Quick 350 question for the engine guys...
Thread Starter
Burning Brakes
Joined: Oct 2020
Posts: 1,149
Likes: 265
From: The frozen wastes...
Quick 350 question for the engine guys...
My 73 came with a 350 from a truck, 78 i believe... about as lame as a 350 came. So far i've read 155HP, 170HP, 175HP, and another guy told me 165HP. I've read 8.2:1, 8.3:1, and 8.5:1 rated CR. What i want to know is, what was the ACTUAL MEASURED CR ov these engines? I ask because once this thing is running, i'll be buying an aluminum top end for the engine... just a bolt on. Cheap (but checked by my engine guy) Chinese aluminum heads, single plane, matching cam, etc. I want to get an idea ov what CC to buy for 10.5:1-ish actual compression.
Mopar 440's were rated at 8.2:1 in the mid 70's, but came in at 7.6ish. Their 400's were rated at 8.2:1, and came in in the low 7's. Was Chevy the same?
Mopar 440's were rated at 8.2:1 in the mid 70's, but came in at 7.6ish. Their 400's were rated at 8.2:1, and came in in the low 7's. Was Chevy the same?
Drifting
Joined: Oct 2020
Posts: 1,566
Likes: 574
From: Wisconsin
I’m in sort of the same boat as you, but I have a low CR crate motor, about 8:1 with 76cc heads as dished pistons. From what I gather, going to the smallest non-Vortec heads on the market (64cc) will only increase CR by about a point. So the best we can hope for is in the low 9’s for CR.
But the good news is a healthy CR is good, but it’s not the end-all for making decent power.
I’ll step aside and let the experts on here give you the details, I just wanted to get the conversation rolling for you. Good luck.
But the good news is a healthy CR is good, but it’s not the end-all for making decent power.
I’ll step aside and let the experts on here give you the details, I just wanted to get the conversation rolling for you. Good luck.
Melting Slicks
Joined: May 2021
Posts: 2,263
Likes: 1,308
From: Fleming Island, FL
Yes, those numbers are all over the place. I don't have a answer to your question, but I remember a article in one of the car magazines called The "Goodwrench quest". The measured compression ratio was about a half point off from the advertised compression ratio.
Thread Starter
Burning Brakes
Joined: Oct 2020
Posts: 1,149
Likes: 265
From: The frozen wastes...
I'd still like an actual number. Someone here knows this.
Old Mopars and Cadillacs, two engines i have experience with, were both typically down half a point from advertised... Mopars even more.
Regardless, looks like i'll need a 58cc head to actually squeeze this engine past 10:1...
Old Mopars and Cadillacs, two engines i have experience with, were both typically down half a point from advertised... Mopars even more.
Regardless, looks like i'll need a 58cc head to actually squeeze this engine past 10:1...
Advanced
Joined: Jun 2019
Posts: 84
Likes: 40
From: Pittsburgh, PA
The only way to know for sure is to take the head of and get some measurements. The deck clearance plays a big part in compression ration as well as if the cylinders have been bored over. On my '74 350 the engine had been rebuilt with shorter pistons and bored .020" over but the engine was never decked so I had a deck clearance of about 0.040! I went to a 64cc head and switched to a metal 0.015" head gasket which got me to 9.25:1 compression ratio. A more normal deck clearance of .015" on the same setup would yield about 9.8:1 ratio. The summit website has a compression ration calculator where you can enter all the variables and it will spit out a CR.
my calculated CR with stock heads (76cc) and head gasket(.042" )before the upgrade was 7.9:1
I have seen it commented on this site in a couple places that the actual compression ration was a full point lower than what was advertised
my calculated CR with stock heads (76cc) and head gasket(.042" )before the upgrade was 7.9:1
I have seen it commented on this site in a couple places that the actual compression ration was a full point lower than what was advertised
Safety Car
Joined: Jan 2009
Posts: 4,000
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From: New York
Burning Brakes
Joined: Nov 2012
Posts: 788
Likes: 100
From: Indianapolis IN
If you buy direct from Brodix they can angle machine the heads for you, Which means they are angle milled and then finished machined as far as the bolt holes go so your bolt holes are at the correct angle and the exhaust & intake flanges also.
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Le Mans Master
Joined: Apr 2011
Posts: 6,284
Likes: 1,171
From: Hermosa
My 73 came with a 350 from a truck, 78 i believe... about as lame as a 350 came. So far i've read 155HP, 170HP, 175HP, and another guy told me 165HP. I've read 8.2:1, 8.3:1, and 8.5:1 rated CR. What i want to know is, what was the ACTUAL MEASURED CR ov these engines? I ask because once this thing is running, i'll be buying an aluminum top end for the engine... just a bolt on. Cheap (but checked by my engine guy) Chinese aluminum heads, single plane, matching cam, etc. I want to get an idea ov what CC to buy for 10.5:1-ish actual compression.
Mopar 440's were rated at 8.2:1 in the mid 70's, but came in at 7.6ish. Their 400's were rated at 8.2:1, and came in in the low 7's. Was Chevy the same?
