Boost and C.R. Question - TCCoA Forums
 
LinkBack Thread Tools
post #1 of 13 (permalink) Old 09-23-2004, 01:10 PM Thread Starter
Seasoned PostWhore
 
Join Date: Jul 2002
Location: Cullman, AL
Age: 56
Posts: 7,758
Send a message via AIM to 94 Daily Driven 4.6L
Boost and C.R. Question

OK, I've done a fair amount of work with roots superchargers and turbochargers, and I’ve done a lot of research on boosted application in general.

So will someone please explain why I keep reading about people building 10.5 – 11.5 compression ratio engines and then running 15 – 20 lbs. (or higher) of boost.

EVERY vendor I’ve ever researched into boosted applications say keep the static C.R. to around 8.5 to 9.5 with max boost around 10 to 15 psi. I know that with aluminum heads/blocks you can usually run about one more point of C.R. than with iron, but still 11.5 C.R. and 20 lbs of boost!!!

Or is this a case where the “boost” being stated is pressure in the intake tract, which may or not necessarily be making it into the combustion chamber. It seems that I mainly hear these numbers with centrifugal superchargers, never with roots superchargers or turbochargers.

So would someone please enlighten me?

97 Lincoln Mark VIII LSC (Chip'd, 3.73 T/L... so far... )
97 Ford Aspire (Slow, but getting 36 mpg (f'n Ethenol!! )
84 F250 Dually w/6.9L Diesel (7.3L IDI pending)
73 Mercury Cougar Convertible w/351C 4V (Partially Restored)
69 F100 LWB w/460 Engine
76 Glastron Carlson 23' Jet Boat w/460 CJ Engine
94 Daily Driven 4.6L is offline  
Sponsored Links
Advertisement
 
post #2 of 13 (permalink) Old 09-23-2004, 03:37 PM
Seasoned PostWhore
 
96bird's Avatar
 
Join Date: Nov 2002
Location: cleveland, TN
Age: 37
Posts: 2,936
Send a message via AIM to 96bird
If you want to know the answer you should read posts by Dave King on the corral in the 4V section. I would just do a search. He is very knowledgeable on building engines. Many people use low compression and a forged bottom end as a way to say that they are invicible, which is just not true. More withstanding is a better word, and with a crappy tune it doesn't matter it will blow anyway, even with billet internals. The main thing about keeping the compression is torque and even though you lose power throughout with lower compression, with a centrifugal blower torque is especially important on the lower end of the drag racing rev range spectrum. I could go on, but just look Dave up.

Russell
96bird is offline  
post #3 of 13 (permalink) Old 09-23-2004, 03:42 PM
Seasoned PostWhore
 
96bird's Avatar
 
Join Date: Nov 2002
Location: cleveland, TN
Age: 37
Posts: 2,936
Send a message via AIM to 96bird
Straight from Dave King:

"You can run as much boost as you want with 10.1 compression as long as you run the correct octane and tuneup. You can run 10.1 compression with as much as 17-18 pounds on pump gas with no problems as long as you tune correctly.

Compression is a highly debated subject.

John Mihovetz did some testing with Jim Bell. They tested different compression levels with different amounts of boost. The bottom line that came from the tests was that theres no substitute for compression.
These motors (4v Cobras) do not make torque, which is unique. Torque is what accelerates your car down the street/track. When you drop the static compression ratio a point down to 9.0:1 or 8.8:1 you loose about 40-50 pounds of torque. That is substantial. Many people belive that simply adding more boost doesn't necessarily make up for the torque in a 4v Cobra motor. Plus your still adding cylinder pressure which is the reason many prefer to lower compression ratio.

Bottom line: Don't go lower than 9.85:1 compression unless you do not have access to 93-04 octane, or live in a region thats 5000 feet or higher above Sea Level.

BTW: I have 10.6:1 compression and run 38 pounds of boost. No problems with the parts holding up."
96bird is offline  
Sponsored Links
Advertisement
 
post #4 of 13 (permalink) Old 09-23-2004, 03:44 PM
Seasoned PostWhore
 
96bird's Avatar
 
Join Date: Nov 2002
Location: cleveland, TN
Age: 37
Posts: 2,936
Send a message via AIM to 96bird
Here is more from Dave:

"Reducing compression a full point means a significant loss in torque.

Example:
Vobra with 12 psi nets 500 rwhp and 440 pounds of torque
8.7:1 compression with the same combo now means the car makes 450 rwhp and 390 pounds of torque.

