Effective compression ratio

[46addict]

Does the combustion chamber care whether I'm pushing 13psi on 11.2 compression or 17psi on 9.8? I currently have a P1SC setup and plan to turn up the wick in the near future. My original thought was to upgrade the shortblock using forged 9.8 comp pistons but I had a crazy idea about using higher compression instead. The latter won't require a blower upgrade as I can pulley down the P1 I have now and hopefully reach the same power level as I would with a D1 making more boost. According to Procharger there is a 200cfm difference in max flow (IIRC) between the two and the higher compression pistons could make up for this, or no?

Also do tighter LSA cam setups deduct some from effective compression? I'm thinking keeping my hot rod cams could give me some headroom if it allows for some boost to "pass through" via valve overlap. Not sure if my logic is correct on that though.

I want to stick with 93 octane and get to 600whp.

 

[RazorbackMustang]

Cylinder pressure does not equal compression ratio.

 

[TheBlackPearl]

It all depends on when the cams close the exhaust valve.

Google a dynamic compression ratio calculator. Play around with that a little.

I found that if I run 10.5:1 static compression. With my cams installed +4 degrees it gives me 9.7:1 ish dynamic compression.

Raising your static CR should help correct the big cam driveability issues also.

Yes LSA (overlap) will allow boost to escape out the exhaust I don't think that's too big of a deal though.

TBP

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[ChewyR]

Personally I am going the other way on compression ratio, I don't think 93 octane is going to like 17psi with 11+:1 compression. At best you will have to pull a lot more timing out, which will hurt HP. I know things were different 10 years ago, but there is a reason Ford kept their factory supercharged motors down around 8.5:1. And those guys with terminator and GT500 motors get 650 rwhp on Pump gas.

 

[46addict]

Cylinder pressure does not equal compression ratio.

If I'm understanding right, static compression and boost are factored in calculating effective compression, with cam timing and altitude being the variables.

Personally I am going the other way on compression ratio, I don't think 93 octane is going to like 17psi with 11+:1 compression. At best you will have to pull a lot more timing out, which will hurt HP. I know things were different 10 years ago, but there is a reason Ford kept their factory supercharged motors down around 8.5:1. And those guys with terminator and GT500 motors get 650 rwhp on Pump gas.

Sorry if I was unclear. I'm considering the use of 11:1 on 13psi as opposed to 17psi on 9.8:1. According to online calculators, both have a similar amount of effective compression. So my question is, are there other factors that are taken into account when figuring out the limitations of 93 octane?

 

[01yellerCobra]

Don't forget the more you spin the P1 the higher the IAT's are going to be. If you have a good intercooler that shouldn't really matter. But something to keep in mind. I prefer the bigger blower turning slower.

There was a guy in socal with a 99 cobra that was built up. I don't remember the full build, but it was basically a race car on race gas. He was overspinning the P1 pretty good and I think he barely cleared 600rwhp. There was nothing left in the blower.

 

[46addict]

I have a 3.40 blower pulley sitting on the shelf and if I pair that with the 7.5" crank damper I should be spinning the blower at 56k rpm which is well below the 62k rpm threshold. I do have a 3 core intercooler and a Spal radiator fan from Procharger but I'm sure I could easily heatsoak the engine sitting in traffic.

 

[TheBlackPearl]

Sorry if I was unclear. I'm considering the use of 11:1 on 13psi as opposed to 17psi on 9.8:1. According to online calculators, both have a similar amount of effective compression. So my question is, are there other factors that are taken into account when figuring out the limitations of 93 octane?[/QUOTE]


Short answer. Yes.

The margin for error in the tune and fuel quality will be smaller.

One benefit you could have is better fuel mileage than the low compression/high boost combo.

Also higher boost means more blower speed. That, in turn, means more energy is used to drive the blower. On top of that it puts more stress on the crank snout.

TBP

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[BruceH]

I like higher compression. It does make the effective compression ratio higher with boost. www.wallaceperformance.com has calculators to try.

Do you have E85 available? If so switch over and don't worry about compression, boost, or spark advance again.

FWIW I made 699rwhp (actual, not corrected) with a .030 over 4.6 with 11:1 compression and Procharger D1 pushing 19psi. This was with FRPP heads and 127400 (112 lsa) cams on E85.

The thing about the P1 or D1 is that they have billet wheel compressors. This means they can spin about 10k more rpm than a similar sized cast wheel centri.

Switch to E85, bump the compression, and make some power.

The Hot Rods will work with your setup but you will make a whole lot more power with 127550 cams. The required spring upgrade shouldn't deter you as they are needed with higher levels of boost anyway.

 

[Department Of Boost]

I want to stick with 93 octane and get to 600whp.

Then you had better stick with lower compression.

