Pbeyer2010

iTrader: (1)

Hi guys,

I am just getting my feet wet in FI and i decided to buy and read the SA Shop book on TC/SC GM LS series engines. I am only about 20 pages in but there is a concept being presented that i cant understand...

- Logically in a N/A Engine Higher compression = Power

- In a TC/SC set up boost = Power. High compression means having to run less boost to avoid detonation. Because of this you decrease compression to increase boost.

Is the moral here that the power lost from decreased engine compression is negated and surpased by the boost produced? Im not denying the existance of power producing TC/SC's . Obviously the things work, producing gobs of power.

Is there an optimal ratio for power gain when it comes to reducing compression to increase boost?

Thanks for the read,
Phil

Forcefed86

iTrader: (4)

If you can use boost to compress more dense air into a larger chamber it will make a bigger “boom”.
Imagine a lady finger (small cc chamber) V.S. an M80 (big cc chamber).

Generally speaking a point of compression is worth about 4% gain/loss of total power. The more compression you drop the more boost you can run. If setup properly 1lb or so of boost should take care of that 4% loss. Any additional boost from that point is all gain.

Optimal ratio depends on your personal application. Fuel available/HP goals/desired power curve etc…

c0ncEpT

iTrader: (4)

Why not just use good fuel and run more boost with high compression?

I never understood why guys liked putting there **** on the edge just to run pump gas....

Sales@Tick

iTrader: (3)

Cylinder pressure is cylinder pressure. Whether you make 700rwtq at 12:1 with 5psi of boost or 700rwtq at 8:1 with 20psi of boost you're generating the same amount of cylinder pressure for a given amount of torque.

That said, boost pressure is NOT a measure of cylinder pressure. Power output is.

Engine is an air pump. Make it efficient, and fill the cylinder in the most efficient way that you can as fast as you can.

NitrOmm

iTrader: (1)

I know this goes against the grain ... But in my case my little MP112 SC can only produce about 8-9 lbs (safely). I have a relatively high compression for boost ( 11.2:1) as I don't have an option to increase boost. I have to say for my application I love it. It's not a track car ... Just fun to drive. That whinny little bitch makes me happy !! If I could have fit a TVS I would have.
At that level of boost, 93 octane , no issues with knock. I later added meth just for extra protection & cooling, either way not tuned for it. I don't know where the threshold lies, nor would I advise to test it. If you have the option to increase boost .... Follow the old school of thought and keep your compression low to avoid knock & boost it up.

Forcefed86

iTrader: (4)

I'd agree cyl pressure is cyl pressure. But boost is more efficient at making power than compression.


Lets say...
12:1 compression being boosted 20psi gives 28:1 effective compression.
7:1 compression being boosted 45psi gives 28:1 effective compression.

While the effective compression remains the same, the 7:1 motor will make much more HP.

For example:
12:1 compression being boosted 20psi gives 28:1 effective compression.
7:1 compression being boosted 45psi gives 28:1 effective compression.

For arguments sake lets say for the "control fuel" being used 28:1 effective compression is the highest level safely achievable in these motors before detonation creeps in.

As the 7:1 compression motor is 5 compression points less, it is already down 16% power potential on the 12:1 compression motor.

Since the 7:1 compression motor needs 25psi more than the 12:1 compression motor to see the same effective compression it is up 87% in power potential. Taking the 16% compression loss away from the 7:1 motor leaves it with 71% greater power potential at the exact same 28:1 effective compression. So it takes 5psi more boost at any given point to make the same power with 7:1 compression than with 12:1 compression. Each pound of boost from there on is pure gain.

SilverSS

iTrader: (30)

Race car or street car? For lower hp (under 550ish) higher comp is better. I'm well over 800rwhp on pump gas at 9:1 compression/F1A. I don't think I could do that at safely 10 or 11:1? E85 or race gas is not an option and I do my own tuning so I'd rather play it safe.

Wnts2Go10O

iTrader: (12)

how long does it take to make up the gap in the compression?

how much longer will it take to spool a turbo because of the lower compression?

how much time (on the staging beam) will it take to get spooled up with one vs the other?

its about balance. id be willing to bet the power band of a high compression/low boost motor makes more usable power than the low compression/high boost motor

Forcefed86

iTrader: (4)

Right, exactly what I said in my first post. Just using two extreme examples to help make a point...



