Is the assembly together and balanced already? If not you should be able to have a machine shop mill the dish in the pistons a bit deeper and maybe a bit wider. From what I have learned with high compression and pumpgas quench should be no more than .045" so if you used a standard .041" thick gasket your pistons should be no more than .004" in the hole. A lower compression engine with too much quench distance will be likely to detonate than one with higher compression and a tight quench. Another thing that will help with higher compression and pumpgas tolerance is to deburr and polish all sharp edges and casting roughness in combustion chamber. If the shortblock is not assembled take the steps I suggest and you should be fine.

 

The engine is already assembled and in the truck. However, I like your solution to lowering my compression. That sounds like a better alternative to buying new pistons with a deeper dish. If I open up the piston that will lower my compression, but then if I use a thinner head gasket to get the "quench" in the range I need it, it's going to raise my compression back up....I guess I could plug the numbers into the formula and see where it will end up. But then like you said, even with higher compression and a tighter quench, it would be less likely to detonate...

Thanks for the advice....

 

All quench does is create turbulance in the combustion chamber, the more turbulance the more tolerant the engine is to detonation. [Google: Sir Harry Recardo] Quench areas can be effective up to about .060" clearance and your setup sounds like you are at that limit. There are other things you can do to prevent detonation, use a timing curve more suited to your combination, limiting total timing to 34-36 deg. limiting the vacume advance if used. Carb adjustment: use a richer mixture to better control cyl. temps. and keep in mind that a whole book could be written on how to set up a carb for particular combinations. Control coolant temp, run the engine slightly on the cool side, say 165-170F and not 195-200F Maintain cool air temp into the carb. by using ducting to the air filter inlet. I have 2 390 FE powered vehicles, one a 66 Fairlane that has app. 9.5 to 1 c/r, zero deck, w/shallow dish pistons, 72cc chambers and it runs fine on 87 octane Calif. gas and the other is a 390 powered F 250 with flat top pistons, zero deck, 72 cc chamber heads and a c/r about 10.3 to 1 and it runs good on 87 octane unless I tow a trailer, then I use 89 octane.

 

Thanks for your reply. Your explanation helped put my mind at ease with regards to detonation....My distributor will be set at 30* max advance due to running SCJ-A aluminum heads (per advice from another guy on here who has experience with these heads), so that should help control the detonation. I'm gonna use a 180* high flow thermostat and I have a MSD timing control unit mounted in the cab so I can control timing if it starts pinging.

So if you're running 10.3 compression with 87 octane gas and don't have issues, then I should be ok with my 9.67 compression (I'm hoping).

So is it better to have less compression and more quench or higher compression and less quench?

 

You can have a motor that's pinging and zero deck the block which raises the compression even more and have the pinging go away.
The quench area is where the flat part of the piston extends past the combustion chamber so there's not all that much area involved. I think around .040" is about what to shoot for.

 

Thanks for that explanation. That helps alot to clarify things...

Ok, your comment about raising compression confuses me....Correct me if I'm wrong, but having too high of compression is one thing that contributes to detonation. So how can raising the compression even higher eliminate detonation??? This makes no sense to me.

You mentioned zero decking the block.....does someone make pistons that come up to zero deck?

So if .040" quench is what I want to shoot for, this leads me to this question....would going for more quench than .040" be even better??? I know more is not always better, but that depends on what you're dealing with. Is there such a thing as "too much" quench to where it doesn't help at all??? Is .040" quench the "sweet spot"???

 

Just an fyi: I've run a 472" with a flat top piston that ended up with .020" piston to head clearance and had more quench area than stock because of having the heads decked .050", on pump gas, with almost 50* of total timing. Of course I detonated the cast pistons after about 15 minutes of running with a load at a block temp of about 150*, but my point is that it wouldn't have lasted as long as it did if not for the amount of quench that was in the cylinder. Had I run this almost 17/1 compression motor on 118 octane leaded racing fuel I truly believe that it would have lasted a good long time. Point is that quench is quite important and compression is the cheapest (initially) and easiest way to make horse power. If you're looking to make up wards of 550 horsepower then you've got two options: compression and the correct fuel or large cubic inches.
And as far as dialing back the timing: when you do the exhaust temp goes up dramically.....and that has its own set of problems like valves not living, exhaust system woes etc.
Rob

 

Just an fyi: I've run a 472" with a flat top piston that ended up with .020" piston to head clearance Rob

Man bet that kept the carbon off the pistons LOL

 

There is alot of reading available on here discussing quench and high compression on pumpgas. I asked alot about the subject before building my 523 and the things I mentioned are some of the things I learned from experienced engine builders and some things that I knew. Who designed this engine combo? Is it a package you put together? Have you figured out your dynamic compression with the cam you are running? A slightly oversized cam may help to lower your dynamic compression and be more tolerable to low octane. I wouldn't say your way out in left field for a pumpgas combo I ran a 9.67:1 466 on 93 octane for 8 years with ALOT of timing both intial and total with cast iron heads and did not have much issue with detonation. Based on my experiences retarding the timing is not a wise choice as mentioned it causes ALOTof heat. Cam phasing can also help lower compression. It's already together and in the truck so stop second guessing yourself and go test it and if needed come back and change things, just keep an eye on it. Check the plugs often.

 

It's already together and in the truck so stop second guessing yourself and go test it and if needed come back and change things, just keep an eye on it. Check the plugs often.

