I am looking to change the heads in my 70 Trans Am for better performance, and I need advice.

The engine is a rebuilt stock bore 1973 455 with the low compression (8.1) stock 4x H1 heads, modified with the Butler Ram Air III "744" hydraulic roller cam and lifter kit (279/291 224/236, .450/.450 HR115), ram air repro exhaust manifolds with Pypes 2.5" exhaust, Holley Sniper Quadrajet EFI and a Hyperspark distributor. The transmission is a rebuilt TH200-4R with 3.42 gears and stock stall speed.

I would like to bump up the compression ratio to the 9.5 to 10 range, while keeping the current Butler kit and the other modifications. Suggestions and comments regarding head alternatives for this purpose are appreciated.

HI Bird Chief - welcome to FP.

Aftermarket Head recommendations:

Butler CNC'd ported heads

Edelbrock cylinder heads

Tiger cylinder heads

For all three of the these heads, I would contact Butler since you already have a Butler bottom end.

Stock cylinder heads:

You 4xs are 2.11" intake/1.66" exhaust valves at 114ccs. The '73 HO 455 heads, 16x, are round port and have 2.11/1.77 valves and a 111 cc chambers. Here are some stock 455 cylinder heads you can look for and perhaps had some porting done to them as well. I tend avoid the heads with a 1.66 exhaust valve since the 1.77s are available. The HO heads, if you can find them, will probably go for the most money on eBay, Facebook Marketplace, or Craigslist.

Here are the 400 heads you can also use.....I have a set of 7K3s on my 455. Again the Ram Air and HO heads will command the premium price if you find a set.

Thanks Frosty for your great info. What do you think about the 6x-4 and 6x-8 heads for my application? Also, given the cam specs of my Butler kit, do you think that this kit is compatible with higher compression heads?

Hi Bird Chief

Here is the stats on the 6x series of heads.

They are a fine head but you are not getting any added exhaust flow compared to the 2.11/1.77 heads. Therefore, my first choice would be to find a set of 2.11/1.77 heads. Now if you can't afford a new set of heads, then using what you have is fine. A lot of guys use these heads.

In order to increase compression, you will need to change your pistons, mill the heads, use a thinner head gasket, or some combination of these strategies. This costs more money and means pulling the engine out and effectively rebuilding it to gain the compression you want. 9.5:1 or 10:1 is a good goal for your engine and it is worth while driving on today's modern pump gas. I do believe that you can re-use your Butler cam. However, if you mill the heads and/or go to a thinner gasket, you will have to double check your pushrod length to make sure they are not too long.

Thanks again Frosty. Excellent info and advice!

Based on your good advice I have purchased a set of #96 heads.

My TH200-4R will probably be the weak link now, especially the torque may be too much for it. I have had no transmission problems with the 455 and the current 4x heads with 8.1 compression. But the #96 heads will probably bump this up to about 9.5 compression, with the resulting hp and torque increases.

Does anyone have any experience with a stock TH200-4R, with stock stall speed, hooked up to a 455?

The 200-4R is a fine transmission. It can be built to withstand your power goals with your 455. However, depending on your power output of your rebuilt 455, you may need a different stall converter. Consult with your engine builder on what he recommends for a stall.

Also the 200-4R case is two inches longer than the 350TH and 400TH cases. You will have to shorten your existing driveshaft or have a new driveshaft made. You may also need to drill new holes in the frame for the transmission cross member since the case is longer.

Hello,

And never forget - the 200-4R will need a properly installed, with correct geometry, and adjusted throttle valve cable to control internal line pressures. Without it, that transmission won't last two miles going down the road. Then it'll be rebuild time all over again.

Rick

Thank you for your comments regarding the TH200-4R transmission. It is already in my 70 T/A, rebuilt and adjusted, and has been running fine, with the 455 with 4x heads, for the 2,000 miles since the transmission rebuild.

When now installing higher performance heads, I am just wondering if this transmission will be able to handle the increased power. (It was rebuilt to stock specs and stock stall speed last year. At which time I had no plans for higher performance heads....)