Usable power for what purpose? On what specific fuel? Whats considered low boost?

Sales@Tick

iTrader: (3)

Every motor we have built lately for boost has been 10:1 plus. My engine is 10.5:1, Dean Goodin's engine that I have the build thread going for is 10.6:1. Both are non intercooled and will be around 1000rwhp.

Most of the heads up motors I design camshafts for have 10:1 with some being 11.0-11.5:1 compression.

They have the option of running larger turbo's in most cases if they please to and run more boost with lower compression ratios yet they choose not to and run higher compression ratios.

It all comes down to engine is an air pump and with added cylinder pressure you are filling the cylinder more efficiently especially at lower rpm's when the size of the camshaft affects how efficiently the cylinder is filled at lower rpm's.

When you have a large cam bleeding off compression, the only way to counteract that is with more cylinder pressure. Boost can fix this, but boost is not always being produced. The comment about usable power range with higher compression is dead on because of this.

Forcefed86

iTrader: (4)

Theres a lot of way to make power... Not saying any of them are "wrong".

Ignore the writing on the top of the page... (wantabe on theturboforums built this setup)

Heres a 1000rwhp before 6k. over 900ftlb from 3k up... Not exactly a lag beast. And I'd say it has a nice usable powerband.

Setup was as follows...

7.2:1 Compression
Methanol,
24 deg timing
Dig 7 ignition
256/245@ 114 cam .510 lift
Oem iron heads 162 cc intake runner 1.9 , 1.5 valves, ferreo 5000 series 148 lb installed
Felpro 1003 head gasket
355 cubes, std stroke
Twin 57mm turbonetics T3 20psi. Easily could have ran more boost. Tiny turbos were killing the top end.

Sales@Tick

iTrader: (3)

Why would you ever run 7.2:1 compression with methanol? No offense, but that's a total waste of octane that could of been accomplished with a lot less octane.

Power it makes under the curve too comes from it's exhaust opening event, 160cc intake runner and 57mm turbo's.

Just not my cup of tea to use that little compression.

itsslow98

iTrader: (18)

There are so many variables to take into account what the best compression is for your setup. Everything comes into play from fuel, power adder, cam, transmission etc. Whats good for 10 people might not be what suits you. I struggled to find out what I wanted and settled on 10:1 for my new engine. 6 speed street car with a smaller 76mm(7668). I run methanol so I am fine with the higher compression.

Twin screw or roots blower cars compression really doesnt come into play as much as it does for turbo/procharger cars.

Forcefed86

iTrader: (4)

Why not? The lower compression is easy on parts (lower cyl pressure), the setup has no lag issues, makes huge power down in the lower RPM ranges, and has a table top torque curve starting before 3000rpm. All that with OEM cast crappo heads. I believe it was also built with the intention of running quite a bit more boost in the future.

I agree meth is a bit overkill (esp at 20lbs). However what are the alternatives? It is is dirt cheap, consistent and readily available. The e85 pumps are all over the board on ethanol percentages and not much cheaper than straight meth. Race gas costs 4 times as much as meth. And I don’t think it would make the same power as reliably or consistently on straight pump.

Again nothing wrong with higher compression. Just an example of another way that obviously worked.

Sales@Tick

iTrader: (3)

You say tomato, I say tomotto...it's all cylinder pressure in the end.

I just prefer more of it

Then manage it with cam timing...that's just me though

ayousef

Martin, thanks for the information but I think you guys (not just you) confused the OP much more than when he originally wrote the thread.

compression ratio mentioned in this thread is static and as you know is only half of the subject, the other half is in the cam timings which will effectively determine the dynamic compression ratio which is more important for this discussion, why?

an 11:1 SCR engine and a 9:1 SCR engine can BOTH have the exact same dynamic compression ratio due to different cam timing events.

what determines the optimum compression ratio vs boost usually comes down to the limitations you face in real life which include (but not limited to) the fuel octane being used, the size of the camshaft, the efficiency of the forced induction used, efficiency of the combustion chamber and the preferred powerband of the engine.

Also to keep the subject focused on compression ratio vs boost, I suggest all you guys assume 93 octane is the fuel used, with a PD blower (to avoid boost lag from the equation etc...)

