Quote:
Originally Posted by Shownomercy
(Post 583910)
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?
|
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,