[khkiley1]

How can a turbo motor run an effective 16:1 - 20:1 compression on pump gas ?

ie. 9:1 static compression with 15psi of boost = roughly 18:1 effective compression.

Even 5psi pops the effective compression over 12:1, which a NA engine wouldn't run on pump gas without a lot of overlap on the cam.

Does the increased turbulance increase the flame speed/decreasing the chance of detonation ?

Does the boost come on later, reducing detonation ?

What other factors are involved ?

The IQ and life expectancy of the average American just passed each other going in opposite directions.

[khkiley1]

How can a turbo motor run an effective 16:1 - 20:1 compression on pump gas ?

ie. 9:1 static compression with 15psi of boost = roughly 18:1 effective compression.

Even 5psi pops the effective compression over 12:1, which a NA engine wouldn't run on pump gas without a lot of overlap on the cam.

Does the increased turbulance increase the flame speed/decreasing the chance of detonation ?

Does the boost come on later, reducing detonation ?

What other factors are involved ?

The IQ and life expectancy of the average American just passed each other going in opposite directions.

[Goatman]

Tom Vaught will answer most of these, I'm sure, but the answer to the first question is:

It can't.

Alot of the issues you are asking about have to deal with the camshaft timing area (duration).

Chance favors the prepared mind.

[FAPhoenix]

My friend has a laser and his only change was bumping the turbo to 15psi so whatever that compression is runs fine on 91. Though I'm just waiting for it to expload, she gets mighty hot after racing. Exhaust manifold is bright red.

Tyler

[GoatRider]

Obviously Tom V. is the man to answer this, but I am bored so will take a layman's stab at it...

When you say the "effective compression" is 18:1 I assume you are basing that on 15psi boost being approximately one extra atmospheres, hence doubling the static 9:1 compression.

But I don't think it works that way, in a your example your compression RATIO is still 9:1. What the turbo does is makes that compression happen on a nice "thick and heavy" charge of air/fuel instead of the "thin" atmosphere that is avialable for people and naturally aspirated cars to breathe.

OK Tom, was that anywhere close? [img]/infopop/emoticons/icon_wink.gif[/img]

"Nothing Beats an Ol' Goat..."

[khkiley1]

This twin turbo SBC runs 93 Octane at 1200 HP.

It has got to be running over 15 PSI boost, with 8.3 static it would be at 16.7:1 effective compression.

I don't believe everything I read, but there are enough stories like this out there to make me curious.

The IQ and life expectancy of the average American just passed each other going in opposite directions.

[Dave A]

I will anxiously await Mr. Vaught's comments, but I am on vacation and free to think about engines instead of brakes for a change, so here goes 2 more cents worth....

Bear in mind that boost and volumetric efficiency are NOT directly proportional. When you boost and engine to 15 psi, you are approximately doubling the inlet pressure, but you are NOT necessarily even coming close to doubling the volumetric efficiency, which is what we're really concerned with when considering 'dynamic' compression ratio.

Consider a turbo motor peaking at 130% volumetric efficiency, and a comparable naturally aspirated engine peaking at 85% VE. The turbo motor has 8:1 static compression, and the NA motor has 10:1 static CR. The dynamic CR (which is really just an indicator of cylinder pressure at the end of the compression stroke) of the turbo engine is proportional to VE x CR, or proportional to 10.4. The dynamic CR of the NA motor is proportional to 8.5. So we're looking at approx 25% difference in cylinder pressures at the end of the compression stroke.

The turbo motor will still make a lot more power, because it's burning 50% more fuel and air than the NA motor. The turbo motor still operates at a higher cylinder pressure after compression, but that's why turbo motor ignition timing is often much less advanced. Increased turbulence generally does help REDUCE detonation and speed up combustion.

It is very important, especially when considering forced-induction engines, to abandon too much emphasis on straight 'static compression ratios', and focus instead on how much cylinder pressure you are building at the end of the compression stroke, which is a function of static CR, inlet pressure, and cam timing at the very least.

[khkiley1]

OK, for lack of a better example, the dyno chart for that SBC shows 986 ft/lbs of torque @ 6200 RPM.

Again, I haven't verified these dyno figures, just one of many examples I found on the Internet.

From HO racing's "Pontiac High Performance Engine Design and Blueprint Assembly"


torque = VE * Displacment.
(I would have thought CR would come into play but I will use this for illustration.)