If our same cars had 5.4's I would say lowering the compression wouldn't be a big deal because there would still be more torque than horsepower. It wouldnt have as drastic affects as it does with a 283 CI 4V Mod Motor. It's the combination of small cubic inch displacement, and cylinder head design that require the higher compression to optimize the efficiency of the motor.

If you can live with less power and slower numbers at the track and added insurance is more important than optimizing the overall performance I would then say lower compression would the way to go. I would recommend an 11cc dish maximum though no larger.

PRO's
1. More protection against detonation (Pre-ignition). This means you may run about 2-3 degrees more timing per giving octane than stock compression. If your tuner does not put the proper fuel curve into the tuneup you will still blow up. If you run out of fuel you can damage the entire engine regardless of compression ratio. Running an intercooler that can reduce air inlet temps about 75 degrees colder will have about the same amount of detonation protection.
2. Running a dish piston allows you to sometimes run a higher lift camshaft and not worry about piston to valve clearance. However with a 4V Cobra engine, you never have to worry about that anyway because you don't run a ton of lift.

Cons
1. 40 to 55 pounds of torque loss can mean as much as 5 tenths and 5 mph in the quarter mile. Or if your racing your buddy it could mean you are at a 5 to 6 car length disadvantage.
2. Less Torque means the car simply will not accelerate as quickly.
3. 40 to 55 pounds of torque is hard to make up. Think about some of the mods it takes to get 40 to 55 pounds of torque back. Some examples are.
a. Adding 5 to 8 pounds of boost (depending upon the efficiency of your power adder)
b. Removing about 500 pounds from your car.
c. Adding an air to water intercooler and adding more timing to your tuneup.
d. Getting extensive head porting and matching billet camshafts.
e. Adding a 30 shot of nitrous.
f. Adding about 3-035 cubic inches."
96bird is offline  
post #5 of 13 (permalink) Old 09-23-2004, 03:45 PM
Seasoned PostWhore
 
96bird's Avatar
 
Join Date: Nov 2002
Location: cleveland, TN
Age: 37
Posts: 2,936
Send a message via AIM to 96bird
Hope this helps you out some. This is the kind of tech I like.

Russell
96bird is offline  
post #6 of 13 (permalink) Old 09-23-2004, 04:14 PM
AED Specialist
 
A-Train's Avatar
 
Join Date: Jul 2002
Location: Somewhere in NJ
Age: 42
Posts: 1,277
Talking

First, what determines the power output of any gasoline engine is how efficiently it burns fuel. Air is a limiting factor in power production.

More compression ratio equals more cylinder pressures. Most of the time, more compression ratio means the engine makes more power and will rev faster (faster acceleration rate) as well as be more responsive. It also means more octane is required.

Less compression ratio means less responsive engine and lower cylinder pressures.

Rule of thumb is with a blower (any blower), stay low with the C/R. Generally 8.5:1 to 9.2:1 is a good number to have with a supercharger and mild boost levels. A supercharger (or turbocharger) will compress the air and force it into the engine. That air will be hot (thermodynamics) and without an intercooler, it will be less dense. Less air density means less oxygen and more spark that is required (and most likely not available). The supercharger also increases the air charge compression ratio and thereby increases the cylinder pressures. So a high compression engine and lots of boost would yield a low durability/longevity engine.

Yes, boost is a measurement of restriction since boost is made when you flow air through a restriction. The more boost you have, the more air is staking up and not being used. You want cfm or airflow.

Nitrous can be used more effectively with a high compression ratio engine. More so than a blower or a turbo application.

A-Train

2008 Acura TSX (5AT)
2012 Honda Ridgeline RTL (w/Navi)

Ex toy: 1995 T-Bird LX - ALLEN supercharged, 2000 Mustang GT 4.6L PI engine, lot's of goodies...
12.74 @ 109.45 mph (BEST E.T. BEST MPH)
325 RWHP/380 RWTQ (SAE) on a dynojet
Tuned by Jerry W. from SCT
A-Train is offline  
post #7 of 13 (permalink) Old 09-23-2004, 09:26 PM Thread Starter
Seasoned PostWhore
 
Join Date: Jul 2002
Location: Cullman, AL
Age: 56
Posts: 7,758
Send a message via AIM to 94 Daily Driven 4.6L
Ok, now I’m really confused!! Are you guys not saying the exact opposite thing:

Quote:
Originally posted by A-Train
Rule of thumb is with a blower (any blower), stay low with the C/R. Generally 8.5:1 to 9.2:1 is a good number to have with a supercharger and mild boost levels.
(Which is what I have ALWAYS learned/read/researched about boost of any kind)

And:

Quote:
Originally posted by 96bird
Straight from Dave King:

"You can run as much boost as you want with 10.1 compression as long as you run the correct octane and tuneup. You can run 10.1 compression with as much as 17-18 pounds on pump gas with no problems as long as you tune correctly.