X2

BTW Eric his elevation from one of his past posts was 1,200 feet which should help some.

 

Oh I forgot to ask, what is the elevation you plan on running this combo at. It can make a huge difference!

 

I'm at 1200' elevation

 

RJP & 68XR7CAT....

Great info, thanks. So with that said, what max timing do you think I should run? Do you think I should bump the timing up to 32*??? or 34*????

 

With SCJ heads? NO

Max power is found with 28 to 30 degrees total timing.

Your intake is done Lance. You can check out the results by going to our FB page link in my signature line.

Since you are local give me a call. The information I posted in your original thread about running your combo as is with some minor changes is still very relevant.

As others here have stated tight quench is essential for detonation resistance even if it raises static c/r.

For every .010" below deck your pistons are you give up about 10 pound feet of torque in addition to lowering detonation resistance for a given static c/r... In your case with a moderate rpm combo I would run quench distance at .035". Scj castings have VERY efficient combustion chambers and are highly detonation resistant.

I have had stock 460's with 7.7 to 1 static c/r with .065" or more quench distance that while towing pinged more than an 8.5 to 1 combo with .039" quench. The devil is in the details...

 

78 150, You have one of the most knowledgeable guys helping you and is local to you, Use his recommendation on what works. Don't take bits and pieces and tid-bits of info from many different people and think you are going to make your particular combination work. If Scotty says this will work good for you you'd be foolish not to take his advice. If you would have said he [Scotty] is helping you and walking you thru this we could have all saved alot of time and bandwidth.

 

In all fairness RJP he just very recently sought my help...

The juice roller you have jason though less than ideal is not a deal breaker for the combo imho. Solid roller lifters would keep the valve train and guides happier. I am going this route on my LTD. I honestly think your combo can work with some minor changes.


S

 

.047 is a hell of alot better than .065! From talking to some of the EMC participants .045 is about max but I don't think .002 is gonna hurt you at 10:1. I was building a 11:1 racing engine on 93 octane. Mine ran great but quite a bit larger cam than you. Have you calculated DCR yet? IIRC my dynamic came out at 9.7:1 which is on the high side but I am at 800' elevation your 1200' elevation should help be a little more tolerant.

 

What is dynamic compression ratio??? The calculations I have run give me 9.62 cr (I thought it was 9.67, but I ran the numbers again last night and found an error). Here's the formula I used to figure my compression ratio. The figures you see below are the numbers for my engine...


COMPRESSION RATIO =

• Cylinder Volume + Clearance Volume + Piston Compression Volume (dish cc / dish piston = positive #, dome piston = negative # / need to convert cc to cubic inches) + Gasket Volume + Chamber Volume divided by Clearance Volume + Piston Volume + Gasket Volume + Chamber Volume
• 58.329 + .227 + 1.343 + .802 + 4.394 / .227 + 1.343 + .802 + 4.394
• 65.095 / 6.766
• 9.62 Compression Ratio
--------------------------------------------------------------------------
FORD 460 Compression Ratio

Bore = 4.392 / Stroke = 3.850 (.030 bore)

Cylinder Swept Volume
• Formula = 0.7853982 x bore2 (squared) x stroke
• 0.7853982 x 4.392 (squared) x 3.850
• 0.7853982 x 19.29 x 3.850
• 58.329
Note: Total Engine Displacement (cid) = Cylinder Swept Volume x # of Cylinders
• 58.329 x 8 = 466.63 cid

Clearance Volume
• Formula = 0.7853982 x bore2 (squared) x deck height (area between deck & top of piston)
• 0.7853982 x 4.392 (squared) x .015
• 0.7853982 x 19.290 x .015
• .227

Piston Compression Volume (Piston Dome---converting cubic centimeters to cubic inches)
• Formula = piston cc’s x 0.0610237
• 22 x 0.0610237
• 1.343

Head Gasket Volume (compressed thickness of the head gasket---usually .100 larger)
• Formula = 0.7853982 x gasket bore2 (squared) x compressed gasket thickness
• 0.7853982 x 4.520 (squared) x .0595
• 0.7853982 x 20.4304 x .050
• .802

Chamber Volume (converting head cubic centimeters to cubic inches)
• Formula = head cc’s x 0.0610237
• 72 x 0.0610237
• 4.394

 

Cometic makes a .027" head gasket that does not require o-rings. Your CR would be 10.14:1...

DCR is supposed to be the effective compression ratio taking into account the valve timing events. Problem is properly determining the correct intake closing point.

 

I just logged onto the Cometic website looking for the head gasket you mentioned and they only listed 3 head gaskets for the 460; all at .040 thickness with different bores. Where did you find the gaskets that are .027" thickness? I didn't see it on their website.

 

http://www.shopengineparts.com/productdetails.aspx?emid=21&cid=40&pid=2705&type=engine

scroll down to the bottom and you can select the bore size and thickness

 

Thanks for that info. I'm gonna order some gaskets and then pull my heads off and have the valves back cut, per Scotty's advice, and throw it back together.

Speaking of which......Scotty.....I talked to the machine shop today and he wants to know if I want just the intake valve back cut or both intake and exhaust. I had no idea and said I'd ask you. So do I have both of them back cut? And how much? 27* or 30*? I think you told me the first time to have them 30* back cut, but you most recently told me 27*.