Original compression for 68 427/435 tripower?
Le Mans Master
Joined: Apr 1999
Posts: 6,134
Likes: 46
From: Raleigh North Carolina
My 1968 Motor's Auto Repair Manual lists these big blocks... some are Vette's some are not...
325 HP 10.25 (396)
350 HP 10.25 (396)
375 HP11.0 (396)
385 HP 10.25 (427)
390 HP10.25 (427)
400 HP 10.25 (427)
425 HP 11.0 (427)
430 HP 12.5 (427)
435 HP 11.0 (427)
Of course, the 430 was "derated".
I posted it just for grins... it's from "Motor's"... so who knows.
325 HP 10.25 (396)
350 HP 10.25 (396)
375 HP11.0 (396)
385 HP 10.25 (427)
390 HP10.25 (427)
400 HP 10.25 (427)
425 HP 11.0 (427)
430 HP 12.5 (427)
435 HP 11.0 (427)
Of course, the 430 was "derated".
I posted it just for grins... it's from "Motor's"... so who knows.
Race Director
Joined: Sep 1999
Posts: 11,361
Likes: 383
From: Plano TX
Note that rated compression ratio is a blueprint spec. Production motors always had lower compression. Normal production tolerances would lower it between 1/2 and a full ratio. So a motor rated at 11:1 would usually be 10:1 to maybe 10.5:1. It will also often have different CRs on the left and right bank!
Race Director
Joined: Nov 2002
Posts: 12,142
Likes: 54
From: Taylor Michigan
Note that rated compression ratio is a blueprint spec. Production motors always had lower compression. Normal production tolerances would lower it between 1/2 and a full ratio. So a motor rated at 11:1 would usually be 10:1 to maybe 10.5:1. It will also often have different CRs on the left and right bank!
This sounds like Ford or Chrysler propaganda!
Melting Slicks
Joined: Aug 2005
Posts: 2,821
Likes: 204
From: Lincoln CA
What configuration was the 385 hp 427? What carb and what size? I am amazed that Chevrolet produced so many different versions of the 427 in 68 and 69. I believe that the 400 hp 3x2 427 is identical to the 390 hp 4x1 427 with the exception of the carbs, air cleaner and intake manifold. Also read that the engine was way over carburated since it used the same cam as the 390 hp which had a single 750 cfm 4V. Any truth in that?
Last edited by RagTop69; Feb 8, 2007 at 07:25 PM.
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Race Director
Joined: Nov 2002
Posts: 12,142
Likes: 54
From: Taylor Michigan
The difference in the 400 and 390 HP is the use of a tri power on the 400. You may be aware of the Berger, Yenko, and COPO 427`s were really 1966 4 bore L72 427`s. And the same L72 is the 1967-69 L71`s as a tri power engine. That L72 really got around.
Burning Brakes
Joined: Oct 2006
Posts: 1,089
Likes: 43
From: Native Californian CALIFORNIA
Army
I believe your thread has been hijacked. Get the head numbers you have and do a search on Mortec or nastyz28 to get the specs on the head. Then it is a combo on the pistons, cam and combustion chamber. Run a compression calculator to get the dynamic compression. This is the number you really want. You probably do not want to exceed 8.3 DCR for 91 octane.
Others more knowledgeable than I will chime in
Others more knowledgeable than I will chime in
Team Owner
Joined: Jan 2007
Posts: 53,648
Likes: 878
From: Southern Cal Ca
St. Jude Donor '22
There was 4 427 avail in 68
435 hp trip duece 11 to 1
430 hp 4v 12 to 1
400 hp trip duece 10.25 to 1
390 hp 4v 10.25 to 1
So comp ratio plus CFM carbs made power.
all had different dist curves
435 hp trip duece 11 to 1
430 hp 4v 12 to 1
400 hp trip duece 10.25 to 1
390 hp 4v 10.25 to 1
So comp ratio plus CFM carbs made power.
all had different dist curves
Race Director
Joined: Nov 2002
Posts: 12,142
Likes: 54
From: Taylor Michigan
Dont forget the camshaft and heads. Rectangle high hp and oval port. The two high hp engines were solid lifter engines wereas the rest were hydraulic. Oh, I changed the compression on the 430 hp engine for you, it is 12.5-1 not 12-1
Team Owner
Joined: Jan 2007
Posts: 53,648
Likes: 878
From: Southern Cal Ca
St. Jude Donor '22
Thread Starter
Intermediate
Joined: Jul 2006
Posts: 31
Likes: 0
Well, the pistons that Summit recommended a few months ago when we put it together they told us were 10-1, but come to find out they were actually 9.5 -1. Needless to say when I put it on the dyno to tune it the numbers were sad. Totally castrated muscle car.
