I hope I am not beating a dead horse, but I have searched this forum on this topic and want to make sure I am not going to blow up my new engine...
For starters, I know this is a truck forum, so forgive me, as I am sporting a 1978 ford LTD Landau 2-door hardtop. IT has a 351M-400 (400 cubic inch) California emissions engine in it, and it is being rebuilt at this time. I had decided to make this engine perform much better than it did out of the factory, and bought it a set of the Aussie heads (closed chamber "quench" design) that has received much attention on this forum...Brimming with excitement, I searched the archives for information, and now I have scared myself. Here is my concern and my questions:
The original compression ratio on my engine from the factory, according to Tom Monroe in "How to Rebuild Ford V-8 engines" is 8.4:1 for my 1978 engine. I read on the projectbronco.com site that I can expect a ratio of 10.75:1. That compression sounded good to me, but then I read numerous posts on this site where guys have said this is simply too much compression for even 93 octane pump gas...My confusion lies in the fact that I seem to recall the 64 Pontiac GTO sporting a compression ratio of 11:1 or so, and it was able to run on pump gas. I may be completely out of line on my facts here...if so, please correct me. Perhaps the GTO had a special design that allowed for such high compression, whereas the 400 with just Aussie heads does not? Please forgive me for the entry-level engine knowledge...I am learning a lot about this motor, so perhaps I am not getting the whole picture. Anyway, the solution I read to the "too much compression" problem (if it is indeed a major concern) was to use dished pistons.
To get straight to my point / questions:
1) Is 10.75:1 too much for this motor, and will it detonate / pre-ignite on pump gas (92 octane)?
2) If answer to above is yes...where can I buy dished pistons for my 400 (any model numbers / brands would be appreciated).
Additional background: Engine will be using Edelbrock 2171 Intake, Edelbrock 2172 camshaft, Edelbrock 7821 timing set, and will be bored out .030 over.
One more thing...
After looking at Summit racing trying to answer my own question...I found a Sterling STL-427P30 piston designed for the 400M (.030 over) that claims a .145" dish and a 7:9:1 compression ratio. Now, I am not sure how to do the math on this mess I have gotten myself into...but if my factory CR before Aussie heads was 8.4:1, and these pistons give a CR of 7.9:1 (Summit stats on their site, assuming factory 2V heads), does that mean these pistons with my Aussie heads would yield a CR of 10.25:1 (Down .5 form 10.75:1)? Does that sound like I am getting back into the proper realm for 92 pump gas without detonation?
Thanks in advance to all you enthusiasts who know all about this fine motor...
The main issue here is the difference in pump gas then and now. In the 60's and well into the 70's you could get octane as high as about 98, plus it had lead in it which allowed you to run much higher compression without detonation. Heck guys were even running 12:1 off pump gas in those days. Some went to airports where they could get 105 octane but that was misleading cause airplane fuel figured octane differently and also compensated for altitude. You would probably need about 95 to run 10.5:1 although I don't have a formula to figure that out. The only other thing I can think of short of additives might be the Sunoco Ultra if you have it in your area. I believe it's higher than 93 but I don/t know how much. It's also $2 a gallon around here.
Unfortunately those heads have a small combustion chamber. I don't see any reasonable way to use them on the street with a heavy vehicle. Anything over 9.5 to 1 is pushing it. You cannot take advantage of the quench effect with going even higher than you plan and detonation will be a problem.
I have given some thought to using them myself but I think you wold need some kind of odd shaped custom piston to make it work.
i went through the same thing rebuilding my 400.
i decided not to use the aussie heads, mostly due to the issue brian brought up: emissions. you could probably make it run on the higher compression provided by aussie heads, opening them up a little bit and using the dished pistons. the big problem is that you'd need a bigger cam with greater valve overlap to bleed off excess chamber pressures, but this wreaks havoc on emissions.
i ended up going about as far as i dare wrt CA emissions. the limiting factor on my motor is now the stock carb, and it'll be a while before i can replace it with a decent 4-bbl. your 78 will be exempt a little sooner, but not yet.
even so, the changes i've made (check my gallery for details) have made a big difference in performance, and i'm pretty sure i'll be able to smog it when the time comes (just did it before rebuild)
I got the same vehicle as you except mine is a four door.
I started out with a 351 and ended up with a 408. I have pretty much the same parts as you. I don't have the aussie heads though. I have flat top pistons instead and my heads have been mill down. Origionally I thought my block was zero decked at the machine shop. Turns out it wasn't, I think in the end I running somewhere around 9.5 to 1 CR. I have problems with pinging on 93 octane, occasionally I get 110 octane from the speed shop. Big difference.
I think you will pleased with the results but you might want to invest in some headers. I was told by Danlee on this site, that without headers the camshaft we have loses its potential because it the exhaust is not as free flowing as it could be.
In the end I wound up with a ill mannered car that scares the hell out of Honda's. Good Luck.
10.5 will take the old "premium fuel" with 98-105 octane. I ran 12.5:1 back in 1970 but that required 115 octane which was available from Phillips as a premium plus "flight fuel" but it was not AV gas. Back in the later 60's a lot of cars ran 10-12:1 compression. Regular gas was 90 octane (R+M)/2. I could buy 130 (R) octane AV gas back then. That old Pontiac required premium fuel at around 100 (R+M)/2 octane.
Shoot for 9.0:1 compression and if you are real good with your measurements and head cc's you can maybe push 9.2:1 on pump gas.
There are several threads here that detail the compression ratio's and other options with aussie heads.
Hey yall I fiured that I'd share some new info with you. For those of you that understand CH. we know that our pistons for a 400 are way to down in the bore. Probe just started making a piston for a Cleveland with a CH of 1.662 instead of the 1.647. This will help with the quench right? I just bought a set and it will be 4 to 5 weeks before I get them as this is a new part and haven't started on production except for mine. I also am using Aussie heads that have been worked. On project bronco it said that I would have 10.51 Comp. I know this is high but on my 302-351W we run that much on the streat its not that bad
I am also going to efi my motor so with the compuer controling the timming I should be ok.
http://smokemup.com/ theres good calculators here for figuring out stuff like compression and what not.
You could get some thicker head gaskets and hope for a .3 point decrease in CR. You could also open up the combustion chamber a bit, maybe.