That SBC displaces 427, so 986/427, VE = 2.31

Static 8.3 x VE 2.31 = Effective 19.173.

I am guessing better swirl/quench with the chevy heads + increased velocity of the inlet flow.

I am still scratching my head.

The IQ and life expectancy of the average American just passed each other going in opposite directions.

[Brian Baker]

"Effective Compression Ratio"?

I've heard this term for many years regarding engines making their own atmosphere. I've yet to hear anyone come forth with a formula that can give us an ECR based on the mechanical CR of the engine, cam timing events, and boost.

I think alot of talk about ECR is conjecture. Sure, we are all aware that cylinder pressure rises with boost, and that the mechanical compression ratio never changes, but what of ECR?

Dave A seems to have a strong grasp on this, but what of a formula? Tom, can you shed some light on this?

[Black 65 GTO]

It is actually very possible to calculate your effective compression.

This is usually based on the intake valve closing point, stroke, bore, and rod length.

I have a program written by my friend, GTO Karl, that calculates this very acurately.

If your good at math, this would be very easy to do. The calculation simply figures compression ratio with the volume of cylinder at the point where the intake valve closes.

For this you would need to calculate, or plot linear piston travel and find at what height the piston is in the bore at IVC.

[img]http://groups.msn.com/_Secure/0PAAAABsTshS*JL3aL9Lt3DTWpiaZQYuTwADv04vUiCSGPh1UW kUzJcyoBHF!u5vkOCDRCP1bN46o1zkzqk3S2WcjuGbw2xV8/Black-65-2-1.JPG[/img]

[This message was edited by Black 65 GTO on December 18, 2002 at 07:01 PM.]

[Black 65 GTO]

As far as boost is concerned:

Your effective compression will remain the same regardless of boost. This is because your intake valve closing point does not change. Therefore, you are not directly increasing the compression ratio.

What IS increased is your VE. Some will argue that pressurizing the intake mixture will give you more than 100% VE. Some will say that once the cylinder is full, you do not acheive or cannot acheive over 100% VE. What a turbo will help you do is completely (or nearly) fill the cylinder. So, by simply running a certain PSI of boost, you do not acheive a perportionate amount of VE. By increasing your boost, you are simply more effecient at filling the cyllinder than is possible with a NA engine.

Now, since you are more efficient at filling the cylinder, you will have more air/fuel mixtre available to burn. This will raise your cylinder pressures or BMEP (Brake Mean Effective Pressure).

This is what makes more power, not the actual compression ratio.

[img]http://groups.msn.com/_Secure/0PAAAABsTshS*JL3aL9Lt3DTWpiaZQYuTwADv04vUiCSGPh1UW kUzJcyoBHF!u5vkOCDRCP1bN46o1zkzqk3S2WcjuGbw2xV8/Black-65-2-1.JPG[/img]

[Tom Vaught]

I have read all of the theories above.

1) You do not double the effective compression ratio by running 15 psi boost.

2) You change the density ratio of the engine with pressure ratio changes. (lb/min of air)

3) You can change the density ratio even further with proper intercooling of the charge.

The comments about valve timing are correct on any engine. Static compression ratio tells you
very little.

An engine with 25 psi boost (gage) normally will
only raise the peak cylinder pressures by 25%
you can see that on most boosted engine graphs.
Corky Bell has a good one in his book. What changes is the amount of oxygen that is available to burn. If you add the appropriate amount of
fuel you keep higher cylinder pressures for a longer period of time which equals more force on the piston/ crankshaft. It is very common to see
boosted engines make over 1000 lb/ft torque at
6000 rpm on a 7500 rpm engine.

Even Boost pressure can be deceiving. Jimmy Keen
never made over 21 lb boost (gage) on his 1600 hp
engine yet there were other racers that routinely
said that with the same supercharger, same engine
rpm, same blower belt ratio, they made 28 psi
with their engine. (BUT THEY WERE SLOWER THAN JIMMY KEEN). They had the wrong cam, the wrong
ports, valves, etc to breath properly compared
to Jimmy. Racing boost pressure is like racing
flow benches or dyno programs, it doesn't work.
Cylinder pressures, air flow (in lb/min), and knock limit of the engine is the real deal. Tom V.

