If you lower the engine compresssion on a non intercooled centrifical blower can you safely increase the about of boost, or is heat heat to a engine?

 

im no guru, but from what i understand yes. i know some vehicles that get forced induction from the factory have a bump down in compression from their naturally aspirated counterparts to help handle the boost.

i hope im right cause if not im gonna sound like a tool

 

sounds right to me just wanted to double check

 

1JT,
I hate to jump in on your post but this is perfect timing...
I am actually trying to find out now what the answer to the CR question with Centrifugal Blowers. There is a difference in opinion from the Roots style blowers.
I am building a new long block to replace my very high mileage engine (because of a coolant leak in the head gasket). I contacted Livernois Motorsports to purchase Mahle pistons. I wanted stock PI piston (15-16cc) which they do not have in the Mahle brand. They are telling me with a Centrifugal Blower I am better off with higher compression and mild boost (not more then 12-14lbs with forged rods and pistons).
With 16cc pistons, stock PI heads and the multi layer boost specific head gaskets; they calculate the CR to be about 9.3. If I install 10cc pistons the CR will be about 10.2. They said with the higher CR, I will have about 45-50 or more HP for normal non-boosted driving and then the boost on top.
I am trying to get a hold of Mike Troyer to get his input since he will be doing the tune.
I have to get this engine done and installed by the end of August so I will let you know what I decided and why.
Any more input?

 

Well I got a round-about answer from Mike (relayed through his counter help) to go with the lower compression. That is not exactly the open dialog I was hoping for but I guess it is best and will leave open further boost options in the future.

 

hi! do your self a huge favor and do not lower your comp. you are going to lose alot of tq.!!!! you should only lower your comp. if you are going turbo or positive dis. blower. even w/ lower comp./ higher boost you truck is going to feel like a pig!!! unless you spray!!! then by all means...


hope this helps you decide.

 

blown330,
See here we go. That is the kind of dialog I was talking about. Torque is of great concern with a curb wt of 5100 lbs.
I thought the higher compression with a tune and moderate boost; say 9 lbs centrifugal, would give me the best everyday performance. I have no intentions to installing a positive displacement blower. Turbo is unlikely because of cost, I already have a blower. I need to get this thing set up for the best performance now.
Anyone else have input before I obligate to these pistons?
Are there down sides to having a CR above 10 like start issues or eradicate performance?
Thanks for your input.

 

With that CR, I think that you have to use super, even if you didn't add the blower. Now add the cent. and I'm not sure you'll be able to boost much before detonation starts in. not sure though. I would call Sal at PSP. I am sure he has had some lightning guys want to go to a higher compression ratio and has dealt first hand with it before.
Jess

 

rocketir,
Thanks for the reply.
Yes I will not run boost until I get a retune. I'm sure it can be tuned for the x-tra CR, just want to know the best way to go. I will see if I can talk to Sal at PSP.

 

I called Sal and he said (again through a 3rd party) if I want to run the blower, I will have to run below 9 CR or I will have detonation issues ????? I'm running 9.5CR (stock) now with no detonation. I don't get it. I guess I will go with the stock pistons. The CR will only be slightly lower then it is now only because of the x-tra head gasket thickness.

 

The reason I asked is I replaced my stock 01 5.4 with a crate long block for a 01 lightning. I did lose some low end, (can hardley spin the tires from a stop) but increased my trap speed by about 5mpg. this is on 7.5lbs of non intercooled boost.

 

Crash,
Thanks for your input. I’m not sure what your end recommendation was. Are you saying to go with stock CR or lower it into the 8’s?
Just to clarify; I am running a Powerdyne at 9+lbs but I am also aftercooled with a water to air Vortec unit that is very effective at removing the added heat (see my signature). I have all the upgrades, 42lb injectors, 90mm MAF, Fuel pump and 4 program chip.
I am very reluctant to reduce the CR below 9.3-1 with a centrifugal blower since 98+% of the time I am running in vacuum which means non-boosted. In fact I wish I could figure out a way to open the by-pass valve at 8-10 lbs vacuum so there would be more usable boost. If I lower the CR into the 8’s then this 4.6L engine would be a real dog during normal driving.
So, what is your recommendation? I need to order the pistons soon.

 

dude, you have more or less the same setup as i do. except that i have a vortech w/ 14+ lbs of boost. w/ a good tune and your amount of boost I think the best thing to do is stay at 9:1 or maybe a little higher. with the right tune you will be able to beat the crap out of it every day and you truck will be fine. the only time i think you should lower comp. is when you pass maybe 14 psi on a cent.






my vote is for 10:1 cr!!!!! then people will ask you if (insert redneck voice here) that thur darn thang got a hemi !!!

 

blown330,
Thanks for your input. Now I'm almost afraid to go with the higher compression since those who I would depend on to do the tune have advised against it. I have pretty much made up my mind to go with the stock CR pistons. I don't want to loose any compression but may loose just a little to the special head gaskets. I will do a compression test before I install it to measure the piston pressures, that way Mike may be able to give me a little more aggressive tune. We shall see.

 

stock cr will be great! had i known a little more about cr a year ago i would have gone a little higher on my stroker setup.


i also just remembered a buddy of mine that has a expedition that i procharged and when it came time to rebuild he lowered it to about 8.5 and he lost 40 - 50 ft lbs of tq. he also lost boost. he later regained it w/ more boost and some spray!

post some results when your done, you should have a screamer!!!