CR issues / engine buildup thoughts / GEN3 intro

[80Stang]

Yes, I read this too: http://fordsix.com/forum/viewtopic.php? ... highlight= which actually triggered to post this for kind of a introductory to the next engine buildup, the GEN3, which has a lot of thoughts on CR.

As always, an engine build up needs a lot of thinking beforehand. Trying to be short, I stumbled with forced induction or N/A, 200 or 250 and such things that dictated for the decisions to come. I was pretty much going to 200/Eaton setup but things went another way...

Got one 250 pretty cheap. Desicion one.
Thought I'll learn more with N/A. Desicion two. No power adders here, I'll leave that for GEN4.

250 N/A it is.

My engine buildup thinking values the proper squench height high in the scale, so I start with getting it right. With a 250 it is a little tricky, though. In one of David's posts there is a clear indication that just decking the block may be impossible, due to huge amount to be decked which makes the deck thin or TOO thin. So need to get the pistons up in the cylinders. Two ways that I can think of (besides of studying offset grinding cranks or such more radical stuff):

- 2.5L HSC 4cyl con rods; gains .110 but hard to get a 6 pcs set of those, needs stock height dished or flattop pistons (2.3L HSC) plus some reasonable milling on the deck. I don't think using flattops would be a good idea as it'll pump up the CR even further.

- 255cid Vee8 pistons; gains .089. Readily available. The rest can be done with milling. Major disadvantage is that 255s are almost flattops; they have 1.5cc valve reliefs for Vee8 valves.

While it is N/A, you better get it going hard. Most probably no daily driver stuff then. Max practical CR, max practical cam, max breathing capabilities...Desicion 3: 255 Vee8 pistons.

Desicion 4: Hot cam. Got out with a single pattern stick with 224 @.050, 109 LSA, .442 with stock rocker ratio. Being a 250 low rpm block I expect to ever use this no more than 6000rpms although the cam allows up to 6500, so I'll study how I'll degree this to move the powerband a little lower.

The block will be bored to +.060. With a Victor Rentz gasket (.045 compr. height) and with 65cc chambers the CR shows 10.39.

Hmmmm, suggestions? Pretty hard to get the CR lower if I want to maintain zero deck. Well, having the pistons dished maybe, 5cc dishes would lower CR to 10, 7cc to 9.75......5cc suddenly sounds good.

I'm not into water injection but if that is mandatory then I'll think again. We have pretty high quality Premium gas here. Vee8 guys run 10.5 CRs with no problems, so I'd assume a little six will survive pretty well if the head and other parameters are up to it. That makes looks to FSPP's new alum head.

Nothing has been done yet but the parts being gathered (C9 block in +.040, full rebuild kit, 255 pistons), so the plan can be changed. I thought of using this brain-grid-technology i.e. fellow FSP member's brains to shape these thoughts up and get the best out of it, so here we are. Thanks for reading!

 

[80Stang]

Did some research and used some "brainware" of an experienced friend of mine.

With Premium gas here up to some 11.0 CR can be run on aluminum head with good quality chambers and properly polished everything in there.

My CR goal is too low. I can go for the ideal squench height 0.030 by milling the block 0.015 more to get the pistons 0.015 up from the cylinders == CR 10.74. Well within "limits".

Current calculations rely on 65cc chambers, but we don't know the size yet.

 

[Funky Cricket]

high enough for premium, but depending on your advance, you could even get away with "mid-grade" or if you can find some ethonal or "E-85", not sure how they do it over there... that would get your octane you would need, just need to rejet for the extra needed fuel.

 

[CZLN6]

Howdy Back '80Stang:

Sorry to be so late in replying, but I had to make a quick trip home to North Dakota for family.

1st, some questions. What was the deck height on disassembling the 250 block? This will give you a ball-park on planning for machining. Be sure to remeasure on mock assembly with the new pistons in place. Most piston manufacturers build replacement pistons .005" to .010" shorter to compensate for machining in the rebuilding process.

Using the 255 V8 pistons will close the quench gap by .085" due to their taller pin height. Decking the top of the block to goal is fairly safe to about .075" to .080". Anymore then that and sonic testing is a good bet.

You will be pushing the limits with a 10.75:1 CR with a cast iron head. Yes, the cam will help, by bleeding off some cylinder pressure. Give yourself every advantage by polishing all of the combustion chamber surfaces and by keeping A/F ratios on the rich side. Given your elevation, I'd be more inclined to shoot for a CR goal of 10:1. I'd also forgo the thought of positive deck height.

