Hey guys, what is the max comp you can run on premium with aussie heads? I keep having a problem of getting something like 10.2:1 running the calcs and that just seems way high for me. Is that gonna cause me problems? I heard somewhere that because they have closed chamber and the quench effect, you can run much higher c/r's. Is that true? Also, does anyone know what the piston volume of a stock 400 piston is? Is it like 22cc? Thanx for all the help.

 

Sorry I think this needs to be added. The problem I am having is that I don't want pinging from too high of compression and I think that's what's gonna happen. The other thing is that I have heard that if you don't run flattops with the aussie heads, you are basically wasting their special closed chamber design. True? If it is, this really sux for me, considering 22cc pistons are needed to stay down at 9.5:1 compression. 13cc cups will give you 10.2:1 and flattops with 2cc valve reliefs give you 11.55:1 compression. I want as high of compression on the street as you can run without pinging or having to use any octane boost. The reason I want the aussie's so bad is I've heard that you can get a lot more performance out of them and you don't have to do near as much porting as you have to on the stock 400 heads. But if I'm not going to get these performance benefits from the aussie's, would I be better off just porting the stock ones and running higher c/r pistons? I was thinking 9.5:1 compression to maintain streetability but if the special design of the aussie's allows more compression, then I would definitely go 10:1. Help me out guys.

 

I'm running Keith Black pistons with Aussie heads. Assuming 62cc and decking the block I came to 9.8:1 compression. I run 91 octain and live in Denver where the altitude is 6,000 above sea level (a good thing for not pinging). I still get a little pinging on very hot days. I plan on installing an air scoop to get the air cooled off. Hope this helps.

 

[updated:LAST EDITED ON 30-Jul-02 AT 06:01 PM (EST)]yes - you DO need flat-tops for the quench effect to work. not only that, but you need to shoot for zero deck clearance. i think you can actually go about .010 either way. anything more that .010 above deck, and the piston might hit the head at high rpm. if it's more than .010 below deck, then the quench effect becomes less effective. at .02 below deck there's no quench and it WILL detonate. so zero deck is considered optimal. this is with .040 head gasket. since you're doing a low-rpm build, you can go above deck with less chance of collision, to get maximum quench.

getting the desired deck clearance depends on available pistons, but you can customize it by either getting the block decked or pistons milled. if you need a dished piston to reduce compression, there are "reverse dome" pistons, which are basically dished, but the dish is the same shape as the chamber, so they have a flat part that matches up with the flat part of the head around the chamber. i think there are some keith black's like that. they seem to have a lot of variety - different tops and compression heights. they look pretty good to me for N/A motors, just make sure to read about the special top-ring gap!

as far as allowable compression - i've heard you can run 11:1 and even higher on premium pump gas, with proper quench setup and some other things (deburr and polish the piston tops. same thing for the chambers, if it's possible to polish iron...). but a lot of it depends on the cam. with a low-rpm type cam, you can't run that much compression. if i had to make up a number, i'd guess your 10.2 would be safe, but going higher would start to push it. 10.5 might require a really lame tune. again, this is with the type of cam that you'd want for low-rpm torque - and ALL of this is just based on what i've been reading... i've been looking into this a lot lately!

 

Here's the deal. Aussie heads with stock 400 pistons give me the 9.5:1 I want. In light of this, would you go with stock 400 pistons and aussie heads or stay with stock 400 heads and use 351C pistons (w/ bushed rods). Either way I am looking at 9.5:1 and the cost is reasonable to me. Bushing the rods and new KB's will be about $75-100 less than buying a pair of aussie's (assuming a cost of $300). I know the aussie's perform extremely well with flat tops but are they worth the trouble (and cost) if I use dished pistons? In essence, the question I'm asking is, are dished pistons on ported aussie heads a better setup than flat tops on ported stock 400 heads? There's one other alternative. There are some other KB's with a 13cc dish that will give me higher compression (w/ aussie's). Would I still just be wasting my time with those pistons since they are still dished, even though the dish is smaller? With Aussie's and those 13cc dished pistons, I am looking at 9.967:1 with no machining, and 10.279:1 with .015 taken off for clean up. Is that much compression even worth all that extra trouble since I am looking for neck-snapping low end? To summarize, I currently have 3 options:

1.) Keep my stock heads and bush the rods for 351C pistons for 9.5:1 c/r
2.) Replace my 400 pistons with stock replacements and use aussie heads for 9.5:1 c/r OR
3.) Get aussie's and use 13cc dished KB's for 10-10.2:1 c/r

Which setup do you think I should go with? Keep in mind that lots of low end is what I am shooting for? In case you are wondering why I don't just use flat tops with the aussie's, it's because that setup gives me 11.165:1, assuming that the heads and block don't require any cleanup. Seems awfully high to run on pump premium, wouldn't you agree? Thanx for all the help so far, you guys have been great.

