72 350 - stock pistons - What heads to get 9.5 compression
 

Hello, I have a stock '72 350 I'd like to bump compression to 9.5-1
Stock heads were 7H1 and 89.5cc
What replacement heads should I be looking for?

Thanks, Mattthew

I'd look for small valve hi-comp 46's that came on 2bbl 400s and 390 428s,for a 350 build, imho

The 1969 #46 heads would definitely be at or near the top of the list, used on medium performance 400s and the standard 360 hp Bonneville 428. Keep in mind that if running headers you’ll need to drill end tap the end bolt holes as they only have 4 bolt holes to attach the exhaust manifolds. They’re pretty much your best small-valve head option.

If you come across some 1968 #17 or 1969 #47 350 heads those would also be good candidates with some milling to get them down to around 75ccs.

I compute your compression at a best of 8.27 to 1 assuming that your Pistons have a valve notch volume of 4 CCs.

If you mill your heads down to 79.5 CCs you will then have a 9.3 to 1 compression.
To get there you would have to mill .060" off of your heads which is doable.

I used a common Fel pro head gasket in my figuring of your compression ratio and it spec's out at 9.4 CCs, but I believe stock replacement types are smaller and might help to up your ratio.

To get the Intake Manifold to play nice with the milled heads you will also have to have the Intake flange of the heads milled at least.045".

If your heads have already been rebuilt I would consider this as a option, if not then look for other heads.

Milling .060" off will not be cheap unless you have a good in with a machine shop, or do some horse trading, hey you never know so there's no harm in having that chat .

If you have a few shops to choose from, call around and get some prices.

Do not toss out you stock heads however, as there a good head for someone with a 400 , 421 or 428 motor.

These 7HI heads are the only Pontiac heads ever made with the 1.96" / 1.77" valve size which makes them a great pick for stepping up to the 2.11" valve and then use them on the above posted motors to get under 10 to 1 compression.

Something in the 72-75 cc range. 12,13,16,62,670. Big valve heads. Adjust CR with head gasket thickness. Check for valve to bore interference.

In addition to the great suggestions below, you could also use the '70 #16 heads that were on the 330 hp 400's (XV code) available in non-GTO A-bodies and more easily found in the big cars. They are in the low 80 cc range uncut. With a truing cut (.006" usually) they will be in the 77-78 cc range. They are small valve with pressed-in studs, so can be converted to screw-in studs easily. I've had no problems with pinning the press-in studs if the valve spring pressures are kept under 300 lbs. open at .500" with milder cam timing (no more than 225 degrees and long ramps...like the Summit 2800/2801/2802 and factory cams).

I'd personally stay with the small valve heads on a 350 Pontiac as they have enough flow to support 350 hp with the right parts/cam and 9.5 compression...not to mention, they are inexpensive to buy.

Dennis

Steve,
Can you post your math?

Stock 350 bore, stroke, with (79.5 cc + 4 cc + 9.4 cc = 92.9 cc) 92.9 cc total volume would be 8.8:1 CR. ► (92.9 + 724.7) / 92.9 = 8.80086

9.3:1 would needed a total volume of 87.3 cc's ► (87.3 + 724.7) / 87.3 = 9.30126

Stan

PS This should cover almost all cases.

Swept Volume = 724.7 (44.226 ci)

_________________Total
_______CR________Volume
_____7.50000_____111.49
_____7.60000_____109.8
_____7.70000_____108.16
_____7.80000_____106.57
_____7.90000_____105.03
_____8.00000_____103.53
_____8.10000_____102.07
_____8.20000_____100.65
_____8.30000_____99.274
_____8.40000_____97.932
_____8.50000_____96.627
_____8.60000_____95.355
_____8.70000_____94.117
_____8.80000_____92.91
_____8.90000_____91.734
_____9.00000_____90.588
_____9.10000_____89.469
_____9.20000_____88.378
_____9.30000_____87.313
_____9.40000_____86.274
_____9.50000_____85.259
_____9.60000_____84.267
_____9.70000_____83.299
_____9.80000_____82.352
_____9.90000_____81.427
____10.00000_____80.522


____Total
____Volume________CR
____80.00000_____10.059
____81.00000_____9.9469
____82.00000_____9.8378
____83.00000_____9.7313
____84.00000_____9.6274
____85.00000_____9.5259
____86.00000_____9.4267
____87.00000_____9.3299
____88.00000_____9.2352
____89.00000_____9.1427
____90.00000_____9.0522
____91.00000_____8.9637
____92.00000_____8.8772
____93.00000_____8.7925
____94.00000_____8.7096
____95.00000_____8.6284
____96.00000_____8.549
____97.00000_____8.4711
____98.00000_____8.3949
____99.00000_____8.3202
___100.00000_____8.247
___101.00000_____8.1752
___102.00000_____8.1049
___103.00000_____8.0359
___104.00000_____7.9683
___105.00000_____7.9019
___106.00000_____7.8368
___107.00000_____7.7729
___108.00000_____7.7102
___109.00000_____7.6486
___110.00000_____7.5882
___111.00000_____7.5288
___112.00000_____7.4705

I have some #17 heads available, I don’t want anything for em, just whatever shipping is:

https://forums.maxperformanceinc.com...d.php?t=868353

Thanks for all the input. The 7H1 heads are on my 455 now. It has stock pistons and runs so good I thought why not wake the 350 up too.

Currently have 17 and 47 heads. A friend has quite a collection and If I knew what other options were more effective I can ask if he has any

How would 6X -4s work? Just curious.

at around 94 ccs not very good on a 350

OP if your friend has a stash of heads take a 6in machinist scale and 6in dial calipers and measure up some chambers on what he has you dont need to measure for cc's just measure to compare

#18s which is a 1968 350 head would be the best if he has them

Thank you. What distance am I looking for?

Hi Stan.
I have to back thru what I may have screwed up at 6 am this morning, but I recall my cylinder cid was 44.338 for a Cc volume of 726.58.

I would look at the chamber scallop when doing a high compression 350. A 400 chamber would create a pocket that wouldn't be ideal in my opinion.

1969 46 heads have a 400 chamber

12, 17, 18, 47, 48, 6x-4 and others have a 350 chamber.

I was thinking of building a 350 at some point and would love to run 12, 18, or 48, but they bring a big price tag.

I will take a couple pics tomorrow on the 2 spots that are tell tale on volume

Here's a handy chart to check the change in power due to a compression change.

Since it's a percentage factor your amount of gain greatly hinges on how much power you have to start with.

Unless your going to run a bigger Cam that calls for a minimum of a 9.5 compression for example I would be happy with a gain in compression of even just 9 to 1, and that could save you a good chunk of money if that's a concern

If you have a pile of heads and are comparing combustion chamber dimensions these 2 places are where to focus on, its obvious the deeper and wider = more CC's 1968 + heads

Mathew, ALL the math is " Static CR" Dynamic can be 1 whole point lower... 9:1 running, is actually 8:1 .
If you run a 71 and up style PS pump, alt set up, there is not a provision for it on 1970 and older heads.
You would need to make a bracket with threads in it.
Don't mill too much or you engine becomes a 1-off

Mathew, ALL the math is " Static CR" Dynamic can be 1 whole point lower... 9:1 running, is actually 8:1 .
If you run a 71 and up style PS pump, alt set up, there is not a provision for it on 1970 and older heads.
You would need to make a bracket with threads in it.
Don't mill too much or you engine becomes a 1-off

Some general info.

The 350 is up top.