Does anyone know the highest compression that can be ran with open chamber heads with pop-up pistons on pump gas(93 oct.)?
I was advised because I'll be running efi, 10:5 would be safe.The computer would be able to advance and make adjustments that a carb engine can't. Carburated, they said 9:1 tops and anymore it would probably ping.

 

10.5 seems too high to me even with a fully computerized EFI system and ECM management. Maybewith closed chamber heads and flat top pistons set to the correct quench distance it would be feasible but on open chamber heads I don't think you could go that high. Assuming you had a closed loop EFI system with all of the sensors controlled by an ECM it still would only be able to adjust the timing within certain limits. Even the most modern computer controlled, fuel injected, OBDII cars can ping even with all of the sensors functioning correctly and the timing set. Drive a 2001 FFV Taurus sometime.

 

When I had my 400 replaced with a 400 longblock I asked for 9.5 pistons. The guy taking my order said that they didn't recomend anything higher than 9:1 because the 400 pings really easily. He also said that if I was using air conditioning that 8.4:1 (stock pistons) would be better. Since I _like_ A/C, I went stock...if I advance the timing just a little too far, it'll knock under load...so buyer beware.

 

Thanks....I guess I'll get some Aussie heads to be safe.I was just wanting to use what I already had.

 

I don't think you will accomplish anything except a lighter wallet.

 

I'll accomplish having a head with quench chambers, high compression, and a better flowing exhaust port when ported than a 4V head, and smaller intake ports for higher velocity for street driving.
Money is not issue when you're getting what you want.

"D"

 

Education is more important than money.

Like Bill mentioned about correct quench distance. A 400 has over .097 stock. Unless you reduce this considerably, the quench chamber has no benefit. 4V heads have much more potential. Up to 340cfm with a stuffed intake and 235cfm with a high port exhaust plate.


Excerpt from
http://www.speedomotive.com/Building%20Tips.htm

"Excessive cylinder pressure will encourage engine destroying detonation with no piston immune to its effects. The goal of performance engine builders should be to build their products with as much detonation resistance as possible. An important first step is to set the assembled quench distance to .035". The quench distance is the compressed thickness of the head gasket plus the deck height, (the distance your piston is down in the bore). If your piston height, (not dome height), is above the block deck, subtract the overage from the gasket thickness to get a true assembled quench distance. The quench area is the flat part of the piston that would contact a similar flat area on the cylinder head if you had .000" assembled quench height. In a running engine, the .035" quench decreases to a close collision between the piston and cylinder head. The shock wave from the close collision drives air at high velocity through the combustion chamber. This movement tends to cool hot spots, average the chamber temperature, reduce detonation and increase power. Take note, on the exhaust cycle, some cooling of the piston occurs due to the closeness to the water cooled head.
If you are building an engine with steel rods, tight bearings, tight pistons, modest RPM and automatic transmission, a .035" quench is the minimum practical to run without engine damage. The closer the piston comes to the cylinder head at operating speed, the more turbulence is generated. Turbulence is the main means of reducing detonation. Unfortunately, the operating quench height varies in an engine as RPM and temperature change. If aluminum rods, loose pistons, (they rock and hit the head), and over 6000 RPM operation is anticipated, a static clearance of .055" could be required. A running quench height in excess of .060" will forfeit the benefits of the quench head design and can cause severe detonation. The suggested .035" static quench height is recommended as a good usable dimension for stock rod engines up to 6500 RPM. Above 6500 RPM rod selection becomes important. Since it is the close collision between the piston and the cylinder head that reduces the prospect of detonation, never add a shim or head gasket to lower compression on a quench head engine. If you have 10:1 with a proper quench and then add an extra .040" gasket to give 9.5:1 and .080" quench, you will create more ping at 9.5:1 than you had at 10:1. The suitable way to lower the compression is to use a dish piston. Dish (reverse combustion chamber), pistons are designed for maximum quench, (sometimes called squish), area. Having part of the combustion chamber in the piston improves the shape of the chamber and flame travel. High performance motors will see some detonation, which leads to preignition. Detonation occurs at five to ten degrees after top-dead-center. Preignition occurs before top-dead-center. Detonation damages your engine with impact loads and excessive heat. The excessive heat part of detonation is what causes preignition. Overheated combustion chamber parts start acting as glow plugs. Preignition induces extremely rapid combustion and welding temperatures melt down is only seconds away!"

 

I'll make note of the above......

The Aussie head will flow what I need for less money. 200 cfm on exhaust without a plate. Just good porting.The 4v exhaust isn't all that good without a lot of help. If I found 4v's for the same price, I may still opt to get 'em.
I would use a flat top piston instead of the dome if I go small chambers.

