Boost question
 

Been doing some reading on forced induction applications. For the bigger applications most guys drop their CR to the 9 area and lower if its a strict race car is what I have gathered. But it seems like you would be taking a step backwards to run boost.... Why not make a solid motor to run boost at a higher CR?

I understand that a lower CR will be much more forgiving to detonation/knock from fuel issues, or tune issues, but can anyone explain why you see more of a power gain from a lower CR compared to a higher CR with the same boost level?

No matter how good your tune, the gas will still predetonate at higher boost. The lower compression lets you run more boost and timing without having to worry about it. Keeping your timing normal is more important than keeping the compression. You are basically multiplying your boost times your compression ratio to get combustion pressure, so higher compression exponentially raises combustion pressure. And another reason for replacing the pistons, if that is what you are concerned about, is that you need pistons that wont develop hot spots under boost and break. Forged is the way to go!

Less compression = capable of more boost on the same tuning and fuel. More boost, everything else being equal = more power. The slight loss in NA power is definitely worth it

Right, but it seems like you are just shifting one curve downwards with the lower CR where as you could limit your boost and make the same power with a higher CR and not lose 25% fuel out the back....

Boost creates more power than compression. So the more boost you can build the motor to take the more power potential it has than a higher CR motor at a lower boost level.

Not all forged pistons are boost friendly. You want low silicone , less brittle material such as 2618 alloy as they are way more forgiving to adding extra pressure of boosted applications.

There are also ways to allow for more boost to a high compression engine without blowing it up such as alcohol injection but if you are planning to run a ton of boost you definetly want to get that compression ratio down.

more boost with less timing.

You guys are not getting the point here.

I know "how" to run a lot of boost on an engine, I was more concerned with why you make more power with a low CR compared to a higher CR and same boost levels.

Tim, I think you prob had the best answer, but any reasoning?

To me, asking why boost adds more power than uping your compression ratio is like asking why Nitrous adds more power than gasoline. It just does, accept it, the more compressed a combustion chamber is the more likely it is to pre detonate due to the extra heat created in there, thats why high compression engines require higher octane levels, the higher the octane the slower the burn. Boost is a horsepower multiplier the more air you squeeze in there without blowing it up, the more power you will create. It always pays to think of engines like vacuums, the more they can suck in there, the better they will perform but at some point when they can't suck anymore you shuv a air compressor down there hose and get a ton more air circulating threw there. The more air passing threw that cylinder, the more fuel you can add and the more power you will create.

I'd guess that the cylinder pressures created in such an application would be a serious issue as well.

If you run the exact same psi and everything else being equal, and you lower the compression, you will actually lose power. But its the benefit of being able to crank the boost up that you are after.

Boost is boost all the time. Compression is just compression on the compression stroke. Raising boost is more than just compression, its raising the volumetric efficiency as well. Thats why its worth more power, if thats what you mean

There are really a ton of factors, Without writing a novel.....

Theoretically you are correct in that it doesn’t really matter if you go N/a with ultra high compression, or high static comp/low boost, or the reverse. What you want to target is the engine's final CR (in this case factoring in boost). And with this you also need to factor in the operating efficiency of the supercharger, such as heat, operating range and when boost occurs. With boost, cam selection is very critical. Unlike a N/A engine, cam/valve lift is a bit less important then duration but LSA is critical, you don’t want the cam to bleed off boost, but you also want to use the pressure to push out or clear the cylinder of the spent gases.

You want to look at along with the type of fuel you using to ensure you don’t go into detonation. But you will eventually run into the limits of compression you can have with any pump gas or even race gas regardless of how you achieve your final CR.

For the SCs, it comes down to larger (race type) supercharger are more efficient then smaller ones, they build boost sooner and more linier, they make less heat, and “should” use less HP to drive them. But because they can move so much air and produce so much boost you need to start with less static compression or you would have too high a compression ratio at max RPMs. You need to match the SC with it’s max RPM efficiency, simply changing pulleys to speed up or slow down the SC to adjust boost is normally not the most optimum.

To actually answer your question.... High boost levels does have advantages over low boost in that with the proper setup you have a more efficient supercharger, and can use the additional boost pressure to better control the air/fuel mixture, clear the spent gases. Since the engine is turning the SC, it takes HP away from the engine as the SC makes it. So the "give back" or wasted HP ratio can be skewed with high comp/low boost engine will likely not make as much power as a low comp high boost. These factors allow the engine to make greater HP and torque then a "like" engine with higher static CR and less boost.

Couple points when talking about boost:

1) Boost is normally measurement at the intake, not within the cylinder, you are somewhat measuring the pressure that you are not using. If you changed to a higher flowing heads (both in the in/and out side, with a cam designed to use the pressure to clear the cylinders), you could have less boost but make more power. Boost can overcome lesser flowing heads only to a certain point, after that you have noting but unused pressure in the intake.

2) The byproduct of the SC compressing air is heat and the heat / expanded air results in less volume, so this is compounded when the piston further compresses it. This is one of the big factors with high boost; you need a serious intercooler (either large exchanger or a chemical intercooler) to manage/reduce this heat.

As an example: I'm currently running the small P600B unit, originally I had a 15lb pullies and a redline on the SC of 8500 RPMs( Supercharger RPM, not engine). With that pulley my engine RPM limit is around 6400. Above that I run the risk of the supercharger tearing itself apart!

But with that pulley and my 385 CI displacment I actually hit max boost of 9lbs at 5900 RPMs, above that the SC simply can't move enough air to feed the engine, and boost drops off. I also see high inlet temps at those RPMs because the SC is out of it's effiecency range. By dropping to a 12lb pulley, I get the same 9lbs at 6200RPMs, but still high inlet temps. But I make a few more HP and I was able to spin the engine higher into the RPMs where the engine/cam were more efficeient.

now, If I go to a larger SC, such as a model D1, I could make the same boost at the same RPMs but because the SC more effiecient I will have lower inlet temps and make more power.

hope this helps,

Well moral of the story is, you need a Ysi. ;)


And thanks, very nice write up!

F1 is what I'd like ( I'm a procharger kinda guy!) The P600B was used and bought for a bargain price from a friend! I nice set of heads and a F1 with about 18 LBs of boost should put me in the 700-750 RWHP which is what I eventually want..... But that has to wait until after I finish paying for my oldest daughter's wedding and my youngest college.... DAMN KIDS!

Aha that sucks having responsibilities!

Can you have them re geared and I guess a new impeller to upgrade or is it just another 5k head unit purchase?

The unit I have is just too small, the P1 can be upgraded to a D1, but I want something just a tad bigger! :rock:

And new spark plug wires.... ;-)

Oh that's just cruel! :cry:

Splain?

When I first put my engine together it developed a miss. It turned out that the wire that run behind the PS pump made contact with the pulley on the pump and cut into them so they were arching causing the miss. So I ordered a new set and "thought" I had them routed well enough.

Fast forward, I drop the car off to Josh last week to have him do a new tune on the car. And out of nowhere, dead cylinder. I was freaking out, thinking something serious! Turns out same thing happened, so another set of wires going in!

Yea, I had to finagle my wires to prevent that from happening, tight space to play with there....