Compression question for the Pro engine builders.. - 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 10-06-2003, 10:40 PM
Member
 
Join Date: May 2003
Location: Metro Detroit
Posts: 51
Wiki Edits: 0

Thanks: 0
Thanked 0 Times in 0 Posts
Compression question for the Pro engine builders..

I have a 327/340 HP which was rebuilt mostly stock except for the following: .30 overbore, the heads were mildly ported and polished with SS valves installed and the combustion chambers were CC'd. The rotating assembly was balanced and the compression ratio was lowered from 11.25 to 9.5 with forged aluminum Speed Pro pistons installed. The cam is the stock specification Duntov grind and is a solid lifter. My question concerns the choice of my compression ratio and whether I will be happy with the resulting performance or if I should change back to the 11.25 to one pistons before reinstalling this engine into my vintage Corvette.

I don't really understand the roll that compression plays. I have been told that this combination won't have the low end performance that it would have with the higher compression 11.25 pistons intended to go with this cam. It was my idea to go with the lower compression. I wanted the car to be able to run on pump gas, yet retain as much of the original character as possible. My builder originally recommended a different cam (a hydraulic) to go with the lower compression and for better performance with this combination. I wanted to retain the solid cam for the sound and character of the engine.

I know there are a thousand and one different combos out there but this is a VINTAGE REBUILD. It has plenty of performance in the stock configuration and I am not looking to re-engineer what Chevy originally intended with this engine. The car will see limited use, (won't be used for towing heavy loads ;-) and should be able to run on premium pump gas. I'd be interested in your educated explanations of cylinder compresson and the roll it plays with a cam of this duration. What are the pros and cons of lowering the compression, as I have already done?

It's not that big a deal to change the pistons now if it's warranted. Once it's back in the car I would have to live with it. I don't want anemic. I want this engine to run as it was originally intended. What do you Pros think? Everyone else is welcome to chime in and learn. Thanks guys.

    Advertisement
Reply With Quote Quick reply to this message
Sponsored Links
Advertisement
 
  #2 (permalink)  
Old 10-06-2003, 11:07 PM
Jmark's Avatar
Hotrodders.com Moderator
 
Last photo:
Join Date: Sep 2003
Location: phoenix
Age: 61
Posts: 4,786
Wiki Edits: 0

Thanks: 0
Thanked 3 Times in 3 Posts
First off, all you will get are good OPINIONS as what to do.

My opinion is to stay with the 9.5 pistons, providing you have the machine shop set the deck height to match the new pistons. That is called "quinch" and i'll post a good link to understanding it later on. The reason I"d stay with the 9.5s is todays gas is nowhere close to the good stuff that was available way back then. With 9.5s, you'll still have a very strong running engine and it'll probably be more streetable than running the 11s.

Setting the "quinch" distance is a must for todays crappy gas and higher compression ratios.

Back in the 60s and 70s, advertised compression numbers were very misleading. Using limited carb throttle openings, high overlap cam lobes killed off most of the supposed high compression. Alot of it was pure hype to sell cars. Some hauled ***, but lots were real slugs with misleading compression ratios.

Here is an excellent read on selecting pistons, setting deck height and cam selection. Take your time selecting parts, it ain't like it used to be! LOL


http://www.speedomotive.com/Building%20Tips.htm
Reply With Quote Quick reply to this message
  #3 (permalink)  
Old 10-07-2003, 07:19 AM
TurboS10's Avatar
Hotrodders.com Moderator
 
Last wiki edit: Auto Terms and Definitions Used in Modern Engine T... Last photo:
Join Date: Sep 2002
Location: Texas
Age: 36
Posts: 3,463
Wiki Edits: 2

Thanks: 0
Thanked 0 Times in 0 Posts
I am no pro, but I know a little. The role compression plays has alot to do with cam selection. Larger cams have more valve overlap than smaller cams. This is also amplified by a tight lsa around say 108-106. What happens is that the exhaust valve stays open longer thus "overlapping" the intake valves opening ramp. This means that you get more reversion or exhaust gas actually trying to enter the intake as the valve opens. This causes a contaminated intake charge and lower cylinder pressures since you get less fresh air and fuel into the cylinder. If compression ratio is higher, it will help with the cylinder pressure in the low rpm when reversion is at its worst. Since cylinder pressure is directly related to power output, you can see the outcome.

