I got a 12:1 compression ratio motor do i have to use a race fuel or could i use super gas or super gas with octane bost? THe only dealer in my area that sells race gas sells some that costs 7 bucks a gal and with my gas hog im not sure i can afford to run her all the time :-staun


thanks much boyz

nate

 

[updated:LAST EDITED ON 11-May-02 AT 03:43 AM (EST)]Were you told you had a 12:1 motor or did you build it? If you built it what is the head cc's, head gaset thickness, deck height, rod length and doom volume of the pistons? Last question, do you know what the duration and over lap of the cam shaft is? Was the cam dailed in on center or setup advanced or retarded?

I am asking all this because a lot of people build a motor with pistons advertised as 12:1 but they don't read the fine print and see that they are 12:1 with 58 cc heads. They then install 76 cc chambers and the compression is only 10.2 (or close to it).

In any case you can have a static compression of 12:1 but really only have the dynamic pressure of 10.0:1 if your cam was installed retarded. That's bandade fix when someone realizes they have to much comp for the street.

If you don't know the ansewer to any of these questions then there are a couple of things you can do to figure out what fuel you need to run. Start with almost an empty tank of fuel and pump in a couple of gallons of 93 test. Take it out and in 2nd gear give it some hell. Listen for the engine to ping or knock. If you have 12:1 you should hear it talking to you real fast. If you don't then try out the lower grades.

The second method is to take an compression tester and check your cylinder pressure. Be sure to crank the engine over for a good 5-10 seconds so you know you have the heighest reading.

150 psi or less use 83
160 to 180 use 87 (if you get ping switch to 93 or use a 160* thremostat)
180 to 200 use 93 (if you get ping use a 160* thremostat)
Above 200 use an 160* stat and octane booster such as 104

I had an 71 Nova SS with a 350LT1. This motor was built factory with 12.1 pistons but had a very radical factory mechanical grind cam that bleed off cylinder pressure at low rpms. I could run 87 in her all day long but really noticed the difference at the track when I would dump 114 in the tank and bump the timing upto 16* advance.

 

I had the motor built by a local machine shop. He said his buddy has a set of 351c pistons that if he put in my 400 would raise the compression to 12:1 i installed a set of 351c 4v heads on it open chamber. He also said if i used a set of closed chamber heads that it would raise the compression even higher i have the exact cam grind because he gave me the comp cams card and it reads as follows:
Camshaft Specification Table
Part Number 32-312-4
Engine 1970-present Ford
351ci-400ci
351C, 351M-400M,
8cyl.
Grind Number FC 280AH-8
Description

--------------------------------------------------------------------------------

Intake Exhaust
Valve Adjustment 0 0
Gross Valve Lift 0.554 0.554
Duration At 0.006 Tappet Lift 280 288

--------------------------------------------------------------------------------

Valve Timing At 0.006
Open Close
Intake 32 68
Exhaust 68 32

--------------------------------------------------------------------------------

These Specs Are For The Cam Installed At 108 Intake CL
Intake Exhaust
Duration At 0.05 232 237
Lobe Lift 0.32 0.32
Lobe Separation 108

--------------------------------------------------------------------------------

Recommended Valve Springs



Im just going on by what the motor guy said that built this but i also told him i would be running it street and race also so maybe he took a count for that and played around with the cam? im really not sure so the only other way of checking out the compression is using a compression tester?

thanks for the info and anything else would be great

 

PN KB 149
C.I.D. 351
CYLINDER V-8
BORE 4.00
STROKE 3.500
ROD LENGTH 5.778
MIN. CLEARANCE .0015
COMP. HT. 1.670
PIN DIAMETER .9122
HEAD TYPE .100 pop up
CRANK CL TO DECK 9.206
WT.PISTON/PIN 351C 600/143g

Head vol.- Comp.Ratio
62cc -12.7
66cc - 12.1
76cc - 10.6

5.778"ROD

1).100 solid dome.

