Compression ratio, how do you change it

[BSB67]

...
The 915 heads I mentioned are closed chamber. In the olden days, racers would swap the 1967 915 heads onto a 1968 HP 440 and get an instant 13:1 compression ratio.

The 915 adds less than one point to the compression ratio over the typical open chamber head like the 906. If you make the swap you describe, the compression ratio will go from about 9.7:1 (actual CR of a 1968 HP 440) to about 10.5. Put a 915 on an 8:1 440, it will raise the CR to 8.7:1.

 

[wyrmrider]

Did it all the time but you get closer to a real 11:1 with 6 pack pistons
you would have to use the domed pistons if you wanted 13:1
BS67 is real close with the other pistons mentioned but late 440's are typically less than a real 8:1 7.5 sometimes
the head ccs given are called "blueprint" or NHRA minimums
as cast they are much larger
always cc both ends of each head then cut them all to the same

 

[451Mopar]

Back to the original question? What are you looking for in a piston? The basic changes that affect compression are the compression height (center of pin to flat top of piston), the the piston might have a dome for higher compression, or a dish for lower compression (compared to a flat top.) The size and depth of the pistons valve reliefs might be important if running a large duration camshaft. This article on custom pistons posted recently at enginebuildermag.com
http://www.enginebuildermag.com/2018/04/custom-pistons-101-everything-you-need-to-know/

 

[BSB67]

.......I have been playing with the compression ratio calculator on Summit Racings website using specs listed for different pistons they sell and getting numbers in the 14:1 range.

That really surprises me. Pretty hard to get to even 12:1 with common off the shelf heads and pistons.

 

[Paul_G]

I had to have pretty much been doing it wrong.

 

[Challenger340]

Now let's really confuse things about your "desired" Compression Ratio, given your statement that PP240 Head Flow is req'd for 500+ hp, and enter into a discussion around actual Dynamic Compression Ratio or the actual Cylinder "pressure" the Engine will see as modified by the Camshaft events.
I mean if you are planning the PP240 Heads, surely you wish to compliment the enhanced Cylinder Head Flow by means of appropriate Camshaft events to exploit ? Thereby modifying your static Compression Ratio target suitably ?
Just pointing out.....
let be strictly a Piston selection criterion discussion, it is the entire combination of parts working together ?

 

[Paul_G]

Now let's really confuse things about your "desired" Compression Ratio, given your statement that PP240 Head Flow is req'd for 500+ hp, and enter into a discussion around actual Dynamic Compression Ratio or the actual Cylinder "pressure" the Engine will see as modified by the Camshaft events.
I mean if you are planning the PP240 Heads, surely you wish to compliment the enhanced Cylinder Head Flow by means of appropriate Camshaft events to exploit ? Thereby modifying your static Compression Ratio target suitably ?
Just pointing out.....
let be strictly a Piston selection criterion discussion, it is the entire combination of parts working together ?

Funny you should chime in here Bob. You started this. I have been wanting to replicate one of your "lowly" engine builds for quite some time. Rest easy though, I haven't screwed up yet. The engine is still in running condition and I haven't bought any parts yet. But I do now own the engine I plan to victimize.

 

[Challenger340]

Oh Crap is that you Paul ? DUH ? Sry bout that.
L2355's will work but they are very heavy, and we're usually stuck around 540 Ft/Lbs and 520-525 hp using them, so there is some parasitic losses to the heavier Bobweight, even using the same/same Ported Stealth's on pump gas with the XE284H Flat Tappet Hydraulic Cam.
Hence why we used to look to other 4032 or 2618 cost effective lighter Forgings ie: the ICONs down in the 900 gram area at 550/550 Trq & hp

Yeah, the "lowly 440" series were fun budget engines..... and we still get requests for them even today !
But alas, good cores are becoming tougher by the year, and well, to be honest ..... the "lowly 440's" were not great money-makers for us compared with other jobs we get compensated for ?

 

[Paul_G]

I havent given up on this project yet. I have the engine tore down to the point that the oil pan is the only thing remaining in place. Overall everything looks really good. There is almost no ridge in the cylinders. I had a friend with much more engine rebuilding experience than me come take a look. He said it looks great, but still convinced me to get the block in to a machine shop for inspection and do a basic block service to prep for rebuild, which I am going to do.

Finding a machine shop in Phoenix is becoming difficult. Those guys must be very busy, don't answer the phone, and don't call back. The one machinist I did talk with didnt like my build plan. He didn't like the Trick Flow PP240 heads, said they are "stupid". Said they are a race piece and will be trouble. I asked him if he had a built a big block Mopar with them before and he said he had not.

