#47 Heads Issues - Pontiac GTO Forum
User Tag List

Reply
 
LinkBack Thread Tools Display Modes
post #1 of 11 (permalink) Old 06-13-2012, 11:55 AM Thread Starter
 
Icefan71's Avatar
 
Join Date: Apr 2011
Location: Winfield, PA
Posts: 240
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
#47 Heads Issues

I picked up a pair of #47 heads for $75 from a running motor for my 350. They are listed as 1.96/1.66 73cc or 8.6:1 compression. I got them home and cc'd them this morning. 83cc on all 8 chambers! What the heck? I have 7h1 heads with 91cc's. I was hoping to go from 7.6:1 up to 8.6:1. With 83cc's I'm looking at a little over 8:1. I drove 3 hrs to get them. That sucks. I did visit some relatives nearby that I haven't seen in a long time. Otherwise it would have been a waste of time and gas...and $$.
Icefan71 is offline  
Sponsored Links
Advertisement
 
post #2 of 11 (permalink) Old 06-13-2012, 01:59 PM
Super Moderator
 
BearGFR's Avatar
 
Join Date: Aug 2008
Location: Springtown, TX
Posts: 5,742
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 122 Post(s)
Garage
Well, 72-73 cc's is what they -should- be, according to all the reference material I can find (and also through my calculations based on them being 8.6:1 on a stock 350).

I don't mean to insult your intelligence or anything, but did you test all the chambers? Seal the valves/seats with grease? Measure multiple times? etc.

Bear

BearGFR
Springtown, TEXAS
BearGFR is offline  
post #3 of 11 (permalink) Old 06-13-2012, 02:38 PM Thread Starter
 
Icefan71's Avatar
 
Join Date: Apr 2011
Location: Winfield, PA
Posts: 240
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Lol, I'm not insulted. I've done lots of dumb things. But yes, I ran never seize around the valves and around the edge of the combustion chambers, laid the plexiglass over it and pressed down and could see that it was sealing around the edges. I used a 60cc syringe. So each one took a full 60cc's, then another 23cc's and stopped at the 37cc mark. I did each chamber 3 times. And I made sure nothing leaked out the sides. Even so, I can't imagine each chamber would leak the exact same amount. I even did 2 additional tests with a different syringe.
Icefan71 is offline  
 
post #4 of 11 (permalink) Old 06-13-2012, 03:01 PM Thread Starter
 
Icefan71's Avatar
 
Join Date: Apr 2011
Location: Winfield, PA
Posts: 240
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Just did a little searching on the subject. I found a post on the PY forums from last summer. The person said he had a set of #17 heads. Small valve, supposed to be 72cc's. They were actually 83.5cc's. Maybe I'm not crazy.
Icefan71 is offline  
post #5 of 11 (permalink) Old 06-13-2012, 03:48 PM
 
lars's Avatar
 
Join Date: Sep 2004
Posts: 445
Mentioned: 0 Post(s)
Tagged: 1 Thread(s)
Quoted: 1 Post(s)
The #47 heads are the 265-horse (low hp) 350 heads that had an advertised comp ratio of 8.6:1.

73 cc heads would produce a comp ratio of about 9.1:1, not 8.6:1, so the #47's can't be 73 cc...

83 cc heads on a 350, assuming .016 deck, .045 gasket thickness, and 5cc valve reliefs, will produce 8.3:1 compression. The factory comp ratings were with all specs at minimal or nominal, so it sounds like you got the so-called 8.6:1 heads with your 47's. If you run a thin .035 head gasket and do a small cleanup mill cut off the heads to remove 3 cc's, you'll be right at 8.66:1 compression, which is what you were after. Sounds like there's no problem.

Lars
lars is offline  
post #6 of 11 (permalink) Old 06-13-2012, 05:18 PM
Super Moderator
 
BearGFR's Avatar
 
Join Date: Aug 2008
Location: Springtown, TX
Posts: 5,742
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 122 Post(s)
Garage
Wellllll...... if I run the numbers
3.785 bore
3.750 stroke
83 cc chamber
.045 gasket
4.16 gasket bore (Fel-Pro Q8518PT)
.016 deck clearance
5 cc valve relief

I get 8.170:1

With an .042 gasket, .020 deck, and 6 cc's of valve pocket it comes out to 8.092:1

Keeping the .042 gasket, .020 deck, and 6 cc's ---- you need 76 cc's of chamber volume to get to 8.613:1. Using .045/.016/5 it takes 77 cc's to get to 8.622:1

(All figures come from using the Excel spreadsheet that's embedded in the attached Word document)

One thing you might try... instead of dumping the whole 50 cc's at once, don't use the whole syringe capacity --- work between two discreet markings on the syringe and dispense/refill it say... 10 cc's at a time or something, just in case something's not marked right.

