Compression Ratio Calculation - 1969 L36
Thread Starter
Instructor
Joined: Feb 2014
Posts: 154
Likes: 1
From: Irvine CA
It all depends on tuning. I ran 11.2 compression on 93 octane with iron heads. You'll want to keep it cool as possible and work out the timing curve. Did you figure out why it blew head gasket previously? Detonation?
Cometic will make gaskets in any thickness you want and have quite a few on the shelf. Go to their website.
Me? I'd do as suggested and find some open chamber heads with 2.19's...or just use what you have. I'd install 2.19 intakes...wouldn't worry about exhausts a lot unless you just wanted larger. I've run awful hard with similar heads on my 427 using a solid flat tappet cam. Actually mine were some weird truck heads from a 366". Large oval ports, semi closed chambers like you have...and had small valves. I installed 2.19's and left exhaust valves alone. Even weirder is they were marked "Hi-Per"...not to be confused with "Hi-Perf" that is cast into rectangular port heads. Ran deep 11's on motor and high 9's with some N20 in my old Camaro.
JIM
Cometic will make gaskets in any thickness you want and have quite a few on the shelf. Go to their website.
Me? I'd do as suggested and find some open chamber heads with 2.19's...or just use what you have. I'd install 2.19 intakes...wouldn't worry about exhausts a lot unless you just wanted larger. I've run awful hard with similar heads on my 427 using a solid flat tappet cam. Actually mine were some weird truck heads from a 366". Large oval ports, semi closed chambers like you have...and had small valves. I installed 2.19's and left exhaust valves alone. Even weirder is they were marked "Hi-Per"...not to be confused with "Hi-Perf" that is cast into rectangular port heads. Ran deep 11's on motor and high 9's with some N20 in my old Camaro.
JIM
Race Director
Joined: Jul 2015
Posts: 13,082
Likes: 3,399
From: SW Florida.
Do you currently have your heads? I just measured my 215 heads, rated at 109 as-cast and they are 100cc's. Get a piece of plexiglass about 6 inches on side. Buy a Walmart flavor shooter. 25cc syringe. Pretty accurate. Drill a hole about 3/8 inch near the edge of plexi. Get some grease and the correct length spark plug. Set head up on a slope so hole in plexi is at the top. Ring of grease between plexi and head deck.
Race Director
Joined: Jul 2015
Posts: 13,082
Likes: 3,399
From: SW Florida.
These are the basics for an under 10 buck cc-ing setup. I cut the point off the needle so I can draw water back out of chamber and double check. With your pistons, the diff between 100 and 106 or 109 is a must-know. Your 1063 heads are very nice heads-for bringing up the CR of a flat or dished piston engine. You have the opposite problem. You have 30cc domes. Especially since your heads either need or got shaved to repair the burn thru. You mentioned period correct. you can't tell a 65 from an 89 big block head unless you show casting number and date codes under valve cover. These heads are 10 months too old to be technically period correct anyway. Sell these heads. Get 781 or 049. Your giant head gasket problems or replace the pistons problems are no longer an issue. 10.1 with standard head gssket.
Last edited by derekderek; May 27, 2018 at 10:46 AM.
Race Director
Joined: Jul 2015
Posts: 13,082
Likes: 3,399
From: SW Florida.
https://orangecounty.craigslist.org/...589272353.html there is also a 454 block with 990 rectangle heads for 800.
Last edited by derekderek; May 27, 2018 at 10:12 AM.
Thread Starter
Instructor
Joined: Feb 2014
Posts: 154
Likes: 1
From: Irvine CA
It all depends on tuning. I ran 11.2 compression on 93 octane with iron heads. You'll want to keep it cool as possible and work out the timing curve. Did you figure out why it blew head gasket previously? Detonation?
Cometic will make gaskets in any thickness you want and have quite a few on the shelf. Go to their website.