Bottom line: Don't go lower than 9.85:1 compression unless you do not have access to 93-04 octane, or live in a region thats 5000 feet or higher above Sea Level.
And don’t get me wrong, I’m not arguing or anything, but just trying to understand (and learn why the last 20 years of my learning about “boost” and their applications isn’t holding true anymore).

So here’s another way to look at it: Lets say I have a Cobra motor with 10.0 C.R… I should be able to slap a roots 6-71 supercharger on top of it and then I can safely run 35 lbs of boost?!?

Or am I getting confused because all the talk about high C.R. and high boost is strictly for racing where super high octane is available and most of my research is for “street/strip”.

Or is this something that is unique to the modular family of engines?

Gawd, I’m giving myself a headache!!!

97 Lincoln Mark VIII LSC (Chip'd, 3.73 T/L... so far... )
97 Ford Aspire (Slow, but getting 36 mpg (f'n Ethenol!! )
84 F250 Dually w/6.9L Diesel (7.3L IDI pending)
73 Mercury Cougar Convertible w/351C 4V (Partially Restored)
69 F100 LWB w/460 Engine
76 Glastron Carlson 23' Jet Boat w/460 CJ Engine
94 Daily Driven 4.6L is offline  
post #8 of 13 (permalink) Old 09-23-2004, 11:31 PM
6th Gear Poster
 
Ashish's Avatar
 
Join Date: Jul 2002
Location: Tampa, FL
Age: 35
Posts: 615
Send a message via AIM to Ashish
The way A-train is putting it is so that you will have longevity. As Dave mentioned up top if overall insurance is more important then your ET then you should lower the ET. There is nothing "magical" about a modular then over a pushrod. Higher Compression and High Boost yields High probability of destruction to your motor. Also like everyone says, "its all in the tune"

As Dave mentioned lowering the CR will lower the overall torque, one way to counter this problem would be a dual power adder like nitrous and a blower.

1997 Birdie, its the good one. Limited Edition Sport Sold
9/6/03-9/7/03-J mod & Mark VIII TC 12/26/03-Apten Chip & 03 GT MAF 3/11/04-3/14/04-PI Cams & PI Intake 8/19/04-FRPP 3.73 Diffy2/26/05-SCT chip (burned by Ken B.)

1998 Z28 M6 5.7 LS1
Ashish is offline  
post #9 of 13 (permalink) Old 09-24-2004, 06:18 AM Thread Starter
Seasoned PostWhore
 
Join Date: Jul 2002
Location: Cullman, AL
Age: 56
Posts: 7,758
Send a message via AIM to 94 Daily Driven 4.6L
OK, now it's making a little more sense.

I’ve been trying to compare apples to oranges (well, ok, maybe apples to pears): For a full-blown race engine, living on the ragged edge, every ounce of power needed, used at a track, that will be “torn into” on many occasions, high boost and high C.R. is obviously the way to go. But for a street/strip application, where fuel octane, longevity/reliability, and price (i.e. lower cost internals) are an issue, it is “safer” to go with the lower C.R. and lower boost, albeit with reduced power.

Dang it, why can’t we have both: 1000 hp w/ 2000 ft. lbs. of torque in an engine that will last 200K miles… without ever needing a tune-up… or oil change… or air filter change…?!?

So like everything: It’s a trade off, max power/short life vs. reduced power/longer life.

Thanks guys... I see the light!!

97 Lincoln Mark VIII LSC (Chip'd, 3.73 T/L... so far... )
97 Ford Aspire (Slow, but getting 36 mpg (f'n Ethenol!! )
84 F250 Dually w/6.9L Diesel (7.3L IDI pending)
73 Mercury Cougar Convertible w/351C 4V (Partially Restored)
69 F100 LWB w/460 Engine
76 Glastron Carlson 23' Jet Boat w/460 CJ Engine
94 Daily Driven 4.6L is offline  
post #10 of 13 (permalink) Old 09-24-2004, 12:43 PM
Seasoned PostWhore
 
