New to me Fox body. Looking for Rough HP and compression ratio

Halford4491

New Member
Oct 30, 2021
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Nebraska
Hi everyone I’m new here so bare with me. So I bought a fox that was build by a couple previous owners before the guy I purchased the car from and there’s a lot of guessing on what the car actually makes for HP. I don’t usually like buying a car that’s already been worked on internally due to the lack of history and or if it was done properly. Car runs strong and sounds good which is why I bought it. Super clean & rust free as well. I think car was set up for future nitrous. So I’m trying to see what you guys have for Ideas on RWHP. Also if there would be any way to figure out what the compression ratio would be on my set up. Planning on getting it dyno tuned next spring. Future plans would be to throw in nitrous at it or possibly blower or turbo. Haven’t decided yet. I’ll list everything I have on the car. I believe the car does have the stock block which I no can be the weak link for high hp numbers over the 500 mark.

1993 Mustang LX hatch 5 speed. 83,000miles.
AC & Smog deleted

331 Ford Stroker forge kit
Custom cam with no specs for it
Dart 195 Aluminum heads
Edelbrock Performer 2 Intake
42lb injectors
Calibrated MAF
Acufab 80mm TB
BBK CAI
MSD coil, distributer, & wires
Walbro 255lph fuel pump
Ford shorty headers
O/R H pipe
Flowmaster 40s out the back
Tremec 3550 tranny
Spec stage 3 clutch
Pro 5.0 shifter
MM clutch cable, quadrant, fire wall adjuster
Porthane bushing
Aluminum driveshaft
3.73 gears
Auburn locker
31 spline axles
UPR upper and lower tubular control arms
Subframe connectors
Boss cowl hood
Line lock
4 core aluminum Radiator with dual electric fans
SVE aluminum Dif cover
17” ROH Snyper wheels
5 lug rear drum brake conversion
Has phantom gauges Oil,Fuel,Water,Nitrous,
& 5” Tach with shift light.

There could be some other stuff I’m missing on here but that’s the bulk of it. Might post this on another fox forum as well. Thanks in advance for all the ideas and thoughts.
 
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90sickfox

Wasn't a pretty sight...and I've got big hands
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There's no way anyone could guess at you compression ratio. That has to do with the combustion chamber area. Things as small as the type of head gasket can change it. Hp can be calculated roughly by track times and weight if the car with the driver. It takes x hp to move x weight at x time. It would be approximate though. Tires and driver skill can change numbers.

I have an engine that wasn't built by me. It was supposedly dyno'd at 423hp. TW 170 heads and systemax intake on top of a 331 stroker, H beam rods, and xe282hr comp cam. I don't belive it put out that much. Most engines I see are between 300 and 370hp with HCI.
 

manicmechanic007

Active Member
Sep 26, 2017
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Check the compression for an estimate of your engines compression ratio
I agree with 90sickfox more info is needed
So much has been done to the car that you owe it to yourself to pull the engine apart and cc it
There is a dyno on every block these days so go get it done if you want
Why wait for spring?
 

Halford4491

New Member
Oct 30, 2021
2
0
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30
Nebraska
Check the compression for an estimate of your engines compression ratio
I agree with 90sickfox more info is needed
So much has been done to the car that you owe it to yourself to pull the engine apart and cc it
There is a dyno on every block these days so go get it done if you want
Why wait for spring?
Hey sorry for the late reply back. Life got busy for sec. Only waiting on dyno till spring so I can save up some cash for a bit. Between funding aftermarket stuff on one fun car and just purchasing this fox i feel Iv spend enough this year. Plus I don’t need to hear it from the wife haha. When checking the cylinder compression to get a rough ratio estimate is there a math formula for once I have the psi number?
 

CarMichael Angelo

my rearend will smell so minty fresh,
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Nov 29, 1999
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Hey sorry for the late reply back. Life got busy for sec. Only waiting on dyno till spring so I can save up some cash for a bit. Between funding aftermarket stuff on one fun car and just purchasing this fox i feel Iv spend enough this year. Plus I don’t need to hear it from the wife haha. When checking the cylinder compression to get a rough ratio estimate is there a math formula for once I have the psi number?
No. You may as well make up a number. W/O knowing how much cylinder pressure is being allowed to escape based on valve timing relative to the camshaft specs ( which you also don’t know), anything you get from that is pure guesswork.
(static) Compression Ratio is not an estimate. It is a VERY accurate number, that has to be calculated, using carefully MEASURED values for combustion chamber size and deck height. You also need to know the exact bore and stroke. These are all hard, solid values, resulting in a hard, solid mathematical Compression Ratio. It's just math, and math is 99.9% precise science.
(lol)
(static) CR is a theoretical value for a compression ratio in a situation where the valves are open at the very beginning of the intake cycle, air fills the cylinder without restriction during the intake cycle, and then valves close at the very beginning of the compression cycle and stay closed until the piston reaches TDC. It also assumes zero leaks. IN real life, this is impossible. BUT, it gives you an accurate, very solid compression ratio value, which is critical when designing and bulding engines. This value will not change, no matter what goes on with the engine. It is always the same, whether the engine is stopped or running at 9000rpms.

That will NOT tell you the actual compression PRESSURE, however.
PRESSURE is just one value that you need to measure with a compression pressure gauge. The result will vary greatly depending on how worn the engine is, and how the test was performed, how accurate your gauge is, how air tight the gauge was attached to the spark plug hole, etc. etc. But a pressure test will still result in a solid value of pressure detected by the gauge. Assuming the gauge to spark plug joint is leak free, you will see exactly what the pressure is inside the cylinder when you take the reading. At the engine speed you measured it at. The pressure will not be the same throughout the rpm range but it is near impossible to measure the pressure of a running engine....
;)



There is also something called Dynamic Compression Ratio. It is not nearly as commonly used, because to be able to calculate that, you would need to add PRECISE valve timing events (bleed-off) into the formula. To my knowledge it still can't account for pressure leaks past cylinder rings.
Anyway, dynamic CR would give you a fairly accurate actual compression ratio for a "running engine", so to speak. Due to valves being partially open during compression stroke, this value is always lower than static CR. If you had all the inputs at a very high level of accuracy, you could get a very accurate, solid value as a result. Like static compression ratio does.

With the help of this formula, you can calculate actual compression Pressure, assuming you know what the athmospheric pressure is. Again, leaks would cause some inaccuracies. Athmospheric pressure varies with elevation and a few other things. Pressure inside the cylinder varies depending on intake and exhaust restrictions. THIS would be an estimate.
 
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