It's kind of a truism that low CR/high boost is a better way to make big power on a forced induction motor than higher CR with lower boost. While I believe this, I am not sure why it's so. David Vizard tries to explain it in one of his books. I came across the section again recently and re-read his explanation. I am not sure it made sense and would like some other opinions.

What Vizard said is basically this: the volume of space for the intake charge is greater with a low CR setup (more area above the piston crown at TDC). As a consequence, the mass of the intake charge is greater, thus the motor makes more power. He says that this factor outweighs the greater efficiency with which the fuel is burned in a motor with a higher CR.

What I don't buy is that it seems like the mass of the intake charge would be a function of flow over time. Obviously, if the flow is into a larger space the pressure rise over time would be less and more flow would occur. But why doesn't the same thing apply to an NA motor? Maybe it does?

I'm confused and hope someone can clarify this.

Rich Krause

 

This variable is not always COMPLETELY true. As long as you have a head unit that is capable of running high boost levels while still being in the good zone of its compressor map, this is true. HOWEVER, if you are beyond the means of your compressor and you are out of its efficiency range, you are going to be blowing a lot hotter air in.

I recommend building your engine around your head unit if you don't plan on changing it. Go to Vortech's site and look at the map for your particular head unit. See if you are in the efficiency range for that level of boost. That should help you make the determination.

AS usual, if someone sees something wrong, feel free to correct me.

Ryan

 

I'll try a simple way of explaining it.

High compression, low boost: A small amount of gunpowder in the open lit won't do much. Put it into a tiny chamber behind a bullet, it'll make a huge amount of pressure. A high compression engine is doing just this. Its packing a big air fuel mix into a tiny space then lighting it. The initial cylinder pressure is big, but decreases quickly as the piston goes down.

Low compression, high boost: The effect of this is a large cylinder pressure that does not fall off as quickly as the piston travels down. So in essence, the piston is being pushed hard for a longer duration.

 

FI = Great increases in average cylinder pressure b/c of the sheer amount of fuel/air without a proportional increase in peak cylinder pressure.

AFAIK offhand anyway. Honestly never put too much thought into it .

 

I think your confusion may lie in that you are thinking about filling the cylinder, as opposed to the fuel/air expanding on the power stroke?

It's true that a larger cylinder volume would allow for a greater mass of intake charge. And since a supercharger increases c/r by the fact of forcing more air into the same space, a lower c/r to begin with (more volume, or area) would conceivably have more fuel/air packed into the cylinder, all other things equal, than a higher c/r cylinder. We're splitting hairs here I believe, as far as the volume difference, though.

I think the limiting factor is the quality of fuel you run. You must prevent detonation, and supercharging increases c/r bringing you closer to the threshold. If the engines c/r is high to begin with, less boost can be made before detonation occurs. Less boost means less fuel air and less power, obviously. A lower c/r will not only allow more volume (slight as it is) but allows for the increase in c/r as a result of boosting the intake charge into the cylinder.

High static c/r makes power at peak pressure which is mostly at the top of the stroke near TDC. A boosted engine, even with a lower static c/r makes more power at all areas of the power stroke due I think to the increased volume of fuel/air expanding as the piston moves downward.

From what I gather, a lower c/r boosted engine, with the same final c/r as a NA engine with a higher static c/r, will make more power at all areas of the power curve than the higher static c/r engine.

YMMV

 

That kinda makes sense. I need to think about it some though.

Thanks.

Rich Krause

 

Let's back up just a little, here. First, assuming you are using a certain fuel in both cases, the pressure at TDC is limited to eliminate detonation.

Let's assume you are using 91 octane fuel, and the pressure at TDC is limited to 150 psi (compression only, no combustion). Now, based on this, which would you rather have. Would you rather have 105cc of air/fuel at 150 psi, or 110cc of air/fuel at 150 psi? I think it's a no-brainer that 110cc of air/fuel will give you more power. By incresing the piston dish volume by 5cc, and increasing the boost to make up for this compression loss, there is more volume of air/fule at TDC.

I know there are impeller efficiencies, etc. to consider, but what I think Vizard was trying to say is for a given fuel, and therefore a given pressure at TDC, the lower c.r. and higher boost application will make more power due to the fact that there is more volume of air/fuel at TDC.

Hope this helps.

Shane

 

also, did ya think about heat management? the top fuel guys run insane compression with insane boost. the diffrence is in the fuel. it burns cooler to allow for the conditions. now take an average FI motor. you're limited to 91 octane (here in TX) or race gas. say 108. even then, droppin the CR is an effective way to get the cyllinder temps down. w/o upping th octane. granted, thats not the only reson why they work better but it's one that i hadnt seen mentioned. ;D happy rodding,

 

Quote "a supercharger increases c/r"

I thought c/r was a number that was given by volume of cylinder with piston up or down. the pressure in the cylinder will change but the c/r will not.

Quote "the top fuel guys run insane compression with insane boost"

They run really lower c/r and really high boost.


I would think lower c/r with high boost would be better because of the heat generated by squeezing the volume of the cylinder into the 58cc camber and 6cc in the pistons. Which would be the cast in a high c/r engine(stock).

If you were to squeeze the more boost into a lower compression engine with 70cc camber and 26cc piston than there is not so much heat generated.


Lewis

 

"If you were to squeeze the more boost into a lower compression engine with 70cc camber and 26cc piston than there is not so much heat generated."

good point.

i was pretty sure that the nitromethane guys were running like 14:5.1 CR with the big blowers.

 

A simple eay of seeing it,

A NA setup can only draw in air equal to (ignoring inertia effects) the swept volume.


A FI setup doesn't care about how the how the volume is achieved, it has pressure to fill the space regardless.

So the effective displacement of the engine is greater with the Lowcomp FI than the equivalent NA. Likewise headers do help to fill some of the unswept volume due to creating a minor vacuum so the effect can be seen NA but not as much so .

 

That makes sense to me.

Thanks (all) for your thoughts.

Rich Krause