96bird's Avatar
 
Join Date: Nov 2002
Location: cleveland, TN
Age: 37
Posts: 2,936
Send a message via AIM to 96bird
Quote:
Originally posted by 94 Daily Driven 4.6L
For a full-blown race engine, living on the ragged edge, every ounce of power needed, used at a track, that will be “torn into” on many occasions, high boost and high C.R. is obviously the way to go. But for a street/strip application, where fuel octane, longevity/reliability, and price (i.e. lower cost internals) are an issue, it is “safer” to go with the lower C.R. and lower boost, albeit with reduced power.
I think you are missing some of the point... I know people running 10 to 1 compression and 15 lbs. of boost. The tune is right on though, but the engine is a daily driver (all of the cars that I know of). It is run on 93 octane, but if he wanted to raise the boost more octane is needed. I would use the most compression that I could get away with and feel safe, so like 9.2-9.4 to 1. If I was using a roots blower, then I wouldn't worry about low speed torque too much anyway so 8.5 to 1 wouldn't be a big deal.

Russell
96bird is offline  
post #11 of 13 (permalink) Old 09-24-2004, 01:15 PM Thread Starter
Seasoned PostWhore
 
Join Date: Jul 2002
Location: Cullman, AL
Age: 56
Posts: 7,758
Send a message via AIM to 94 Daily Driven 4.6L
No, I understand. And 15 psi and 30 psi are way apart in the scheme of things.

Also, the aluminum blocks/heads will "handle" a lot more C.R. with the same level of boost. From everything I've learned over the years an aluminum engine can usually handle about one more point of C.R. than an iron engine, then polish the combustion chambers and add another .5 C.R.

So I guess if we start at 9.0 - 9.5 and 10 - 15 lbs of boost (which is easy with a good forged bottom end and nothing special), convert to aluminum and polish the cc's, 11.0 C.R. is really not that wild.

I just needed to get up to speed on aluminum vs. iron vs. computer tuning vs. carburetors.

Thanks for the info!!

Now riddle me this: Is 15 psi at 3000 from a roots blower going to provide the same hp and torque as 15 psi at 3000 from a centrifugal (assuming delta T is not a factor).

Just kidding!!! Lord knows I don’t want to start a roots vs. centrifugal “discussion”!!!

97 Lincoln Mark VIII LSC (Chip'd, 3.73 T/L... so far... )
97 Ford Aspire (Slow, but getting 36 mpg (f'n Ethenol!! )
84 F250 Dually w/6.9L Diesel (7.3L IDI pending)
73 Mercury Cougar Convertible w/351C 4V (Partially Restored)
69 F100 LWB w/460 Engine
76 Glastron Carlson 23' Jet Boat w/460 CJ Engine
94 Daily Driven 4.6L is offline  
post #12 of 13 (permalink) Old 09-24-2004, 01:18 PM
TCCoA Founder
Administrator
 
Sir William's Avatar
 
Join Date: Jul 2002
Location: Mountain Home, AR
Age: 49
Posts: 4,161
That would depend on the parasitic loss each type of blower would induce on the engine. Of course 15lbs from a turbo would produce more.

God Bless and Fly Low!

Sir William
TCCoA Founder


97 Thunderbird LX - Smoothed

99 Expedition Eddie Bauer

04 Grand Cherokee Overland
Sir William is offline  
post #13 of 13 (permalink) Old 09-24-2004, 01:22 PM Thread Starter
Seasoned PostWhore
 
Join Date: Jul 2002
Location: Cullman, AL
Age: 56
Posts: 7,758
Send a message via AIM to 94 Daily Driven 4.6L
Dang it... after I posted that about the delta T, I was going to add disregard the parasitic losses also... But you beat me to it.

97 Lincoln Mark VIII LSC (Chip'd, 3.73 T/L... so far... )
97 Ford Aspire (Slow, but getting 36 mpg (f'n Ethenol!! )
84 F250 Dually w/6.9L Diesel (7.3L IDI pending)
73 Mercury Cougar Convertible w/351C 4V (Partially Restored)
69 F100 LWB w/460 Engine
76 Glastron Carlson 23' Jet Boat w/460 CJ Engine
94 Daily Driven 4.6L is offline  
Reply

Quick Reply
Message:
Options

Register Now



In order to be able to post messages on the TCCoA Forums forums, you must first register.
Please enter your desired user name, your email address and other required details in the form below.

User Name:
Password
Please enter a password for your user account. Note that passwords are case-sensitive.

Password:


Confirm Password:
Email Address
Please enter a valid email address for yourself.

Email Address:
OR

Log-in











Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)
 
Thread Tools
Show Printable Version Show Printable Version
Email this Page Email this Page



Posting Rules  
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is On
Trackbacks are On
Pingbacks are On
Refbacks are On

 
For the best viewing experience please update your browser to Google Chrome