UGH... the problems of building a rare engine

bluestang65

New member
I hate to say how much more I love building SBF, Modular and even the new Coyotes. This is a rant. If you can get through it, I'm actually asking for help.


Build is a 250 with the CI head and EFI.

I ordered a set of custom pistons and they came out with the compression distance at 1.600 instead of 1.511. When you buy custom pistons, you sign a waiver that you own them. Fault is on me because I just send them a link and asked them to make me a forged stock piston for a 4.1L Ford. Gave them the specs from summit racing including the 1.511 compression distance, but there's no way for me to prove that.

According to my calculations, that makes distance below deck .034" using 9.469 distance to center, 5.881 rod length and 1.955 (half of stroke). The machine shop kissed the block deck, so it's probably closer to .030" if I'm lucky.

Plug it all in and assuming the small side of CI's estimates for the heads of 48cc and I'm over 11:1. I haven't cc'd the heads yet.
I am building a cool-sounding, EFI, fun, street engine and I don't care about performance too much, but clearly the temperament of this engine doesn't match what I'm trying to do.

I'm pissed and venting a little. Help me come to some working solution...

New pistons - $500 sucks and of course the same 'no guarantee we do our job right' exists.

Copper head gasket - .094 gets me in the 9.5:1 but I've called 2 places and keep getting "huh, what motor... oh here it is... it's a 300, right?" I don't think I can explain to another human being what this engine is. I'm not keen on using these, but if somebody knows a source, lmk.

MLS - expensive but cheaper than new pistons, I guess. Did CI ever get Cometic to start making gaskets for these motors or is it a straight custom job?

Custom rods?

New cast pistons - bite yet another bullet and snag a cheap set of cast pistons just to get this job done. It's an NA build, but the idea of cast pistons isn't exciting. Plus, the block was machined to the piston's specs so it's set right at 3.710 instead of adding a couple thousandths to accommodate the .030" oversized pistons.

I could shave a little off the pistons like maybe .010 but this relies on also being able to squeeze a little more out of the heads. Anybody know if it's possible to get a few cc's out of the heads? I'll have to double check what's left to work with in the combustion chamber anyway.


I know we are building a dying breed here, but they built more than one of these motors, so it shouldn't be a total one-off. I'm about to put this whole mess piece by piece in the classifieds.
 
It sounds like a really cool build, I hope you get it all built and running to your satisfaction.
The truth lies in the metal, ya gotta cc the head first, then you can figure out the game plan.
250 pistons are the same as 200's, those pistons may have some resale value ?
Would you mind saying which piston company ?
Would you say "they" made the mistake, given what you wanted, is that something that happens often with custom pistons ?
I'll probably going to go with custom piston in a few months so I'm really curious about your experience.
Good luck, hang in there, " If it was easy, everyone would have one".
DannyG
 
Just curious, have you picked your cam? If you cc the head you can figure the dynamic compression. I wonder how far out you will be? Good luck...you'll get it!
 
:unsure: If they are forged you should be able to cut them down some or have the dish depth increased to get to the compression you want to be. With that Aluminum head and a good radiator 9.5 to 10 to 1 should be workable. The piston company should help you out some if they didn't machine them right to your plan specs by fixing or remaching the dish. The FelPro permaTorque head gasket is thicker then the Victor so that may help some too. Sounds like it would work great as a race motor if I had the extra money I'd sure buy it for my drag car project. Good luck :nod:
 
You need another 5-7 cc to get close to 10:1

The combustion chambers in the aluminum head are not finished and the user is expected to complete the chamber work and to make sure all the chambers have the same volume
You should easily be able to pull a few CCs out of the chambers.

Bolt the head to the block and scribe the cylinder bore on to the head surface.
The chamber wall area that was machined for the valves needs to be blended into the un-machined wall area approaching the spark plug hole.
That same area around the valves needs to be angled back to the scribe line representing the cylinder bore.
Then the entire chamber needs to be polished.

If you still need more after the chamber work then you can work in the piston dish.

Please post a picture of the combustion chamber and the piston dish.
 
You're deck height is .030" or better w/ the new pistons. Are the pistons flat tops? If so, call Cal at Cal Spec in CO and have him mill some D dishes in them to your volume. Imo, if you were having custom pistons made you should have brought the deck to near zero and had them mill D dishes to whatever was called for. Best of all worlds. But whatever to late now.

Measure the chambers after the head is done, measure a piston in the hole and have them dished. Cal is very reasonable. Assuming .030" now, w/ 55cc chambers, Corteco gasket @ .053" you're @ 10:1 w/ 7cc dishes, 10cc's w/ a .035 oem composite, 8 w/ a Victor.