2)Designated as O.E. replacement Boss engines manufactured in 1972.

3)* All other years.

4)Not recommended for street use because of high compression and today's low octane fuel.

SIZES Std.,.020,.030,.040,.060 Check piston dome-to-cylinder head clearance.

This is what I was talking about when I mentioned that head CCs make a difference on how much compression you are running. This is right off K/B's parts book for the 351C. I believe the 4V open C head is around 78CC's. With the open heads you shouldn't have to much trouble
running 87 or even 83 with a 160* stat.

As for the cam it's a good grind for the street/strip RPM range should be around 3000 to 6500 rpms. With the cam you should be using a 3000 to 3500 rpm stall and a 750 CFM or larger carb. But I don't know what he dialed into it. I honestly cannot say if he dailed the cam one way or the other. You could ask him though.

 

I came up with 9.2 to 1 comp with your set-up, If you didn't have the block decked .091 and your using the 78cc 4V open chamber heads.If you had the block decked to Zero then you'd have 10.7 to 1.

ShaZam

 

well that sounds a bit better I was hoping that my compression wasnt that high because this is also a daily driver.. but what confuses me is why would he tell me it is when it isnt if he was off by that i wounder what else he is off by that kinda makes me feel awkard when i just dumpt like 4k into a motor and he says all this stuff that isnt true .. well im kinda upset but kinda glad but im going to buy a compression tester to make sure and see wahts up but thanks for the advice

 

No:
What I think he probley told you was he had a buddy who had some 351C 12 to 1 comp. pistons lying around that would fit into your 400 easy enough..
And your brain heard 12 to 1 comp. 400 Cool ;-) Kick Butt.

Most people just don't do there home work, It looks like the 351C pistons will just drop into a 400 because the Comp. height is almost the same as the 400 and you have to bush the pin , which is easy enough.
OK here's where the home work comes in.
Deck Height... You have to know what your deck height is, it changes through out the years the 400 was made, Comp. height of pistons they changed through out the years too.
But the standard 400 is 10.297 and the deck height for a 351C is 9.206. So OK it looks to be 1" taller right.. but it's that .091 that will cost you a few points in your final compression numbers.

 

the year of my motor is a 1977 i belave it was a 351m and i converted it to a 400. The motor was given to me so i had it built i dropt it off to a local machine shop and said do me the works so I have no idea what the deack hight is or the pistons so im the stupid boy with a pocket full of money taht said do it up lol so im really not sure what i have all i know is i thought he said 12:1 but like yo usaid my brain was doing over time he said his buddy was going to use them in his 400 build up but never got around to it so i dono i guess i could call him up and ask him

thanks for the advice

 

but if i swapt out the heads i could bump the compression higher right? to an aussie head or a 2v or a closed chamber type?

 

Yes you could if you went to the aussie heads 2V heads.

And Hey don't feel bad because you had a pocket full of money and you wanted a bad *** 400. My very first 400 build didn't work out at all like I wanted to. But as they say You learn from your mistakes and I had more mistakes on that first 400 then you could shake a stick at. But now on my 3rd build up I got it All right, and I've even learned more about making a 400 stronger and tougher.

Persistance does pay off.

Beside not everyone has all the tools to do a rebuild in there Garage.

 

>Beside not everyone has all the tools to do a rebuild in
>there Garage.


Nor a wife that is understanding.

 

Yes your so right about that .. and I'm Lucky enough to have both ..

 

well the comp ratio down a bit I hope that it will still rock and like you said i have learnd a lot about motors that i never knew. so this is deft a learning experince and the next time oh yes there will be another motor done i will do it a bit difernt well thanks a lot and i still hope she hums he said it will produce 400-500 horse i hope that is still a factor with that low of a comp and the cam i got if noting else the next one will be bigger when i get this one all paid for thanks a lot boyz

 

 

We use race fuel in are race car and it runs way to hot for street use! We mix race fuel and high octane gas (50/50) it works nice!