compression ratios

[dailydriver]

Originally Posted By: asand1
They use nitro-methane fuel and run a 14-71* blower at 56-74 PSI boost.


Thanks for that info.

Somehow, I always took for granted that they were up in the 100 PSI range for boost, considering their power outputs.

 

[hsd]

2014 Lexus V6 CR=10.8 and runs on 87 octane thanks to ECM computer

 

[artificialist]

Originally Posted By: mjoekingz28
Is it just the crankshaft and piston tops that determine this, and also the head portion of the combustion chamber?

GDI has allowed higher compression than what was possible in the past. Some newer turbo engines use 87 octane and still make full boost.

Anyway:
In the 2 cars I remember:
2002 VW 1.8T 9.5:1
2010 Mitsubishi Lancer Ralliart 9:1 and turbocharged.
Both use 91 or higher octane.

 

[gonefishing]

The 5.3 V8 in my Sierra is 11:1

The available 6.2 V8 is 11.5:1 and premium fuel is recommended.

 

[Clevy]

Originally Posted By: mjoekingz28
I suspect top fuel dragcars are way up there to achieve the power they do.

Is detonation, heat or metallurgy (strength) the holdback of high CRs? Or what?


Fuel in a gas engine.

The more compression in a cylinder the more octane is required to prevent pre-ignition.
You can retard timing to 0 but it won't help if fuel doesn't have enough octane and explodes on the upstroke.
The spark lights off the fuel and the explosion happens as the piston is on the way down. If the fuel explodes to early it's like hitting the piston with a sledgehammer while it's still moving upwards,which is bad.
Ignition timing in modern vehicles use the knock sensor to figure out when to spark. Most advance spark til it senses knock then will slightly retard from there to maximize power output. That's why some vehicles have different hp ratings depending on fuel.

 

[artificialist]

Originally Posted By: Clevy
Originally Posted By: mjoekingz28
I suspect top fuel dragcars are way up there to achieve the power they do.

Is detonation, heat or metallurgy (strength) the holdback of high CRs? Or what?


Fuel in a gas engine.

The more compression in a cylinder the more octane is required to prevent pre-ignition.
You can retard timing to 0 but it won't help if fuel doesn't have enough octane and explodes on the upstroke.
The spark lights off the fuel and the explosion happens as the piston is on the way down. If the fuel explodes to early it's like hitting the piston with a sledgehammer while it's still moving upwards,which is bad.
Ignition timing in modern vehicles use the knock sensor to figure out when to spark. Most advance spark til it senses knock then will slightly retard from there to maximize power output. That's why some vehicles have different hp ratings depending on fuel.

Also, some turbo engines raise or lower boost depending on fuel octane.

 

[Jetronic]

Originally Posted By: Clevy
Originally Posted By: mjoekingz28
I suspect top fuel dragcars are way up there to achieve the power they do.

Is detonation, heat or metallurgy (strength) the holdback of high CRs? Or what?


Fuel in a gas engine.

The more compression in a cylinder the more octane is required to prevent pre-ignition.
You can retard timing to 0 but it won't help if fuel doesn't have enough octane and explodes on the upstroke.
The spark lights off the fuel and the explosion happens as the piston is on the way down. If the fuel explodes to early it's like hitting the piston with a sledgehammer while it's still moving upwards,which is bad.
Ignition timing in modern vehicles use the knock sensor to figure out when to spark. Most advance spark til it senses knock then will slightly retard from there to maximize power output. That's why some vehicles have different hp ratings depending on fuel.


Petrol doesn't explode, that's what happens in a diesel. Petrol burns, with a flame front moving from the spark plug(s) and chasing the piston. Low octane fuel can explode ahead of the flame front due to temperature and compression rise, after the plugs have sparked.

 

[CarbonSteel]

16.2:1 = 2015 6.7L Powerstroke Diesel
10.8:1 = 2014 3.5L Explorer
8.8:1 = 2010 Kawasaki Brute Force 750i

 

[RoadRaceJosh]

2015 Nissan Frontier 2.5L, 9.5:1, 152hp
1998 Honda VFR 800, 11.6:1, 108hp
1997 BMW M3, 3.2L, 10.5:1, 240hp
1954 Dodge pickup, 241 ci V8 (4.0L), 7:1, 123hp net, 140hp gross

 

[lexus114]

my rx is: 10.8
wife`s c300: 11.3

 

[mjoekingz28]

16.2:1 PSD


Is that the main reason diesels are so heavy? To handle the stresses ofhigh compression....also could be the high amount of low rpm torque.

 

[Jetronic]

the diesel fuel explodes, pretty much a detonation event. that's why they are so heavy. You already know what happens to a gas egine with continued detonation

 

[RoadRaceJosh]

Originally Posted By: Jetronic
the diesel fuel explodes, pretty much a detonation event. that's why they are so heavy. You already know what happens to a gas egine with continued detonation


It's not that simple. Older diesels knock at idle because they inject a significant portion of fuel before the ignition event begins. Once ignition begins the small cloud of fuel burns in a very short period of time. Newer diesels inject a small bit of fuel, pause while it ignites and then inject more. This quiets them down massively.

Diesels are heavy because of the high compression ratio, yes, but also because they are asked to operate near their rated output for a much larger percentage of time than a light duty gas engine. If you look at a 300 hp gas engine and a 300 hp diesel the differences are massive. But look at an older 300 hp gas truck engine and things look a lot more like the diesel.

 

[Jetronic]

Originally Posted By: Joshua_Skinner
Originally Posted By: Jetronic
the diesel fuel explodes, pretty much a detonation event. that's why they are so heavy. You already know what happens to a gas egine with continued detonation


It's not that simple. Older diesels knock at idle because they inject a significant portion of fuel before the ignition event begins. Once ignition begins the small cloud of fuel burns in a very short period of time. Newer diesels inject a small bit of fuel, pause while it ignites and then inject more. This quiets them down massively.


the newer diesels are built a lot lighter aswell.

 

[punisher]

The old Winston Cup/Nascar engines ran compression in the 16.5-18 to 1 area pre 1996. The engines ran on the edge for the entire race. All it took was the tiniest oil leak (ring or intake valve guide), or lean condition to cause detonation, and let the smoke out. Once detonaton or sometimes pre-ignition began, engine life was measured in seconds.

EDIT: 116 octane fuel was the rule.