I'd strongly encourage you to reconsider milling a "D" shaped dish into the top of the 255 pistons that mirrors the profile of the combustion chamber in the head. There are several- this will give the best quench-to-bore ratio and take advantage of your efforts to achieve a zero deck height. It is also a theoritical fact that dished pistons reduce the tendency of spontanious combustion and that, all else being equal, an engine with a dished piston will produce more power than with a flat-topped or domed piston.

The only downside is finding a machine shop with CNC milling capabilities and the added cost.

Ironically, my brother Dennis is doing the "brain games" on his next engine- a 250. He will be searching for the .100" longer HSC 2.5 rods. If he finds a reasonable source, we'll post it for all to see.

Adios, David

 

[Anonymous]

brainiacs!

too much math...

 

[80Stang]

David, that's ok. I really appreciate your moments here with me.

I have not yet measured the block, although it is still assembled. I don't actually find it worth the time to measure it now with old pistons. First I'll have the bottom end done at the shop but not decked; then I'll do the mock assembly with 255 pistons and measure the deck height. Then I'll roundtrip it another time to the shop to be decked the right amount.

Ford Racing engine specs catalog lists a 250 pin to top height 1.511, and 255 1.600, thus the 255 piston closes the gap by .089. If the std deck height is around .130-.150 it is needed to mill .041-.061 to achieve zero deck, .015 more to achieve .030 squench height (optimal safe?). I'd like to maximize the squench effect, that's why I would not stop at .045 at zero deck. Well, if there would be a .030 gasket available, then of course I'd go that way.

I'm not going with a cast iron head. It'll be Mike's new alum head. Said that, but I'll retard if he puts a price on it way higher that predicted. That's why we raised the max reasonable goal to 11:1.

Milling the pistons is something that has been considered. There are shops that'll do it but I think it will cost several hundreds to do it. What would you say; is the gain worth the money? For example 3cc milling will loose 0.4 in CR, and 3cc dish is actually pretty small.

I'd be interested in 2.5HSC rods. Then the pistons need be stock and the milling on the block would be less. I guess the ultimate fit would be 2.5HSC rods, 2.3HSC flattop pistons D-milled - for strength with stock parts (rods & pistons), for ease of fitting, for least milling of block, and for power.

Thanks David and others. Let's plan this good to make it run goooood.

 

[rbohm]

8) 80stang, if you have a dish milled into the piston tops, it will be more like 20-30cc's rather than 3cc's. the best piston design to avoid detonation is a perfectly flat piston top, but you have to balance that with compression ratio as well. the next best piston design for detonation resistance is the "reverse dome" design. basically you do as david said and have the piston tops milled with a dish that mirrors as closely as possible the combustion chamber shape. either way what you are looking for is a smooth flame front that doesnt boost cylinder pressure at the far end too much, that way you avoid detonation. done right you can actually use 87-89 octane with better than 10:1 compression. one more thing, the longer rods will also help avoid detonation, so search up a set.

as far as cam selection goes, wait on that untill you have decided just what compression ratio you are actually going to run, then select a cam with the right amount of duration and overlap to bring dynamic cylinder pressures either up or down as needed to prevent detonation. again the right cam will let you run a lower octane fuel, just dont over, or undercam the engine.

 

[80Stang]

I haven't measured but how thick are the piston tops anyway? How big a dish could be milled still preserving enough strength in it?

Just an example calculation: 20cc dishes in pistons would need chambers 52cc to get to 10:1. If you could mill 20cc dishes on the pistons we still don't know if the new alum head will support milling so much that it could have 50-52cc chambers. Probably it has.

 

[rbohm]

8) piston tops are thick enough that you can mill .020-.030 off the top and not affect strength.

 

[80Stang]

Ok.

3.74" (+.060 oversize) pistons top area is 70.84cc2.

If a D-shape dish would be milled, I assume it would cover about 50% of the area. 35cc2.

0.030" deep dish for a 35cc2 area would give 2.667cc.

So I went to garage and measured the .040 oversized dished pistons that are in the 250 block at the moment. Round dish right in the middle is about 2 3/16 wide (average, the edges are beveled, gives 24,25cc2 area) and it is about 0.11" deep thus probably being a 6,5cc dish piston. If that depth could be milled to a 255 piston (35cc2 area) that would give 9.4cc dishes, which would ram CR down to iron head bearable 9.66:1.

This may be totally fooled, I was not too good in mathmatics.

 

[80Stang]

Max practical CR, max practical cam, max breathing capabilities...

This CR thing still awaits for final decisions. In addition to determining the target purpose, this thing should be driveable as a normal hobby car; a few thousand miles from April to Oct, be a grocery getter, maybe some highway duty to work and back, some track and strip action as well. Show finish, and with max practical performance.

