about it

[dadnova]

Go to Wallace racing calculators to due the math. All the Calcs. you will need.

And the cam open and closing points will have a lot to due with dynamic compression as well.

You are starting off with a very low and safe static compression factor at 8 to 1.

I am at 11 to 1 with my Turbo and at only 15 lbs boost I am at 17 to 1 on my 427 cu. If I change the intake closing sooner it makes it 20 to 1, close it later and I am at 12 to 1, so the cam makes a big differance.

[wikd69]

I think you numbers are way off. Go to Wallace racing calculators to due the math.
With a blown application your dynamic comp. will be a lot more, your 14 to1 is more of a static compression number.

And the cam open and closing points will have a lot to due with dynamic compression as well.

You are starting off with a very low and safe static compression factor at 8 to 1, Im at 11 to 1 with my Turbo

As far as meth injection, look into E85 for it burns colder then most fuels and nothing to break once it in the tank its there. 105 octane acts like 115 due to the cool and slow burn to help stop pre- detonation.

I think you numbers are way off

Wait, who is that directed at, me ? <WIKD69> or Bill <sc68z28> ?

I'm easily confused, so go slow.

I'm certain my table derived dynamic compression numbers are way off, but I'm still trying to digest how the cam open/close event timing affects the realized compression values. If it's as simple as determining effective stroke, that is, from the moment you actually begin a true hermetic compression cycle until you achieve TDC, then the numbers that Bill quoted from the tool he was using are probably pretty accurate. However, I'm still trying to figure out what my exact build is doing with the way I degreed the cam in (either 2 or 4 degrees advanced). I'm looking at the size of my combustion chambers, what my actual bore x (effective) stroke will yeild, and from that, try and nail down a realistic dynamic compression value.

Bill, I'm actually running a .030 over with this block but still the standard 427 stroke, so the numbers will vary somewhat.

I'm just trying to get my head completely around the full valve event cycles and boost curve profile.

As a starting point, I used the standard Weiand charts for estimated boost and extrapolated dynamic compression.

to be continued....

[sunsation540]

[quote="supernova"]I noticed you don't mention the cam specs. The cam will also have impact on the dynamic compression mainly the intake velve timing I beleave. So you need to keep that in mind. Picking the wrong cam can be bad very bad.[/quote]
followed by dad nova And the cam open and closing points will have a lot to due with dynamic compression as well.. we were considering the((intake valve )) in relation to dynamic compression the exh along with overlap changes the dynamic compression .. again i'm no expert but the intake whether @ 50 or 200 lift has intake manafold pressure IE boost at it's back side dynamic compression raises really don't apply to blower mtrs...thats why you can give a dynamic number with added boost in the first place.. it's math not physics 10%=8lbs boost example only as for compression this applys to the fuel chosen PS turbos are a diffrent animal than a belt driven supercharger as the exh duration moves more air and in turn makes more boost and the given area of the fan determins the boost... all valve timing aspects make adiff on a NA engine and don't carry over to roots stuff unless the cam //intake//valve is so small that the boost backs up in the manafold the intake has 12 lbs boost and the chamber sees 8 lbs due to restriction..

[sc68z28]

, but I'm still trying to digest how the cam open/close event timing affects the realized compression values. If it's as simple as determining effective stroke, that is, from the moment you actually begin a true hermetic compression cycle until you achieve TDC, then the numbers that Bill quoted from the tool he was using are probably pretty accurate. However, I'm still trying to figure out what my exact build is doing with the way I degreed the cam in (either 2 or 4 degrees advanced).

"If it's as simple as determining effective stroke"

You got it Harry, it's that simple! The intake valve closing/after bottom dead center, effects DCR. Shortens the stroke, less stoke, less compresion.
The 52* ABDC is @ .050 lift. Your cam actually does not completely close the intake untill 83* (seat to seat) ABDC Similar to mine (80*)
83 degrees of crank rotation, moves the piston 1.36", leaving you only 2.40" of effective stroke.

