Inliners International Forums General Discussion Hi Performance Porting an 848 head for a 261?

Can any one give me a ballpark $$ ? And...Is it worth it? I won't be running high RPMs often.

The main thing to do would be just do some bowl porting,and working on the short turns.

Should be about $150-$200 range??? Porting alone.
Any type of porting will help.

Is it worth it,,,,,It all depends on what you want to get out of it.

You can skip the porting to save some money & buy a good cam.
Not sure what you are looking for???

This will be going in my 62 pick-up. I'm building a 261 stroker with caddy pistons and 292 rods. I have a clean 848 head. I'll be using powerglide intake valves, 261 springs. The cam will be a mild grind 261 cam. If it doesn't make a difference in the 1000 - 3000rpm range, I probably won't do it.

For that type of set up I would just leave youe cyl head alone,just make sure you get a good valve job!!!
I would change to a turbo 350 trans if you can.

You would feel more improvement from a trans swap (powerglide to turbo 350)than you would from porting your head.

Hank, I'm just using the larger powerglide valves - I don't have the tranny. Mine is an sm420 with a Gear Vendors O.D. The rear is a posi 3:90
I will still be using it as a truck so I'll stick with this. I guess what I'm going for is truck that will look original, still be used as a truck and flow with modern traffic. I'm pretty much there except the 235 doesn't like the hills. Especially if I have a load.

Old Bert,,
Your intake/carb & exhaust manifolds are your big restriction.

But if you want to look original I guess your stuck w/that?

A nice set of multiple carbs & split exhaust manifolds will wake it up.
Esp, if you can put side draft Webers on it.
MBHD

I guess when I say look original I mean at least an inline that looks like the one that came with the truck. I am currently running fenton headers(repops) with stock, waterheated intake and HEI. The intake on the 261 stroker will be an Edelbrock 2x1 w/ Rochester carbs. I also may split my original exhaust manifold for a better and more reliable fit.

Old Bert,
The way you described your set up you won't be turning enough rpm to warrant the trouble of porting your head. There are only three things that make your engine perform: time (cam timing) temperature (cylinder pressure) and turbulence (how well the fuel is mixed before combustion). It's possible at low rpm to make the ports so smooth ( or big) that you don'r get good fuel atomization and actually decrease performance.

Old Bert,

Sounds like you are building a fine engine.

Have you read the May 1955 article in Hot Rod magazine about Frank McGurk's dyno test of several mods to a 261.
261 article in HRM
One of the mods tested was changing from the stock 261 head, with a 6.75 compression ratio, to a 235 head, ported and milled 0.030" to give a 7.85 compression ratio. The torque at 3000 rpm went up about 7%; however, at 2000 rpm the torque only went up by 2%. How much was due to compression ratio and how much to porting? Theoretically, increasing the compression ratio from 6.75 to 7.85 should increase to peak torque by about 5.5%, indicating that the porting may have provided 2.5% boost at 3000 rpm, but hurt at 2000 rpm. With your increased displacement, there might be a little more benefit for the porting, as the engine will want more air. I have always felt that one of the major problems with the Chevy head design was the 90-degree turn that the intake flow has to make to get from the siamesed port to the intake valve. Just increasing the port diameter will reduce the velocity but it will not eliminate the turn.

Hoyt, II#922

Dear Bert;

NO, It's not worth it.

Reworking a 216/235/261 head car run a couple thousand. Helps for the "all out full race" type is all.

Spend that for a nice paint job & enjoy etc.

Hppy trails.

Hoyt, I have that article, it's awesome. The 2 things my motor will have that that one didn't is the HEI and the increased stroke. My block will be bored .050 over instead of .125 over like what McGurk did. I'm wondering what benefit the increased stroke would look like on those charts! Anyone care to speculate? Anyway, thanks for the info, I'll keep posting progress. The block just got decked and is now at the machine shop getting bored.

Howdy Bert, one thing you could do is tell us the OD ratio and rear tire diameter. One thing I noticed about the article is that with the exception of test 6 (which was an example of what not to do) every one showed an increace of power. Even down at the bottom end it was up 30+ hp. It sounds like your building a stump puller. A port job would help but is not needed without a cam and carburation to take advantage of it. My 2 cents on speculating a stroke increace would be an increace in torque with a decreace in RPM.

Tire diameter is 30" and the O.D. is .28 - I think. I know that running in 4th over is equivilant of running with a 3:08 rear end. I'm running it now with my 235. I can cruise 70mph @ 2400 rpm.

Bert,

I calculate that the Cadilac 261, with a 4.125" stroke and 3.750+0.050" bore will be 281 cubic inches, about 1% above the 278 for Tests 7-9 of the McGurk article. At the rpm range that you will be running, a cubic inch is a cubic inch. A few months ago HRM reported on some dyno tests, on a BBC, where they tested different combinations of bore and stroke that gave the same total displacement. As I remember, there was very little difference in the performance.

If you plot the torque versus RPM for the McGurk tests, you will see that you really want a better than stock cam. See what Tom Langdon (or other members of this board) recommend. There must be something better than the McGurk No. 18 grind now. I've always liked to look at torque vs rpm rather than HP vs rpm, as the curves are not distorted by the rpm effect on horsepower. It is much easier to see where the torque starts to fall off, and what combinations of cam, carburetors, etc. really do for your desired operating range.

Keep us posted on your engine - it is a great project.

Hoyt, II# 922

As per Tom's recommendation, I'll be using a 261 cam reground to his "bulldog" specs which I don't have in front of me now. When I have time, I'll campare the numbers vs the McGurk cam. Unless someone here beats me to it

"The cost of porting the head and polishing the combustion chambers is $35, plus $12 for seating, lightening and assembling the valves and $9 for milling and recessing the valve seats." -that is from the May 55 article on the 261. Anyone have a time machine?

I do not know what McGurk was paying his help in 1955. I was making $1 per hour as a service station helper in 1955. I got up to 2.25 per hour as a "mechanic" in 1962 when I went back to school. A 12-port head assembly cost $405 in 1960. One could go on forever.

Hoyt, II# 922

Cam specs are: McGurk # 18 - intake duration 245, .421 lift. Exhaust duration 284, .410 lift.
Mr. Langdons specs - intake duration 254, .435 lift. Exhaust duration 264, .435 lift.

Have the Langdon "Bulldog" cam in my .040 over 261 with Fenton headers and dual Carter/Weber carbs. Has great low-end through mid-range power. My truck is a 55 1st with the original 4-speed hydramatic and a 3.90 gear and a 28.5" tall rear tire. Pulls great at 50 to 65 mph. I don't have a tach but my truck really likes to cruise at 55 to 60.

The McGurk 3/4 cam (aka #18) specs:
Intake: opens 10 degrees before TDC
closes 55 degrees after TDC (245 degrees duration) 0.421" lift

Exhaust: opens 72 degrees before BDC
closes 32 degrees after TDC (284 degrees duration) 0.410"lift

BTW just for comparison:

Stock Cam:

Intake: opens 11.5 degrees before TDC, and closes 52.5 degrees after TDC Duration - 244 degrees/ 0.405" lift

Exhaust: opens 51 degrees before BDC, and closes 13 degrees after TDC - 244 degrees duration/0.414" lift