383 Stroker build questions - Hot Rod Forum : Hotrodders Bulletin Board
Hotrodders.com -- Hot Rod Forum



Register FAQ Search Today's Posts Unanswered Posts Auto Escrow Insurance Auto Loans
Hot Rod Forum : Hotrodders Bulletin Board > Tech Help> Engine
User Name
Password
lost password?   |   register now

Reply
 
LinkBack Thread Tools
  #1 (permalink)  
Old 09-09-2010, 07:59 PM
55effie's Avatar
Registered User
 
Last photo:
Join Date: Mar 2010
Location: Colorado Springs
Posts: 45
Wiki Edits: 0

Thanks: 0
Thanked 0 Times in 0 Posts
383 Stroker build questions

Soooooo recently I took my truck to a show in Cali, where the elevation was only about 1500 foot above sea level. The truck was a rocket ship. I mean, clearly faster and more responsive than here in Colorado Springs. I got to experience fiorst hand how elevation affects engine performance, because four days later, I was driving it here....and it felt like a slug. So, I had some thoughts.....which led to questions.

1) Would a blown motor work better here than a naturally aspirated high compression engine? It would seem that having forced induction would return the the air fuel mixture to a more reasonable level.
2) What is a good compression ratio for a 383 stroker with a blower to keep the truck at about 425 HP?
3) Will a Road Demon 625 suffice for a carb?

    Advertisement
Reply With Quote Quick reply to this message
Sponsored Links
Advertisement
 
  #2 (permalink)  
Old 09-09-2010, 08:34 PM
techinspector1's Avatar
Senior Curmudgeon
 
Last wiki edit: DynoSim combinations Last photo:
Join Date: Oct 2005
Location: Hemet, California, USA
Age: 72
Posts: 13,306
Wiki Edits: 326

Thanks: 836
Thanked 1,151 Times in 950 Posts
Quote:
Originally Posted by 55effie
1) Would a blown motor work better here than a naturally aspirated high compression engine?
Probably.

Quote:
Originally Posted by 55effie
It would seem that having forced induction would return the the air fuel mixture to a more reasonable level.
It's not about the mixture ratio of fuel to air, that has to remain very close to ideal all the time in order for the motor to operate at all. It's about the density of the mixture. At altitude, you have fewer oxygen molecules per cubic foot of air than you do at sea level, so you have to use less fuel molecules to mix with the fewer oxygen molecules to keep the ratio within certain bounds in order to burn at all. If you can pack more oxygen molecules into the cylinder with a pressure-feed apparatus such as a blower, then you can also use additional fuel molecules via larger jets (in a carbureted system).

Quote:
Originally Posted by 55effie
2) What is a good compression ratio for a 383 stroker with a blower to keep the truck at about 425 HP?
If I were building a blower motor today, I would use a forged, D-cup piston that would produce 8.00:1 static compression ratio with the chamber I was going to use. That would allow me to use 8 lbs of boost safely on pump gas with no aftercooler.

Quote:
Originally Posted by 55effie
3) Will a Road Demon 625 suffice for a carb?
In a word, No.
Let's say that the atmospheric pressure at your location is 12.5 lbs/sq/in. A 383 turning 6000 rpm's and rated at 100% volumetric efficiency will pass 665 CFM of air. If you set the blower up to push 8 lbs of boost into the motor, you would divide 8 by 12.5, add the whole number 1 and come away with a factor of 1.64. Now, if you multiply 1.64 times 665, you will find that the motor will be passing 1,090 CFM of air at 6000. So, you will need a mixing device of at least 1,090 CFM to feed the motor at that speed. Most guys would use two 600's.

You can play with this math any way you want to. Let's say you just wanted to puff 4 lbs into the motor at 5000 rpm's. The motor would pass 554 CFM at 5000. Dividing 4 by 12.5 and adding 1, we come away with a factor of 1.32. Multiplying 554 times 1.32 will find that the motor will be passing 731 CFM at 5000 and 4 psi and so probably a 750 carb would feed it.

If I were thinking about the lesser boost, I might be looking at one of the smaller Weiand blowers that would mount one carburetor (like the 750 mentioned). Now, the smaller blower will still produce big pressure, but with higher boost temps. To use one of them at 4-5 psi just to get back to a reasonable power level at altitude makes a lot of sense to me.

