WHY DOES BOOST LIKE LOWER COMPRESSION ??

[2002_Viper_GTS_ACR]

Ok people,

I need help understanding why Boosted (either turbo or super charged) cars like to START with lower compression ?

I know its TRUE, I just need to understand at the scientific level, WHY !

If we look at ultimate power, one would think that the HIGHEST compression motor with the MOST forced induction you could put into it, would produce the MOST power. Yes, I know the limiting factor is probably the Fuel.

But why is it better to take a 8.5 to 1 motor and put 10 psi into it, VS a 9.5 motor and just do 7 psi ?? What is the big advantage of lowering compression ?

Wouldn't equal amounts of pressure produce MORE power in HIGHER compression motors ?

I have always known from my Porsche turbo days, that 7.5 - 8.5 to 1 compression is the "sweet spot" for turbo and super charged cars. Now tell me why.

Save us both time by not explaining the simple stuff like you need at least 7.5 for the motor to run when its not on boost. I know this.. Please provide educated and knowledgeable response to all this. I am really looking to expand my knowledge !! Thank you.

Jon

 

[RedGTS]

It's not complicated. Lower compression resists detonation, the archenemy of forced induction. You can pump huge amounts of boost into a properly built low compression motor, and you just can't do that on a high compression motor, because other methods of resisting detonation (higher octane fuel and charge cooling) can only help so much when the motor is prone to preignition to begin with due to the compression ratio.

 

[2002_Viper_GTS_ACR]

Still doesnt answer my question.

I bet the cylinder pressures on a 8.5 motor with 10 psi is HIGHER then the cylinder pressures on a 9.5 motor running 7 psi.

Yet the 10 psi w/8.5 comp is probably safer then 7 psi with 9.5 comp.

IF the ultimate goal is to get the highest amount of cylinder pressure, then why not just do 11 to 1 compression and only pump in 3 to 4 psi of boost ?

I am not understanding how boosted pressures in lower compression motors are more stable or safer ? Are they ?

Assuming you aquire 200 psi in a compression stroke, does it make any more power or is any more stable if you did it by supercharging a 8.5 to 1 motor with 7 psi then doing a 10 to 1 motor with 5 psi ?

I am just asking for a given cylinder pressure, both being equal is it safer to have gotten there by using more psi from the super charger, or more compression of the piston, then explain WHY !

 

[utahviper]

A properly built engine with 10psi/8.5:1 compression will make more power than a 3-4psi/11:1 motor with the same compressor because higher compression doesn't make as much power as higher boost. I would rather have the 10psi motor because it would make more power and you could still run even more boost. Also, timing is power and a viper motor with 11:1 compression would probably have to run some race gas with no blower to prevent detonation. In order to run a 11:1 motor with a compressor (blower,turbo,etc) you would have to take out so much timing, so you wouldn't detonate, that it probably would make less power than the same motor with no compressor. Also, in order to make decent power from a compressor you have to run at least 5lbs of boost otherwise inefficiencies come into play.

HOrse Power from a 15psi compressor is in a bell curve, meaning that the first 1-3 and the last 12-15 lbs of boost do not give you as much power as the middle 9lbs of boost. So in order to at least get into the meat of the curve you have to run somewhere between 4-11lbs of boost. All compressors are different bell curves,starting and ending points, but they all are basically have the same curve.

The ultimate setup would be to run 11:1 compression with 10lbs of boost but you have to run race gas (118 octane), water/methanol injection, huge intercooler (w/nitrous spraying the intercooler),an incredible pcm with knock sensors and I am sure some other equipment to even run this setup. However, this would make incredible power as long as you could get it to run without taking out huge amounts of timing.

Let me take it one step further. Start off with a super built motor that would be able to run lets say 20+ lbs of boost(turbos) with timing advance at 32-33 degrees from 3000rpm and upto 7000rpm. As long as you could keep the compression above 7.5-8:1 that would probably make the most power but you would have to run the things mentioned above to make it work. You would probably have around 1300-1500rwhp with that setup. This is getting into supra territory 30-35lbs of boost with 800-1000rwhp out of a 3-4.5 liter engine.

We have only just begun to tap the potential of the viper engine. There is a long way to go and lots of unchartered territory. How much boost can a stock motor take? Can you build a reliable motor to run 20+ lbs of boost? How much timing can you run at 10,15,20,25 lbs of boost with a reasonable setup, not like the one mentioned above. Intercooler and water/methanol I think is reasonable but that is about it. At what point do you have too much power 850,900,1000,1100,1200 rwhp? Where do you stop? What about using a turbo and increasing boost the faster you go? For example, start with 8psi upto 60mph, 12psi upto 90mph, 15psi upto 110mph, and then 20+ psi until the traps.

 

[Sean Roe]

I only have a couple of minutes to answer, but basically, you have to think about what is happening in the cylinder. The stock Viper Gen2 piston is compressing the air in the cylinder 9.6 times to one. At top dead center, you end up with a given cylinder pressure (dependent on camshaft design, etc.). Now, lets say we reduce the compression ratio down to 8.6:1. The piston is not compressing the air as much in the cylinder (lower pressure at TDC). Well, that means we can force more air and fuel in the cylinder to start, then end up with the same total cylinder pressure when compressed. It's the additional air and fuel that allows a lower compression with higher boost engine to make more power. The more fuel there is to burn, with the right A/F ratio, the harder the piston will get pushed down when that mixture is ignited. You're getting more air and fuel in the cylinder with a lower compression / higher boost engine.
Regards,
Sean

 

[Bugeater]

Good Q Johnny!

That answers it for me. Past explanations typically focused on simply the PSI number itself, and not how it was derived (with more or less air/fuel).