Pick your cam then decide what you need for the dishes. Base it on DCR not SCR
 
It is still my fault for not sending them information. I asked for a 'stock' piston. They acted like it was impossible. I sent them a link to the stock cast piston on Summit Racing's website to match it up. All the specs matched, but not the compression height. It came out 1.600 and should have been 1.511.

Racetec was the supplier.

Pics forthcoming. Need to figure out a host for them.
 
[image]https://scontent.fsnc1-1.fna.fbcdn.net/t31.0-8/13147503_10206574706266364_7009828066094665448_o.jpg[/image]

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That's a groovy dish ;) I bet you're going to be fine with the available gaskets once you know where you stand w/ all the variables. Quench is not going to be there but the aluminum head will help. What are your goals for the engine? Idle quality, HP, transmission, use?
 
It's a fun street motor. I've had the car fast before with a 351W. Now I just want it EFI, reliable and running right. Power isn't a concern but the when it had an L6 in it the car was unbearably slow.
T5 transmission.

I've read the articles on SCR vs DCR and just curious what I will get out of this:
http://www.summitracing.com/parts/hrs-280938-10

http://www.summitracing.com/parts/hrs-280998-10

I much prefer to build proven, trusted combos. I'm out on a limb here. I
 
The calculator download at the bottom of the page on this link for Patrick Kelly is the one I've been using. With a T5 you'll be able to have a pretty sweet ride and run the freeway. I don't think you need the duration to be that long, but you'd be fine I bet C/R wise with either of those cams. 208 @ .050" is probably doable if you want more power down low. You're 250 build isn't to different than mine except the head and whatever your chamber size is. I'm @ .020" deck, ~61cc chambers, 8 cc dishes. I'm going to be running 8.2 DCR iirc w/ an iron head and the vistor gasket. Still waiting for my new cam. It's 204/208 duration @ .050". I'm running an AOD w/ 3.7:1 in the rear. It's all theory till it hits the road though.

http://cochise.uia.net/pkelley2/DynamicCR.html
 
That definitely looks like an Autotech piston.
The dish looks like 2.300" diameter by .100" deep.
If the dish diameter was increased to 2.8" the dish volume would be increased to 10 cc.
That would still leave close to a .500" wide squish ring around the top of the piston.

Here are the two areas that can be angled out closer to the edge of the cylinder bore which also unshrouds the valves.
Click on photos to enlarge.



You will also notice the ridge around each valve where the valve seats were installed.
The entire chamber surface can ground or polished until those ridges are gone and the surface is smooth.
As a reference here is a photo of a finished Jeep Six head chamber which is similar in shape.
If you duplicate this chamber finish you will be good to go.



If the Static compression ratio gets close to 10:1 then the Howards 275* cam installed 4 degrees advanced will yield a dynamic compression ratio just over 8:1 which will work fine on pump gas.
 
One step at a time.

CC the head and the pistons to determine the volume of each.
If necessary, the head volume can be increased by unshrouding the valves and smoothing the chamber.
There should be enough meat in the piston to increase the diameter of the recess to gain a few more cc's.

Definitely check the head and piston volumes first to determine what path to take.

Good luck.
 
Thanks a lot for the tips. I'll run the head to the machine shop and ask them about unshrouding the valves.

I will update on progress.

Here's the crank going in it

[image]https://scontent.fsnc1-1.fna.fbcdn.net/t31.0-8/13198553_10206574692826028_5838590554821907356_o.jpg[/image]

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[image]https://scontent.fsnc1-1.fna.fbcdn.net/t31.0-8/13161881_10206574692106010_2262354044124770036_o.jpg[/image]
 
If they just took a light pass on the deck I'd be surprised if you're much less than .050" in the hole. I had .035" taken off my deck, 1.556" compression height pistons and ended up @ .020"
 
BCOWANWHEELS":2anla3xc said:
who made your pistons for 500.00 ? all I,ve priced is 100.00 per piston / pin.
thanks
bob

AutoTec makes a custom forged 4032 alloy piston which is less expensive than the 2618 alloy piston.
The 4032 alloy requires less piston to wall clearance than the 2618 piston.
The spiral machine work in the piston dish is a telltale sign of their work.
The piston also includes an accumulator groove in between the 1st and 2nd ring groove.
 
OP, if you are still having issues getting your compression ratio down, you can always spec the cam lobe separation angle at 108 degrees. this increases the valve overlap and reduces cylinder pressure at low speeds. this reduces the engines dynamic compression ratio at low speeds helping to prevent detonation.
 
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