NOW, this sounds like big cams are bad. Not always true. As the engine RPM goes up there becomes less time for that exhaust gas reversion to happen. As things start happening faster, the engine starts "cleaning up" and hits the so called "power band" at which the cam starts to come into its own. If everything else is matched, the large cam will start to pay off in the higher rpm.

Remember, most people try to run too much cam cause it sounds cool. Big cams are not always best for street driving.

It comes down to where you want the power.....

Chris
Reply With Quote Quick reply to this message
  #4 (permalink)  
Old 10-07-2003, 08:07 AM
Member
 
Join Date: May 2003
Location: Metro Detroit
Posts: 51
Wiki Edits: 0

Thanks: 0
Thanked 0 Times in 0 Posts
Quote:
Originally posted by Jmark
First off, all you will get are good OPINIONS as what to do.

My opinion is to stay with the 9.5 pistons, providing you have the machine shop set the deck height to match the new pistons. That is called "quinch" and i'll post a good link to understanding it later on. The reason I"d stay with the 9.5s is todays gas is nowhere close to the good stuff that was available way back then. With 9.5s, you'll still have a very strong running engine and it'll probably be more streetable than running the 11s.

Setting the "quinch" distance is a must for todays crappy gas and higher compression ratios.
[/
Jmark, the engine is already done. Thanks for your opinion and info. I am afraid you're right about OPINIONS. I guess the only way to know for sure is to run it in on the dyno before installing. However, the cost of a dyno run is a gamble because I could just switch out the pistons and go with the 11.25's, as stock for approximately the same money.

The engine was built by a respected professional who has since sold out and retired. I don't know if he set the deck height to match the 9.5's but I would bet that he did. He did recommend a hydraulic cam to replace the stock Duntov solid lifter. His logic was that it would perform better with the lower compression and make more usable HP at the low and mid ranges without giving up too much top end. I wanted to stay with the stock grind because that's what gives the engine it's character. It is the "correct" cam to use for my VINTAGE engine. Given the reality of what's available at the pump I figured I'd be better off with a lower compression.

Quote:
Originally posted by Jmark
Back in the 60s and 70s, advertised compression numbers were very misleading. Using limited carb throttle openings, high overlap cam lobes killed off most of the supposed high compression. Alot of it was pure hype to sell cars. Some hauled ***, but lots were real slugs with misleading compression ratios.[/
As it has been explained to me, the higher compression is designed to work with the higher overlap cam. I don't have the specs of the Duntov 327/340 grind at my fingertips but it is commonly referred to as a 30-30. Given quality control and castings technology at the time, I am sure there was a variance between stated and actual compression in the cylinders. However, what would have been the lowest? No way to know but I am sure it would be higher than 9.5. So supposing my engine has a true 9.5 CR how much HP loss might I have? Or, more importantly will it run well or be a neutered pig?

Quote:
Originally posted by TurboS10
I am no pro, but I know a little. The role compression plays has alot to do with cam selection. Larger cams have more valve overlap than smaller cams. This is also amplified by a tight lsa around say 108-106. What happens is that the exhaust valve stays open longer thus "overlapping" the intake valves opening ramp. This means that you get more reversion or exhaust gas actually trying to enter the intake as the valve opens. This causes a contaminated intake charge and lower cylinder pressures since you get less fresh air and fuel into the cylinder. If compression ratio is higher, it will help with the cylinder pressure in the low rpm when reversion is at its worst. Since cylinder pressure is directly related to power output, you can see the outcome.

NOW, this sounds like big cams are bad. Not always true. As the engine RPM goes up there becomes less time for that exhaust gas reversion to happen. As things start happening faster, the engine starts "cleaning up" and hits the so called "power band" at which the cam starts to come into its own. If everything else is matched, the large cam will start to pay off in the higher rpm.

Remember, most people try to run too much cam cause it sounds cool. Big cams are not always best for street driving.

It comes down to where you want the power.....

Chris
Chris, Your explaination is how/why it was explained to me that the lower compression ratio of 9.5 won't perform as well for me as the stock 11.25 with my stock specification solid lifter cam.

The stock cam does have a nice lope to it and the solids have their unique sound. I like the power this engine made before being rebuilt, more than enough for my use. Very streetable too, although I wouldn't want it as a daily commuter.