 

Actually the 4V exhaust port needs very little help. Their only problem is the intake to exhaust flow ratio. The intake just flows too much.

Don't go by Powerheads numbers, they have been exaggerated to promote sales. Independent testing shows the 4V exhaust flows more at lower valve lift and almost the same at .500. The only substantial increase over the 4V was @ .600 lift.
http://home.isoa.net/%7Emharrisj/fordhead.html
Primary tubes on headers for the 2V ports are usually limited to 1 3/4" Sometimes that's not big enough. I only mention it because air flow testing is only done on the heads, but that's not where the port really ends. The port begins at the valve and ends at the bottom of the primary tube when it enters the collector.

The reverse is true on the intake. The port velocity and port volume is affected more by the intake manifold selected rather than the head. I'd rather partially fill/stuff a 4V port than enlarge the 2V.

Check the compression ratio with the flat tops before purchasing. The stock 400 flat top will have about a 4cc valve relief and sit .057 below deck. With a 302C 62cc chamber that works out to 10.7:1. The KB 351C piston has a 2cc valve relief and sits .037 below deck. Compression is about 11.4:1.

 

Thanks.....

 

What is the deck clearance on a stock 351M? I was told it was better than a 400 and I am planning on trying fuel injection and aussie heads on a 351M to make a high performance but hopefully better milleage motor than a 400. BTW I am also building an all out 400 so dont tell me to forget about the 351M and go for a 400 because I am doing that too. I also have another 400 crank but I am thinking about using that to make a stroker for a 351W to put in my tbird.

As for quench chambers and piston design I thought flat tops and domed pistons were better for the aussies? But the statement above says that "Dish (reverse combustion chamber), pistons are designed for maximum quench, (sometimes called squish), area. Having part of the combustion chamber in the piston improves the shape of the chamber and flame travel. " This dish piston they are talking about, would that be the same thing as whats in a stock 351M, so then the stock pistons would be "designed for maximum quench"?

Just trying to get my facts straight before I put hand to wrench.

 

The deck clearance is about the only thing a 351M has over the 400. The 351M .016"DC plus a .040 thick head gasket can benefit from a quench chamber. Aussie heads will increase compression 1.3 points or more on a 351M.

Don't save a 400 crank for a 351W stroker. You can buy a new aftermarket 351W stroker crank for less money than modifying the 400 crank. A 351W, 4.17" stroke, iron crank from Speed-o-motive is $264 delivered.

A reverse dome for quench chambers is best, a D shape port that matches the outline of the chamber. A stock dish is not ideal because they are circular, not D shape.

 

Thanks Brian S... back to engines 101 for a second... let me get this straight, a D shaped reverse dome is best for aussie heads. Is "reversed dome" a dish into the piston or a raised protrusion from the piston. And assuming for a second that I was going to use my 400 crank after all, is there a 400 piston or a 351C piston that is close to this D-shaped reverse dome? What do the stock 302C aussie pistons look like?

Thanks for the heads up about the 351W storker crank. I didnt realize they were that cheap... most stroker kits I have seen were over $1500.

 

http://www.kb-silvolite.com/page45.htm#SECT2
Look at the image for the KB148, the 13cc D cup is for a Cleveland closed chamber head. Aussie 302C or 351C 4VCC.
A dish is a "reverse dome", but often "dish" is used when describing a circular shaped area like most stock pistons and a "reverse dome" is used for an irregular shape.

A few companies make a Cleveland D shape cup, but I've never seen one for a 400. That's probably because 400s never came with close chamber heads. A Cleveland piston will work in a 400, but the pin is .912 vs .975 so a 400 rod has to be bushed for a Cleveland pin. Also, pay attention to compression height variations. The KB148 mentioned has a 1.67" CH which is .020 taller than the 400 @ 1.65CH. This is not a big difference but it will change the quench distance and compression ratio slightly.

A 302C piston is the same as the 351C. They use longer connecting rods in the 302C so the pistons interchange.

Keep in mind, the price I gave for the 351W crank is only a crank. Complete kits will have rods, pistons, rings and bearings and are usually balanced. There are still companies that modify 400 cranks for a 351W, but that involves cutting the snout .75", removing the timing gear spacer, cutting down the counterweights, and expensive rebalancing with mallory metal slugs.

 

I built a 9.7 to 1 400 using 351C forged speed pro flattops the chambers in the 400 heads are fully polished, milled .030, deck is milled .010 . This motor runs fine on 93 octane with 10-12 degrees initial timing but does ping occasionally. The piston pin bores were drilled out to the 400 diameter to fit the rods. These and I'm sure some others can be done the same as well. the "meat' under the pin is .013 less than a stock cast 400 piston after being opened up. This motor has been run daily for almost a year now and no problems.