Still looking for a machinist. I want to get ahold of IMM in Indio California. I think they will have a good idea of what I am after. They are on the way for me when we go to California to visit the kids. Still trying to contact them.

 

[Larrry B.]

Not sure if this is out of the way for you but you can check them out.
http://www.valleyhead.com/

I had my motor built there with no complaints.

 

[33 IMP]

Paul G, did you measure the deck height of those low-compression piston before you broke down the engine? As 451mopar said, stock low compression piston can be WAY down in the hole ( an actual 7.5 to 1) . If they are that far down, chances of getting to 9 to 1, even decking the block, head swap, and shim head gasket will be difficult, 10.25 per your post utterly impossible without a piston swap. Your engine can be made to run very well with the piston you have, It just won't have your desired c/r, and part selection must match the c/r you choose. With a piston swap you can have almost anything you want.
Also, I heartily endorse IMM, they saved a pair of antique ported max wedge heads for me,beautiful work, very pleased.

 

[Paul_G]

Paul G, did you measure the deck height of those low-compression piston before you broke down the engine? As 451mopar said, stock low compression piston can be WAY down in the hole ( an actual 7.5 to 1) . If they are that far down, chances of getting to 9 to 1, even decking the block, head swap, and shim head gasket will be difficult, 10.25 per your post utterly impossible without a piston swap. Your engine can be made to run very well with the piston you have, It just won't have your desired c/r, and part selection must match the c/r you choose. With a piston swap you can have almost anything you want.
Also, I heartily endorse IMM, they saved a pair of antique ported max wedge heads for me,beautiful work, very pleased.

The rotating assembly is still in the block. I have not measured how far down in the hole the pistons are. I will do that, it would be nice to know, but I don't intend to use those pistons anyway.

I will state again, my goal is a street engine, off the shelf parts, nothing extravagant other than spending the most money on a set of good heads. I chose the PP240's because of what I have read about them. They are good to go out right out of the box, no porting or any fancy work required to make great power.

At this point I need to get the block to a machinist. After he decides if the cylinders need to be bored and the block decked, we can determine the piston choice, cam, and I can start buying parts.

 

[Paul_G]

I checked the piston to deck clearance. I measure 0.138 in the hole.

 

[33 IMP]

By my calculations,using numbers from 451mopar, post 17, and using some of my own assumptions (head volume, flat tops with no valve reliefs,) I have you currently at 8.08-1. With a swap to 75cc heads in place of 452s, 8.9 to 1 is possible. Steel shim head gaskets,decking the block, shaving the heads, can all alter those numbers.

 

[wyrmrider]

FYI my 915 heads are closer to 80 than 74
I opened the chambers some more to get some more cc's- opposite of your quest
I had to deck my block so much to clean it up that my 6 pack pistons were out .015
so I had to use thick "marine" gaskets I'm at about .030-.035 quench
relatively impervious to the kind of gas true 9:1 Jones Cam with a slightly longer exhaust

 

[Challenger340]

The rotating assembly is still in the block. I have not measured how far down in the hole the pistons are. I will do that, it would be nice to know, but I don't intend to use those pistons anyway.

I will state again, my goal is a street engine, off the shelf parts, nothing extravagant other than spending the most money on a set of good heads. I chose the PP240's because of what I have read about them. They are good to go out right out of the box, no porting or any fancy work required to make great power.

At this point I need to get the block to a machinist. After he decides if the cylinders need to be bored and the block decked, we can determine the piston choice, cam, and I can start buying parts.

"Good" Heads can be relative to the HP Goals ?
Again, and just as a reminder, if you wish to use the PP240 Heads to best advantage by selecting sufficient camshaft to make the expense worthwhile, versus, far cheaper Heads ?
Then be sure to consider the duration of any Cam choices effect on the "Static" Compression Ratio, then called Dynamic Compression Ratio.

"Rule of thumb" being "1 cfm per 2 hp" ? for whatever lift/duration range at this early stage in the planning process, may be to consider your final HP & Trq Goals ?
For example:
and again, just a Ballpark at this EARLY stage in planning ? But if you are looking for 500 hp, the brain can quickly demise a minimum 250-260 cfm requirement @ Lift.
FAR MORE TO IT OBVIOUSLY than this,
but easy to see at the outset that a 340 cfm @ .600" Lift OOTB PP240 Head, might be somewhat overkill for the slightly 500+ hp Goal ?
Just say'in
A set of Edeljunk E-Streets, Stealth's, or Sidewinder's OOTB can easily supply sufficient AirFlow for 525hp for less than half the cost, leaving room for Forged Pistons/Rings, Bearings, Cam/Lifters inclusive.