Searching the web for reference material, it seems that there's some discrepancy out there with regard to what volume they were, nominally. I found them listed at 73 cc's, 72 cc's, and several other sites that just said "8.6:1" which was a little unusual. I do know that Pontiac heads can vary from the factory specs, and the factory specified "compression ratio" especially was always misstated by quite a bit, but the difference you're finding does seem to be excessive (depending on which, if any, web site is 'right').

Bear
Attached Files
File Type: doc CompressionRatioTool.doc (80.5 KB, 211 views)

BearGFR
Springtown, TEXAS
BearGFR is offline  
post #7 of 11 (permalink) Old 06-13-2012, 06:37 PM Thread Starter
 
Icefan71's Avatar
 
Join Date: Apr 2011
Location: Winfield, PA
Posts: 240
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
8.6:1 was not my goal. I just thought thats what the 47's would give me. I thought for $75 I could bump up my compression. I was really surprised when they were not the 73cc's I was expecting. I did also get a 69 4 bbl intake from the same guy. So even if its not exactly what I was expecting, I'll still get some improvement. I'm up a 1/2 point in compression, 2 bbl - 4 bbl swap, Chevy qjet carb, and I'm looking at cams.
Icefan71 is offline  
post #8 of 11 (permalink) Old 06-13-2012, 07:42 PM
 
lars's Avatar
 
Join Date: Sep 2004
Posts: 445
Mentioned: 0 Post(s)
Tagged: 1 Thread(s)
Quoted: 1 Post(s)
Quote:
Originally Posted by BearGFR View Post
Wellllll...... if I run the numbers
3.785 bore
3.750 stroke
83 cc chamber
.045 gasket
4.16 gasket bore (Fel-Pro Q8518PT)
.016 deck clearance
5 cc valve relief

I get 8.170:1
Bear -
The difference is in our assumed gasket bore diameters. You ran the "big bore" Fel-Pro, and I run the numbers with the "tight bore" Flatout 921543950 gasket at 3.9". This gasket compresses at .040, and produces the 8.3:1 comp ratio stated. You also indicated a 3.785 bore, which must be a typo, because I ran your numbers, and you must have used the correct 3.875 bore size in your calculation.

When I was running Super Stock, we found that Pontiac Compression ratios were often over-stated by a full compression point. I.e., the GTO engines advertised at 10.75:1 were actually 9.75:1 if you measured everything out. The only way to get them to the advertised 10.75:1 ratio was to mill the heads to the NHRA minimum allowable chamber volume. This would "blueprint" the heads to the advertised comp ratio. You can bet that the "47" heads are the same way: they are not anywhere close to the advertised 8.6 ratio unless they are milled to minimum allowable volume.

Lars
lars is offline  
post #9 of 11 (permalink) Old 06-14-2012, 08:22 AM Thread Starter
 
Icefan71's Avatar
 
Join Date: Apr 2011
Location: Winfield, PA
Posts: 240
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
So you're saying its pretty common to have bigger combustion chambers than what they should be? How come I don't read more about this happening? I cc'd my heads because thats what I learned from forums like this one. People always says to check for yourself so you know exactly what comp. ratio you'll end up with.
Icefan71 is offline  
post #10 of 11 (permalink) Old 06-14-2012, 10:34 AM
Super Moderator
 
BearGFR's Avatar
 
Join Date: Aug 2008
Location: Springtown, TX
Posts: 5,742
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 122 Post(s)
Garage
I've always "heard" that Pontica chamber sizes can vary quite a bit from the published factory specs, but I don't have hard numbers on just how much "quite a bit" is. In your case, with 47's, I'm also having a hard time findind a factory spec that I trust as being correct. Not sure why that is...

Bear

BearGFR
Springtown, TEXAS
BearGFR is offline  
Sponsored Links
Advertisement
 
Reply

  Pontiac GTO Forum > The 1964-1974 Pontiac Tempest, Lemans & GTO > 1964-1974 Tempest, Lemans & GTO Complete Engine Compartment Discussion

Quick Reply
Message:
Options

Register Now



In order to be able to post messages on the Pontiac GTO Forum 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
Display Modes
Linear Mode Linear Mode



Similar Threads
Thread Thread Starter Forum Replies Last Post
Has anyone else done this could this cause issues?? WV_GTO Intake and Exhaust Discussions 4 01-14-2011 06:46 PM
edelbrock heads or 6x heads bobby326 1964-1974 Tempest, Lemans & GTO Complete Engine Compartment Discussion 10 08-23-2010 12:40 PM
Will I have issues? fusser87 Wheels and tires 7 04-10-2009 11:22 PM
More strut rub issues rock421 Service, Maintenance and Technical Discussion 8 12-18-2006 03:22 PM

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

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