Your .030 over bore did not change things, but 4* advance added .25 to DCR, 11.66:1 .

---Bill.

[wikd69]

, but I'm still trying to digest how the cam open/close event timing affects the realized compression values. If it's as simple as determining effective stroke, that is, from the moment you actually begin a true hermetic compression cycle until you achieve TDC, then the numbers that Bill quoted from the tool he was using are probably pretty accurate. However, I'm still trying to figure out what my exact build is doing with the way I degreed the cam in (either 2 or 4 degrees advanced).

"If it's as simple as determining effective stroke"

You got it Harry, it's that simple! The intake valve closing/after bottom dead center, effects DCR. Shortens the stroke, less stoke, less compresion.
The 52* ABDC is @ .050 lift. Your cam actually does not completely close the intake untill 83* (seat to seat) ABDC Similar to mine (80*)
83 degrees of crank rotation, moves the piston 1.36", leaving you only 2.40" of effective stroke.

Your .030 over bore did not change things, but 4* advance added .25 to DCR, 11.66:1 .

---Bill.

Geez louise !!! This motor currently makes this sort of horsepower generating less than 11.5 to 1 compression at full tilt boogie, on only 7 or 8 lbs of boost ? Holy moley !!!!!

I have a new cam, still in the shrink-wrapped box and identical to my last one, sitting on my bench. I wonder if I might want to pick something else up instead, something with a narrower intake window and more exhaust duration. Moving up to 10 lbs of boost with a longer effective stroke could really help my realized DCR (thanks for the new acronym ). Hard to imagine that much more horsepower, wow !

Ok then: Dummy that I am, help me understand the benefit of this cam in a naturally aspirated motor. How does closing the intake 83* ABDC help a NA motor ? You'll note that this cam is symmetrical, that is, lift and duration is identical between intake and exhaust, tho I don't believe the timing of the events is symmetrical. Lots of lift, lots of duration, but the short stroke would affect a higher compression motor even more than it does my blown motor in the end. Shorter effective stroke means a smaller fuel charge which means less horsepower / torque in the end. The huffer compensates for this fact somewhat with the overpressure factor, the NA motor cannot.

This cam was advertised to be good in the 3500 to 6500 rpm range. I would guess the higher bottom end rpm starting point is due to the relatively large lift and duration of the grind.

Hmmm... this gives me something to consider. I may have to go back and take another look at available hydraulic roller cams.

Man, it's still a bit spooky to consider what I could gain from just a minor change in my cam grind here. The horsepower and torque numbers are so big with a blower motor that even minor changes in this cam should really change the old blink rate

[wikd69]

i didn't read the whole thing about the alch injec but as for the dynamic comperssion this is also related to the efficiency of the blower not just the drive if you have a known boost at RPM and the pulley ratio you can figure your increase or decrease in dynamic compression form the base given for the blower it it should make 8 @9under and makes 7 then your decrease is 1.14% make sense however as the boost goes up so does the heat and the pumping loss.. further the heads and intake valves become restrictions at boost, and the carbs square area vers the blowers cfm per revolution can kill boost as well,, for the valve opening thing on a blower mtr it operates at boost read manafold pressure the valve is open or is it closed boost is not changed by the intake valve opening .. now the exh valve is a diffrent thing as with to much overlap the boost ends up in the header.along with high egts P.S. for a cool street deal put a nitrous plate under each carb with just the fuel side hooked up run to a seprate fuel system IE pump 1 gallon cell seprate reg use a hobs pressure switch set at 8 psi fill the 1 gallon cell with c16 or some other race gas run the engine on pump swill and hook up a retard on an rpm switch set at ??? what ever rpm your engine sees boost over 8 psi and enjoy. tune on the rich side till you can read the plugs and jet up and down do mtr first. our pump gas 496 made 751@6000 and on the gas tune it pulled 935 with plates.. oh i forgot we quit burning head gaskets and won the blown gas class 3 years in a row... last thing a carb was built to run @ atmospheric pressure not with a mechanical sucker under it it will reach a point that it has no more fuel vers air volume to flow and that's why blown gas is a tight rope walk...note blow through carbs are kept small for this reason fuel flow vers air flow..think about it

Ya know, the more I learn about blower motors the less I know about them.