Give me all the specifics on your cam, heads, exhaust, etc and I'll run up a DynoSim with the blower on the motor.
Reply With Quote Quick reply to this message
  #3 (permalink)  
Old 09-09-2010, 08:55 PM
55effie's Avatar
Registered User
 
Last photo:
Join Date: Mar 2010
Location: Colorado Springs
Posts: 45
Wiki Edits: 0

Thanks: 0
Thanked 0 Times in 0 Posts
Whoa....I figured this would get somewhat technical....but this is a whole new level for me.

What are D-Cup pistons?

Here are the specs on what I do know:
GM 383 balanced and blueprinted stroker, 6 rods, Hypereutectic pistons
Compression ratio:10.5:1
Dart Heads: 64cc chambers, 200cc intake runners with large valves
Cam shaft with .495 of lift and roller rockers
625 CFM Road Demon carburator
MSD ignition
Autolite AP25 plugs
7127-1W Gm alternator
2.5 dual exhaust
Reply With Quote Quick reply to this message
  #4 (permalink)  
Old 09-09-2010, 08:57 PM
55effie's Avatar
Registered User
 
Last photo:
Join Date: Mar 2010
Location: Colorado Springs
Posts: 45
Wiki Edits: 0

Thanks: 0
Thanked 0 Times in 0 Posts
BTW, I meant density in regards to the air/fuel mixture, not ratio. Thanks for pointing that out.
Reply With Quote Quick reply to this message
  #5 (permalink)  
Old 09-09-2010, 09:43 PM
techinspector1's Avatar
Senior Curmudgeon
 
Last wiki edit: DynoSim combinations Last photo:
Join Date: Oct 2005
Location: Hemet, California, USA
Age: 72
Posts: 13,306
Wiki Edits: 326

Thanks: 836
Thanked 1,151 Times in 950 Posts
Scroll down here a page or two to where it says HEAD DESIGN on the right side of the page. You'll see several piston crown designs there, each with a different type of quench pad or "squish" pad. As the piston ascends in the bore, the large, flat squish pad comes up against the underside of the cylinder head, squishing out the fuel air mixture that is there and jetting it across the combustion chamber. This turbulence thoroughly homogenizes the mixture and eliminates any dead zones, allowing the use of lower grade fuels in some instances and the use of pump fuels without detonation in other cases where a high static compression ratio combined with a cam that has an intake closing point that is too early and traps too much volume of mixture for the combination to operate on pump gas without rattling like shaking a can full of rocks.
http://www.kb-silvolite.com/assets/a...ve_catalog.pdf

Normally, if a guy was going to build a motor to stay together with a blower, he would choose a forged piston with a D-cup and it would be specified as a "blower piston", which would have a thicker crown to resist yielding under the additional heat and pressure of a compressed air/fuel mixture.

Now, if I were going to build a mild blower motor, I might try to squeeze by with a low compression piston with a standard crown thickness, such as one of these 31cc dish SRP forged pistons in conjunction with a 70cc combustion chamber.
http://www.jepistons.com/Catalogs/SR...0-400_inv.aspx
Here's a price list showing such a piston at $563.36 for the set. Now, this supplier does not show a 6" rod piston for a 383, but I'm just trying to show you some different pistons that I might use if I were starting from scratch and wanted to spend a little less on pistons because of using low boost, like 4 or 5 psi. Call up these guys or your favorite piston manufacturer and talk with them about it. The guys who make the pistons always have the best information.
http://www.jepistons.com/PDFs/OrderI..._Sportsman.pdf
Reply With Quote Quick reply to this message
  #6 (permalink)  
Old 09-09-2010, 11:12 PM
55effie's Avatar
Registered User
 
Last photo:
Join Date: Mar 2010
Location: Colorado Springs
Posts: 45
Wiki Edits: 0

Thanks: 0
Thanked 0 Times in 0 Posts
That answers that. An engine has to built to accommodate any sort of forced air induction. I was wondering if I could simply swap out the heads and intake on my 383 to receive a blower and such. I knew that forced induction required a lesser compression ratio, but I was shocked at the ratio around 8.0:1. That makes sense to use the D-cup pistons then for a blower motor.....more space in the compression chamber equates to lower compression ratios for a blower. I have no idea what head design my pistons have. Probably something that works better for high compression.....opposite of what I want.