Mopar 440's were rated at 8.2:1 in the mid 70's, but came in at 7.6ish. Their 400's were rated at 8.2:1, and came in in the low 7's. Was Chevy the same?
Just got done ccing the heads. I cc'd three of the head volumes one of them twice to see if I could get repeatable results.
The results are as follows;
78.5 cc
79.5 cc
80.0 cc I did this one twice to make sure. It was 80cc's
Keep in mind these measurements were without cleaning any carbon out of the chamber.
So GM's 76 cc heads are not 76 cc heads.
In the Manual it states a CR of 8.5:1 for the L-48... that's optimistic.
Using the CR calculator at Summit racing if I negate the area of taper on the circumference of the top of the piston and use 79 cc head volume I get a CR of 7.91:1.
However if I include the area around the top of the piston which I measured this morning and computed at 3.9 cc's,79cc for the head .017 gasket then the compression for this 1977 L48 works out to be a wopping 7.67:1.
Here of some pics of how I cc'd the heads.
Pic of air hole in plexiglass. I fill until it comes out this hole.
Use this one twice. 70cc's
Use this one to fill remainder. total 80cc's
[/IMIn this thread I cc'd the pistons in the hole.
https://www.corvetteforum.com/forums...er-size-2.html
Your's are likely similar or the same.
Melting Slicks
Joined: Dec 2009
Posts: 3,225
Likes: 654
From: Thunder Bay
I was helping a friend with his '69 350 the other day. The chambers measured 76 cc. The pistons in the block measured 23 cc. With a .040" head gasket that works out to 7.7:1. A steel shim head gasket will get that up to 8:1. A 65 cc. head yields 8.5/8.9 respectively. You need to take your own measurements, but you're unlikely to see 10.5:1 without changing the pistons.
Race Director
Joined: Jul 2015
Posts: 13,082
Likes: 3,399
From: SW Florida.
which truck 350? some were 4-bolt forged pistons and crank. some were biscayne wagon 350's. get a borescope from amazon or ebay. see what pistons, crank, main caps you have. head casting numbers will tewll you what the chambers were supposed to be.
Le Mans Master
Joined: Apr 2011
Posts: 6,284
Likes: 1,171
From: Hermosa
I was helping a friend with his '69 350 the other day. The chambers measured 76 cc. The pistons in the block measured 23 cc. With a .040" head gasket that works out to 7.7:1. A steel shim head gasket will get that up to 8:1. A 65 cc. head yields 8.5/8.9 respectively. You need to take your own measurements, but you're unlikely to see 10.5:1 without changing the pistons.
Must have had better quality control in 69 vs 77.
Le Mans Master
Joined: Apr 2011
Posts: 6,284
Likes: 1,171
From: Hermosa
I'm just surprised his 69' 76 cc heads (?) actually cc'd out to 76cc's.
Thread Starter
Burning Brakes
Joined: Oct 2020
Posts: 1,149
Likes: 265
From: The frozen wastes...
Hmmm... Well, i would have guessed i was in the high-7's. Seems i was probably right. The thing has NO bark at all. Sounds like a mid-70's engine. I'll drive it around for a little while, get a feel for its condition. If it impresses me, i'll scrounge up a tight-chamber aluminum head/single plane top end for it.
I made great power out ov a high mile 7.6:1 440 with just headers, exhaust, ported intake, Holley 750, and some good tuning. Engine had a stockish "HP" cam (read: tiny, it had to be cold for you to hear it). Had i dumped even junk aluminum heads on that thing and cammed it, it'd have been a monster. I'm sure with the right top end, even this plebian 350 could make some power...
I made great power out ov a high mile 7.6:1 440 with just headers, exhaust, ported intake, Holley 750, and some good tuning. Engine had a stockish "HP" cam (read: tiny, it had to be cold for you to hear it). Had i dumped even junk aluminum heads on that thing and cammed it, it'd have been a monster. I'm sure with the right top end, even this plebian 350 could make some power...
Pro
Joined: Jun 2021
Posts: 505
Likes: 230
A 78 engine it's probably been rebuilt at one time, so it's hard telling what you have without measuring. The HD pickup 350's were 4 bolt, forged crank, cast pistons and about 8:1. You can shave down a 64cc head and gain a bit of compression, but using a thing head gasket without decking the block in playing with fire. Even at 9:1 you can run a small cam, but you will be limited. Good long tube headers will help a lot. I would run a dual plane manifold, you won't really be able to run enough cam or RPM to take advantage of a single plane. With cast pistons you might want to keep the RPM's under 5500, although I've spun them to 6k frequently and never broke one.
Racer
Joined: May 2020
Posts: 498
Likes: 167
Maybe he was trying to say the old stock deck may not be straight and a thin gasket could cause issues.
Race Director
Joined: Jul 2015
Posts: 13,082
Likes: 3,399
From: SW Florida.
not just straight. the shim gasket wants a way better finish. the blue felpro is much more forgiving. plus, you need to see what you have for piston crowns and depth in the hole before buying gaskets. hell, could be 30 or 40 over and decked. and the heads could have been worked and put on the 78 truck block. or they could be 305 heads.