Static vs Dynamic Compression Ratio Questions

[8ton]

How much dish do the replacement pistons have? I see in pictures that they have valve reliefs, but maybe no real dish. I recall my 360 pistons having the reliefs and a fairly large dish.

[DuckRyder]

Yep, best value with the cam is to buy the whole kit springs and all 10330700K. I believe Lunati is now owned by Comp Group (Crane also now has an Olive Branch MS address and Olive Branch is very close to Memphis TN (and along US78/I22) so it makes sense that they have gone to Comp’s “K Kit” concept.

There is always Crower... Street roller with HIPPO lifters for the win ...

Not having much luck finding the cc for the valve reliefs on the KB pistons 7-10cc is probably a good guess, but who knows for sure... I’m getting more in the range of 8.5:1 using some online calculators though. My 360 had steel shim head gaskets on it when I tore it apart (with ford on them) so you’re already going down with composition gaskets...

If you were to start with a 360/390 block it opens your piston choices up ...

[8ton]

My 360 had the steel shims from ford as well. I found these: http://scegaskets.com/store/ford/ford-f ... er-534252/
$22 ea, .020 thick. They need to be sprayed with copper coat and retorqued after first warm up.
I wouldn't think 9 or 9.5-1 static would ping on regular, especially with some of those larger cams and good quench (that you would almost have to have to get Cr up that high on all stock parts).
Many aftermarket pistons have a shorter compression height than OEM to allow the block to be decked and heads milled flat without the compression going up.

[DuckRyder]

I would try to actually measure the deck clearance and cc of heads and pistons - particularly if the engine appears to have been into before because there is just no telling what has been done.

Oh and actually starting with a 360/390 block (or boring yours to std 390) doesn’t help you much if you stick with the 3.5 stroke and 352/360 rods.

Will Diamond or Ross make a custom 352 piston?

[colnago]

The pistons don't have any dish. I _assume_ that the eyebrows are ~8cc. I'm sure the machinist didn't deck the block or heads, but I'll have to pull out the paperwork. I can't easily measure the piston compression height, because it's all buckled up and is now my daily driver. Looks like my options are:

1. Install shims instead of head gaskets to increase the compression, or

2. Build up another block as a 390.

Either way, get a new cam/springs/etc for the final solution.

Remember a couple of months ago, when I said this was my classroom on wheels? Well, I think that I learned a valuable lesson ...

Joseph

[colnago]

Crud! I just realized that in order to put in metal shims, I'll have to remove the exhaust manifolds (I don't have headers yet, and Mama will flip if she sees what this is already costing). Also, I'll need shorter push rods.

Time and money. Well, you can't take it with you, I guess.

Hey, does anyone have any experience with the metal shims? What to do, what not to do, anything?

Joseph

Whoops, no I won't! For some reason, I mentally placed the shim on the BOTTOM of the piston! But I will need shorter push rods.

Whoops, yes, I will! Oh, I need a drink.

[DuckRyder]

I was thinking you had the engine apart. Being that it is together probably making the best of what you have is the way to go. If you’re willing to pull the heads and do the thinner gaskets and change the cam you should end up with decent power. I wouldn’t think the pushrods would have to be replaced if they are stock length.

If you want me to run it on DynoSym send me all the details (including your current cam) on the engine via PM and I will build it in the program and see what it comes up with - just give me several days, it is crazy busy at work and will be a busy weekend.

[8ton]

I vote #2. Build an extra 390 or 410, in your spare time with your spare cash. You already rebuilt your 352 right? It isn't knocking and belching smoke, or unable to crest 45 mph? You have an extra 390 shot block, right?
You can spend a little at a time to make steady progress, and in a year or so have a 350+hp 390 that gives as good of mileage as a 352 does with less power, and sell the good 352 for $700-$1000.

What EXACTLY are you unhappy with about your current engine?
Mileage? What are you getting now and what do you WANT to be getting?

Power? You have stated that you don't race, don't rev over 4k and haven't talked about towing anything crazy. For light towing and around town keeping up with traffic, displacement is your friend.

[colnago]

Robert,

Interesting about the push rods. I would have thought that since the distance is closer, I would need shorter rods. Do the lifters compensate (to a limit)?

