Hey guys I've been thinking about next years setup and have a basic question.

How do you all feel about the boost vs compression thing. Some seem to like
higher compression and less boost others feel the opposite.

Can we compare a 8 to 1 engine with say 40 pounds of boost to a 10 to 1
engine with 30 pounds. Turbo's are matched to each setup...

Thanks Steve

 

This thread wont get far. u will have ppl who like both types of setups. then you will get one guy saying, just go in between and have the best of both worlds.

 

The type of fuel you want to run will answer your question. Do some research on the characteristics of fuel. Is this a street car, race car or both?

 

This is a street class car, but really spends almost all of it's time on the track.
Fuel is C16.

Steve

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Boostage &raquo;</TD></TR><TR><TD CLASS="quote">This thread wont get far. u will have ppl who like both types of setups. then you will get one guy saying, just go in between and have the best of both worlds.</TD></TR></TABLE>

lol. hilarious, but so true. I could take either side and find a way to argue to its death.

 

Here goes a piece of a article hijacked from Sept 04 MM & FF testing compression vs boost...they are comparing two 4.6 2V short-blocks with all the same top end components(cams,intake,heads,etc) removed and installed on the other.Only difference is one short-block has 8:1 compression and the other has 10:1....

"To see how the high compression and low compression motors responded to boost,we ran both combinations with the 1.7L twin screw Autorotor(Kenne Bell).The pulley ratios were not changed betwen the two. On the high compression short-block,the Kenne Bell supercharger pumped out 600hp@6300rpm and 539tq@4400rpm.The pulley ratio produced 10.2psi@3800rpm and a final boost reading of 9.3psi@6300rpm. We mentioned the boost readings since the pressure actually changed from one motor to the next, despite identical pulley ratios. We attribute this to the fact that more flow was needed to fill the volume in the low compression motor.
The Kenne Bell 1.7L blower assembly was then applied to the low compression 4.6. The blower produced 533hp@6300rpm and 500tq@4400rpm.Compared to the high compression supercharged motor,the peak power was off by 67hp while the peak torque suffered just 39ft-lbs.Note that the change in compression reduced the power across the board from 3000rpm to 6300rpm.It is interesting to note that the boost pressure was slightly lower on the low-compression motor than the high compression version. The peak boost registered on the low-compression motor was 9.3psi@3800rpm,while the boost finalized at 8.7psi@6300rpm.Remember,we ran the same pulley ratios on the two motors,so the compression was the only variable responsible for the loss in power and boost pressure.
The same scenario was repeated on the high- and low-compression versions with a Vortech Supercharger, Equipped with a 6.5" crank pulley and 3.33" blower pulley, the Vortech pumped out 655hp@6400rpm and 556tq@5600rpm on the high compression motor,The peak boost registered 12.6psi @6400rpm.Installing the same set up on the low compression motor,the 4.6 pumped put 607hp@6400rpm and 518tq@5600rpm.The peak boost registered 11.5psi,again down compared to the high compression motor..It is obvious that the change in compression ratio had a major effect on power production."

 

That article is great and all, but what it tells is common knowledge. We all know a higher compression motor will make more power at the same boost if it doesn't self destruct. The benefit of running lower compression is to be able to run higher boost and ultimately make more power before it all goes boom. If your ecu has the capability to run more than 30psi and have decent resolution, then I believe that around 8.5:1 or so and ALOT of boost is the way to go. Alot of people stop at 30psi, that's almost a standard place to stop it seems like. All I can figure is this is because of a 3 bar map sensor. Why stop at 30 if you don't have to?

 

superchargers work a bit different, but the main thing you need to look at is your power goals and what type of fuel you have avalible.

lower compression means you can make more power on said fuel, 10:1 motor will only make about 350-375 on pump gas before you have to start pulling all sorts of timing, the last 7.8ish:1 motor i did made over 500 on pump gas.. we're talking 2 liter b-series here.

it all depends on what you want, if you are running c16 then dont worry about the compression, design the piston and combustion chamber to have the best flow and make the best power, then worry about compression

 

This has been a long debated issue. It all depends on the type of fuel you are running and whether you have the ability to map out your timing.

In the end I truely think its a matter of prefference. Nutty tuner like me believes in the 10:1 minimum principal. However, I have seen both schools of thought work. For a full blown racecar I really don't see the advantage of running lower compression ratio if you have the choice to run alcohol or super high octane race gas. Octane is a whole seperate issue in itself.

For street application running high compression with boost is probably not the best idea. Chances are you are going to detonate or have to run extremely retarded timing to the point of running the EGT too high under boost.

Super charged application..... you can run higher compression and more timing than turbocharged application with the same compression ratio. You will not believe how much timing a roots style blower can take... scarry.. It all has to do with efficiency of the blower. Less efficient more timing, more efficient less timing. On twin screw style blowers they are very efficient compared to standard roots blowers.