I'm looking for a replacement that will do better but stay pump gas friendly. If these are the best we can do & use 91, then so be it, but I thought I would check you you guys to make sure.
I'm looking for a replacement that will do better but stay pump gas friendly. If these are the best we can do & use 91, then so be it, but I thought I would check you you guys to make sure.
Race Director
Joined: Nov 2002
Posts: 12,142
Likes: 54
From: Taylor Michigan
Well, the pistons that Summit recommended a few months ago when we put it together they told us were 10-1, but come to find out they were actually 9.5 -1. Needless to say when I put it on the dyno to tune it the numbers were sad. Totally castrated muscle car.
I'm looking for a replacement that will do better but stay pump gas friendly. If these are the best we can do & use 91, then so be it, but I thought I would check you you guys to make sure.
I'm looking for a replacement that will do better but stay pump gas friendly. If these are the best we can do & use 91, then so be it, but I thought I would check you you guys to make sure.
Burning Brakes
Joined: Oct 2006
Posts: 1,089
Likes: 43
From: Native Californian CALIFORNIA
Army
Tech Contributor
Joined: Aug 1999
Posts: 14,379
Likes: 6,390
From: At my Bar drinking and wrenching in Lafayette Colorado
Note that rated compression ratio is a blueprint spec. Production motors always had lower compression. Normal production tolerances would lower it between 1/2 and a full ratio. So a motor rated at 11:1 would usually be 10:1 to maybe 10.5:1. It will also often have different CRs on the left and right bank!
With this in mind, an actual comp ratio of 9.5:1 is not bad. That would equate to a factory advertised comp ratio of 10.5:1. You can run that on pump gas.
Racer
Joined: Apr 2007
Posts: 413
Likes: 4
From: Marshall MI
FWIW -
When I took delivery of my 68 L71 roadster about 3 years ago, the seller offered me a gallon can of Tetraethyl lead additive. Not wanting to turn my home into a superfund site, I graciously declined.
In order to run the L71 on the street, I found that a mixture of Torco and Shell Premium, about 1 - 3 ratio worked well to avoid detonation. Also, I had the carbs rebuilt and ensured proper jetting to avoid a leaning out. A 160* thermostat and Water Wetter is another plus, and stick to the conservative side on distributor advance, unless running higher than pump-gas octane.
Also, watch your plugs. If you spend a lot of time just cruising or idling the car, the plugs will foul, causing preignition.
In a pinch, I can run 93* premium, but I have to really loaf the car.
When I took delivery of my 68 L71 roadster about 3 years ago, the seller offered me a gallon can of Tetraethyl lead additive. Not wanting to turn my home into a superfund site, I graciously declined.
In order to run the L71 on the street, I found that a mixture of Torco and Shell Premium, about 1 - 3 ratio worked well to avoid detonation. Also, I had the carbs rebuilt and ensured proper jetting to avoid a leaning out. A 160* thermostat and Water Wetter is another plus, and stick to the conservative side on distributor advance, unless running higher than pump-gas octane.
Also, watch your plugs. If you spend a lot of time just cruising or idling the car, the plugs will foul, causing preignition.
In a pinch, I can run 93* premium, but I have to really loaf the car.
1st Gear
Joined: Mar 2025
Posts: 1
Likes: 0
430 HP 12.5 (427) derated?
Derated How was this done? derated to what?
My 1968 Motor's Auto Repair Manual lists these big blocks... some are Vette's some are not...
325 HP 10.25 (396)
350 HP 10.25 (396)
375 HP11.0 (396)
385 HP 10.25 (427)
390 HP10.25 (427)
400 HP 10.25 (427)
425 HP 11.0 (427)
430 HP 12.5 (427)
435 HP 11.0 (427)
Of course, the 430 was "derated".
I posted it just for grins... it's from "Motor's"... so who knows.
325 HP 10.25 (396)
350 HP 10.25 (396)
375 HP11.0 (396)
385 HP 10.25 (427)
390 HP10.25 (427)
400 HP 10.25 (427)
425 HP 11.0 (427)
430 HP 12.5 (427)
435 HP 11.0 (427)
Of course, the 430 was "derated".
I posted it just for grins... it's from "Motor's"... so who knows.