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Old 12-22-2019, 12:19 AM
Mcronk Mcronk is offline
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Default 69 350 HO heads

Question on the cast 48 heads used on 1969 350 HO Firebirds. I assume they had the cast 48 on center exhaust ports but did the 350 heads have any other differences? From what I can find combustion chamber was 68cc. The 69 400 motor with cast 48 heads was advertised as 10.75 compression ratio. The 69 350 HO motor was advertised as 10.5 compression ratio.

Mark

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Old 12-22-2019, 08:09 AM
thews thews is offline
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I have two 69 350HO converts and dug around the interweb about this question. I can tell you that from the outside, the #48 350HO heads are the same as the 400 heads. While I didn't cc the #48's that came with the 350HO engine I bought, the chambers look the same when sitting next to the 400 #48's I have. Poking around the web I've read that the factory shaved the 350HO heads down to 68cc, but I highly doubt it.

The 69 #48 and #62 heads are for all intent and purposes exactly the same as the 1970 #13 and #12 heads. I would think that if the 350HO required a 68cc chamber, then they would have cast the head with a different number like the 68 #18's.

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Old 12-22-2019, 09:11 AM
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ponyakr ponyakr is offline
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If the chambers were anywhere near 70cc, then Pontiac reaally lied about the CR of the '69 350HO.

http://www.wallaceracing.com/cr_test2.php

Enter some numbers into the calculator.

It has been posted here that most Pontiac pistons were .023 in the hole.

I don't know the exact size of a factory 350 head gasket.

Also don't know the 350 piston valve relief volume.

But, even with estimating the numbers, I can't quite come up to 10:1 CR, even using 66cc chambers.

This site shows the chambers to be 64cc. I don't know what they were.

http://classicpontiac.org/FAQ.asp?myPage=V8HeadInfo

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Old 12-22-2019, 11:15 AM
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necdb3 necdb3 is offline
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Mark, remember, a 68 cc chamber head will have higher compression on a bigger displacement block, thus lower compression on a smaller displacement.

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Old 12-22-2019, 12:52 PM
Mcronk Mcronk is offline
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Thanks for the information.

On some of the heads from the mid 70's Pontiac stamped a small X near casting number to indicate a different chamber size when the same casting number was used for different applications. I was not sure if anything like that was done for the 48 heads used on 350 HO.

I agree that if both the 400 and 350 engines used an identical head, and everything else equal, then the factory advertised compression ratio is wrong. I have also heard it said that the factory compression ratings were about .5 point too high than actual. So 10.75 becomes 10.25 and the 69 350 HO becomes 10.0.

I have a spreadsheet that a friend made (mechanical engineer who works in aerospace industry and a Pontiac guy). It is very similar to the calculator on the Wallace site. It allows to enter variables such as deck height, piston mill, valve reliefs, milling piston dish, head cc, etc. It also lists all the more popular engine sizes from 326 to 455. You can enter data and it will calculate static compression ratio for all engine sizes as well as overbores. Whatever data I enter for calculation the 350 engine is almost a full point less than the 400 engine. For a 68 cc head, deck height of .023, stock bore, .038 crushed head gasket, and valve relief of 7.9cc
I get 10.2 for 400 and 9.3 for 350. Point is with all things equal it seems the 350 engine with the same head as a 400 is almost a full point less compression.

Also interesting to note that some literature including the factory sales information list the 68 350 HO and the 69 350HO as the same HP at 320. However the 68 version used a smaller valve head. McCarthy book lists the 69 350HO at 330HP.

I am looking to purchase a set of 48 heads for a 350. These will be a low dollar upgrade on a 350 2bbl motor. I already have a excellent shortblock with original 49,000 miles, factory 4bbl intake, and will use 1.65 rockers on the factory 2bbl cam, as well as recurve the distributor. Not looking to "race" just improve upon the factory 265 HP.

Mark

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Old 12-22-2019, 01:13 PM
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That's not the changes I'd make.

But good luck with it !

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Old 12-22-2019, 03:29 PM
thews thews is offline
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Quote:
Originally Posted by ponyakr View Post
That's not the changes I'd make.