I did a fair bit of research on carbs and superchargers and discovered that a lot of blown motors are killed due to extreme lean conditions at high boost. I found that a lot of old school blower guys were building motors that ran really fat on the bottom end but balanced out cleanly at high boost. This was the problem I had with my original build which used Dominators. Here's an old shot of the top end. Looks like an old river rat motor, doesn't it ?



These were 750 CFM Dominators with really large shooters, large square jetting and 4 standard 6.5" power valves. They were not boost-referenced.

I had to run this thing really fat on the bottom end or I'd experience very high heat and severe detonation under high boost. I had some really scary backfires through the carbs, and prolly only the fact that this old blower is not super tight is what kept it on the manifold.

So, to address the lean top end I had to load up the bottom which meant I also had to play with a lot of different heat ranges in my plugs before I found some that would not foul out or burn up. I ended up with NGK -7's. At the advice of Rex Hutchinson (a motor builder here in Sacramento) I swapped the 6.5" PV's for 10.5" PV's in all 4 metering blocks, ran much larger jets and shooters and advanced the gee whiz out of the timing. That really helped my top end out but you could not run it in the shop more than a few minutes without needing resucitation. <gack>!!!!

So, after doing my homework, I finally ended up swapping the old Dominators for a pair of Holley 750 CFM 4150 HP boost referenced carbs. Oh man, what a difference !!! Now I'm running much lighter 3.5" PV's in the primary blocks and none in the secondaries. I'm using small jets in the primaries, large jets in the secondaries and was able to back off a bit on the size of my shooters. Now I just tweak the jets depending on current track conditions and the motor seems to like things just fine.

The reason I'm boring you with all of this is in reference to your suggestion of a nitrous plate under each carb, spraying higher octane juice at full boost. I don't have a clue as to how to tune for that. It was my understanding that the carb setup I'm running should handle my fairly conventional boost pressures without much problem, keeping me in safe fuel/air mix conditions. How did you compensate for the overrich condition when spraying, without knowing the actual resulting air/fuel mix ? O2 sensors in the exhaust ? Or is this one of those 'go for the gusto' things where you want as much horsepower as you can get in the short term wthout sweating plug life and whether or not you make bystanders pass out from the fumes ?

[wikd69]

Go to Wallace racing calculators to due the math. All the Calcs. you will need.

And the cam open and closing points will have a lot to due with dynamic compression as well.

You are starting off with a very low and safe static compression factor at 8 to 1.

I am at 11 to 1 with my Turbo and at only 15 lbs boost I am at 17 to 1 on my 427 cu. If I change the intake closing sooner it makes it 20 to 1, close it later and I am at 12 to 1, so the cam makes a big differance.

Thanks for the information.

I'm betting that building and tuning for a turbo is a lot tougher than it would be for a blower.

[sc68z28]

help me understand the benefit of this cam in a naturally aspirated motor. How does closing the intake 83* ABDC help a NA motor ?

Air speed (port velocity) @ 3500+ rpm, the air speed is greater than the small amount of compression that takes place at the begining of the stroke.
Cylinder filling is still positive at that time, for this reason, you don't want to have less intake open time after ABDC.
True you would have a higher DCR, but less air/fuel volume to burn, (less VE) volumetric efficiency at higher rpms.
That is the way I understand it anyway.

I do agree more exhaust duration would help, open the exhaust sooner and longer with 112 to 114 LS

Gee that sounds just like my Comp blower/turbo cam, In 255* Ex 262* @ .050, 114 LS, .613 lift on both.

BTW 11.5 :1 DCR is a lot! That's 98+ octane race gas territory. Most the NA guys try to stay in the 9:1 DCR, (10.5 static) range, for pump gas.

---Bill.