raising compression on a 351m/400
 

[updated:LAST EDITED ON 06-Jan-02 AT 04:50 AM (EST)]whats the easiest way to raise the compression on a 351m id like to get 10:1 out of it

raising compression on a 351m/400
 

>whats the easiest way to raise the compression on a 351m id
>like to get 10:1 out of it

Two viable options that I can suggest: Higher compression pistons, or quench heads. There are only one or two sources for higher compression pistons for the 351M/400 - I don't recall who has them but search the list (badger?). In 1980 I had a set of 9.5:1 made by Jahns and they were real pricey as they were custom made. You can also have the rods modified (floating pin) to accept 351C pistons - where you have a greater selection.

The second alternative is what I have been considering. The Australian heads are a hybrid of the 2v and 4v heads: they have the large 4v valves and quench chamber of the 4v heads but the smaller ports of the 2v heads. For most street use, the 2v ports are desirable since the 4v ports are too large to maintain port velocity at low RPM, so low-end torque suffers. The quench combustion chamber will raise your CR to 10.0:1, and the larger valves will provide a nice kick. Due to the quench chamber, you should be able to run high octane pump gas.

These heads are occasionally available on Ebay for $250-$350 unrebuilt - expect to spend more than that having them rebuilt. You can buy them rebuilt for about $800 or rebuilt and ported for $1300. I thought the website was:
http://www.phpmotorsports.com/heads.html
but I just checked and there are none listed - perhaps they are out right now or I have the wrong site. To get a similar effect, you could get a set of early 351C 4v quench chamber heads, and get a set of port plates from:
http://www.mpgheads.com/port_plates.htm

The port plates effectively reduce the port size, increasing port velocity and low-end torque.

raising compression on a 351m/400
 

you can buy 351c psitons and bush the rods, the problem is you will ping with open chambered heads.

get flat tops from badger and clsoed chambered heads

you can mill the block and heads

thats pretty much it

raising compression on a 351m/400
 

I don't think you'll find too many 351c pistons that'll work in a 351m - they'll be way too short.

351c pistons will work out right in a 400 but I don't think you can make use of the closed chamber heads with the 4" stroke and only a 4" bore. You can't get the piston close enough to the head to make the quench effect work. It's possible, but the compression ratio will end up very high.

raising compression on a 351m/400
 

>I don't think you'll find too many 351c pistons that'll work
>in a 351m - they'll be way too short.
>
>351c pistons will work out right in a 400 but I don't think
>you can make use of the closed chamber heads with the 4"
>stroke and only a 4" bore. You can't get the piston close
>enough to the head to make the quench effect work. It's
>possible, but the compression ratio will end up very high.


The compression height of the 351M piston is larger than the 351C and the 400.

raising compression on a 351m/400
 

dom, he said 351m, sorry, i must have been on crack that day.

raising compression on a 351m/400
 

>I don't think you'll find too many 351c pistons that'll work
>in a 351m - they'll be way too short.
>

sounds like a good reason to swap in a 400 crank! Oo.

>
>351c pistons will work out right in a 400 but I don't think
>you can make use of the closed chamber heads with the 4"
>stroke and only a 4" bore. You can't get the piston close
>enough to the head to make the quench effect work. It's
>possible, but the compression ratio will end up very high.
>

how close SHOULD the piston come to the head for quench to work?
are we talking like zero deck & an .035 gasket, or...?

thanks,

raising compression on a 351m/400
 

>
>how close SHOULD the piston come to the head for quench to
>work?
>are we talking like zero deck & an .035 gasket, or...?
>

Yes, that's my understanding. Too much clearance and you'll have worse problems than with open chambers. Compression ratios start to become unreasonable in a 4" bore and stroke engine with the piston this high.

raising compression on a 351m/400
 

actually, **NO**. the 'Aussie' 302C heads have small closed chambers, 2V valves and small 2v ports. There was sometimes a sticker on the air cleaner which said '4V-High Performance' but this simply means there was a 4 bbl 2V manifold and 4 bbl carb on top of the engine, as opposed to 2 bbl manifold and carb.

raising compression on a 351m/400
 

bbtkd,
First off, you're right about the Aussie heads, They are th ebest bet. I actually have a few sets laying around, I have used them on a 400 and they make one hell of a difference. Second, the website you were looking for is www.powerheads.com. They market complete Aussie heads for about $900 or more, ported, bigger valves, etc. But it's best to get your own, do a basic valve job and get springs matched to your cam. A little porting wouldn't hurt, either. - Don

raising compression on a 351m/400