Me? I'd do as suggested and find some open chamber heads with 2.19's...or just use what you have. I'd install 2.19 intakes...wouldn't worry about exhausts a lot unless you just wanted larger. I've run awful hard with similar heads on my 427 using a solid flat tappet cam. Actually mine were some weird truck heads from a 366". Large oval ports, semi closed chambers like you have...and had small valves. I installed 2.19's and left exhaust valves alone. Even weirder is they were marked "Hi-Per"...not to be confused with "Hi-Perf" that is cast into rectangular port heads. Ran deep 11's on motor and high 9's with some N20 in my old Camaro.
JIM
Cometic will make gaskets in any thickness you want and have quite a few on the shelf. Go to their website.
Me? I'd do as suggested and find some open chamber heads with 2.19's...or just use what you have. I'd install 2.19 intakes...wouldn't worry about exhausts a lot unless you just wanted larger. I've run awful hard with similar heads on my 427 using a solid flat tappet cam. Actually mine were some weird truck heads from a 366". Large oval ports, semi closed chambers like you have...and had small valves. I installed 2.19's and left exhaust valves alone. Even weirder is they were marked "Hi-Per"...not to be confused with "Hi-Perf" that is cast into rectangular port heads. Ran deep 11's on motor and high 9's with some N20 in my old Camaro.
JIM
Race Director
Joined: Jul 2015
Posts: 13,082
Likes: 3,399
From: SW Florida.
No, your chambers are still 15 cc's too small and there are zillions of better heads for your purpose. You are fixated on the 68 build year. 049 or 781. And the replacement won't have a head gasket burn.
Thread Starter
Instructor
Joined: Feb 2014
Posts: 154
Likes: 1
From: Irvine CA
I'm not fixated on the 69 year for the heads. I'm just frustrated at potentially taking a brand new set from my car that I just got restored, bubble wrapping them up and then putting it in storage for perpetuity. Then having to buy another set for a few hundred bucks and spend another $400 on a valve job for those heads. Basically all the work I just got done was pointless and now I need to redo the same job on another set. Doing that puts me at basically $1,200 in and at that point the question ultimately becomes why don't you just rebuild the whole damn motor in the first place.
Race Director
Joined: Jul 2015
Posts: 13,082
Likes: 3,399
From: SW Florida.
I'm not fixated on the 69 year for the heads. I'm just frustrated at potentially taking a brand new set from my car that I just got restored, bubble wrapping them up and then putting it in storage for perpetuity. Then having to buy another set for a few hundred bucks and spend another $400 on a valve job for those heads. Basically all the work I just got done was pointless and now I need to redo the same job on another set. Doing that puts me at basically $1,200 in and at that point the question ultimately becomes why don't you just rebuild the whole damn motor in the first place.
Last edited by derekderek; May 29, 2018 at 08:42 PM.
Corvette Stories
The Best of Corvette for Corvette Enthusiasts
Top 10 Most Expensive Corvettes Ever Sold on Bring A Trailer
Brett Foote
10 Things Every Corvette Owner Needs (2026 Edition)
Michael S. Palmer
8 Most "Only Corvette Owners Understand" Quirks and Problems
Pouria Savadkouei
10 Reasons the C6 Z06 is Still A Performance Benchmark After 20 Years
Joe Kucinski
How Much Horsepower Every Corvette Engine "LOST" in 1972
Joe Kucinski
Top 10 DOs and DON'Ts for Protecting Your Convertible Top!
Michael S. Palmer
Top 10 Most Explosive Corvettes Ever Made: Power-to-Weight Ratio Ranked!
Joe Kucinski
150 hp to 1,250 hp: Every Corvette Generation Compared by the Specs That Matter
Joe Kucinski
8 Coolest Corvette Pace Cars (and Replicas) of All Time
Verdad Gallardo
Race Director
Joined: Jul 2015
Posts: 13,082
Likes: 3,399
From: SW Florida.
See if he can open that lump either side of the spark plugs any. Also help flow. If he hasn't started cutting the ports and seats bigger, he hasn't started the expensive work yet. Ask him what he thinks of 119 chambers with your pistons.