Apologies for throwing a wrench into the plans ?
but costs being what they are these days, for std Stroke 440's between 500 to 550 hp/Trq on F.T Hydraulic Cams and Pump Gas, we still just default to the cheaper heads like Eddy E-Streets, Stealth's if the stock appearance is wanted, or even Sidewinder's which are ALL easily capable of those power levels using the right Camshaft, and far cheaper than the PP240's.

 

[Paul_G]

"Good" Heads can be relative to the HP Goals ?
Again, and just as a reminder, if you wish to use the PP240 Heads to best advantage by selecting sufficient camshaft to make the expense worthwhile, versus, far cheaper Heads ?
Then be sure to consider the duration of any Cam choices effect on the "Static" Compression Ratio, then called Dynamic Compression Ratio.

"Rule of thumb" being "1 cfm per 2 hp" ? for whatever lift/duration range at this early stage in the planning process, may be to consider your final HP & Trq Goals ?
For example:
and again, just a Ballpark at this EARLY stage in planning ? But if you are looking for 500 hp, the brain can quickly demise a minimum 250-260 cfm requirement @ Lift.
FAR MORE TO IT OBVIOUSLY than this,
but easy to see at the outset that a 340 cfm @ .600" Lift OOTB PP240 Head, might be somewhat overkill for the slightly 500+ hp Goal ?
Just say'in
A set of Edeljunk E-Streets, Stealth's, or Sidewinder's OOTB can easily supply sufficient AirFlow for 525hp for less than half the cost, leaving room for Forged Pistons/Rings, Bearings, Cam/Lifters inclusive.

Apologies for throwing a wrench into the plans ?
but costs being what they are these days, for std Stroke 440's between 500 to 550 hp/Trq on F.T Hydraulic Cams and Pump Gas, we still just default to the cheaper heads like Eddy E-Streets, Stealth's if the stock appearance is wanted, or even Sidewinder's which are ALL easily capable of those power levels using the right Camshaft, and far cheaper than the PP240's.

I understand what you are saying. I am going to have much more head flow than what I need to build a 500+ HP/TQ engine. This power level can be achieved with a less expensive head, correct?

So this makes me ask, with the PP240 head, how much more will the engine build require to take full advantage of the PP240 head? If we do that, and use the head to it's potential, will the engine still be streetable enough for good vacuum for power brakes and run cool enough to survive the desert heat with the A/C running?

 

[wyrmrider]

while you are playing with the deck chairs on the titanic
a necessary re-iterative drill till you get it right
keep pricing the stroker kits if you need pistons
easiest way to get lots more torque
true a big cam that maxes out the heads airflow will give about the same ho stock vs stroked- but at a higher rpm the torque below maximum horsepower is greater and you get to maximum hp at a lower rpm
NOT THAT YOU SHOULS GO FOR MAX HP AND RPM except for a heads up trailer queen.
measure twice- cut once
try the controlled induction package and run the simulators

 

[Paul_G]

I have the engine completely disassembled. Taking it to the machine shop tomorrow. The crank journals have very slight scratching that can barely be felt with my fingernail. I will find out tomorrow if a polish can clean that up or will it require to be cut down. Once I hear back from the machinist I can start ordering parts.

I have the main caps back in place, all the freeze plugs removed. The pistons and rods are marked and in a box. I will have them press the old pistons off the rods.

If the crank requires machining, I must ask myself at what point do I trash the old rotating assembly and go with a stroker? The 440 Source 440 to 512 kit is $2250 balanced and delivered. Comes with the crank, rods, pistons, rings, pins, locks, and bearings.

 

[Paul_G]

keep pricing the stroker kits if you need pistons
easiest way to get lots more torque

Since the crank did need to be turned along with new pistons, and the need to have the piston swap done at the machine shop and probably rod sizing too, I priced out the rough cost of all that vs. a 440 source 440 to 512 kit. My numbers come out to be around $800 difference. So I put the old crank, rods and pistons back in my truck and made the decision to go with a stroker.

The bores will need to be cut .030 for sure. The rest of the block looks really good as per the machinist. His rough estimate for the machine work which is basic prep for rebuild, hot tank, crack check, and clearancing the block for the stroker, final fit of the pistons in the bores, assembling the pistons and rods, basically having it ready for me to assemble is $800.

This changes everything. Probably going to use a roller cam now too. Price goes up, just means it adds time to the build. The way I see it now, there are no Mopar big blocks in Heaven. Got to get it while I am here.