Compression and Boost

[loots06]

I am getting ready to for a budget short block build and will running slightly higher compression. My build will consist of Wiseco piston 9.5:1, .020 over, Boss connecting rods with a stock 4.6 crankshaft. I know there are several forum members that are running higher compression ratios and boost using various blowers or turbo set ups. While I was speaking with a sales representative and preparing to order the pistons they actually encouraged me to go with a lower compression ratio. They simply explained that higher compression and boost does not work well. I explained that the new 5.0 has a higher compression and that Whipple makes a blower kit for the car which produces tons of power. I didn’t understand their argument and really thought that low compression and higher boost was an old school thought given the numerous documented successful builds out there, several of which have been well documented on this forum.
Does running a different type of power adder such as positive a positive displacement or centrifugal blower really matter when it comes to a higher compression set-up? They thought it did given that one is instant while the other builds boost. A turbo set-up is obviously different and seems to make a ton of power. Is there huge difference in the amount of power a high compression motor will make using a different power adder? I would image the power would be more linear but nothing more. Any how let me know what power adder you running, compression ratio, amount of boost and of course the total power your combo makes. I never ordered the pistons but will over the weekend. Maybe I'll should look at a different vendor!

 

[psfracer]

You are missing a few very important pieces to the puzzle.

Fuel.

The high compression boosted applications I know of have had great success running E85.

How much boost is also another question to be asked. Are you going to run 8psi or 20 psi? You said budget build so I am assuming the lower side.

Camshafts, differences in lobe separation, and / or how they are degreed in can play a huge role in cylinder pressure in addition to compression ratio and lbs of boost.

 

[loots06]

You are correct, I will be running pump gas 91, California what can I say.
I'm looking at heads but haven't made up my mind but lets say stock for now with some 127500 cams. I'm looking at 10 psi at the moment but may go up a little bit down the road.

You are missing a few very important pieces to the puzzle.

Fuel.

The high compression boosted applications I know of have had great success running E85.

How much boost is also another question to be asked. Are you going to run 8psi or 20 psi? You said budget build so I am assuming the lower side.

Camshafts, differences in lobe separation, and / or how they are degreed in can play a huge role in cylinder pressure in addition to compression ratio and lbs of boost.

 

[dysan]

I would say on pump gas go with stock compression which is 9.8:1. Also, the 127500 cams are N/A and I seriously doubt those cams will work well with a blower or turbo. you would want the 127550 which are for blower setups.

Myself and a few others are 11:1 + compression and boosted and it works great but we are all on E85. My normal boost is 9-11 psi and with 25* timing my car made 630rwhp at that boost and at the end of last race season was in the low 10.30's at the track.

The nice thing about running higher compression is it will take less boost to make the power you want which in turn will have lower IAT temps. To me, that's the biggest plus about it.

 

[JeremyH]

One thing about the coyote is with tivct effective compression can be changed and tuned.

They are not wrong, lower compression will support more power at the same overall effective compression/cylinder temp since you will have more fuel and oxygen in the cylinder. The benefit of forcing the air into the smaller space all the time in the end can not outweigh putting more fuel and oxygen into the motor. Being able to safely run more boost will make more power every time.


Based on your budget short block build and fuel choice of 91 octane. I would not get crazy with the compression 9.5:1 is a great choice.


I made over 600rwhp at 10psi with 9:1 compression on my "budget" short block build.


Here is a very good read on compression and boost.

http://www.xcceleration.com/cr-boost%20101.htm

 

[loots06]

I would say on pump gas go with stock compression which is 9.8:1. Also, the 127500 cams are N/A and I seriously doubt those cams will work well with a blower or turbo. you would want the 127550 which are for blower setups.

Myself and a few others are 11:1 + compression and boosted and it works great but we are all on E85. My normal boost is 9-11 psi and with 25* timing my car made 630rwhp at that boost and at the end of last race season was in the low 10.30's at the track.

The nice thing about running higher compression is it will take less boost to make the power you want which in turn will have lower IAT temps. To me, that's the biggest plus about it.

Thanks for the clarification on the cams.

One thing about the coyote is with tivct effective compression can be changed and tuned.

They are not wrong, lower compression will support more power at the same overall effective compression/cylinder temp since you will have more fuel and oxygen in the cylinder. The benefit of forcing the air into the smaller space all the time in the end can not outweigh putting more fuel and oxygen into the motor. Being able to safely run more boost will make more power every time.


Based on your budget short block build and fuel choice of 91 octane. I would not get crazy with the compression 9.5:1 is a great choice.


I made over 600rwhp at 10psi with 9:1 compression on my "budget" short block build.


