low compression for blown motors

[02stangGT]

I was just wondering why it is better to have low compression when running a supercharger?

[WildBill]

I was just wondering why it is better to have low compression when running a supercharger?

That's not necessarily so. Your combo is a perfect example.

With a centri, it's better to have relatively higher compression since you don't build boost in the lower rpms.

But to answer your question, it's all about effective cylinder pressure. Superchargers add cylinder pressure. It's EFFECTIVE cylinder pressure that mandates timing allowed and fuel requirements.

[02stangGT]

So the stock compression is best because im not running a lot of boost?

[WildBill]

So the stock compression is best because im not running a lot of boost?

In your particular case........yes

[02stangGT]

my car just hit 87xxx miles. what is the max hp i would run with out blowing up the inside? (still have stock internals) have seen same set up with 420-450rwhp.

[WildBill]

my car just hit 87xxx miles. what is the max hp i would run with out blowing up the inside? (still have stock internals) have seen same set up with 420-450rwhp.

TOTALLY depends on how well the car has been maintained and beat on in the past, fuel system and the quality of the tune.

I've seen 02 GTs run 430 to 450 for long periods of time.

[El Jefe]

my car just hit 87xxx miles. what is the max hp i would run with out blowing up the inside? (still have stock internals) have seen same set up with 420-450rwhp.
Most I ever saw on a stock motor was right over 500rwhp. Fuel/Amazing tune I guess but the max on most is right around 450

Some great reading here:

http://www.modularfords.com/forums/showthread.php?t=51059&highlight=static
http://www.modularfords.com/forums/showthread.php?t=34527
http://www.modularfords.com/forums/showthread.php?t=9673

[wickedcobra]

I'd say it would depend on how you drive it. Balls to the wall all the time = motor doesn't last.

[02stangGT]

thanks for all the advice! I dont drive my car hard that often. but i love knowning that i have the power if i want to. its my DD so i dont drive it hard alot cause it has to get me from point A to B in one peice

[02stangGT]

just wondering kinda new at this.. if i was going to get a forged short block what compression would i want with my set up?? would i raise it more than stock??

[BurnTire]

Lower to run more boost.

[WildBill]

Lower to run more boost.

Not with a centrifugal style blower

[BurnTire]

You take a motor with a lower Static CR and one with a Higher Static CR then adjust boost on each motor so they the same Effective CR. Which car will make the most power?????

I guess the ultimate question is how much boost does he want to run. If he wants to crank it up by no means does he want to increase the static cr.

[WildBill]

You take a motor with a lower Static CR and one with a Higher Static CR then adjust boost on each motor so they the same Effective CR. Which car will make the most power?????

I guess the ultimate question is how much boost does he want to run. If he wants to crank it up by no means does he want to increase the static cr.

You're missing the point. A centrifugal style blower needs and can use more compression.It needs the higher compression for the low boost bottom end with that style of blower. With a roots or twin screw, higher than stock compression is not needed due to the boost being made in the low rpms. This is common and well accepted practices. At the very least he should keep the stock compression ratio, NOT lower than stock.

[BurnTire]

You're missing the point. A centrifugal style blower needs and can use more compression.It needs the higher compression for the low boost bottom end with that style of blower. With a roots or twin screw, higher than stock compression is not needed due to the boost being made in the low rpms. This is common and well accepted practices. At the very least he should keep the stock compression ratio, NOT lower than stock. Again it depends on how much BOOST he wants to run. High Boost + High CR = Boom. I believe his car has about 9:5 to 1 so it by no means is high compression, but I wouldn't run that in a higher boost application.
I see you bypassed the question I asked above:hi:

[Stanger00]

I would keep stock compression and keep boost below 20...Ideally 15-18 on street tune...anything more and the power is semi useless below 50 MPH. This is just me though. I doubt that P1-SC can even be spun to 15 psi and make power. The V2-SQ blowers fall on their face around 16 psi with stock compression, heads and cams. That much boost still makes 500-525 to the wheels though. These 2 valves kind suck at the making power thing..but with the right build its possible to make more. We have the little blowers so they lose steam before the motors do. Upgrade the blower then we have FUEL issues.

[WildBill]

Again it depends on how much BOOST he wants to run. High Boost + High CR = Boom. I believe his car has about 9:5 to 1 so it by no means is high compression, but I wouldn't run that in a higher boost application.
I see you bypassed the question I asked above:hi:
I didn't address the question above to refrain from making you look less credible.
The higher compression engine with lower boost will make more power over the curve and more peak HP than the lower compression and higher boost combo. Parasitic losses will be more with the higher boost combo as well losses due to higher inlet air temps.