Thanks Sean! This question always lingered in my mind as well.

 

[utahviper]

Thanks Sean. I missed the point.

 

[2002_Viper_GTS_ACR]

Sean is hitting it the closest.. I think he is on the right track !!

So I think I understand everything you said, just need a bit of clarification.

So lets say (these numbers are TOTAL hypothetical pulled out of the air) just used for an example:

With lower compression (and more boost) you could put 2 Cubic Feet of air and compress it into 120 PSI TDC Cylinder pressure

OR/BUT

If you did higher compression pistons and lower boost, you might only get 1.5 Cubic Feet of air pushed into that SAME 120 PSI TDC Cylinder Pressure ??


Is this correct ?? IF so, THEN I finally understand why lower compression with more boost is better, as you get more REAL AIR in there.

SEAN, please respond to this last question, so I can finally get a good nights rest ;-)

Jon

 

[SapphireGTS]

NEVER Lower the compression. I cant tell you the details of lowering compression on these engines but it is not a good idea. Again, this is a very bad idea and you will really regret it if you lower the compression.

I cant tell you why its bad, yet, but its bad.

 

[jrkermode]

2002GTSACR,

That's my understanding of Sean's post.

Sean, TorqueMonster,
How about a related question?

At what point does the lower compression cylinder's lack of quench reduce the combustion efficiency such that the lower compression, despite more air, does not burn as efficiently?

I would suspect a mega-boosted low compression cylinder more than offsets the lower efficiency burn, but I'm wondering if there isn't a boost / compression point of compromise.

I ask because I seem to recall reasonably high compression ratios being used in the Turbo F1 days.

 

[joe117]

The lower static compression isn't going to mean that the boosted actual compression isn't going to be as high as possible when you are on sc.

It's not the 10:1 or 8.5:1 that you have to think about. It's the final pre spark pressure in the chamber that decides if it will detonate or not.

The nascar guys running on a restrictor plate track, can set up for a very high static compression because at racing rpm the engine can never get a full breath. They don't start with a full cylinder so they can compress what they have lots more.
Just don't give it too much throttle until you get the rpm up.

The NA cylinder at BDC starting compression stroke is starting from about normal air pressure or less.
The SC cylinder in the same place is starting to compress a charge that is already compressed.
You can only end up with so much chamber pressure when you light it if you don't want to detonate.

There are squish areas in a chamber that will get things swirling and hold down detonation a little.
You can lower the compression without getting rid of the squish areas.

 

[glenn hofstetter]

Consider a mildly (5 lbs. boost) supercharged viper with a crank HP of 650. If you decrease the compression of a cylinder from 9.6 to 9.0 you have lowered it 6.25% and will lose about 8% of your crank Hp about 50 ponys.You now have 600HP at the crank. BUT you can now force air into the chamber in a higher volumn than before. Lets say you move your boost up from 5 lbs to 10 lbs. You are now increasing your boost 100% which equates to about 85% more additional boost ponys than the original setup. This does not give you 85% more power because 450 of the HP being put out is stock HP that was always there.But it does give you 85% more than the gain you had from the 5 lb setup less the 50HP lost in lowering the compression. So you have 600HP less 450HP stock engine HP (150HP X 185% =280 additional hp added to the 450 stock output. Your new total is 730HP a gain of 80HP. But there is more to it. To safely run your 5 lb setup you ran the engine rich and pulled lots of timing from the advance. Both of these decrease HP and increase RPM runup time. With your new compression ratio you can lean out some and add back some of the lost advance. This can be worth another 50 to 100 HP. Hope this helps some

 

[Torquemonster]

1 - Boost adds to static compression. There are tables that can show you exactly how much compression will raise (other things being equal) if you add specified boost levels to a given static ratio.

For this reason - you really push the envelope when you add boost to a high static compression - it might be 9.6:1 static but 14.6:1 under full boost....

OUCH on pump gas!

2 - There is a way of dropping compression and improving quench massively, but that is proprietory info I'm sorry... anyone who wants to achieve this will need to be prepared to spend extra to do some blueprinting and a total engine rebuild. They'll also need some proprietory parts only available from one source. If you are willing to do that - pm me and I'll point you in the right direction but please do not expect trade secrets on how it is achieved.

I was free with the water/methanol injection info several months before anyone was running it in the Viper community. Now it is a popular mod (and that's great) - but this one I'm keeping, sorry about that.

Just dropping in lower compression height pistons or a thicker head gasket will drop engine efficiency off boost... however for a high boost engine this is better than adding boost to a high compression engine on pump gas....

so lower compression is not as bad as some think if you want to run high boost on limited octane. Besides - the Viper engine has no effective quench in stock form - so lowering cr makes little difference to that, and although lower cylinder pressure off boost will drop efficiency - in most cases not enough to feel if tuned well. Mpg will suffer a bit as will emissions - but not hugely.

Combining high compression and high boost is best left to the experts as they are a grenade with the pin out if the smallest thing goes wrong.

It is possible to run 7.5:1 on a well built turbo engine that has good street manners - but as the V10 is so big and powerful stock - this is unnecessary. Although a 7.5:1 cr could handle 25lb on good pump gas it is impracticle for many reasons - one being that running 25lb boost would make it a 5th gear wonder and a death machine in all lower gears.

For this reason - running from 8.5 to 9.6:1 is the best range for the Viper on boost, with the higher compressions limiting boost or requiring expensive compensations to make it work and hold up.

 

[pauljkopick]

Whatcha talkin bout??????????

 

[2002_Viper_GTS_ACR]

Thanks Guys !! Very information

Jon