Port and polishing.Is it worth doing?

[1badgtx]

I have a low compression 440...Is porting and polishing my heads gonna give me any advantages???Gonna try a cam change,so I figured maybe while it,s apart this may help in low end torque..
Petty Blue 67 gtx

 

[Stumper]

It can never hurt ..... BUT, if you are going to leave it low compression and use a corresponding mild cam then you really won't notice any difference. Not enough to justify the cost / time at any rate. Of course if you go after the combustion chambers themselves to any extent, you can lower compression further and hurt performance.

 

[67 B-body]

What is the working compression? Do you have stock cast iron heads? A lot of guys using the newer aftermarket heads are intentionaly using dished pistons to keep the comp down to 10.0:1. If you have cast Factory type heads and Santa was good to you, I would buy Aluminum heads with low chamber volumes. That will show you a HUGE increase in power over stock cast heads! Even if you spend the money to have stock heads ported and decked, you still wont be even close to the same work invested into a good set of modern Aluminum heads. There are several manufactures that have out of the box flow numbers that blow away max ported cast heads! If you look at mine, they hav NO pinch from the valve stem as the stock design has. I use the 440 Victor style head for your referance. I just seen a set advertised on B-bodies, and they were sold FAST! I'm not a fan of the aluminum versions of a factory head, because they don't improve on the pinch caused by valve stems location in the ports! Look at some of the Indy heads that are along the same design as the Victors. They move things around a bit too and also require the offset rocker arms to reach the valve stems.

 

[Cranky]

It can never hurt ..... BUT, if you are going to leave it low compression and use a corresponding mild cam then you really won't notice any difference. Not enough to justify the cost / time at any rate. Of course if you go after the combustion chambers themselves to any extent, you can lower compression further and hurt performance.

I disagree on what you say about not noticing any difference. I've been a part of many low compression builds and using stock heads and with just a pocket port made a noticeable difference! A buddy has a low compression 72 318 pushing a Demon mid 13's with a set of 360 heads that only have a pocket job done to them...and 360 heads lowers the compression even more because of a larger combustion chamber but they have larger ports and valves.

What is the working compression? Do you have stock cast iron heads? A lot of guys using the newer aftermarket heads are intentionaly using dished pistons to keep the comp down to 10.0:1. If you have cast Factory type heads and Santa was good to you, I would buy Aluminum heads with low chamber volumes. That will show you a HUGE increase in power over stock cast heads! Even if you spend the money to have stock heads ported and decked, you still wont be even close to the same work invested into a good set of modern Aluminum heads. There are several manufactures that have out of the box flow numbers that blow away max ported cast heads! If you look at mine, they hav NO pinch from the valve stem as the stock design has. I use the 440 Victor style head for your referance. I just seen a set advertised on B-bodies, and they were sold FAST! I'm not a fan of the aluminum versions of a factory head, because they don't improve on the pinch caused by valve stems location in the ports! Look at some of the Indy heads that are along the same design as the Victors. They move things around a bit too and also require the offset rocker arms to reach the valve stems.

I agree with you on what you said. My out of the box Eddy RPM's posted similar flow numbers as my old max ported 906's!

 

[1badgtx]

I,m using edelbrock rpm heads..84cc...Thinking also about shaving the heads to gain a little compression...Right now it,s 8:1...Went to comp cams site...they made a few suggestions on cam...Funny they don,t ask about heads in their 10 questions to find the right cam...
Thanks for the input folks!!!
Petty Blue 67 gTx

 

[67 B-body]

I,m using edelbrock rpm heads..84cc...Thinking also about shaving the heads to gain a little compression...Right now it,s 8:1...Went to comp cams site...they made a few suggestions on cam...Funny they don,t ask about heads in their 10 questions to find the right cam...
Thanks for the input folks!!!
Petty Blue 67 gTx

The machine shop I had bore my block told me that he had to shave alot from a set of RPM's to come close to the advertised chamber CC's!

 

[1badgtx]

Sounds like I need to have more than a few things done to my heads to get were i want it...
Petty Blue 67 gTx

 

[Cranky]

The machine shop I had bore my block told me that he had to shave alot from a set of RPM's to come close to the advertised chamber CC's!

I checked mine when I got them and they were on the money.....

 

[lewtot184]

I,m using edelbrock rpm heads..84cc...Thinking also about shaving the heads to gain a little compression...Right now it,s 8:1...Went to comp cams site...they made a few suggestions on cam...Funny they don,t ask about heads in their 10 questions to find the right cam...
Thanks for the input folks!!!
Petty Blue 67 gTx

sounds like the heads are probably o.k.. you need a piston change, and yes i know its a lotta work.

 

[1badgtx]

So go from flat top to dome is what your getting at..
Petty Blue 67 gTx

 

[Cranky]

So go from flat top to dome is what your getting at..
Petty Blue 67 gTx

A flat top with a 0 deck height would be better than a domed piston imo. The 84cc heads have a closed chamber design and with a .040 gasket, you'll have .040 quench distance which is pretty good. All you need to do is machine valve reliefs into the tops.