I would keep stock compression and keep boost below 20...Ideally 15-18 on street tune...anything more and the power is semi useless below 50 MPH. This is just me though. I doubt that P1-SC can even be spun to 15 psi and make power. The V2-SQ blowers fall on their face around 16 psi with stock compression, heads and cams. That much boost still makes 500-525 to the wheels though. These 2 valves kind suck at the making power thing..but with the right build its possible to make more. We have the little blowers so they lose steam before the motors do. Upgrade the blower then we have FUEL issues.
You are correct on all points, hence my recommendations to the OP.

[Bandit]

[WildBill]

[BurnTire]

I didn't address the question above to refrain from making you look less credible.
The higher compression engine with lower boost will make more power over the curve and more peak HP than the lower compression and higher boost combo. Parasitic losses will be more with the higher boost combo as well losses due to higher inlet air temps.


You are correct on all points, hence my recommendations to the OP.

Bill with all due respect you have those backwards.

For example my car has 8.5:1 at 20 PSI that is an Effective CR of 19.8

If my car was 9.5:1 it would take 16 psi to have an Effective CR of 19.8

The 8:5 20 PSI combo will produce more power across the curve and more peak HP.

Worry about your credibility not mine:hyper:

[BurnTire]

[WildBill]

WHatever Jeff!
You're talking apples and oranges with centri and twinscrews or roots.

What part of parasitic losses don't you understand?

[BurnTire]

WHatever Jeff!
You're talking apples and oranges with centri and twinscrews or roots. Blower Type is irrelevant. The original questions was about why a lower CR is used in Forced induction applications. If you peak at 20PSI with a centrifugal is creates the same Effective CR as a screw blower at 20 PSI. I can understand your theory on having a higher CR to help the centrifugal setup move along better at lower RPMS, but when you get into higher boost applications a lower CR and forged internals are the best choice for power and longevity.

[BurnTire]

What part of parasitic losses don't you understand? Bill time to move on. Subject over.

[WildBill]

Blower Type is irrelevant. The original questions was about why a lower CR is used in Forced induction applications. If you peak at 20PSI with a centrifugal is creates the same Effective CR as a screw blower at 20 PSI. I can understand your theory on having a higher CR to help the centrifugal setup move along better at lower RPMS, but when you get into higher boost applications a lower CR is the best choice for power and longevity.

Wrong Jeff.

I don't know what else I can say to you for you to grasp basic concepts.
With a higher boost and lower compression combo you have the following working against the combo.
1. Higher boost pressures will result in higher parasitic losses due to spinning the SC harder.
2. With higher boost pressures, the inefficiency of the following come into play more and reduce overall efficiency.
(a) Higher boost requires the supercharger to spin at a higher rate. This raises the temperature of the air. This means less timing you're able to run.
(b) Higher boost means the air spends less time in the heat exchanger to remove the heat from the already hotter air.
(c) Higher boost will bring out further any inefficiency in the intake manifold and the heads with cylinder filling.

Bottom line is that a higher compression/lower boost combo will be quicker and more responsive on the street and when the time comes to "up the ante", all that would have to be done is up the octane of the fuel and pulley up the blower.

[silver03cobra]

I SHOULD READ ALL OF THIS BUT I AM NOT GOING TO......

having built the FASTEST 3 time or 4 time SCFCA (Southern California Funny Car Association) spelling? Funny car (chill factor) 7.272@ 160.58 only time i recall.


FOR YOU CAR!!!!!!! low comp high boost... the static loss wont matter with the boost. the d1sd would be better(over kill for street car) I REALLY DONT WANT TO START BASHING... but i have built OVER 40 Motors (still runing today..) go with 8.5:1 or 9.0:1 (with aluminum (spelling?) heads i would go with 9.0:1 and run 10-15 psi... you WILL make more safer power....) ask kieth... he tuned your car... he is a vender here and the only one I have tune my car's... and soon my diesel....:woot2: Now if the final CR 's are the same the lower CR more boost will STILL make more power it has beed proven time and time again.... end topic....... If you need you motor built i would be glad to talk to you just pm me. the real way to go is high CR's AND high BOOST... (not a street car by any means..)

[02stangGT]

im 20 years old just trying to learn a little.. thats what this is about. i am also saving for a block though too. the car will still be my DD i just wanted to know if i did get a block what compression to run and be safe for my DD.. i would like to run about 12-15 psi but this would only become when i have a new block that could handle it.