Coyote motors are 11:1 and you're limited to about 10.5-11psi and 600ish RWHP on 93. And they have twin variable cam timing to blow pressure off through the midrange. On the flip side I've made 775rwhp on 93 with a 9.25:1 3v pushing 19.5psi.

IMO if you have enough blower to make the power don't go crazy with compression.

 

[TheBlackPearl]

Just FYI, I plan on a 3.7" bore stock stroke 4.6. With 10.5 CR and 127550 cams. Blower will be an F1A. The compression is for pump gas drive ability. 12 psi on pump gas and 22-24 on race gas/e-85. The compression will help keep it from being a pig on low boost.

TBP

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[46addict]

I like higher compression. It does make the effective compression ratio higher with boost. www.wallaceperformance.com has calculators to try.

Do you have E85 available? If so switch over and don't worry about compression, boost, or spark advance again.

FWIW I made 699rwhp (actual, not corrected) with a .030 over 4.6 with 11:1 compression and Procharger D1 pushing 19psi. This was with FRPP heads and 127400 (112 lsa) cams on E85.

The thing about the P1 or D1 is that they have billet wheel compressors. This means they can spin about 10k more rpm than a similar sized cast wheel centri.

Switch to E85, bump the compression, and make some power.

The Hot Rods will work with your setup but you will make a whole lot more power with 127550 cams. The required spring upgrade shouldn't deter you as they are needed with higher levels of boost anyway.

There is only one E85 station near me and they are usually out. So unfortunately it will be strictly 93 for me. Since this is primarily a street car I want to keep the stock phasers, if possible, and use a milder cam like a 127350. I haven't made a final decision yet on cam choice.

Just FYI, I plan on a 3.7" bore stock stroke 4.6. With 10.5 CR and 127550 cams. Blower will be an F1A. The compression is for pump gas drive ability. 12 psi on pump gas and 22-24 on race gas/e-85. The compression will help keep it from being a pig on low boost.

TBP

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Will you be using a wastegate to control boost? I assume you will have two separate tunes for this. Are there systems that let you switch on the fly, or would you just upload from a handheld? I've always wondered this. I would also think it would be a PITA to go from gasoline to E85 and vice versa. Unless you have a separate fuel cell for ethanol.

 

[TheBlackPearl]

No waste gate. IMHO that's the worst idea ever.

Yes two separate tunes for two different pulley combinations.

The only switch on the fly ecu I know of is the aem infinity. I haven't really looked though so there may be more.

Fuel swapping is a pain, but I'm willing to deal with it. It's not for everyone.

TBP

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[Pentalab]

Then you had better stick with lower compression.

Coyote motors are 11:1 and you're limited to about 10.5-11psi and 600ish RWHP on 93. And they have twin variable cam timing to blow pressure off through the midrange. On the flip side I've made 775rwhp on 93 with a 9.25:1 3v pushing 19.5psi.

IMO if you have enough blower to make the power don't go crazy with compression.

## If you look at the typ boost vs rpm graphs for the coyote with a pd blower, I never see the boost dropping off in the midrange.... but that is for 500-550 rwhp.

Having said that, the coyote + pd blower appears to have peak tq numbers that are way down from max hp numbers. With the 3v, tq + hp are very similar, with tq being slightly less. And sometimes the same, and sometimes the tq is a hair higher than the hp.

 

[702GT]

No waste gate. IMHO that's the worst idea ever.

Yes two separate tunes for two different pulley combinations.

The only switch on the fly ecu I know of is the aem infinity. I haven't really looked though so there may be more.

Fuel swapping is a pain, but I'm willing to deal with it. It's not for everyone.

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Why do you feel waste gates on a centri is the worst idea ever? My WG is second to the only thing that has kept my stock bottom end alive. First being a good tune. On a higher power application, it makes a high boost car more streetable. 600rwhp+ is no joke to try and lay down on the street. A simple re-tune and boost controller adjustment can have you track ready for the weekend or street safe(er) for a road trip. Useful as well if you are higher compression and want to swap between e85 (or other race fuels) and pump gas, you can turn the boost down and re-tune.

As far as fuel swapping, it's still far less of a PITA to run a tank down and fill with your fuel of choice than it is to swap blower pulley's for a desired boost level.

I still find the biggest advantage to WG a centri for me, I get to have all the lower powerband a smaller pulley has to offer without the risk of the high end boost that would undoubtedly be the end of my bottom end. I've played with my MBC, my WG starts to open around 4600rpm (7psi). If I dial it up I'm making 10psi around 5,500rpm. When I sized my pulley I wanted it capable of 14psi, and it looks like it would be bang on. Gotta love torque.

 

[TheBlackPearl]

With a waste gate setup, the blower is still moving the same amount of air just less of the air is going into the engine. The blower is still spinning just as fast and getting just as hot.

