351W Edelbrock RPM head porting?
 

Why have I heard that it's not worth it to have Edelbrock RPM heads ported? I have the #60259 heads. I have an old tappet 351W block with dished .060" pistons. A shop here in town milled the heads at their "max" (?) to try and give me 9.5CR because the owner of the shop says my motor had 8.5CR pistons. I have the Comp XE274H cam with RPM air-gap intake and Holley 750Vac carb. I also have a modified manual C4, 11" - 2500 converter and a 4.56 rear. Because of my dished pistons - which that shop also flycut so the 2.02" valve reliefs "worked" - and since the current heads are already milled - rather than spend a couple of thousand $ to get better heads and then get them milled - I was hoping it would be feasible to have these Edelbrock RPM heads seriously ported so that they flow better than 251CFM at .500" lift if I'm quoting Edelbrock's flow chart correctly. I am looking for more top-end HP while keeping it under 6000RPM.

I have only ported one set of SBF Edelbrock RPM heads. As cast these heads
are no better than average. There is plenty of room for improvement with
out going hog wild.

OK - any tips for someone like me who has never ported before? Tools and how you did it? Thanks!

For a 170cc head your heads are flowing pretty good. (251@.500" 255@.550") Yes they will flow more with porting. This makes the port larger. Larger ports want more rpm. Most all of the porting flow improvement will be at high valve lift.
If you want more "top end" you need to think along a total systems approach.
Your can is small for "top end power"
with 4.56:1 gears why are you limiting rpm to "under 6000rpm?"
more airflow will want more rpm. (same cid)
If you really want to see the gain from porting your heads: I would:
change the camshaft
.550"+ lift 244ish@.050" (hyd) 108 LSA
248 to 255 (solid lifter)
something like this
Victor JR manifold 750+cfm carb. (modify your carb or get a larger carb)
9" ATI converter 3500-4000rpm stall
And port the heads.
I would also improve the airflow of the exhaust system.
This engine will want around 6400rpm and will still drive well on the street.
You'll have a whole new opinion of your 351W.
You need to pull your heads and actually check your real compression ratio by measureing the variables accuratly yourself. 9.5 minimum required.
With the heads off its not that hard to do.

Even as they sit now your RPM heads are not the big choke point.

have you had this car to the track? ET?? MPH???

This is a daily driver 11 miles Monday thru Friday - doesn't see track time. It has stock cast internals so I keep it under 6000RPM. I do drive it like it's stolen - and it's alot of fun. I forgot to mention that I also have a 1" four-hole spacer on top of a 1" open spacer on top of the intake. The exhaust is good - headers with dual 3" Flowmaster's with a "H" pipe connector. I am looking for more acceleration from 4000 to 6000RPM.

Vic Jr and the lunati cam. leave the heads alone.

Get a cam closer to 290 advertised duration, and a single plane intake...forget the spacers. Stay under .525" lift unless your going to put some serious springs in the heads.

Might want to do the bottom end before you do this, the stock pistons crack the skirts off and have a habit of throwing the rod through the block from the inbalance at 6500 rpm.

I have a nice collection of 351W rods that I had to dig out of the pavement from back in the day when for $75 you could pick up another engine from the wrecker in good running condition and throw on your parts and do it again!

The 1969-71 rods are good for 7000 rpm, the stock pistons are good for 5500rpm max. She won't bang shifts at 6000 rpm for long before you crack the skirts dude, the oil control ring drain slots on the factory cast pistons are a huge weak point. Factory springs will valve float at 5700 rpm with a stock cam and with a .500" cam...5500 rpm. Likely your tachometer is off, Ford tachs are notoriously inaccurate.

Been there done that. :D

As for porting...spend your time on the exhaust and intake bowls and get the ports matched. Most of the heads potential is restricted due to these pinch points. Unshroud the exhaust valve on the chamber side and leave the rest alone. Factory heads can have the valve support hump ground off in the intake and exhaust ports if you bronze guide the hole. Doing much more will just be cosmetic. Replace the factory pushrods, they are noodles and flex like crazy at even 5000 rpm.