Last edited by Axelrod; 10-07-2003 at 08:07 AM.
Reply With Quote Quick reply to this message
  #5 (permalink)  
Old 10-07-2003, 09:14 AM
johnsongrass1's Avatar
Race it, Don't rice it!
 
Last photo:
Join Date: Jul 2002
Location: Columbia, Mo
Age: 37
Posts: 4,078
Wiki Edits: 0

Thanks: 0
Thanked 9 Times in 9 Posts
There's about a 6 percent gain/loss in power for every point of compression on a typical street engine up to around 14:1. The increases are reduced to around 6-8 percent change after the 15:1 mark. You will notice the change. However with careful tuning in cam timing you can get back some of that loss in the upper range and increase the power in the lower range. A hydraulic cam will also help with power on a lower compression engine. By reducing the character ramp inclines and closing the valve faster on the down side of the lobe the cam will work better lower and hold a range longer than the solids, but will reduce peak power. I think you will appreciate what the machinist did.
Reply With Quote Quick reply to this message
  #6 (permalink)  
Old 10-07-2003, 09:40 AM
NorthStar's Avatar
Member
 
Last photo:
Join Date: Feb 2003
Location: Minnesota
Age: 50
Posts: 847
Wiki Edits: 0

Thanks: 0
Thanked 0 Times in 0 Posts
If you compare the two combinations the higher compression motor will make more power. But in order to run high compression piston and pump gas you would have to pull the timing back so far to keep it from detonating that it would run like crap.

Id stay with the 9.5 to 1 pistons.
Reply With Quote Quick reply to this message
  #7 (permalink)  
Old 10-08-2003, 02:10 PM
Member
 
Join Date: May 2003
Location: Metro Detroit
Posts: 51
Wiki Edits: 0

Thanks: 0
Thanked 0 Times in 0 Posts
I have the solution!

I was given the solution for my application which I believe will work very well. I am posting it here for the benefit of others. This solution was provided to me by a former GM Engineer, a person with a long history of automotive experience and knowledge.

Replace my cam with the '70-'72 LT-1, "the best all-around solid lifter cam" and "the culmination of 15 years of Chevy solid-lifter cam development." He informed me that this cam gives 20% more torque in the normal driving range than the 30-30 and loses nothing on the top end. Further it is designed to run through stock exhaust manifolds and use plain vanilla production valve springs. It has a soft action profile that is easy on the valve train.

This way I don't lose any performance on the top end, gain usable torque at the lower end and can run it on the available premiums at the pump today. I also save big because I don't have to switch out pistons.

The more radical profile of the stock cam I have in the 327/340 likes higher compression to build cylinder pressure. To make it work properly I'd need the higher 11.25 compression ratio over the 9.5 it was rebuilt with. That would leave me blending off road fuel with pump premium to get the necessary octane, (93 today is equal to 97 from yesteryear with the changes in the way it is measured), or living with the more radical cam profile and retarding the timing to prevent detonation.

Thanks to all that replied and a BIG thanks to the un-named GM engineer.
Reply With Quote Quick reply to this message
  #8 (permalink)  
Old 10-08-2003, 06:31 PM
New Member
 
Join Date: Nov 2002
Location: texas
Posts: 28
Wiki Edits: 0

Thanks: 0
Thanked 0 Times in 0 Posts
My .02:

Every cam size jump, for a street engine, needs more compression, and we routinely build and tune 11-12:1 EFI motors with large cams, in the area of 230-250* of duration at .050 valve lift. There are a few more variables to consider, but that's the general rule. granted, we custom tune the EFI controls for them, and we do run higher compression than most would recommend (10-10.5:1 on GM TBI's, etc), but these guys all have a point in their arguments. On the cam type, I do not use solids of any type for street motors under 6500rpm operating range, for maintenance reasons, and not that all old cam grinds are bad, but there are much better, smoother operating ones to chose from now, that work much better, in all ranges.
Reply With Quote Quick reply to this message
  #9 (permalink)  
Old 10-08-2003, 08:14 PM
Member
 
Join Date: May 2003
Location: Metro Detroit
Posts: 51
Wiki Edits: 0

Thanks: 0
Thanked 0 Times in 0 Posts
Quote:
Originally posted by oemfisystems
My .02:

On the cam type, I do not use solids of any type for street motors under 6500rpm operating range, for maintenance reasons, and not that all old cam grinds are bad, but there are much better, smoother operating ones to chose from now, that work much better, in all ranges.
OEM, I would agree with you on the solid cam except in the case of a VINTAGE rebuild like mine. No doubt they require more maintenance than a hydraulic but nothing sounds like the mechanical valve train on a well tuned solid. I love it!! If my car didn't have the original motor I would go with something entirely differant. I'd probably just install a ZZ4 crate motor and call it a day.