The Atmospheric pressure here in Colorado Springs is closer to 11.7-12. Roughly that of 6500 feet of elevation. Not exactly ideal for hotrods...which makes the PPIHC all the more amazing of a race.

How did you come up with the volumetric efficiency of 665 CFMs for 6000 RPMs? How about 554 for 5000 RPMs?
Reply With Quote Quick reply to this message
  #7 (permalink)  
Old 09-10-2010, 12:25 AM
Landshark928's Avatar
FrankenPorsche
 
Last photo:
Join Date: Feb 2010
Location: Pensacola, Fl
Age: 45
Posts: 459
Wiki Edits: 0

Thanks: 0
Thanked 13 Times in 9 Posts
For $400 you can put a 75-150 RWHP shot of giggle gas on it. HP on demand. When you're off the spray it's a mild mannered daily driver. When the ricer pulls up, you get the extra ponies to keep up.
Reply With Quote Quick reply to this message
  #8 (permalink)  
Old 09-10-2010, 03:06 AM
techinspector1's Avatar
Senior Curmudgeon
 
Last wiki edit: DynoSim combinations Last photo:
Join Date: Oct 2005
Location: Hemet, California, USA
Age: 72
Posts: 13,306
Wiki Edits: 326

Thanks: 836
Thanked 1,151 Times in 950 Posts
Quote:
Originally Posted by 55effie
The Atmospheric pressure here in Colorado Springs is closer to 11.7-12. Roughly that of 6500 feet of elevation. Not exactly ideal for hotrods...which makes the PPIHC all the more amazing of a race.
My mind works off math most of the time. I didn't look it up, I just know that for each 1000 ft increase in altitude, you lose about 3% power, so I figured if you are at 5000 ft, that's 15% loss, so 14.7 times .85 would be 12.49 psi.

So, let's split the difference on what you said here and call it 11.85 psi. If we divide the boost, 8 psi, by 11.85, we get 0.675....now if we add a whole number 1 to it, we get 1.675 as a multiplier. Multiplying 665 times 1.675 shows the motor will pass 1,113 CFM at 8 psi boost and 6000 rpm's at your 6500 ft altitude.

Quote:
Originally Posted by 55effie
How did you come up with the volumetric efficiency of 665 CFMs for 6000 RPMs? How about 554 for 5000 RPMs?
6000 times 383 divided by 3456 = 664.93 CFM. 5000 times 383 divided by 3456 = 554.10 CFM. The 100% efficiency is theoretical, what the motor would pass with decent heads and cam. If you are talking a dog poo motor with junk heads, you will want to begin factoring the efficiency down by some factor. For instance, if you figure the heads are only worth about 85% of what a decent head will flow, then you might multiply the 665 times .85 and wind up with 565 CFM. I run quite a few DynoSims and have seen efficiencies from 80% to 115%. Most of the combinations that I see that exceed 105% make so much cylinder pressure that they would not operate on pump gas without detonation.

Last edited by techinspector1; 09-10-2010 at 03:12 AM.
Reply With Quote Quick reply to this message
  #9 (permalink)  
Old 09-10-2010, 09:37 AM
55effie's Avatar
Registered User
 
Last photo:
Join Date: Mar 2010
Location: Colorado Springs
Posts: 45
Wiki Edits: 0

Thanks: 0
Thanked 0 Times in 0 Posts
Quote:
Originally Posted by techinspector1
Most of the combinations that I see that exceed 105% make so much cylinder pressure that they would not operate on pump gas without detonation.
So, how do figure out if the combination I have is correct? I think it works fine, but I have been wrong before....but only once.

Was the information that I gave you enough for me to figure it out (the cam duration and head information)?
Reply With Quote Quick reply to this message
  #10 (permalink)  
Old 09-10-2010, 09:45 AM
55effie's Avatar
Registered User
 
Last photo:
Join Date: Mar 2010
Location: Colorado Springs
Posts: 45
Wiki Edits: 0

Thanks: 0
Thanked 0 Times in 0 Posts
Oh yeah, I wanted to ask where you came up with the "3456" that is being divided into the Engine displacement x RPMs? And how does one determine what RPMs the engine need to be at for 100% efficiency? I guess that my interest in building another 383 is greater than I thought, considering all my questions.
Reply With Quote Quick reply to this message
  #11 (permalink)  
Old 09-10-2010, 10:18 AM
Siggy_Freud's Avatar
Hotrodders.com Moderator
 
Last photo:
Join Date: Dec 2003
Location: Portland, Oregon
Age: 29
Posts: 2,371
Wiki Edits: 0

Thanks: 0
Thanked 1 Time in 1 Post
8:0-1 isn't required to run a blown setup. Whatever static compression you do run will, to an extent, determine how much overall boost you can run though. On my mustang I started with 8.7:1 compression, and am currently running 13lbs.

