Go Back   Corvette Forum > C3 Corvettes, 1968 - 1982 > C3 Tech/Performance
Sign in using an external account
Register Forgot Password?
Register Search Today's Posts Mark Forums Read FAQ Vendor Directory
Search
C3 Tech/Performance
V8 Technical Info, Internal Engine, External Engine, Basic Tech and Maintenance for the C3 Corvette Sponsored by
VBP

Welcome to Corvetteforum.com!
Welcome to Corvetteforum.com.

You are currently viewing our forum as a guest, which gives you limited access to view most discussions and access our other features. By joining our community, at no cost, you will have access to post topics, communicate privately with other members (PM), respond to polls, upload content and access many other special features. Registration is free, fast and simple, join Corvetteforum.com today!


Corvette Store
 
 
C7 Parts & Accessories
C6 Parts & Accessories
C5 Parts & Accessories
C4 Parts & Accessories
C3 Parts & Accessories
C2 Parts & Accessories
C1 Parts & Accessories
Wheels & Tires
Sponsored Ads
 
 
Vendor Directory
  
Reply
 
 
 
 
Thread Tools Search this Thread
Old 11-25-2002, 09:29 AM   #1
SpyderD
CF Senior Member
 
Member Since: Sep 2002
Location: near Memphis TN
Default 58 cc heads and flat top pistons - too much compression?

I am currently having my engine rebuilt. I am using my high performance top end parts to bolt to the rebuilt lower end. My heads are corvette aluminum heads with 58 cc chambers with (I think) a current compression rating of 9.7 to 1. The current cam, which I plan to keep, is a comp cam magnum 270 with 224 duration at .050 which in theory will bleed off some compression. The cylinders will probably be rebored.
If I rebuild with flat top pistons will this raise the compression ratio to above pump gas friendly levels? I don't mind pushing the limits of pump gas but not if it results in retarding the timing to prevent detonation.
Is there any other benifit to flat top pistons (increased torque in a lower RPM range, more efficient fuel burn in cylinder, etc) besides increased compression? In other words, I know that higher compression will bring more power, but do flat top pistons in itself improve performance beside the fact that it raises compresson?
thanks

P.S. - the bottom end is a stock 350 '81 engine, never rebuilt previously



[Modified by SpyderD, 2:52 PM 11/25/2002]
SpyderD is offline   Reply With Quote
Old 11-25-2002, 09:54 AM   #2
joeveto
CF Senior Member
Cruise-In IV Veteran
Cruise-In V Veteran
St. Jude Contributor
 
joeveto's Avatar
 
Member Since: Mar 2001
Default Re: 58 cc heads and flat top pistons - too much compression? (SpyderD)

With aluminum heads, I would not go higher than 10:1 compression. You can help with a lower thermostat (assuming your running stock). But this is not a cure all.

I'm running iron heads (Vortecs) and 9.7:1 compression. With this, I'm limited to 93-94 octane. I've run less, 91, and I think I heard some displeasure from the engine. Also, my timing is less than optimal.

If I were you, I'd go conservative with the compression. Unless this is going to be a bragging numbers only car or a track car that you will need to squeek every last ounce of performance from, do yourself a favor. The car will be fast. But it will also be streetable.
joeveto is offline   Reply With Quote
Old 11-25-2002, 10:09 AM   #3
MotorHead
CF Senior Member
 
MotorHead's Avatar
 
Member Since: Apr 2000
Location: Never argue with Morons. If you do, you'll lower yourself to their standards, and they'll win, based on experience !!!
Default Re: 58 cc heads and flat top pistons - too much compression? (SpyderD)

Many things to consider here, first you have to know the deck height of the block, tne cc of the cylinder head and what head gasket you are using. Then you have to
find out what the manufacture says he CR is for the piston with your deck height and whatever gasket they recommend.

