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Old 01-08-2021, 08:13 PM
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Default 6X-4 heads on a 455

I am considering using 6X-4 heads with my 455. So for you guys who run this street combo what gas octane do you run? Will it live on 87 octane?

bt

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Old 01-08-2021, 08:56 PM
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I don’t know where Banana is?...Is 87 your only available octane?
The heads on Jim Hands wagon are 6X-4’s, but the attention to many details concerning his engine build and tune are very well done. His compression ratio is/was like 10.1:1. If you have his book, it’s pretty well detailed.

Cam degreeing will need to be done, proper cam selection. Removing sharp edges on pistons and combustion chamber will have to be done. Your heads won’t need the port work his have. Good valve stem seals and piston rings will be a must.

A properly tuned carb will have to be had!

There’s more. Some will disagree. I’m getting closer to building a very similar engine to his ( Thanks Rocky!) I am using a 041 blueprint cam. Iimused one in his 455 for a while, with success!

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Old 01-08-2021, 09:04 PM
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Default 6X-4 heads on a 455

I had 6X-4’s on a .060 over 455 with TRW flat tops. Had a few different cam, carb & intake combos. Never had any issues, I’d say go for it. I just wouldn’t run too small of cam. The first & smallest I ran was a 744 cam.

ETA. Sorry I missed the 87 octane bit. I don’t know about that. I ran 91 in mine.

Good luck!
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Last edited by Murf; 01-08-2021 at 09:43 PM.
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Old 01-09-2021, 03:30 AM
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I have this combo in my 71 GTO.
455 @.040 over, 6x-4 heads, Melling spc-8 (041 clone), 1.5 stock stamped rockers, Rhoads lifters, stock Q-jet, rebuilt using Cliff's parts and suggestions; stock iron intake, RA manifolds, M20, 3.73 gears.
I use the 91 octane that's available here.
Runs fantastic. Very tame until you stomp on it.

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Old 01-09-2021, 03:39 AM
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I run 6x4’s on a 455. The heads have only been milled slightly to clean the surfaces up. The engine is .030 and I would estimate compression to be 9.5:1. The engine has a Summit 2802 cam and runs fine on cheap gas.

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Old 01-09-2021, 07:56 AM
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I always run 91 or 93 on my 6X-4 455 with a roller cam rest of specs in signature , compression is 9.5

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12.075 @ 112.70
12.092 @ 112.43
12.128 @ 111.71

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Old 01-09-2021, 08:43 AM
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BIG difference between 6X-4 heads at 92cc and 6X-8 heads at 101cc. With flat top pistons and tight quench you are looking at 9.2-9.3 to 1 vs 9.8-10 to 1 compression. That's nearly a full point more and the cam for the higher compression engine needs to have close to 10 degrees more seat timing to maintain a safe margin for pump gas considering you are making good choices right to start with. I see folks get into a LOT of trouble with the big 455's when it comes to this topic. The smallest cam I'd ever put in any 455 would have 230 @ .050" for seat timing and even then you need to be careful about closing the intake too early or messing with short seat timing cams on tighter LSA's.

My first 455 had 6X-4 heads on it, right at 10 to 1 compression and Crower RAIV clone cam. It managed pump fuel w/o any issues, would even run fine on 87 octane but I typically used 92-93 in it for some margin of safety. With 232cfm 6X heads I was also nicely rewarded with 1hp/CID. With untouched heads and the Crower 60919 cam we made 440hp/530tq. When I mean untouched they didn't even get a port match at the intake gaskets! So a RAIV cam is a very good choice as it also makes enough vacuum for power brakes, good throttle response/street manners and enough power to put a 4000' car solidly into the low 12's and even down into the 11's if you are really using the power well.

The camshaft is going to be a HUGE player in that deal. I've seen 455's as low as 9.3 to 1 compression with the wrong cam in them pound the rod bearings down to the copper before they made it off the dyno! Have had at least half a dozen 455's in here now to custom tune the absolutely HATED the small cam that was in them and pinged pretty hard on pump gas without pulling out some timing.