10.5:1 is high, but you could still get away with some mild boost (4-5lbs) and run on premium fuel. Now, is it worth it to build a blower system for 4-5 lbs? Thats a question for you to decide. Keep in mind that you will make more out of those 4-5lbs of boost than 4-5lbs starting at 8.0:1 compression though.

You should have no issues getting 420'ish HP running 4lbs of boost.

My Stang made ~435hp on 4 lbs. Yes, it has fuel injection and advanced timing features, but at the time it was nearly 100 cubes less than your setup.
__________________
Bringing history and technology together.
Reply With Quote Quick reply to this message
  #12 (permalink)  
Old 09-10-2010, 11:29 AM
Member
 
Last photo:
Join Date: Aug 2010
Location: Missouri
Posts: 466
Wiki Edits: 0

Thanks: 0
Thanked 0 Times in 0 Posts
A boosted engine will beat a NA engine- its why you see so many racers running boost or nitrous.

For a blown engine Techinspector has it outlined fairly well, except in a crunch a 625 road demon can be made to work just fine, though you'll be giving up about 35hp up top. There's also a few slightly skewed math assumptions but for 95% of street engines his guidelines are simple to understand and work pretty well.


Also, if you only want 425hp a blown 350 set up right will get the job done and will save you some money. A 383 is nicer but not needed at all for 425hp with a decent amount of boost (7-10psi).
Reply With Quote Quick reply to this message
  #13 (permalink)  
Old 09-10-2010, 12:21 PM
ChevelleSS_LS6's Avatar
Jeep XJ and a Javelin
 
Last wiki edit: Compression test
Last journal entry: August 9, part III
Last photo:
Join Date: Dec 2001
Location: Grand Rapids, Michigan
Age: 29
Posts: 1,614
Wiki Edits: 7

Thanks: 6
Thanked 0 Times in 0 Posts
Aluminum cylinder heads (like modern cars) tends to help being able to run a higher CR with boost as well. I'm sure modern combustion chamber design is a factor as well.

I think techinspector had EXCELLENT advice, and is a good generalization of what to run. I think it's better to run too low compression than too high. Race gas isn't cheap, and especially if you don't want to pull the heads off to fit a thicker head gasket in hopes of being able to run pump gas.

Nitrous has it's ups and downs, but is the budget power king when compared to forced induction. The main downside to a blower is price and if you go turbo, plumbing as well.

I'd personally go with a street oriented roots blower. Centrifugals are more efficient, but they don't move as much air at lower rpms as the 'ol twinscrew. Turbos... it depends on what you have and how it spools and whatnot, but then there's the plumbing issue that, personally, I'd rather not deal with. Then again I don't have a lot of welding experience either.
Reply With Quote Quick reply to this message
  #14 (permalink)  
Old 09-10-2010, 03:39 PM
techinspector1's Avatar
Senior Curmudgeon
 
Last wiki edit: DynoSim combinations Last photo:
Join Date: Oct 2005
Location: Hemet, California, USA
Age: 72
Posts: 13,306
Wiki Edits: 326

Thanks: 836
Thanked 1,151 Times in 950 Posts
Quote:
Originally Posted by 55effie
Oh yeah, I wanted to ask where you came up with the "3456" that is being divided into the Engine displacement x RPMs?
It is the constant in the formula.
You're starting off with cubic inches and ending up with cubic feet, so you have to convert. One cubic foot equals 1728 cubic inches. Then you have to realize that for each time the cylinder fills, the crankshaft has made two revolutions. There are 720 crank degrees to a complete cycle. So, you can double the 1728 and call it 3456 or you can just begin solving the problem by cutting the revs in half and dividing by 1728.
6000 X 383 / 3456 = 664.9
3000 X 383 / 1728 = 664.9


Quote:
Originally Posted by 55effie
And how does one determine what RPMs the engine need to be at for 100% efficiency?
I was just using reasonable street rpm's in the formulas. You don't know what the efficiency of the motor will be until you run a DynoSim on it or strap it to a real dyno and pull the handle. The beauty of the DynoSim software is that you can change any part of the build and verify results in seconds.