Until you know these numbers the CR is a guess
MotorHead is offline   Reply With Quote
Old 11-25-2002, 10:30 AM   #4
Smokehouse69
CF Senior Member
Cruise-In VI Veteran
Cruise-In VII Veteran
 
Smokehouse69's Avatar
 
Member Since: Aug 2000
Location: Semper ubi, sub ubi
Default Re: 58 cc heads and flat top pistons - too much compression? (SpyderD)

Here is a link to a website that figures what your compression is based on information you put in.
http://www.rosspistons.com/CR%20Calculator.htm

I entered in information for sort of a standard setup and came up with these numbers.
Quote:
Bore .in 4.030
Stroke in 3.48
Head cc's 58
Gasket Thickness .in .039
Deck Height (Put .0001 for zero deck) in .01
Dish or Valve Pocket [-] or Dome [+] cc's -6 (flat tops with 4 valve reliefs)
Number of Cylinders 8
Displacement Cubic Inches 355.12
Compression Ratio 10.8 to 1

Compression Ratio Formulas
GV (Head Gasket Volume) = Bore(in) X Bore(in) X 12.87
X (Head Gasket Thickness in Inches)


DV (Below Deck Volume) = Bore(in) X Bore(in) X 12.87
X (Inches Below the Deck)


HV (Head Volume) = CC's


VV (Dish, Valve Pocket, Dome Volume) = CC's
(Minus for dish or valve pockets, Plus for dome)


PV (Volume displaced by Piston) = Bore X Bore X Stroke X 12.87


Compression Ratio = (GV+DV+HV-VV+PV) / (GV+DV+HV-VV)

Cubic Inches = Bore x Bore X Stroke X Number of Cylinders X .7854

Convert Cubic Inches to CCs = Cubic Inches X 16.386
Convert Cubic Inches to Liters = Cubic Inches X .016386

Engine in Liters = (Bore(mm) X Bore(mm) X Stroke(mm)
X Number of Cylinders X 12.87) / (16386 X 1000)

Engine in CCs = (Bore(mm) X Bore(mm) X Stroke(mm)
X Number of Cylinders X 12.87) / 16386

Convert CCs to Cubic Inches = CCs / 16.386
Convert Liters to Cubic Inches = Liters / .016386
So it looks like your compression ratio is going to be somewhere around 10.8 to 1 give or take a little.

Smokehouse69 is offline   Reply With Quote
Old 11-25-2002, 11:06 AM   #5
SpyderD
CF Senior Member
 
Member Since: Sep 2002
Location: near Memphis TN
Default Re: 58 cc heads and flat top pistons - too much compression? (Smokehouse69)

Thank you in particular to Smokehouse for doing the math for me. That is exactly whatl I wanted, an approximation of compression which sounds like it will be in the 10.5 to 11 range. I agree that is too high. Now for the second question - any benifit to flat top pistons besides raised compression ratio?
SpyderD is offline   Reply With Quote
Old 11-25-2002, 11:09 AM   #6
Fevre
CF Senior Member
 
Fevre's Avatar
 
Member Since: Mar 2001
Location: Living the Surreal Life in Ypsitucky MI
Default Re: 58 cc heads and flat top pistons - too much compression? (SpyderD)

With alum heads you can get by with 10.5 to 1 and run 93 oct, and depending on cam overlap and duration, you might be good with 11 to 1.
Fevre is offline   Reply With Quote
Old 11-25-2002, 11:18 AM   #7
gkull
CF Senior Member
 
gkull's Avatar
 
Member Since: Apr 1999
Location: Reno Nevada
Default Re: 58 cc heads and flat top pistons - too much compression? (SpyderD)

I have heard that flat tops promote the best flame travel and they are less prone to have carbon buildup than dished pistons.

I don't have the program, but you can input the seat to seat valve events for any cam and static compression ratio and it will give you you dynamic compression. Which is the value that you need to be concerned about.

I don't see any problems with running 10.8 and aluminum heads on 91 octane.


[Modified by gkull, 10:51 AM 11/26/2002]
gkull is offline   Reply With Quote
Old 11-25-2002, 11:34 AM   #8
Wardog
Junior Member
 
Wardog's Avatar
 
Member Since: Sep 2001
Location: St. Louis MO
Default Re: 58 cc heads and flat top pistons - too much compression? (gkull)

Your cam choice will be critical here. If you use a short duration, high lift (almost like an RV cam...) for ultimate low-end torque, then you have the full effect of that high CR - pinging, or worse... If you use a longer duration cam, that will drop your effective CR a bit - maybe as much as a full point. When I rebuilt a different 1973 454 years ago I went the short duration route with 9.5:1 forged pistons. That car jumped! Because of the crappy gas we get today, you may want to get a set of heads with a little more chamber volume - go with open or semi-open chambers and flat-tops. Get all that in an effective CR range of 9.5 to 10 and you'll be happiest.
Wardog is offline   Reply With Quote
Old 11-25-2002, 11:41 AM   #9
john's '81 mouse
CF Senior Member
 
john's '81 mouse's Avatar
 
Member Since: Dec 2000
Location: Des Moines Iowa
Default Re: 58 cc heads and flat top pistons - too much compression? (Smokehouse69)

