Boost: What will it do if I raise CR?

[BruceH]

Bruce, which of the 2 results that the wallace racing calculator spits out, do you use ?

Not sure I understand the question. When I'm "what iffing" with the boost compression ratio calculator I use a compression and boost for comparison, then I start adjusting the numbers. Let's say I want to see what stock 9.86 compression with 10psi gives for a boost compression figure. I'll use that figure to compare to say 11 compression and less boost to see how much less boost will be required to obtain the same cylinder compression.

It's not an exact science, it's more of a ballpark calculator. I can reference that I had an 11 compression motor with a Paxton and e85 that made around 570rwhp with 10psi iirc. I also had the same motor (285 cu) with different heads producing a calculated 10.76 compression along with 19psi from a D1 producing 699rhwp. This was also with e85.

I reference those numbers just for ballpark comparisons to other builds. How much does 10psi and 19psi usually make with e85 and a .030" over stock stroke motor and the same blowers? I don't know. I do know that a friend of mine needed about 23psi+ to make about 660rhwp with gasoline and meth. IIRC his compression was around 9.5.

I've also had a 9.4 compression stock displacement motor that made 599rhwp with e85 and about 16psi from a whipple. The whipple is limited on a 3v due to the air inlet design, 16-17psi is where the inlet reaches it's maximum air flow so anything more results in diminishing returns.

Lito mentioned the fact that with a blower you have more atmosphere in the cylinder. That's something I didn't think of before. More atmosphere means more o2. E85 also means more o2.

If I compare my current 12:1 motor to a stock 9.86:1 for cylinder compression my 12:1 motor has the same cylinder compression as the stock compression motor with 3psi.

Now that I used the calculator I see what you were asking. Previously there was only one result and it was the top one. That would of been a whole lot shorter answer.

 

[eighty6gt]

138 is pulling a LOT of wool for 560, isn't it!!?

 

[swflastang05]

138 is pulling a LOT of wool for 560, isn't it!!?

I was thinking the same thing! Was the 560 on a Mustang Dyno maybe?

 

[eighty6gt]

Would have been at brenspeed. Maybe they're conservative!

 

[Pentalab]

Lito mentioned the fact that with a blower you have more atmosphere in the cylinder. That's something I didn't think of before. More atmosphere means more o2. E85 also means more o2.

If I compare my current 12:1 motor to a stock 9.86:1 for cylinder compression my 12:1 motor has the same cylinder compression as the stock compression motor with 3psi.

Now that I used the calculator I see what you were asking. Previously there was only one result and it was the top one. That would of been a whole lot shorter answer.

Yes, the newer version of the wallace racing calculator results in 2 answers. The 2nd (bottom) answer it spits out is lower in value than the 1st one.

Using that wallace racing plug in formulae... if I used a CR of 12:1 + 2 psi boost, I could end up with an effective boosted CR that is identical to what I now have ( 9.8:1 CR + 5.8 psi boost).

At that point, one could toss the blower away lol.

Increasing the CR just a bit may well alleviate the need for higher boost.....which in turn means lower iats, and more hp when in NA mode..and less hp to run the blower.

I think Bruce managed to run 12:1 CR on 91 pump gas...and no doubt with a bunch of timing pulled. Still, even if 10.5.....to as high as 11.5 were used for CR, you could still reduce boost a whole bunch.

I mean why run 5.8 psi boost, when you can get the same results with just 2-2.5 psi.

Another example. 10 psi boost and 9.8:1 CR is the same as 5.5 psi + 11:1 CR . That's a huge difference in boost. That would be a 27.5-44 deg F drop in IAT right there.

It's almost like 'free hp'. Or am I delusional ?

 

[TheBlackPearl]

Sorry for digging up an oldie, but I have been mulling this idea over lately.

With a stock static cr of 9.7:1 and the stock cam closing the intake valve 19 degrees ABDC that gives me a dynamic cr of 9.52:1.

With my 127550 cams, if I raise my static cr to 10.4:1 then run the same calculations I get a dynamic cr of 9.54:1. this is because of the later closing intake valve (41 degrees ABDC).

That being said shouldn't bot those engines handle 12-15 psi of boost (pd or centri) if the bottom end is built for it?

Edit: Shouldn't drivability, on the cammed motor, be similar to the stock motor out of boost?

Again sorry for reviving, derailing, or applying bad grammar to this thread.

TBP

 

[46Tbird]

I realize that this conversation has drifted toward boost versus compression, but that was not your original question. It was:

Question:

My stock (9.8:1 CR) shortblock is fed 9 PSI by my TVS.

If I put in my 298 stroker with 11.5:1 compression and make no other changes to the build (no pulley changes), what will that do to my boost pressure?

I will actually be adding:
Comp 1273509 cams w/ Livernois or Comp Limiters
Ford Racing Performance Products twin 65MM CobraJet throttle Body
JLT 127mm Carbon Fiber BIG AIR intake
VMP tuning TVS high-flow inlet elbow

I want to stay around 9 PSI with the above mods; I'm wondering if I'll need to change blower snout pulley sizes.

It is difficult to say what the manifold pressure will be with all of those changes.

Let's say you're boosting your bathroom to 9psi. Then you decide to boost your bedroom to 9psi while changing the size of the door and how long you keep it open.

That's not an easy problem to solve.

Intake manifold volume and valve timing have HUGE impacts on boost pressure. If the engine underneath is more efficient at ingesting air from the manifold then that will also play a part. It is almost a guarantee that the changes to your combo will require more airflow to arrive at 9psi in the manifold. So you'll need to spin it faster or pick a blower with a different efficiency map.

As for the commentary about boost versus compression, there are excellent points made by many of the previous posters. But I would say for your "90% street" comment, you would be better off with moderate to low compression (around 9:1) and add boost to your desired power level. As lito said, you don't have the advantage of computer-controlled valve timing that would bleed off static compression when it's not needed. Plus it's just easier on parts when lugging around.

It is very easy to overpower a street car chassis/tire so I don't see where the advantage of a hard initial "hit" is on the street. But this is just one opinion, you have a lot of options to build it just like you want.

 

[lskiller]

Aren't cyl pressure the issu, no matter how one gets there?

It is all about cylinder pressure. When I bought my shortblock in 2011 I went with stock 9.8:1 CR for the 3v. If I were doing it over, I'd go at least 10.5:1 and maybe up to 11:1. ESPECIALLY if you're on E85.

 

[05stroker]

It is all about cylinder pressure. When I bought my shortblock in 2011 I went with stock 9.8:1 CR for the 3v. If I were doing it over, I'd go at least 10.5:1 and maybe up to 11:1. ESPECIALLY if you're on E85.

E85 is the shit if you want high CR and high boost. I have 12:1 CR and even at 21 psi and with lots of timing there is no detonation. Just limits of the parts and build.

 

[DiMora]

Im going to keep it high CR with the existing pistons and run E85 and 9 PSI and see what it does.

 

[DiMora]

Just bought some used ID1000's...E85 conversion will happen first before I swap engines. I want the fuel system solid before I get it tuned with the high compression and boost.

 

[05stroker]

Just bought some used ID1000's...E85 conversion will happen first before I swap engines. I want the fuel system solid before I get it tuned with the high compression and boost.

Good move, I can tell you from experience, to much at one time and you will pay for it in the long run.

Just like I told my kid when he asked for the DOB setup to go in with the new higher CR motor, we are going to run the new motor for 2-3 weeks before we add boost.