All the mentioned parts are here except the new head. The block is machined and new cam brgs changed in, but it is not decked yet until I know how much it needs decking, which can be measured after the new pistons are mated with conrods and mock assembled to the block, which can be done after the new head is here and the dishes are machined to the pistons to mirror the chambers in the head...nice domino-effect here.

3.74 bore, zero deck, 3.81 gasket 0.045 thick, 58cc chambers, 6cc dish
== 10.72CR

If the new head doesn't quite get 58cc and is 56cc for example
== 10.99CR

58cc chambers, 8cc dish == 10.46CR.

How will my cam cope with this?
My assumption is that the chosen cam already is too low lift with stock rockers to utilize the head to the full.

For this buildup I can't backup anymore to STD size Chevy 305 pistons (to avoild need to dish the pistons but it still would run high CR with their 5cc dishes), the AMC big 21cc dish pistons would be doable if the block was bored again. Cams can always be swapped, and the other block below the table brought into light...I'm flexible.

One piece of info: a custom cam cost 40% of the high lift rockers. I'm not necessary thinking of going to rollers yet.

Sorry long posts just seem to keep on coming. Bear with me, and thanks all for your help! Any thoughts appreciated.

 

[80Stang]

Wow, time flies. Isn't a 8-year update appropriate at this point?

Since my last post to this thread I managed to get an alum head from Mike, install it into the 1980 notch 200cid car I HAD, and you can read about that saga here at the forum and also at my oldish website.

Placed the 1980 for sale, without the engine, in purpose to keep the alum head for myself for these further "studies". First person who came to see the car wanted to buy it as whole, so bye bye Classic Inlines alum head. Sad.

The new owner of the car swapped the engine with a buddy with 1965 HT Mustang. So the alum headed engine went to the '65, and its stock engine went to the '80 in return. This '65 guy has been around here at the forum.

Earlier this year the '65 guy swapped the powerful L6 to a lazy V8, and offered me to buy the L6 back. Guess if I answered YES?

OK, my faithful L6 sits back in my garage as a whole engine. For my future purposes I only need the upper half of it, the rest is for sale locally.

What comes to the GEN3 engine buildup, it doesn't need much (well....)...

I've got the 250 block bored and honed, cam bearings installed, cleaned, and it has a coat of primer on it
The 250 crank is done to -010, ready
The rods are ready with ARP bolts
The new 255cid pistons are milled down to achieve 11.0 CR once the block is decked to zero, without touching the 58cc chambers of the alum head
I've got two cam choices on the shelf, a 224 hydraulic, and a 228 solid. Will choose the latter
new gaskets, oil pump, water pump, t-stat housing etc etc small things are here
Vintage Classic Inlines thermal coated exhaust headers; I've got the single outlet as well as the dual outlet models. Probably will dyno both once the engine is being tortured, just out of curiosity how the compare

Needs:
mock-up to determine how much the deck needs be milled & then of course it has to be done at a machine shop
balancing
solid lifters
balancer (in plans of adapting a 302 SBF balancer there)
pushrods
inspiration
time
a car to put it to (not mandatory)

Now as E85 has become readily available, I regret the piston milling to lower the CR from 12 to 11. I might still run this with E85, at least it doesn't keep running after ign has been turned off...

Let's see if the next update will take place in 2022, on Friday maybe.

To have something to do in year 2030 and after, I have another 250 block, and 300cid L6 rods. I'll get something together with those then.

Until the next time,

Teppo

 

[JackFish]

Hey nice to see you back here with an update.

 

[80Stang]

Surprise...almost yearly update!

The "needs" list:

mock-up to determine how much the deck needs be milled & then of course it has to be done at a machine shop >>>>DONE! About 0.033" milled, yes MILLED
balancing >>>> DONE!
solid lifters >>>>> YES, I've got them!
balancer (in plans of adapting a 302 SBF balancer there) >>>>YES, I have it
pushrods >>>>>> Yet to be measured
inspiration >>>>>> we have some evidence of its presence
time >>>>>>> a resource that seems to pass me by
a car to put it to (not mandatory) >>>>>> no sight of such

As a bonus:

The CI aluminum head was inspected, pro ported and milled. It was a banana and all the valves were leaking, but now it is top-notch.
New valve springs, seals, Titanium retainers (bling bling), locks...
New spec for camshaft. Those two mentioned earlier have been shelved, and they will remain there. Now I've got a Schneider Racing Cams 230/230 .480/.480 mechanical cam, but maybe I should have taken 10 more degrees anyways...let's see if the cam spec changes once more.

Now something should happen in the garage to assemble the engine. Only the pushrods are to be purhased anymore, and the vintage CI dual out headers modified by a guru.

 

[bubba22349]

congrats on your progress, it does sound like your 250 has a very good combo of parts. I will be looking forward to hear how well it does! Good luck on finishing up the rest of the details.