My current cam specs (intake/exhaust):

Lift: .485 / .511
0.050 Duration: 204 / 214
Advertised Duration: 282 / 292
LSA: 112
ICA: 73 (I don't remember the others)

8ton,

Realistically, the truck is my daily driver. I'm no longer 17, so I don't have a need for speed. I don't need to tow on a regular basis, but I don't want to be worried about towing. This is a truck, so I want it to be a truck. I may get a small camp trailer, or an in-bed camper.

As far as mileage, I get a bit over 9MPG in town. I'd like to see 10; I'd be ecstatic with 12. Not that I need it (my daily commute is under 20 miles, round-trip), but I think it could.

Performance-wise, she's soft. Yes, she can scoot, but not tire-screeching scoot. My vacuum is also a bit low (16" at an altitude of 2000', with a small vibration in the needle). When I did the rebuild, I bought a kit that looked like it would give me stock-ish performance. The only change was putting in an "RV" cam. More of that low-end grunt, right? Well, now that I'm smarter, I can see that there's nothing outstanding about this particular cam (see specs above). Because my deck clearance is bigger than stock, my SCR is lower than stock. Because of the cam specs, my DCR is really low.

I figure that I can do the cam/shim swap for $500 - $600, unless I go with the Lunati cam. That would probably add $300, so realistically I'll probably go with the Crane 901. I'll never make up $600 in fuel savings ... so will it make me $600 happier?

I still want to put Sanderson headers on, but that will probably be a year or two.

Thanks,

Joseph

[DuckRyder]

The Lunati cam and lifter kit is about the same price as the crane cam and lifter kit.

10330700LK https://www.summitracing.com/parts/lun-10330700lk

is about the same as a Crane cam and lifter kit

343902 https://www.summitracing.com/parts/crn-343902

I suspect when you start adding springs and retainers the complete lunati kit (Check the included parts)

10330700K https://www.summitracing.com/parts/lun-10330700k

Is going to start looking like a pretty good deal.

[Ranchero50]

I'd just do the thinner gaskets and build another engine for later. Low compression is going to effect any cam selection adversely and the more radical cam the worse it will perform.

[DuckRyder]

I feel like the difference in ICA between the VD250 and the existing cam would make a big difference in the dynamic compression (have you run it through a calculator)

Raising the DCR through the cam and thinner head gaskets (which raise the SCR and thus the DCR) would make a difference in the “softness” IMO.

If the pushrods are stock they should be for the thinner gaskets anyway... but yes I think the lifters would tolerate that kind of difference.

I’m not sure where 300.00 more for the Lunati cam is coming from...

[colnago]

Well, I figured out why the Lunati was appearing to be so expensive. I did a search for 10330700K (the whole kit: cam, lifters, springs, retainers, timing set), and was shown 10330700LK (cam and lifters), but with the price for K. So, it looked like cam and lifters were $480-ish. I figured that if the cam/lifters carried such a premium, so would the springs. That's where the $300 delta came from. Now that I know that I can't add, the Lunati is looking pretty good.

I've done some basic runs through a calculator on the net. It looks like if I go with the 0.020" head gasket/shim, I get an SCR of around 8.4:1. If I then install the Crane, I get a DCR of around 7:1. The Lunati gives me a slightly higher DCR. My current set-up is significantly lower, both in SCR and DCR.

Currently, I'm leaning towards the 0.020" shim, with the Lunati kit. After that, maybe I'll rebuild a 390, but do it smarter from scratch. I think that the worst part is that I have to remove so much non-engine stuff off the front end just to get to the cam.

Joseph

[71cc]

All of this seems like an exercise in diminishing returns. Have you considered just building another short block at your convenience. That way you start with desired compression, deck height, cam. Add the cubes of 390. I would recommend degree ing cam. When you are ready. Yank the other motor, swap heads and sheetmetal in a weekend. Hp per $ is going to be better

[Jacksdad]

I agree - biding your time and building the engine you want seems like the best option. You only put money into one motor and that one gets built the way you want, rather than doing a bandaid fix on the one you have. Plus you give yourself time to get the best deals on parts.