I agree you can pulley the crap out of the blower and bleed the boost off with a waste gate to get a more positive displacement like power band, but you're trying to do something that style of blower isn't designed to do.

These blowers use more HP the harder you spin them. So you're going to also loose more HP to you Super Fast blower that you're only using part of the benefit of.

In a nut shell. Parasitic loss of power and IAT increase are the reasons why I don't like a waste gated centrifugal blower.

If you want a PD style powerband, get a PD blower.

TBP

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[702GT]

With a waste gate setup, the blower is still moving the same amount of air just less of the air is going into the engine. The blower is still spinning just as fast and getting just as hot.

I agree you can pulley the crap out of the blower and bleed the boost off with a waste gate to get a more positive displacement like power band, but you're trying to do something that style of blower isn't designed to do.

These blowers use more HP the harder you spin them. So you're going to also loose more HP to you Super Fast blower that you're only using part of the benefit of.

In a nut shell. Parasitic loss of power and IAT increase are the reasons why I don't like a waste gated centrifugal blower.

If you want a PD style powerband, get a PD blower.

TBP

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Whether you run a WG or not, a centri will never deliver a PD powerband. My reference to the low-mid range powerband was only in reference to being able to take advantage of a smaller pulley without destroying my stock internals. If I wanted PD-like powerband, I'd hook the bottle back up. The only reason one could even compare a procharger to a PD is due to its step up gear ratio being generally more aggressive than other centri's. You could call it the PD of centri's, in that regard I suppose.

Your arguement is regarding technical efficiencies, and by the numbers. As soon as you strap a belt driven blower to the crank you suffer parasitic loss. After that, pulley size isn't as relevant in regards to blower pulley. I hadn't realized a 4.00" pulley was that hardcore. My personal belief is that if you have to pulley the crap out of a blower, you're doing it wrong. Buy a bigger blower.

You are going to suffer the same IAT's from a 3.4 pulley regardless of whether you WG it or not, the only difference will be if you want to make 700hp or something more manageable. The blower is doing exactly what it is designed to do, a WG just allows you to throttle the boost. A WG doesn't harm your IAT's, and the parasitic differences between pulley sizes is marginal at best, otherwise no one would pulley down. If you pulley down and make less power, due to parasitic loss, you're probably far beyond over-spinning the blower as it is. The effects of IAT's on the top-end are non-consequencial for someone running a WG, as the entire intension of running the WG is to reduce power output at the top. Whatever IAT's you suffer while building boost pre-WG is what you would suffer without a WG.

The OP had relevant concerns about how much boost to push into a higher compression combustion chamber on pump gas. If he's going to live on the cutting edge of pump gas octane rating, high compression, and boost, it's to his advantage to be waste gated. It will allow him to benefit from the lower powerband a smaller pulley has to offer and tailor the top end to keep the boost in check. Personally, as per usual I agree with Bruce's post. Run e85, and you won't need to run a WG.

 

[TheBlackPearl]

First, I agree with you. Parasitic loss isn’t as noticeable in this situation, but I was asked why I don’t like WG’s on centri blowers.

Again this is why:

A blower pullied for 18 pounds of boost is still going to make more heat and use more power from the crank than a blower pullied for 9 pounds of boost regardless of the fact that the waste gate limits boost the motor sees to 9 pounds.*
Let’s think of it this way. You take a motor that makes 400 HP naturally aspirated. You put a Procharger on it and a pulley set up that that makes 700 at 12 pounds of boost.*
If we do the math:
14.7+12=26.7
26.7/14.7=1.82
1.82*400=728
728-700=28
You lose 3.8% of your produced power to parasitic loss.
Where does the extra 28 HP go? To drive the blower.
Let’s keep going with the math, but let’s waste gate the boost down to a safe 8 pounds.
14.7+8=22.7
22.7/14.7=1.54
1.54*400=618
618-28=590
You lose 4.5% of your produced power to parasitic loss.
Now this is just an example. The engine specs are only to use as an example.
28 HP loss is extremely efficient. Most AC compressors use more than that. Some of the big Prochargers and Vortechs can use upwards of 200 HP to drive them on race motors.*
Is that the case in the OP’s situation? No, his blower might only use 50 HP to turn.*
I’m just trying to explain the fact that if you spin a blower harder and bleed boost off you will make less power than in identical setup spun to a safe max boost pressure. On top of that you put less wear on the blower spinning it slower.
Again I will say if you want push-button adjustable boost. Get a turbo. If you want boost to come on quicker get a PD blower. If you want traction off the line coupled with a strong top end charge. Use a centri.
This is why I won’t use a waste gate on a centri.

TBP



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[wabirch]

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