But good luck with it !
I agree. If you're going to go with the #48 heads, you really should do the 350HO cam as well.

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Old 12-22-2019, 08:06 PM
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I had a 68 350 HO I changed it from a automatic to a stick, changed the cam and put in headers. It was the comp cams high energy 268. It definitely woke it up. I might not use the same cam for a high compression engine but it worked very well. It would pull to 6000 but I would shift it at about 5300 to get it back into all that torque. If I shifted to soon the tires would just light up.
What in trying to say is seriously consider putting a different cam in it. Your leaving a lot on the table especially with the 2 barrel cam.

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Old 12-22-2019, 08:54 PM
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What is this going in? Did you just buy something else?

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Old 12-22-2019, 09:13 PM
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Quote:
On some of the heads from the mid 70's Pontiac stamped a small X near casting number to indicate a different chamber size when the same casting number was used for different applications. I was not sure if anything like that was done for the 48 heads used on 350 HO
There was a thread (probably more than one) where this was talked about.
From what we found was not enough evidence yet?

Basically could find no extra stampings. Not sure on the cc's of each.
Main problem is most did not have definitive proof that the head(s) were from a 350 or the 400 engines.

HFR probably has a link. I haven't searched for it. I was in the thread though.
Seems Rocky was in one of them also.


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Old 12-22-2019, 10:44 PM
Mcronk Mcronk is offline
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Ok let me explain a little better why I am doing this.

The original 1968 350 2bbl. I got with my 68 Tempest Custom only has 49,000 miles and in excellent condition. It was replaced by a 501 cubic inch motor that dynoed at just about 600HP. I am just using what I have to make a nice little motor to drive around with. Probably will go in a project car for my son. Not wanting to spend a lot of money. As for the cam I can gain 10 to 15% additional lift and a little more duration with just a rocker arm change either factory 1.65 or aftermarket 1.6. The 48 heads, or as it seems, any large valve head will increase flow capability even with stock ports over the 2bbl heads. My concern about the 48 heads was if there was any actual difference between 350 application and 400. I want to make sure it will run on pump gas and wanted to calculate compression. My gut feeling is there is no difference therefore I could choose other large valve heads with proper chamber size. I would probably cc one chamber just to check since heads could have been milled over the years.

My reference point are two engines I raced. The original 400 HO motor in my 67 GTO was built with low compression pistons and a replacement cam that was equivalent to the 066 cam before I got the car. I tuned engine, suspension, and ran a 2800 stall convertor and 3.90 gears. All I did to engine was 1.65 rockers, HO VS11 valve springs, recurve distributor, rebuild and tune quadrajet. The car ran stock intake and exhaust manifolds. It weighed 3960 with me in the car and was loaded with A/C, power steering, power brakes, etc. Car ran best ET of 13.46 at 98 mph. With 3.23 gears it ran 13.66. I did use a Ram Air pan that sealed to the hood and open factory scoop. I won NMCA Stock ET IN 1995 (basically a bracket racing class). Engine compartment looked factory correct with date coded spark plug wires!

Second engine was a.030 over 428 I built with RA IV heads. It was under cammed with a old Crane Fireball hydraulic cam and Rhoads lifters. If I remember specs were .488 - .505 with 232- 242 duration at .050. I wanted to drive it on the street and have a good idle. It would easily idle at 800 rpm and had very quick throttle response. People said it did not sound like a Pontiac because it revved so quickly. The Chuck Gulledge prepared Holley 800 probably had a lot to do with that. I was running cast rods and heavy TRW pistons so rev limiter was set at 6000 and I shifted around 5600 to 5800. The 3200 pound back half 67 Lemans ran 11.18 at 118 mph. It probably could have used more cam and I would not use Rhoads lifters again (gave up some max lift of cam). Point is being under cammed a little is a lot better than being over cammed especially street driving a small cubic inch motor.

I am not looking for max perforomance just trying something different.

Dave I have not purchased anything yet. Sean and I have talked about a 69 A body project or first/second generation Firebird.


Last edited by Mcronk; 12-22-2019 at 10:52 PM.
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