Suppose you could have a good machine shop perform a little work on the heads and 'un-shroud' the area around the intake valve, which may open up the combustion camber area in the heads from 58 ccs to 60 to 62 ccs, which will give you a little more 'comfrort' room in your compression ratio.........

I understand that you want the tightest/smallest 'quinch' area which promotes fast flame travel, so you don't want to use a thickner than necessary head gasket, and you do want to use flat top pistons if possible. Perhaps the 'new generation piston, which is flat on the quench side of the piston is something you should look into. (I believe I read something about these new pistons in Chevy High Performance magazine a while back.)
john's '81 mouse is offline   Reply With Quote
Old 11-25-2002, 11:50 AM   #10
SpyderD
CF Senior Member
 
Member Since: Sep 2002
Location: near Memphis TN
Default Re: 58 cc heads and flat top pistons - too much compression? (Wardog)

Hmmm, more things to think about. I don't really want to replace or machine the heads just to fit flat top pistons. Here are the cam specs copy and pasted from a web site:

Gross valve lift: .470" intake/ exhaust with 1.5 to 1 rockers

Duration at .006" Tappet Lift: 270 degrees intake/ exhaust

Duration @ .050": 224 degrees intake/ exhaust

Lobe lift: .313" intake/ exhaust


open close
intake 29 BTDC 61 ABDC
exhaust 69 BBDC 21 ATDC


Lobe Separation Angle is 110 degrees with 106 degree intake centerline

STRAIGHT UP = 106 degrees; cam is ground 4 degrees advanced

...so I think that duration is relatively high, not sure about the overlap. Is it high enough to bleed down the 10.8 to 1 compression?
SpyderD is offline   Reply With Quote
Old 11-26-2002, 11:19 AM   #11
SpyderD
CF Senior Member
 
Member Since: Sep 2002
Location: near Memphis TN
Default Re: 58 cc heads and flat top pistons - too much compression? (SpyderD)

I am letting the pro decide - My engine builder is measuring the head combustion chamber (he thinks it may be larger than 58cc), entering it into a computer with all the specs including my cam and seeing what pistons I can use to get the absolute highest compression ratio I can get away with on pump gas. He says 10.5 up to 11 should be OK, and the car will fly!

Well that's why I pay him the big bucks (hopefully not to big), to figure all this out. The engine will also have a 3 year warranty so if my car detonates to destruction they will replace it.
SpyderD is offline   Reply With Quote
Old 11-26-2002, 07:06 PM   #12
Budman68
CF Senior Member
Cruise-In VII Veteran
St. Jude Donor '07
 
Budman68's Avatar
 
Member Since: Sep 2000
Location: DFW TX
Send a message via AIM to Budman68
Default Re: 58 cc heads and flat top pistons - too much compression? (SpyderD)

I'm running 52cc with factory pistons and it calculated to about 9.5 to 1. I memory serves me correct, flat top would have brought it up to 10.5 to 1.
You should be safe with that duration.
Budman68 is offline   Reply With Quote
Old 11-26-2002, 09:24 PM   #13
candyapplevette
CF Senior Member
 
Member Since: Apr 2002
Location: Ringgold GA
Default Re: 58 cc heads and flat top pistons - too much compression? (Budman78)

Dave, with the flat top pistons you get a better flame travel and this cuts down on detonation to some degree.
With the aluminum heads you can run 10.5-1 compression with no problems on 91-93 octane gas.
You will not have a zero deck height on the piston unless your engine builder decks the block for a zero deck height. Ask him if he plans to do this.
The engine should be fine with the 10.5 compression but I think you will be a little below that unless he does deck the block for zero deck height.
candyapplevette is offline   Reply With Quote
Old 11-26-2002, 09:47 PM   #14
SpyderD
CF Senior Member
 
Member Since: Sep 2002
Location: near Memphis TN
Default Re: 58 cc heads and flat top pistons - too much compression? (candyapplevette)

