I have a 89 Mustang 5.0 with the following parts installed and could use some help with trying to figure out what the compression ratio this motor has..

A9s computer
5.0 short block
F303 cam 288/.512 lift , roller rockers
GT40P cylinder heads
Trick Flow upper and lower intake man.
Professional Products 70mm throttle body and EGR
C&L 76 mm MAF housing, #19 sensor
36 lb Ford racing Injectors
Powerdyne BD-11 supercharger, at 9 psi boost .

I do not know if the heads are shaved, I was told the it have all forged components in the block with flat top pistons. I will be doing a compression check this week end , the motor has about 400 miles on this build..

 

Do an intro and I'll tell you that none of that info is used to figure your compression ratio.

I might explain how you need to know the chamber size of your heads, the distance below the deck your pistons have at TDC, and how much your head gasket choice can screw the pooch.

 

Assuming that all of your components are complete stock and within OEM spec, your compression ratio (static) would be about 9.5:1

Scotty just hasn't been around long enough to know that the ~59-61 cc chambers on the GT-40P heads will bring your CR just under 9.5:1 with a stock shortblock.

 

I am trying to figure out what octane gas i can use in this motor i have been putting in the 91 but was wondering if i could use the 90 or 89 ?

 

the best way to determine that is to power time it at the track. with whatever octane you have in it....run it starting low where you ARE CERTAIN you dont have any detonation. like even 0* for a benchmark.

run it and increase 2* per run.

when you see your mph slow down...your too much. depending where that is, the temp, humidity and all else factored in, then you can determine if you'd rather run 89 and 12* or 91* and 14*

fyi i spray 80 on my motor wth 12* timing and 87 octane, as long as its cool out that is.

in your situation....i would run the lowest octane that would let you keep it about 10-12* when it's hot . that way you can adjsut with octane OR timing

 

I was just gonna make him work it out for himself.....


.... after he posted an intro....

 

I see that you are in Idaho.
Most of that place is at altitude, you can get away with a little more timing as the air is thinner than at sea level.
Again, run it at the strip to really find out......

 

I am new to Ford and know not a thing about them ... I am trying to learn and I will not be dragging the car.. This is just something I am making sound and run good or the best I can... I was told to unhook the blower and get a aftermarket computer so the motor will not run lean and blow up, Is this true?

 

No. Leave it stock and keep it well maintained.

 

who sells that guy his meth ? they must be making a killing on him

im a bit confused tho....if you dont ever care to race it...why not just dump ALL the timing out and call it a day.

the blower makes heat so you need to compensate for it by removing some timing.

removing it sounds like the dumbest idea ive ever heard of tho...other than trying to fish your pen!$ into a vacuum cleaner.

but drop timing or raise octane. timing is free tho