Lots of fellows talk down the value of the DynoSim, but I did a lot of research before I bought it and found out it was programmed by some pretty high-powered minds in the high-performance industry. One thing that I do though, is to factor cylinder head flow before I enter it. Usually, I will multiply the published flow at each valve lift by 0.90 or 0.95, depending on how much I think the manufacturer is fudging on the figures. Here are some of the things I look for....
1. What size pipe was used for testing. Was it real-world like 4.030" for a small block Chevy head or did they use a 4.200" pipe to skew the results in their favor? There is one mfg who does this for sure.
2. Did they use a pipe on the exhaust side or not?
3. Did they use reduced stem diameter on the valves?
4. Did they use fully radiused valves and seats or just a standard 3 or 5 angle valve job?
5. What was the ambient temperature and relative humidity in the shop at the time of the test? (I have never read that any of them use a standard set of test parameters as far as temp or humidity).

As far as not needing 8.00:1 static compression ratio for a blower setup, that's true, depending on the boost you are planning. Blower Drive Service says you need 8.00:1 for 8 lbs of boost on pump gas. They also say you can run 2 lbs of boost at 11.00:1 on pump gas. These guys have been doing this stuff for decades, so I tend to believe what they say. Now, it is true that you can run more boost on pump gas with a centrifugal blower because it will not beat and heat the air like a Roots unit.

The pump gas range is shown here in the shaded figures. Make your own determinations as to whom you want to trust.
http://www.blowerdriveservice.com/techcharts.php

Last edited by techinspector1; 09-10-2010 at 03:46 PM.
Reply With Quote Quick reply to this message
  #15 (permalink)  
Old 09-10-2010, 03:53 PM
techinspector1's Avatar
Senior Curmudgeon
 
Last wiki edit: DynoSim combinations Last photo:
Join Date: Oct 2005
Location: Hemet, California, USA
Age: 72
Posts: 13,306
Wiki Edits: 326

Thanks: 836
Thanked 1,151 Times in 950 Posts
Quote:
Originally Posted by ChevelleSS_LS6
Race gas isn't cheap, and especially if you don't want to pull the heads off to fit a thicker head gasket in hopes of being able to run pump gas.
Bad juju bwana. You don't want to be installing a thicker head gasket to lower static compression ratio. By doing that, you increase the thickness of the squish and thereby contribute to less octane tolerance and the increased possibility of detonation on the same quality of fuel you have been running. Engineer the motor with a 0.035" to 0.045" squish when you are putting your parts wish list together. If, later, you need to lower the static compression ratio, either change heads or change pistons.
Reply With Quote Quick reply to this message

Recent Engine posts with photos

Quick Reply
Message:
Options

Register Now

In order to be able to post messages on the Hot Rod Forum : Hotrodders Bulletin Board forums, you must first register.
Please enter your desired user name (usually not your first and last 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:

Log-in

Human Verification

In order to verify that you are a human and not a spam bot, please enter the answer into the following box below based on the instructions contained in the graphic.




Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)
 
Thread Tools

Posting Rules
You may post new threads
You may 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 Off
Trackbacks are On
Pingbacks are On
Refbacks are On


Similar Threads
Thread Thread Starter Forum Replies Last Post
383 stroker built...now need cam suggestions Astrodokk Engine 7 01-31-2009 03:53 PM
383 stroker build 84berlinetta Engine 3 12-01-2008 07:48 PM
How to build a 383 stroker JAKE 68 GMC Engine 15 05-30-2006 02:28 PM
383 Stroker questions Pre-StrokerC1500 Engine 9 05-23-2006 02:53 PM
I want to build a 383 stroker and i don't know how...need info stud_duck77 Hotrodders' Lounge 5 05-23-2004 05:19 AM


All times are GMT -6. The time now is 09:49 PM.


Powered by vBulletin® Version 3.8.7
Copyright ©2000 - 2014, vBulletin Solutions, Inc.
Search Engine Optimization by vBSEO 3.6.0 PL2
Copyright Hotrodders.com 1999 - 2012. All Rights Reserved.