[BurnTire]

Now if the final CR 's are the same the lower CR more boost will STILL make more power it has beed proven time and time again.... end topic..... I rest my case.:yes:

[BurnTire]

im 20 years old just trying to learn a little.. thats what this is about. i am also saving for a block though too. the car will still be my DD i just wanted to know if i did get a block what compression to run and be safe for my DD.. i would like to run about 12-15 psi but this would only become when i have a new block that could handle it. 9.0:1 as recommended by Silver03Cobra the one with Real Engine Building Experience.

[WildBill]

Yeah.......funny car.........street car.:icon_rolleyes:

How do you figure?
If it is the SAME effective cylinder pressure, how can the higher boost make more HP in over the curve and peak, when it has MORE LOSSES with a centri?

Please get technical as you like.

[silver03cobra]

all the same........ they both need to last. AND I didnt just build funny cars....

[WildBill]

all the same........ they both need to last. and AND i didnt just buil funny cars....

Listen
I'm not here for a pissing contest. I want the young man to be happy with his car IF he builds his engine. And I don't really care where he gets the work done. I want him to have valid and accurate info to make a decision on what direction to take his car and be happy with the hard earned money he will have spent.
It's very simple engineering basics.
The car will run better on the street with higher compression when not in boost. The blower will last longer when operating at lower boost pressures. He will still have room left in his combo if he want to up the HP at a later date without having to go into the engine and spend more money.
Given the same effective cylinder pressures in both systems as stated by Jeff, there's just no way that the system that has higher inlet air temps and higher parasitic losses will make more HP.

If you think so, please explain and be as detailed as possible, in how your skills will overcome those losses, be a better street combo, all while maintaining the same effective cylinder pressures.

[silver03cobra]

low CR + (big boost pulley) with a blow off valve set to the desired PSI will be the best of both worlds AS long as the blow off valve is before the mass air... Blower will last no matter how much boost... EX.

281ci 9.0:1CR + D1SC pulleys set to 25psi (guess, must tune for right combo) blow off valve set to 10-15psi
BEST SET UP!

or
281ci 10.0:1CR + D1SC boost set to 5psi (or 8psi) wont even come close to the first set up.... DO YOU THINK I AM WRONG???a???? WHY DO PISTON COMPANIES SELL FORCED AIR PISTONS AS A LOWER COMPRESSION THAN STOCK??? i bet they know something..... I bet they Have A LOT OF MONEY IN THE "engineering basics".......

[BurnTire]

Ok Bill here is the deal.

1 Point of CR = 2% to 3% Gain in Horsepower
1 Pound of Boost = 15-20 HP

So my car makes 625 RWHP 20 PSI 8.5:1.
If I upped the CR to 9.5:1 I would put down 644. 625 x .03= 18.75+625 = 643.75

So now with 9.5:1 I lower the boost to 16 PSI so I have the same effective CR as 8.5:1 with 20 PSI. I will be super conservative and use only 15HP per pound of boost.
15 x 4 = 60HP 644-60 = 584

8.5:1 with 20 PSI and an Effective CR of 19.8 = 625HP
9.5:1 with 16 PSI and an Effective CR of 19.8 = 584 RWHP

Boost is your friend when used correctly.

[silver03cobra]

Ok Bill here is the deal.

1 Point of CR = 2% to 3% Gain in Horsepower
1 Pound of Boost = 15-20 HP

So my car makes 625 RWHP 20 PSI 8.5:1.
If I upped the CR to 9.5:1 I would put down 644. 625 x .03= 18.75+625 = 643.75

So now with 9.5:1 I lower the boost to 16 PSI so I have the same effective CR as 8.5:1 with 20 PSI. I will be super conservative and use only 15HP per pound of boost.
15 x 4 = 60HP 644-60 = 584

8.5:1 with 20 PSI and an Effective CR of 19.8 = 625HP
9.5:1 with 16 PSI and an Effective CR of 19.8 = 584 RWHP

Boost is your friend when used correctly.

been drinking too much 100proof southern comfort to do math but that looks right..

[WildBill]

Neither one of you guys are addressing the young man's problem.
He has a PS1C. It won't make or carry the boost numbers you two are wanting to use in your scenario. And you are not using the same effective cylinder pressures in each combo.

Say what you want. Shout as loud as you like. Cast as many dispersions as you wish. It still won't change the facts.
Pretty ridiculous that either of you guys want to conduct yourself in such a fashion to TRY and prove a point.

[BurnTire]

Neither one of you guys are addressing the young man's problem.
He has a PS1C. It won't make or carry the boost numbers you two are wanting to use in your scenario. And you are not using the same effective cylinder pressures in each combo.

Say what you want. Shout as loud as you like. Cast as many dispersions as you wish. It still won't change the facts.
Pretty ridiculous that either of you guys want to conduct yourself in such a fashion to TRY and prove a point.