 

[1badgtx]

Lots of great info guys...
Thanks alot!!!!
Petty Blue 67 gTx

 

[Cranky]

A '0' deck piston in a .030 over 440 with 84cc heads, .040 gasket and no valve reliefs will net a 10.7-1 Compression. In my book, that's a great ratio for an aluminum headed street moder with a cam that bleeds off some cylinder pressure in the lower RPM range. Valve reliefs that usually accounts for about 5cc will put that around 10.25-1.

 

[67 B-body]

A '0' deck piston in a .030 over 440 with 84cc heads, .040 gasket and no valve reliefs will net a 10.7-1 Compression. In my book, that's a great ratio for an aluminum headed street moder with a cam that bleeds off some cylinder pressure in the lower RPM range. Valve reliefs that usually accounts for about 5cc will put that around 10.25-1.

Cranky is right,,, If you can get the compression your looking for with flat top pistons you're better off! Think of it like this, if you have X amount of pressure or strength available in your arm and you press against a scale you'll get X amount of value showing. If yout take some of that same force and start placing it lateraly on your scale "side to side", you will actualy read a lower number on your scale right??? Well the piston acts the very same way. If your expanding gasses are placing a force directly down on the entire piston top equally in the same direction of travel, you'll have the most efficent usage of those expanding gasses. As soon as you start adding domes, you start taking away that "even" force applied across the entire top of the piston traveling in the same direction as the expanding gasses.
Now then, you might ask "why does anyone ever go with domed pistons then wise guy"? Well the fact of the matter is, the potential loss in efficiency of gasses expanding pressing against the piston in a direction other than the pistons direction of travel is usualy less than the performance gain by increasing compression by two or even three points.
I happen to be one of those "domed piston guys" and did so knowing the pros and cons of doing this. My pistons are machined flat on the domes "this helps hide the dome to the expanding gasses" and still provides nearly the same flat surface as a flat top piston would. I have 14cc's of dome and that gets me around 13.5:1 compression with my aluminum heads, and allow's me to run a bigger cam lift and higher duration but not feel the bottom RPM range fall to nothing! "Yes I run high octane fuel".
I hope this provides some insite to the workings of piston design and compression.
BTW, the rule of thumb is that 10:1 comp on aluminum heads is about the same as 9:1 comp on a cast iron head. This is a simple rule of thumb, but gets you close when concidering octane levels of fuel. This is a result of aluminum being very inefficient at holding heat and reduces those nasty little hot spots that cause pre ign in higher cyl pressures. :werd:

 

[lewtot184]

So go from flat top to dome is what your getting at..
Petty Blue 67 gTx

i'd use an Icon IC822 piston. they're not expensive, zero deck is easy, about 250 grams lighter, and should give 10:1 compression. the compression increase by itself is probably 25hp. adding a quench adds more torque thru out the power band and you can step the cam up some. just use a conventional 1/16-1/16-3/16 ring pack, nothing fancy.

 

[Stumper]

You guys are talking about a lot of other modifications and aftermarket components. My comments about the porting not yielding much performance were assuming from the original post that the motor sported the OEM low compression pistons and that the configuration would stay the same and that the cam would match the low compression motor. If you gain 20% over stock on this set up I don't think you will feel it.

 

[Cranky]

You guys are talking about a lot of other modifications and aftermarket components. My comments about the porting not yielding much performance were assuming from the original post that the motor sported the OEM low compression pistons and that the configuration would stay the same and that the cam would match the low compression motor. If you gain 20% over stock on this set up I don't think you will feel it.

I think the biggest improvement to that little 318 I mentioned did more for the engine than not doing the valve pockets. The factory back cut really does suck and working it improves flow big time. Take a good look at a set of heads that have a good pocket job and compare them to a set of bone stock heads and you'll see what I'm talking about. The teen's compression ration is like 7.8-1 because of the 360 heads but they do flow well and the mods to this engine is a .480 lift cam (don't remember the other specs), Torker intake, 600 DP, headers, STOCK converter and lots of gear, 4.88's to be exact and 29" tires. We turn the engine 6200. It's a dog off the line with a 2.2 60 foot but it runs 13.51 @ 102. If you crunch the numbers, the car should be running low 13's with that 102 number. The engine doesn't turn on until 3000 rpm. Yeah, it's a sorry combo but we threw it together with left over stuff. On these big block heads, you can see the back cut that's just under the valve seat. That ridge will really slow down the flow.

On these out of the box Eddys, you can see how smooth is it just past the seats. Big difference. And notice the guides are not even teardropped yet. Just working that area helps a ton even if you don't touch the rest of the port. Matching the intake the heads will also help.

 

[Meep-Meep]

If I had to choose an upgrade to your engine and the choices were port heads or increase compression I would go with compression. You need compression to make real power plain and simple. Porting has it's advantages, but if you already have Eddy heads, what can you possibly do to improve them for a street motor? I pocket port my iron heads but I also know that many people make serious power with unmodified heads in stock class racing. Get your cam and compression right and you will be happy. Leave the heads alone.

 

[rumblefish360]

X's2!

 

[Cranky]

Oh I agree that raising compression will be a big benefit but doing other things will help too but a piston swap is a major operation while reworking a set of heads isn't so major....and going with new pistons will probably dictate a rebalance unless you can work them down to what the old ones were if the new ones are heavier....