question: Low CR vs. High CR for forced induction?

[JustCallMeOrlando]

The VVTi 1JZ runs only 9-9.5:1 compression doesn't it? I don't remember it being double digits. It's combination of VVTi and smaller single turbo worked wonders for it's torque though yes, max torque at like 2400rpm.

[Nim]

The VVTi 1JZ runs only 9-9.5:1 compression doesn't it? I don't remember it being double digits. It's combination of VVTi and smaller single turbo worked wonders for it's torque though yes, max torque at like 2400rpm.

However, getting decent boost into it would cause problems. That's why you want VVTLi on a boosted engine. Could run big(ish) boost with higher dynamic compression down low for your economy/emissions/low end torque. As the turbo(s) spools up, swap cams.

[mic*]

this kinda sums it up...

Just looking quickly at this chart, if the peak pressure is the same for both comp ratios drawn, then the 2:1 hoypothetical line would require 7.5 X the intake pressure of the 15:1 line. This is supported by the position of the 2:1 line at/near BDC compared to the 15:1 line.

So in the real world, on a street vehicle, spooling up massive intake pressures with a turbo just to acheive (EDIT: power) = to a high comp N/A motor isnt very practical for stop / start, low rev driving. So a setup like the BA Turbo is more favored; higher static comp, less boost.

In a race engine where you would never let the revs get low, and the turbo should be boosting constantly, running massive (well cooled) boost and low comp gets the same air/fuel in and reduces the piston speed required to do so. As mick said tho it gets harder to drive coz the usable rev range is narrower.

High boost, low comp + usable rev range = lisholm

[kingmick]

lisholm, good for street, usless for circuit racing.

[oldcorollas]

Just looking quickly at this chart, if the peak pressure is the same for both comp ratios drawn, then the 2:1 hoypothetical line would require 7.5 X the intake pressure of the 15:1 line. This is supported by the position of the 2:1 line at/near BDC compared to the 15:1 line.

your kinda missing the poitn tho (a little).. it's not peak pressure that makes torque, it'sthe area under the curve...

if you compare the two graphs with same peak pressure (and heaps of boost for 2:1), the 2:1 will have much more torque, as it has greater force acting on the piston for longer...

read the article that the diagram came from

but basically...

lower CR needs higher boost to reach the same peak pressure.
peak pressure is kind of one of the limiting factors due to the fuel properties.
low CR and high boost will give more torque when on boost.. but be worse at all other times

[Nim]

That diagram also shows how crap a 2:1 compression would be unboosted. The green is the difference between an NA 15:1 and 2:1.

[mic*]

I guess i jumped loosely from the graph reference to the real world situation. They dont really relate very well.

My understanding is the graph is demonstrating (impractically) the effect on torque by altering boost/comp with the same maximum cylinder pressure as the limiting factor.

I was trying to suggest very vaguely how when looking at these variables if you took two cars, one N/A and one boosted, and say that the N/A car was making slightly higher peak torque, you could drop the static comp of the boosted motor pump up the boost to get the same dynamic comp again, and you may now have the torque over the N/A, but it sucks to drive. So dont build a street car like this...

I think . Leave me alone. Im sick .

[Nim]

The graph is working on dynamic compression already. And that's not peak torque, that's peak pressure... I think you're reading the graph wrong.

[gearb0x]

And what about diesel. What if you shove more boost into a diesel. Take for example an older mercedes turbo diesel that doesn't have an ECU of any sort. If i force more boost into a diesel am i limited by the strength of the motor? too little fuel and too much boost in a diesel what will happen? will the combustion tempratures soar thru the roof? i'm not sure but i'd like to try with a spare turbo diesel i have. (and the effect of higher boost on a turbo diesel running straight vegatible oil)

Turbo diesels are a different animal again The more boost you add, the more diesel you need. But if you add a heap of boost without upping the fuel u generally wont see much if any benifits, deisel EGT's actually get hotter with MORE diesel, so if your overfueling your likely to crack combustion chambers etc.. Then with older style diesels with "pre combustion" chambers you are limited by their size and how much actuall air you can get through such a little hole. A friend is experimenting with non turbo diesels, specifially an SD33 on SVO

Im definatly not an authority on this subject, what i know about turbo diesels came from all the researching i did when i was going to turbo a non turbo landcruiser for a tow car.

[mic*]

The graph is working on dynamic compression already. And that's not peak torque, that's peak pressure... I think you're reading the graph wrong.

Ok where am i wrong;

Peak pressure = dynamic comp - it is constant for both examples "1000"
Torque = area under the curve - it is greatest for 2:1 boosted line

I realised when typing this ive disregarded the bottom line - the N/A 2:1 so truly its;

LOW CR vs HIGH CR vs BOOSTED, and subsequent effect on torque.

Which is practically the title of the thread which is why Stu is like a million rep...

[Nim]

Okay, perhaps I'm ust reading your post wrong then.

My brain is dead today.

[AE82FROG]

Hey guys. I think with this graph, and you can correct me if i am wrong, the most important thing to note is 30PSi 4AGTE's comment about combustion chamber size.

The 2:1 (static) compression ratio engine's combustion chamber size is going to be half of the entire cylinder volume (measured when the piston is at BDC). The 15:1 (static) compression ratio engine's combustion chamber will be 1/15th the size of the cylinder (measured as above).

Considering that the cylinder pressures that can be tolerated by both engines will be limited by the fuel they are using, all things being comletely equal in terms of peak cylinder pressure (the article that gave us that graph states that the top curve is what would be achieved with a supercharger making the cylinder pressure in the 2:1 engine the same as the 15:engine), of course the turbo setup is going to supply more torque (area under teh curve)

I think to main reason for going with low comp pistons on a turbocharged engine is to make the most of increased combustion chamber size.

I have done some calculations and a 12.7:1 static comp ratio Formula Atlantic 4AGE has a combustion chamber of 33.8cc

A 4AGE with 8:1 (static) compression pistons has a combustion chamber of 56.67cc.

That's a 67percent larger chamber in the tubo engine which will explain their inherent abilities for making more power than NA.

This is only my thoughts so please correct me if you don't think this is correct.

[Nim]

Yeah, that's one way to look at it. Although I find that way of explaining it confusing, but that's okay.

[AE82FROG]

Sorry, didn't mean for it to be confusing. In the work i do i come across heaps of people that always state "use low comp pistons if you are going to turbocharge, otherwise the engine will blow up"

But it is not the combination of boost and high static comp that will blow the engine up, but rather making the cylinder pressures too high - as they try and chase the sort of power outputs that other people have achieved by using low comp pistons.

High comp turbocharging can be completely acceptable and safe, but you have to realise the ceiling of achievable power will always be lower than using a low static comp setup because you are working with a smaller combustion space.

[30psi 4agte]

well put!