 

Geeze... my brother runs 10.3:1 compression (stock 1970 4v 351W) on 95 RON petrol with no problems at all. Seems like you are worrying more than you ought to.

 

[updated:LAST EDITED ON 01-Aug-02 AT 04:04 PM (EST)]>are dished pistons on
>ported aussie heads a better setup than flat tops on ported
>stock 400 heads?

if the compression ratio comes out the same, i don't see how it would make MUCH difference - with the aussies you might have SOME quench on the part of the piston that doesn't have the dish, where with the open chamber 400 heads you have NONE, and i think the exhaust port on the aussies has a little better shape than the 400/351M heads.

>There's one other alternative. There are
>some other KB's with a 13cc dish that will give me higher
>compression (w/ aussie's). Would I still just be wasting my
>time with those pistons since they are still dished, even
>though the dish is smaller?

KB has some "reverse dome" type dished pistons - THOSE would be the ones to get, so i'd say option #3 - you still get quench, but with the lower compression. check the KB "strokers" web page - i'm not 100% sure KB is the one that makes those pistons, but if it's not them, it's somebody else that makes high-performance hypereutectic pistons at a similar price.

yeah, over 11:1 with the kind of cam you'll be running is too high - the info i've found indicates that will only work with a higher rpm cam.

***XYfalcon500 - the highest octane we have at pumps in the US is 93 ((research+method)/2) and some places just have 92 or 91 (altitude?) BTW - i was looking at aussie falcons last night - those cars are COOL!

 

actually the highest octane we have here in the u.s is 130 octane. ita called turbo blue and is about $4.50 a gallon last i checked.

any how the highest pump gas i've seen is 94 octane and it's usually carried by exxon. some places have it some don't. guess it depends on your locale.


later.......:P

 

where do you live? in houston, premium is either 92 or 93...

 

Up here in Green River, WYO (6600 feet) premium is usually 91 maybe 92 octane if you really look. I don't know how good that is but I'm not gonna worry too much because it is only $1.42 at present. My brother's not too happy about that though because he knows that the lower the price of gas is, the closer the pedal is to the floor.

 

>actually the highest octane we have here in the u.s is 130
>octane. ita called turbo blue and is about $4.50 a gallon
>last i checked.
>
>any how the highest pump gas i've seen is 94 octane and it's
>usually carried by exxon. some places have it some don't.
>guess it depends on your locale.
>
>
>later.......:P

Excuse me ... But Turbo Blue is 104 Octane at $4.50 A gal.
I'm using Phillups 66 B33 Which is there highest octane fuel at 116 and cost $9.40 A gal.
And then there's are other fuels out there .

 

[updated:LAST EDITED ON 02-Aug-02 AT 06:39 PM (EST)]Fastcat - do you remember the part# or specs of the pistons you used, and how much you took off the deck?

if it knocks a little up there, it would be damaging down here - unless our gas is better here. anyway, i'd still like the info just to compare... and who knows - maybe you found a part# i haven't found!

 

with flat tops and aussie heads you should come out to around 11-11.2:1, this is good on 93 pump gas. you are worrying for nothing

 

>Fastcat - do you remember the part# or specs of the pistons
>you used, and how much you took off the deck?
>
>if it knocks a little up there, it would be damaging down
>here - unless our gas is better here. anyway, i'd still
>like the info just to compare... and who knows - maybe you
>found a part# i haven't found!


Looking at my recipt it looks like the piston # is K13148. According to the recipt the block was decked to "0" and bored .030

 

>Looking at my recipt it looks like the piston # is K13148.
>According to the recipt the block was decked to "0" and
>bored .030

that part no. looks like the 12cc d-cup piston with the 1.670" compression height - KB-148. i guess the "13" at the front is for .030 over.

not sure what decked to "0" means - maybe 10.290 vs. 10.297?

if that's the case, it might explain why you have 9.8:1 instead of the 11:1 or so that you should have if it was decked far enough (.047) to get zero deck clearance. it would also explain why you're getting some knocking at 9.8, since a number of people here have said that 11:1 will run on pump gas.

i realize that you're running 91 octane instead of 93, but it seems to me that your altitude should more or less cancel out the octane difference. when i rebuilt my 351w with 10:1, it ran great on regular in durango CO (5200 ft.) but when i got down to TX, it started knocking like crazy. i had to set the timing way back until i started running 93...