So tread softly in that direction with your cam choice IF you want a good end result..........FWIW.......Cliff

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Old 01-09-2021, 09:24 AM
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Here’s the cam I’m running on my car and the flow sheet for the Heads that Paul Knippen ported for me
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12.075 @ 112.70
12.092 @ 112.43
12.128 @ 111.71

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Old 01-09-2021, 10:39 AM
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71-72 D port 455 4bbl with 8.2 CR and 067 cam, called for premium fuel from the factory.

It wasn't entirely because of the compression ratio. More about how far the combustion process pushed the piston down the bore.

Performance suffers, ring seal suffers, when the expanding gas quits too soon. You can get detonation type noises at cruise speed from pistons unloading and the slack in rod bearings changing directions too soon before BDC.

I've refreshed a few of them that were "used up" before their time.

Clay

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Old 01-09-2021, 10:44 AM
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Form74, did you do any dyno testing with that combo?......

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Old 01-09-2021, 11:33 AM
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6x#4`s on mine. TRW flats. No problems. But, I run 93 octane. Runs 170 to 175 psi on cranking compression. 246/253 @ 050 cam. 108 sep. Been like this since 1996 or so. Runs 11.80`s to low 12`s with 3.55 gears. 1.70 60`s.

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Old 01-09-2021, 12:17 PM
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The 6x-4 heads I have on the 400 in the '81 I pulled off an original 350 in a yard. They were 92.5cc. I smoothed the ridge in the chamber and then milled the 0.050 and they were still 92.5.

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Old 01-09-2021, 02:29 PM
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I think these "simple" iron head combos are the cats meow.

High 11's to low 12's on basic parts.

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Old 01-09-2021, 02:58 PM
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Quote:
Originally Posted by ponjohn View Post
I think these "simple" iron head combos are the cats meow.

High 11's to low 12's on basic parts.
Cast rods too. I might sell that engine soon. Runs good today. Good oil pressure.

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Old 01-09-2021, 04:33 PM
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On 93 octane with only 175 psi of cranking pressure along with a good tune, knocking and or ping should not be a issue !

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Old 01-09-2021, 06:46 PM
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For my 455 I am using Speed Pro flat top pistons, Voodoo Hydraulic Flat Tappet Cam - Pontiac V8 276/284 ;Duration @ .050 (Int/Exh): 233/241 ;Gross Valve Lift (Int/Exh): .504/.527 ;LSA/ICL: 110/106 and Rhoades V max lifters. I think I will play it safe and use 6x-8 heads. I already have a set ported.

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Old 01-09-2021, 07:25 PM
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Quote:
Originally Posted by PunchT37 View Post
6x#4`s on mine. TRW flats. No problems. But, I run 93 octane. Runs 170 to 175 psi on cranking compression. 246/253 @ 050 cam. 108 sep. Been like this since 1996 or so. Runs 11.80`s to low 12`s with 3.55 gears. 1.70 60`s.

What weight?

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Old 01-09-2021, 08:20 PM
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Quote:
Originally Posted by ponjohn View Post
I think these "simple" iron head combos are the cats meow.

High 11's to low 12's on basic parts.
Jim Hand style..

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Old 01-09-2021, 11:25 PM
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Quote:
Originally Posted by Skip Fix View Post
The 6x-4 heads I have on the 400 in the '81 I pulled off an original 350 in a yard. They were 92.5cc. I smoothed the ridge in the chamber and then milled the 0.050 and they were still 92.5.
Exactly what I was thinking Skip. Smoothing the ridge and assuming pistons will be in the hole .010 or even more will put you in the "happy zone" even on 87. A valve job may add even more chamber volume. With a fairly big cam I would not think twice about it.

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Old 01-09-2021, 11:36 PM
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Quote:
Originally Posted by Cliff R View Post
Form74, did you do any dyno testing with that combo?......

Cliff , No I wish I would of , I just wanted to get in the car.

Estimate is around 450HP

One of these days I need to take it to my neighbors chassis Dyno

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12.075 @ 112.70
12.092 @ 112.43
12.128 @ 111.71


Last edited by form74; 01-09-2021 at 11:50 PM.
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