Rob - I just emailed you. After he told me he was checking the heads with a straight edge I was imagining Bubba with a ruler eyeballing the heads (this is North East Tennessee afterall), turns out he plans to magnaflux test the heads and a few other things and running all my specs in a computer program to determine parts and pistons and so forth. I think he did say he was decking the block. I will keep you up to date...
SpyderD is offline   Reply With Quote
Old 11-26-2002, 11:44 PM   #15
jackson
CF Senior Member
 
Member Since: Sep 2001
Location: Timmonsville SC
Default Re: 58 cc heads and flat top pistons - too much compression? (SpyderD)

Quote:
Rob - I just emailed you. After he told me he was checking the heads with a straight edge I was imagining Bubba with a ruler eyeballing the heads (this is North East Tennessee afterall), turns out he plans to magnaflux test the heads and a few other things and running all my specs in a computer program to determine parts and pistons and so forth. I think he did say he was decking the block. I will keep you up to date...
Bubba from Tennessee or Hans from Stuttgart ... there's plenty BS everywhere. Now y'all correct me if you disagree, but my understanding is that aluminum (heads, blocks etc) cannot be magnafluxed (aka eddy current testing). Use of a fluorescent penetrant dye (aka Zyglo) can expose surface cracks in aluminum ... but the ability of magnaflux to indicate sub-surface abnormalities cannot be used on aluminum. Click the image to open in full size.
jackson is offline   Reply With Quote
Old 12-01-2002, 01:42 PM   #16
The Money Pit
CF Senior Member
 
The Money Pit's Avatar
 
Member Since: Jun 2002
Location: Orrtanna Pa.
Default Re: 58 cc heads and flat top pistons - too much compression? (jackson)

In days of old they had to run the cr high because nobody made cams with tighter lobe centerlines than 114 degrees. With todays grinds 108-110 degerres is more the norm. This effectivly increases the dynamic cr,making the engine think it't got more compression. In fact it does. Back in the 60's it took a big block running domed pistons to break 450 lbs. of torque. I pull almost 500 lbs out of my 406,running flattops.
Save the money you'll spend on pistons,take the wife out to dinner,you'll both be happier.
The Money Pit is offline   Reply With Quote
Old 12-01-2002, 01:53 PM   #17
candyapplevette
CF Senior Member
 
Member Since: Apr 2002
Location: Ringgold GA
Default Re: 58 cc heads and flat top pistons - too much compression? (jackson)


Quote:
Bubba from Tennessee or Hans from Stuttgart ... there's plenty BS everywhere. Now y'all correct me if you disagree, but my understanding is that aluminum (heads, blocks etc) cannot be magnafluxed (aka eddy current testing). Use of a fluorescent penetrant dye (aka Zyglo) can expose surface cracks in aluminum ... but the ability of magnaflux to indicate sub-surface abnormalities cannot be used on aluminum. Click the image to open in full size.
You are correct, You cannot magnaflux aluminum. I am sure there are other methods to check aluminum but the one I am familiar with is to pressure test the heads for cracks.
candyapplevette is offline   Reply With Quote
Old 12-01-2002, 01:53 PM
 
Go Back   Corvette Forum > C3 Corvettes, 1968 - 1982 > C3 Tech/Performance
Reload this Page 58 cc heads and flat top pistons - too much compression?
 
 
 
Reply

Tags
10, 21, 58, 58cc, cc, cmpression, degree, flat, heads, low, makes, piston, pistons, rebuilt, top


Thread Tools Search this Thread
Search this Thread:

Click for Advanced Search

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 Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off

Forum Jump

Similar Threads
Thread Thread Starter Forum Replies Last Post
Question about .015 head gaskets - FelPro 1094 surfshark C3 Tech/Performance 15 04-01-2006 10:57 PM
11.25:1 compression...bad news? nastee383 C3 Tech/Performance 92 05-12-2005 01:54 PM
Anyone running L98 Aluminum cylinder heads ? dman535 C3 Tech/Performance 10 02-02-2003 09:18 PM
New head compression ratio question, How much is to much? KJL C3 Tech/Performance 12 09-24-2002 11:25 PM
Pistons: Compression ratio differences. conv90 Engine Mods 4 06-25-2002 06:51 PM


All times are GMT -4. The time now is 12:35 AM.


Emails & Password Backup