Reading Comprehension Bill
I used the same effective CR in the math problem two post above.

What next c16 isn't a supercharged fuel?
A steel cage is only good to 11.0?

Oh wait you already made those claims. :rollingfloorrlol:

It is amazing the lengths one will go to cover the fact that they don't know what they are talking about.

[About2Bite]

:popcorn:

[BurnTire]

:eat:

[Stanger00]

HAHA...this has turned into should I dare say..."Will a plane take off from a treadmill" discussion.

LMAO.

This discussion seriously made me laugh.


One question though, how do you get 19.8 for effective cylinder pressure? Or are you just using your car and your cars compression ratio as a base line to prove a point?

The point being that a car with 8.5: 1 compression with 19.8 effective cylinder pressure at 20 psi will make more power over a car with 9.5:1 compression with the same 19.8 cylinder pressure.

BUT like you mentioned above if said motors run the SAME boost at a HIGHER compression ratio then of course more power will be made...but then we have other issues like cylinder heat, IAT's fuel delivery BLAH BLAH...stuff I the consumer doesnt really need to think about.

but really all of this info should be typical R&D for engine builders and a simple phone call will have you on your way.

[BurnTire]

Here is a chart.
Source
http://www.blowerdriveservice.com/techcharts.php#final

3829

[Stanger00]

sweet...my cylinder pressure is somewhere between 16 and 17.3.

[BurnTire]

sweet...my cylinder pressure is somewhere between 16 and 17.3. I assume your blower is intercooled?

[silver03cobra]

PSD1, D1SC,V1,V2, SO ON SO ON...... his blower WILL be just fine.. this is not a heaton (eaton).... come on bill. this is dumb SOME ONE CLEAR this thread...


PROBE
JE
KIETH BLACK
SRP
DIAMOND
SPEED PRO
AND EVERY OTHER PISTON PRODUCER SELLS FORCED INDUCTION PISTONS AS LOWER COMPRESSION RATIO'S RIGHT????????
IF YOU ANSWER YES THEN NO MORE DEBATE. or show me differently..... :pinkthumb:
not trying to be and a$$. but CR's must be wicked to see a true gain. same motors same EVERYTHING with 2.0 more compression wont make much more power... BUT MIGHT MAKE THE OWNER PAY MORE AT THE PUMP...... I AM DONE. this is why my mustangs are all parked and i dont care....... who ever started this thread look at FORD's Aluminator short blocks...... no matter the boost if you are running boost (you are) FORD says lower compression ratio..... I THINK FORD KNOWS MORE THAN ALL OF US!!!! I,m druck gooid night! :hellothere:

[Stanger00]

sure do...paxton 2 core.

[BurnTire]

On a lighter note

:xmas-smiley-037:

[BurnTire]

sure do...paxton 2 core. Cool is Cool. LOL
How much power did your car put down?

[Stanger00]

93 octane 400

91 octane my best guess is 380.

I have a AODE in my car. I lost my x-cal that had my tunes so now I have to start over if Modular Powerhouse doesnt come through with my x-cal 2 files. sorry to hijack

merry x-mas

[silver03cobra]

93 octane 400

91 octane my best guess is 380.

I have a AODE in my car. I lost my x-cal that had my tunes so now I have to start over if Modular Powerhouse doesnt come through with my x-cal 2 files. sorry to hijack

merry x-mas

go see Kieth at Arizona dyno chip... he will hook you up.... he is a super nice guy.... DONT CALL! lol go see him.

[BurnTire]

93 octane 400

91 octane my best guess is 380.

I have a AODE in my car. I lost my x-cal that had my tunes so now I have to start over if Modular Powerhouse doesnt come through with my x-cal 2 files. sorry to hijack

merry x-mas Those are some decent numbers.

[Stanger00]

yup and its been about 30 thousand miles since install as well...only problem was my stock AODE got lazy then finally broke.

[Lucafu1]

Reading Comprehension Bill
I used the same effective CR in the math problem two post above.

What next c16 isn't a supercharged fuel?
A steel cage is only good to 11.0?

Oh wait you already made those claims. :rollingfloorrlol:

It is amazing the lengths one will go to cover the fact that they don't know what they are talking about.


and thats the ball game:pepper:


why chance it with higher compression and less boost when you can keep the boost and do a gear swap to keep the low end. thats pretty much common practice especially with a centri blower.

[4Jenna]

I always did my boosted street motor's low compression for safety. 8.5-8.8 is what i ran mine at (pushrod)

Can't we all just get along...haha.