Get the exhaust right and carb tuned and you should see close to 400Hp with that combo with a peak rpm potential of 6500 rpm.

Remember what I said about the factory pistons...they WILL crack if you keep abusing them.

I can show you what I would do. The port size is very close to being correct
for your combination so I would concentrate on increasing flow with out
increasing port volume by much. The two places to modify are the push rod
pinch and the short turn. The exhaust ports can be widened. The rest is just
getting rid of casting flash and blending everything in.

As for tools you will need a 1/4 grinder, (if electric) a router speed control,
a 1/4 shank tear drop shaped 3/8" bur with a 4" long shank, 60 grit cartridge
rolls and a 4" long mandrel. Don't forget safety equipment, goggles, dust
mask and ear plugs.

I too believe the XE274H cam is too small for a 351 at 6000 RPM, what’s the
duration @ 0.050"?

Thanks for the advice, man. @ .050" duration is split 230*/236* with .519"/.523" of valve-lift.

With the tight converter, I'd leave the cam alone. I'll try to find some
pictures tonight to show you where to remove material from the ports.

How much does the car weigh?

Here’s a picture of a Trick Flow SBF port. It’s a lot different from the RPM
but gives you the general idea.


http://members.cox.net/dnjunk/trick%20flow%20r4.jpg

The Merc weighs 3800LBS with me in it. I really appreciate your help.

You really need better pistons. You can mill the heads over dished pistons till the cows come home and not gain anything worth chasing. The problem with circular dish pistons is a lack of squish/quench in the combustion space. This results in a less than ideal burn with a tendency toward detonation and preignition problems. You can port and cam forever but you'll never get out of this engine what it should deliver because you've got a fundamental engineering flaw in the combustion chamber because of the piston crown shape. To get the compression up and get some squish/quench going you need to run a piston with as much flat space as possible to get under the flat portion of the head as close as reasonable somewhere in the zone of .050 to .060 inch including the distance the piston sits below the head deck, and the head gasket thickness. Starting with the combustion chamber volume, you need to add the head gasket volume and the volume the piston has from it's crown to the head deck. These will establish the base volumes of the combustion space. The compression ratio is then trimmed with the piston crown, which can be a dome, flat, or if a dish is need a D shaped dish under the valve's and sparkplug side of the chamber. As the cam gets into longer durations and or shorter LSAs, the compression should go up, an XE274 with an aluminum (Edlebrock) head will easily support 10 to one on 92 octane unleaded for the street.

Also when computing head gasket volume with an aluminum head, remember that you can't use thin steel shim gaskets, you'll be looking at composition gaskets that will have about .050 inch thickness already, so it doesn't have much allowance for the piston to be down the hole very far at TDC. This can be a problem with replacement pistons as they often come with a shoter pin to crown distance as the manufacturers assume the block will be decked, so they're trying to build in some cushion. You need to talk directly to them to insure you get the properly sized stuff or all the work going into getting the chamber right goes out the window.

Bogie

Again using the trick flow head I highlighted the places I remove material.

On the left the port turns around the push rod. I open up this spot to slow
the air speed as it makes the turn. You must be careful here or you will break
through. Measure the width of each port at the turn to get them all the
same.

On the right in at the beginning of the short turn I widen the turn on both
sides. I only put the arrow on one side but I do both sides. Again this is to
slow the air speed at high RPM.

On this head I worked the guides, on the RPM heads it's just the bronze
sticking up so there is nothing to do there.

The rest is just blending every thing in. Notice I don't try to get the surface
smooth, just eliminate rough casting and imperfections. In all I only remove
about 5 cc of material.

http://members.cox.net/dnaquin/Trick%20Flow%20copy.jpg

Why would you want to slow the air speed down at high RPM?

Two reasons, air doesn't turn well when its moving too fast and the port will
go into "choke" as the air starts and stops with the valve action.