As you mentioned, in your example, you are tuning the EFI systems for the higher compression and can set them to allow for pump gas. With an AFB and a dual point distributor I am limited.

My builder made the same point on cam choices but I wanted to go with the original solid. The LT-1 cam would have made a better choice but I didn't know enough about cam design to think of that option and my builder didn't suggest it. Given my goal of retaining as much of the original character of the engine as possible, yet being able to gas it up at the pump and go, the LT-1 cam seems to be the ticket with my compression at 9.5 to 1. Compromise with the least give up is the order of the day. If I went back to 11.25 to 1 pistons it would obviously cost me more money and then I would be tethered to blending fuel to get the desired octane.
Reply With Quote Quick reply to this message
  #10 (permalink)  
Old 01-15-2007, 03:18 PM
Member
 
Last photo:
Join Date: Nov 2006
Location: pacific NW
Posts: 74
Wiki Edits: 0

Thanks: 0
Thanked 1 Time in 1 Post
compression

was wondering; 377" chevrolet with 14 cc dish piston and 64cc heads---what is my compression ratio ?? would that be like a flat top piston with 78cc heads and appx. 9.5-1 ???
Reply With Quote Quick reply to this message
  #11 (permalink)  
Old 01-15-2007, 03:54 PM
NorthStar's Avatar
Member
 
Last photo:
Join Date: Feb 2003
Location: Minnesota
Age: 50
Posts: 847
Wiki Edits: 0

Thanks: 0
Thanked 0 Times in 0 Posts
Use this to figure it out.

http://www.rosspistons.thinkhost.com/calculator.php
Reply With Quote Quick reply to this message
  #12 (permalink)  
Old 01-15-2007, 05:38 PM
NAIRB's Avatar
Hotrodders.com Moderator
 
Last photo:
Join Date: Sep 2002
Location: cimarron, ks
Posts: 1,658
Wiki Edits: 0

Thanks: 0
Thanked 1 Time in 1 Post
ooooppppsssssss!!!!!


3 year old thread!!!!
Reply With Quote Quick reply to this message
  #13 (permalink)  
Old 01-15-2007, 10:42 PM
techinspector1's Avatar
Senior Curmudgeon
 
Last wiki edit: DynoSim combinations Last photo:
Join Date: Oct 2005
Location: Hemet, California, USA
Age: 72
Posts: 12,599
Wiki Edits: 326

Thanks: 701
Thanked 886 Times in 753 Posts
I didn't notice that it is a 3-yr old thread, but I will throw in my 2 cents worth anyway.

The relationship between static compression ratio and the cam has to do with the intake closing point of the cam, not with reversion and overlap. The motor cannot begin compressing the mixture until the intake valve closes at some point after bottom dead center. With a lower c.r., you will need to use a shorter cam with an early closing point to capture enough fuel/air mixture to make any power. With a higher c.r., you will need to delay the intake closing point and bleed off some of the mixture in order to limit cylinder pressures and use pump gas. The fellows who do not understand this and want the rump-rump of a long cam will install a long cam with the stock c.r. and then not understand why the motor is a pig. It's because the dynamic compression ratio is in the tank. The flip side of the coin is the fellows who have higher c.r. and will install a short cam. The result is a motor that will not operate on pump gas and sounds like a can of rocks being shaken (detonation) because of excessive cylinder pressures. Choosing a cam for a motor is a science and is determined by math, not your emotions.

The best thing to do is to call your cam grinder and tell them the EXACT static compression ratio of your motor, then tell them what you want your motor to do. Cams with relatively short duration can be ground on a shorter lobe separation angle to provide rump-rump if that's what you want. You don't have to screw everything up by using a long cam with low c.r. just to get the sound you want.
Reply With Quote Quick reply to this message
  #14 (permalink)  
Old 01-16-2007, 06:10 AM
Registered User
 
Join Date: Dec 2006
Location: So Cal
Posts: 128
Wiki Edits: 0

Thanks: 0
Thanked 0 Times in 0 Posts
Compression question for the pro engine builders..