Here is a very good read on compression and boost.

http://www.xcceleration.com/cr-boost 101.htm

I really don't have much choice on fuel considering I'm in CA, 91 is the best I could get. Thank you for the link, tons of info on there.
I'm gonna order those pistons and look at blower cam options, but 127550's are my first choice.

 

[mach828]

Running e85 or using a centri = higher compression

PD blower or turbo on 91 = lower compression (because its really easy to just swap a pulley/wastegate spring and add more boost, its safer to make power with more boost and less timing/compression then less boost and a lot of timing and high compression) of course you will want to read your compressor map to be sure you are still in the efficiency range of the turbo at higher boost levels

I'm piecing my budget build together as well. Boss rods, stock crank, and flat tops. But I need the higher compression to help low-mid power and I'm already near the limits of the centri I have. So lower compression and spinning the blower harder isn't really an option. More timing and compression is the only way I'll make more power.

 

[JoshK]

^the car in your Sig your current car? This a 197 forum, you wouldn't happen to be one of the new paid posters would you? Forgive me if you do have a 3valve and that is the car you are speaking of.

 

[mach828]

^the car in your Sig your current car? This a 197 forum, you wouldn't happen to be one of the new paid posters would you? Forgive me if you do have a 3valve and that is the car you are speaking of.

There are paid posters? And no, I do not own a 3v but I have worked on quite a few 3vs (cam install, headers, gears, boost, etc). Car is my sig is current. Would like to add a 2015 2.3 to the garage in a year or two though.

I have the Mach 1, a 71 Comet, a Ranger truck, and a 05 CVPI. I've had a variety of other Fords as well. There isn't really any job I haven't tackled. I am on this forum mainly for tech knowledge and generally being a Mustang enthusiast.

 

[JoshK]

 

[01yellerCobra]

I'll vouch for mach828. He's on a couple other forums that I'm on as well.

Personally I like a little higher compression. It helps when out of boost. Although that's more for a centri then a PD or turbo.

 

[jodadejss06gt]

i think I am one of only a few on here that has done high compression w/o e85. I'm having motor issues no currently and I have diamond flat tops which are about 11:1(compression and motor issues are unrelated). I've run up to 8 lb on my setup with 93 w/o any issues, but for a very very short time. My plan was to do 93 as an every day low boost type deal and then e-85 for high boost. If I rebuild, I will be in the 9's compression. Most likely something with a 6.5 - 10 cc dish. Car does run well with the higher compression though off of boost!

 

[jodadejss06gt]

I'll vouch for mach828. He's on a couple other forums that I'm on as well.

Personally I like a little higher compression. It helps when out of boost. Although that's more for a centri then a PD or turbo.

why would high compression not be for turbo?? I'd think it would help with out of boost performance for a turbo and centri the same, where PD blower doesn't need out of boost performance because its instant boost?

 

[01yellerCobra]

why would high compression not be for turbo?? I'd think it would help with out of boost performance for a turbo and centri the same, where PD blower doesn't need out of boost performance because its instant boost?

Bit of a brain fart on that one. I was thinking about how fast a turbo can spool to make up the torque. Never mind the fact that I told a friend to go with a 9.5:1 compression in his turbo engine to help with spool and off boost performance.

 

[mach828]

why would high compression not be for turbo?? I'd think it would help with out of boost performance for a turbo and centri the same, where PD blower doesn't need out of boost performance because its instant boost?

Helps off boost performance in all. The PD blower's bypass bleeds off the boost just like a centri at part throttle since its driven by the crank. You spin the blowers based on only rpm, vs a turbo which is driven based on exhaust gasses which increase based on throttle input and rpm.

High compression is good for turbos and PD blowers too up until you start reaching your detention threshold. If you can't cool the air anymore, your really pushing the limits of the power adder (generating a lot more heat), you restricted to the limits of 91/93 octane fuel, you have the right plugs, you are running low timing, etc. then the other option in the formula is to run less compression. Higher compression puts more stress on the engine since it compresses the air/fuel mixture more during ignition generating a strong down stroke = more power but also more heat during the process. If you leave more space so the air/fuel isn't compressed as much then you make less power (everything else constant) because the energy generated during ignition is less powerful. But since its not being compressed as much you have more room for detention prevention because its not generating as much heat.

 

[jodadejss06gt]

I was simply pointing to the fact that yeller pointed to high compression being better for Centris only and not turbo or pd. I understand the concept I was just giving him a chance to correct himself.

 

[mach828]

I was simply pointing to the fact that yeller pointed to high compression being better for Centris only and not turbo or pd. I understand the concept I was just giving him a chance to correct himself.

Whoops!

 

[01yellerCobra]

I was simply pointing to the fact that yeller pointed to high compression being better for Centris only and not turbo or pd. I understand the concept I was just giving him a chance to correct himself.

I do actually have an idea about boost and such. But my mind was on work for once. Go figure.