Last edited by derekderek; May 29, 2018 at 09:28 PM.
Thread Starter
Instructor
Joined: Feb 2014
Posts: 154
Likes: 1
From: Irvine CA
Spoke with him yesterday and we both agreed I cannot escape physics so a rebuild is happening. He'll do the machining and I'll do the reassembly in my garage with a co-worker to save some money. From what it looks like, I think I'll probably snag Speed-Pro Forged Pistons L2300NF30 OR something in the same ballpark in terms of size/head volume. The next thing I need to figure out is what cam I'm going to put in this motor. I may be able to use the existing one but honestly who knows since I have no idea what's currently in it.
Race Director
Joined: Jul 2015
Posts: 13,082
Likes: 3,399
From: SW Florida.
Get a degree wheel. You can download and print for free. Get a dial indicator and stand. 25 bucks harbor freight. Now you have what you need to get lift, duration and open-close events. Another possibility to lower CR is get builder to machine 5 or 10 cc off tops of pistons and/or machine 5 or 10 cc's out of the chambers. I cant use the pistons. Mine are all 454. Diff pin height. I gotta say it 1 more time. It is cheaper to buy heads with 119 chambers and put the 2.19 valves in them. Also, that big lump either side of the spark plug is a flow-power restriction. You will get more power with true open chamber heads with same compression ratio.
Last edited by derekderek; Jun 1, 2018 at 02:40 PM.
Thread Starter
Instructor
Joined: Feb 2014
Posts: 154
Likes: 1
From: Irvine CA
Get a degree wheel. You can download and print for free. Get a dial indicator and stand. 25 bucks harbor freight. Now you have what you need to get lift, duration and open-close events. Another possibility to lower CR is get builder to machine 5 or 10 cc off tops of pistons and/or machine 5 or 10 cc's out of the chambers. I cant use the pistons. Mine are all 454. Diff pin height. I gotta say it 1 more time. It is cheaper to buy heads with 119 chambers and put the 2.19 valves in them. Also, that big lump either side of the spark plug is a flow-power restriction. You will get more power with true open chamber heads with same compression ratio.
Race Director
Joined: Jul 2015
Posts: 13,082
Likes: 3,399
From: SW Florida.
Still need a degree wheel to tell what the cam is. You can measure lift with it out, but must have indicator and wheel to get open-close events, duration at .050, overlap, lobe sep angle, all that good stuff.
Last edited by derekderek; Jun 1, 2018 at 09:43 PM.
Had a 1976 L-82, 4-sp
Joined: Mar 2011
Posts: 5,338
Likes: 1,213
From: Some days your the dog and some days your the hydrant.
Royal Canadian Navy
The real root of your problem is not knowing the cam specs. Once you determine the cam specs, plug in the numbers into a dcr calculator and for pump gas, you want somewhere between 8.0-8.5 as the result. If its not in that range, then play with numbers such as gasket thickness, head cc's, intake valve closing point, static cr, to get in that range. If you're going to spend some money, spend it on a nice set of aluminium heads with the correct cc's based on your dcr number and a new cam. Keep your pistons if the dcr number can be attained with the dome volume and head volume.
I had a similar situation to yours when I did my upper end as in I already had domed pistons that I wasn't replacing. So I worked with the dcr calculator and got 8.4 for my setup by changing the head gasket thickness, cam and AFR heads. It's running on 91 octane with a scr of about 10.7:1. No pinging, but it would be hard to hear over the sound of the side pipes, lol! Engine is still in one piece and it's revved quite often to 6200+.
I had a similar situation to yours when I did my upper end as in I already had domed pistons that I wasn't replacing. So I worked with the dcr calculator and got 8.4 for my setup by changing the head gasket thickness, cam and AFR heads. It's running on 91 octane with a scr of about 10.7:1. No pinging, but it would be hard to hear over the sound of the side pipes, lol! Engine is still in one piece and it's revved quite often to 6200+.
Race Director
Joined: Jul 2015
Posts: 13,082
Likes: 3,399
From: SW Florida.