Hi,
I know there is a lot on discussion on this topic, and I am quit lost now!
I just start the assembly of my 390 engine (stock crank and rods) and when installing the pistons, it looks like they sit a bit recessed at TDC!

It looks like I have a deck clearance of 0.135 which bring the CR around 8:36 if I did the math correctly! I know I will be able to use premium gas, but I thought I could still use cheap gas with a CR around 9.5:1?

The shop told me to use the Sealed Power 381NP piston over the Keith Black 1131-H030 I proposed (maybe because he already have the S.Power in stock??).

The Sealed Power 381NP30 have a compression distance of 1.660 so deck clearance is 0.135.
The KB 1131-H030 piston have a compression distance of 1.760 and deck clearance will be 0.035.

So here is the question:
Will it make a significative difference changing the Sealed Power for the KB since It will be easy to do the swap now or having a CR of 8.36:1 is good enough?



- Heads have 2.090 Intake Valves, 1.650 Exhaust Valves with a volume of 72cc.
- Lunati hydraulic roller Voodoo 20330710, lobe separation at 112°, ICL:106° lift:0.559 for both IN. and EX. with 211for IN. and 219 for EX.
- Performer RPM intake.
- 3 speed manual.
- 3.25 gear with stock size tire on a F100 from 1974
- I will use the cheap headers from Magnaflow and I didn't make my mind yet for the carb.
- Deck height 10.165
- I couldn't find the volume of the dish piston from Sealed Power???

 

I would run the KB's and deck the block .035 with an .040 gasket so long as I could still run pump gas.

.135 in the hole is way too much...will be a detonation pig.

you want piston to be flush with deck

 

Thanks Somethingclever,

I wasn't sure at first, even if I was a little sceptical.
But I contacted the machine shop who did the work on the block and sold me the pistons and they say they don't understand why this set of piston have a compression height of 1.660??? They are going to call Speed Pro.
I am just hoping they will swap the 381Np to the Speed Pro L2291F30, those have a dish with +10cc dish (Iwill have a CR of 9.56:1) and a compression heigh of 1.776 and the deck clearance will only be 0.019 so with a 0.040 gasket I will have 0.059 squish! yeah I just learn the squish ideally it should be betwen 0.030 and 0.045.


So will it be really bad to have a squish of 0.059??

 

As you mention, .040" is ideal squish or "quench". 9.5+ CR with .059 quench sounds like it might detonate or ping... a bit high even with premium fuel. 8.2 sounds low. Any way to get somewhere in the middle? Thicker head gasket and around 9.0 compression or "perfect" .040" quench and low 9's CR?

 

Thanks Blue and White,

I guess I won't be able to get the squesh at 0.040 unless i have some special order piston!
Can I get thinner gasket? Is the minimum thickness gasket is 0.040?
And if I understand what you are saying Blue and White I could keep the same squesh but I will need a lower CR, around 9:1 and I will avoid detonation?

Even if it is a situation I didn't expect, I am glad it happen because I am
learning so many interesting things right now! Thanks to you guys.

So a lower CR with a slightly bigger squesh will not produce detonation over a higher CR and smaller squesh???

 

If it's coming back apart for different pistons then deck the block the additional 0.019.

 

As a ballpark figure, I guess 9:1 without squish or 9.5:1 with ideal squish wold be OK. A "little too much" compression is no fun. You will be fighting pinging and retarding timing to try to stop it.

Cometic MLS (multi layer steel) gaskets can be ordered down to about .027". This is a very good gasket and requires a finer finish on the deck to seal vs a conventional gasket. Also a bit expensive.

Ideally, you would use a piston with compression height to dial in squish or deck the block to dial in squish... and dial in compression with correct size dish in the piston. Sometimes this is possible with off the shelf parts or it may require a custom piston. JE, Ross and others would be happy to make a forged piston to your individual specs. Last time I did this it was ~$700/set.

Its a choice... go with compression in the 8.5 to 9.0 range with shelf parts and open chamber (no squish) or spend the extra time, effort and probably money to get it dialed in for squish and higher compression. As you know, the higher compression squish engine can make a bit more power, burn a bit less fuel and handle a bit bigger cam without loosing low end torque.

 

Thanks Blue and White

My deck clearance was calculated with a rod length of 6.48, so I decided to use the filer gauge to get an accurate result. It appears that the clearance is only 0.130 with the 381NP piston, which mean my rods are 6.485 I will assume.

I did some research and I have the manufacturer replied to my email.
I have now the weight for the Sealed Power:
- The L2291F30, the piston is 650 grammes +/- 10gr. and the pin 151gr +/- 2gr. They weight a little bit more than the 381Np (784gr. piston+pin) so it could be an easy match.
And the compression height is 1.776 which will bring the deck clearance to 0.014''.

I went to see the machine shop yesterday and the guy told me he could match the weight so I will not have to balance everything again. I ask him if I should mill the deck of the block to reduce squish and he told me it will not be necessary. But when I ask if I will be able to use premium gas he told me I should use some high octane gas mix with "marvel mystery oil"???

I disassembled all the pistons and rods, and if I have to reduce the deck height I will have to take the damper, timing cover, oil pump, fuel pump out, which is not that big of a deal! But should I really do it or it will be fine?
It is hard to understand when I have to use high octane gas or when pinging can occur!!

I still really want to use cheap pump gas so do you guys think I will be ok?
If I use those piston I will have 0.014'' deck clearance and with a 0.040'' compressed gasket I will have a squish of 0.054'' and the CR will be 9.66:1.


By the way the first shop who sold me the piston is willing to get the 381NP back and reimburse me.

Thanks

 

Still trying to figure it out.
My other solution will be to use the KB150-030 it will bring my deck clearance to 0.01''
CR will be 9.09:1. They will be lighter in weight so I will have to redo the balancing.
I could avoid milling the deck by choosing those 0.027 gasket and have a squish of 0.037''.

More I am looking at those number and more I like it.

What do you guys would choose?