When I picked my static compression ratio of 9:1 on my Whipple 4.0 RHS 427", I knew that 93 octane is all im ever going to use on this car, I knew that I didnt want more than 3 degrees of overlap in the cam at .050" and I wanted to spin the blower at xxxx rpms to keep it in within its efficiency range. I ended up with a very late intake valve closing event to lower dynamic compression ratio enough for the 427 motor to be able to handle whatever boost the Whipple decides to throw at it at that blower speed. I estimated boost to be 16psi, it ended up being 17.5psi at an elevation of 650feet, which will inevitably be more at sea level when I get the car in a few days. Car did 930rwhp with 16 degrees of timing and we did not even try to push the timing since the clutch was slipping.

Then theres the comfort zone of tuners (which arent the smartest people on the planet either). Assuming equal static and dynamic compression ratio and equal set-ups one tuner might be comfortable running 12psi of boost and 16 degrees of timing, vs another one who thinks its okay to run 15psi of boost and 12 degrees of timing etc...

At the end of the day I think there is more than one way to make decent power from an engine, however if you were to find the absolute limit of a set-up based on specific limitations (as discussed above), then you would have to blow ****-loads of money trying out various different set-ups and comparing the results, which is something that a large manufacturer can do but nothing anyone of us want to attempt.

I hope this explains some things?

Pbeyer2010

iTrader: (1)

I appreciate all the great feedback guys. It's a bit over my head but with some more reading I will be making use if it soon. If only I spent 4 years in mechanical engineering instead of civil...... I doubt they will give me a refund!

ddnspider

iTrader: (26)

Something that I think gets overlooked quite often is margin. You can make just as much power with high CR and low boost as you can with low CR and higher boost. It is always a balance but for a purely streetcar running strictly 93 octane, lower the CR and give yourself more margin for error....due to a cold day or a worse tank of gas then when tuned or a momentary over boost due to an issue, etc. the lower CR setup has more margin unless both setups are right at the limit. Lower CR setups usually can run milder cams since the boost does the work so it may have better drivability too. Take a look at the ZR1's compression......9.1:1

smokeshow

iTrader: (17)

You guys get too wrapped up in cylinder pressure lol. OP was asking about horsepower. In comparing boost levels and SCR that net the same cylinder pressure, lower compression wins the horsepower contest every time. Horsepower comes from mass flow rate, not pressure. In the end, the lower compression setup has more air in the cylinder both because it allows for more boost before detonation and the larger chamber and/or piston dish allowing slightly more air in.

I tuned a 5.3 with 317 heads with a 75mm on 16psi, in a 5400lb truck with a crappy launch it was still trapping in the 120s. Straight garbage Arizona 91 octane, nothing more. No chance a higher CR 5.3 would have kept up with that on pump gas.

Sales@Tick

iTrader: (3)

Are you telling me this or the OP?

Title of the thread was decrease compression to increase power. It was never stated as static or dynamic. You stated static or dynamic.

In the end it is all just cylinder pressure.

Cylinder pressure is about cylinder fill. The more atmosphere that fills the cylinder the more pressure produced. Air mass stacked on top of each other equals higher pressure. More atmosphere(boost more pressure), less volume in the chamber (more pressure).

It's all about volume.
The ZR1 is not a maxed out race car or even a 1000rwhp performance vehicle. It's a production car meant for longevity and reliability.

I don't know how many times I have to say it but cylinder pressure is cylinder pressure. Regardless of which road you took to make 1000rwhp, the cylinder pressure is the same whether high or low compression high or low boost. If both cars make very close to equal power(the two cars in this case are low compression high boost vs. high compression low boost) and you over boost, you end up with the same cylinder pressure whether you overboost to 15psi from 10psi in the high compression set-up or 30psi from 25psi in the low compression set-up. No added protection there. Only if both cars are very close in hp as we are comparing them in the sense of this thread.
You realize air can be compressed?

A dish piston or larger chamber has no effect on the amount of air that can be compressed in a cylinder.

If this were true, that would be the same thing as saying that a higher compression motor cannot fill the cylinder like a low compression motor. Totally false.

Read what I said above. Mass flow rate is power and not cylinder pressure? Seriously? Of course the mass flow rate of the compressor will dictate horsepower, but mass flow rate and how much the compressor flows or cylinder head flows contributes to a higher amount of air mass.

Do you not know that the more air mass that enters the cylinder the more pressure created? Cylinder fill is directly related to cylinder pressure.

Cylinder pressure is torque.

Torque=horsepower.