[Dave A]

Brian B, I do share your apparent frusteration with all of the terminology and how loosely it is applied sometimes! 'Effective Compression Ratio' in an of itself is not a terribly useful figure, but as others have pointed out, what you are really interested in is cylinder pressure throughout the engine cycle.

The way folks use the term 'effective compression ratio', it is probably interchangeable with 'dynamic compression ratio'. It IS your mechanical compression ratio with valve timing taken into account. Please note that I MISUSED the term in my post above. Whatever is in the cylinder at the point of the intake valve closing is squeezed according to the dynamic compression ratio.

When we're taking into account volumetric effeciency, we should probably abandon use of any term containing the words 'compression ratio' - we are really trying to model the compression stroke at this point and figure out what cylinder pressure is at the end.

For the twin turbo 427 making 987 ft-lbs of torque - is this on pump gas???? Not saying that it can't be done, but it's impressive if it is (I'd suspect race gas, though). Also, note that the torque (and therefore the Volumetric efficiency) peak at a lofty 6200 RPM - the risk of knock is decreased at higher engine speeds.

[Goatman]

I would also caution all of you reading magazines out there that when they say a motor like that runs on pump gas, they are both lying and telling you the truth. When you are dealing with EFI and computer engine management, you can indeed run pump gas "around town" as you actually have a 8.5:1 motor. With the computer and EFI, you limit the injector pulse width and timing and its a normal motor. When it comes track time, I will guarantee that the street gas is drained and that at least 116 Octane is added for big boost numbers.

Rodney Butler's motor in the PE article was dyno tested on pump gas and then on race gas. Look at the differences in power. I think he made around 1000 HP on pump gas (low boost, aluminum heads). 2200 HP on race gas!

Chance favors the prepared mind.

[Tom Vaught]

I personally am not sure if the dyno that was used to test Rodney's motor has the ability to measure 2200 hp. I have seen the dyno facitity in Huntsville, Alabama where they run the normal engines and it doesn't have that capability. The
dyno at Wayne Youngs's shop may have that HP
capability. The largest dyno I have seen that
was capable of 2000 hp was John Meany's. It had
two 1000 hp water brakes. I will have to talk with Wayne and confirm that. Tom V.

[Goatman]

Sorry, that was a bit misleading Tom. On that dyno, it made 1800HP. The Butlers are now claiming the engine is making 2200HP. Not sure if that is dyno confirmed or not.

Chance favors the prepared mind.

[nickg]

i'd have to wonder about that one since most diesel's run at around that range or higher?

[Taff]

KH, some other factors involved could be the use of sequential fuel injection and individual cylinder ignition timing systems. The timing computer in conjunction with a knock detector will retard the amount of ignition advance until best performance, just prior to detonation, is realised. This will occur with whatever fuel you use,with the computer retarding timing more for low octane fuel, but more power will be produced with higher octane fuel. I doubt too, that the dyno figures that were achieved were on low octane pump gas.

[TCSGTO]

As far as runing 15psi on pump fuel it can and is done often. As a matter of fact I did it earlier this week [img]/infopop/emoticons/icon_smile.gif[/img] Actually it was 17psi in a 78 datsun 280z I swapped a 83 ZX turbo motor(7.4-1 CR) into. The turbo is a t3/t4 hybrid with a T4OE 50 trim compressor side that is quite efficient at that pressure which lets it transfer less heat to the intake charge. The Spearco IC drops the inlet air temps further and at 17psi my total timing is 20deg. The 420cc injectors controlled by a SDS programmable system are more than adequate for fuel delivery. Cool the air,add enough fuel,knock the timing back enough to keep detonation away,and run the lowest restriction exhaust possible and there are no problems. I've never put race fuel in the tank and have never had any detonation. This is on a 90,000 original mile engine that has been abused by me for over 20,000 of those miles without a rebuild. Believe me, I've tried to hurt this thing and it's no worse for wear. Best ET is a severly traction limited 13.43 @ 108.
I know of others with larger more efficient IC's running more boost than I am on pump gas. With 110 octane race fuel I hear 25psi is not uncommon and lots of fun.

[Goatman]

As Tom and others have said, boost isn't the same across engine families and combinations. Combustion chamber design alone has alot to do with boost tolerance.

You can run a 11:1+ small block Chevy with cast iron heads all day long on pump gas and it won't ever detonate. Same deal here. Apples to Oranges.

Chance favors the prepared mind.