Hey Im new ? I dont understand 3 year old post thing ? Good read ! But head Flow, timming curve , and exhaust pulse, elevation, type of head's << Per say Hemi ? play a good part in how hi you can go on CCR too. Along with what cam you run etc. . But Just like too add that 9.5 -1 in my opinion is about it on pump gas . Not too mention your Engine will last considerably longer ????? Now I have another, coupla question's ? Is there a Puter on this site that figures, or computes CCR ? Why is this a 3 year old Post ? Im still feelin the place out . Cant find 1/2 my Post ? Im just a lookin around, makin my usual funn of Chevies ? But dont understand 3 Year old Post ? You dragg this up from Archives ? Delete my post cause I kidd about Chevy ? Or is it the Vw thing ? Knew I should have kept my big mouth shut ? Now im hated Ostercized even ? Built few v8's in my youth . But last few year's mostly been doin Vw thing . Been flavour for over 10 year's now . Curious here if any one takes time and trouble too Degree there Camm's ? Im lookin too get back intoo some American Muscle. Lookin Hard at a Willys Jeep pick up . Mainly cause it's Light, cheap . That and Im thinkin Chop top Possibly . More Areo Dynamic ? Remember year's ago Guy across street was plantin a 327 in one . Never got it goin ? But im open for option's ? Mainly Pick up's . Capable of Full size Loads . Yett light and Aero dynamic . But Good read for 3 year old post . Sean
Reply With Quote Quick reply to this message
  #15 (permalink)  
Old 01-16-2007, 10:29 AM
techinspector1's Avatar
Senior Curmudgeon
 
Last wiki edit: DynoSim combinations Last photo:
Join Date: Oct 2005
Location: Hemet, California, USA
Age: 72
Posts: 12,599
Wiki Edits: 326

Thanks: 701
Thanked 886 Times in 753 Posts
Posted by Sean:
"I dont understand 3 year old post thing ?"

Sean, if you'll look in the gray line at the top of each post, it'll tell you when that post was entered. This thread was begun by axlerod on 10-06-2003. I'm sure that by now, he has figured out that the long GM cam won't work with the 9.5 static compression ratio. The motor would be so soggy on the bottom that it wouldn't be any fun at all to drive.

I can appreciate your opinion that 9.5 is about it on pump gas, but you can run on up to about 11.0 if you build a tight squish into the motor and use the correct cam for the static compression ratio you're running. If you don't understand squish, it is the clearance between the crown of the piston and the flat underside of the cylinder head opposite the chamber. On a small block with tight-fitting pistons such as cast or hypereutectics, you can adjust this clearance down to 0.035" to 0.040". Squeezing the mixture out of this area jets it across the chamber and helps to homogenize the mixture for a better burn. It also eliminates the dead space in this area of the chamber where the mixture would otherwise not be ignited and would contribute to dirty emissions and possibly detonation. Of course, there is some stretch in the rods, the crank will bend a little and there will be a small stretch in the piston from the pin to the crown as the piston ascends to the top of the bore. All these will add up to put the crown very close to the underside of the head with the motor at speed. That's the trick, to get it as close as you can without a collision of the piston against the head. On a bigger bore motor or one with forged pistons where the piston can also rock in the bore on the wrist pin axis, more initial clearance may be called for. With aluminum rods on a small block, the builder may go to 0.060" or more squish clearance because aluminum rods will stretch more than steel ones.

I've wanted one of the early "Jeep" pickups for years.
I lived in Wisconsin during last summer and found a '51 Willys Overland pickup sitting behind a house there. It was complete, but with two rods hanging out of the side of the block. I bought it for $100 and the guy towed it to the house for me. Being an engine swap and frame swap guy, I began looking around for a suitable frame donor. I located a S15 Jimmy with a 4.3 V6, 5-speed, 4WD truck sitting at a local garage and attempted to get in contact with the owner. The motor was toast and I figured I could get the whole thing for a couple of hundred dollars. I could never get in touch with him and left Wisconsin to come back to Phoenix for the winter, but that would be a perfect project if I go back.

Last edited by techinspector1; 01-16-2007 at 10:37 AM.
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:
If you do not want to register, fill this field only and the name will be used as user name for your post.
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



All times are GMT -6. The time now is 05:53 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.