Power difference guesses? 73T

[That Guy]

Hi all, I bought a 73 911T a few months ago, great car goes well.

I always thought for a T it was really strong, but as the previous owner had the engine rebuilt at great expense (I've got massive stack of invoices) a few years ago I figured it was just newer parts working well. However I recently found out while sifting through the volumes of invoices that while the engine is a matching numbers engine, when it was rebuilt they changed it to higher compression with new pistons/cylinders to 9.8:1.

I'm not a fan of putting old cars on dynos so not keen on that, but was thinking you guys would have some good educated guesses out there as to what that would do to the power output?

cheers

[tobluforu]

Does it state what cams were put in it?

[RWebb]

divide the 2 compr. #'s

[RSTarga]

same displacement? CIS or MFI?

[regency]

I rebuilt my 73 T MFI Coupe as follows:
E cams
MFI Pump worked over by Gus @ Pacific Inj to E spec.
Throttle Bodies rebuilt to E spec by Matt Blast @ Eurometrix
85mm cylinders from EBS Reno
J & E pistons @ 9.5:1
Replaced the smaller T stacks with E stacks.
MSDAL6

Therefore, I must be makin about 285hp..........Errrrrrr, probably around 165-175. Really happy with the engine & it's power, on ramps are a blast.

Steve

73 911 T MFI Coupe, Aubergine

[regency]

Opps, wrong pic.....911 not Diesel

[That Guy]

Its a carb model (euro). According to all the paperwork, same displacement and cams etc everything else was standard, although carbs were retuned to suit the higher compression...

What does dividing the two compression numbers achieve? Does that mean 7.5/9.8 x standard HP?

thanks!

[Flieger]

The percentage change in power should be similar to the percentage change in compression ratio.

[That Guy]

Originally Posted by Flieger

The percentage change in power should be similar to the percentage change in compression ratio.

ok...that's a big uplift then, 130% of standard HP = approx 170HP (assuming it had 130 to start with). I used to have a 73E and it does feel stronger than that but I figured that was because the E was a bit tired...maybe not...

[Flieger]

I am not sure what the actual correlation is. It is non-linear but in the region we are talking it may be about 1:1. This is empirical, not based on strict physics, so it is an approximation.

[Steve W]

Generally each additional point of compression increases the power output of a motor by 6%. So assuming you had 130hp to begin with at a CR of 7.5:1, going to 8.5:1 increases the output to 130*1.06 = 137.8 hp. Going from 8.5 to 9.5 CR is another 6% or 137.8*1.06 = 146 hp. Going from 9.5:1 to 9.8 is about another 2% power increase, or 146*1.02 = 149 hp.

Or you can calculate it by the equation for Otto cycle efficiency:

where is the compression ratio, for air fuel combustion is approximately 1.3.

[RWebb]

I wonder if the original CR of 7.5:1 is correct - seems low

[Peter Zimmermann]

Originally Posted by RWebb

I wonder if the original CR of 7.5:1 is correct - seems low

1973 "T" w/Solex-Zenith carbs (per Tobias Aichele) = 7.5:1
1972/73 "T" w/MFI (per Spec book) = 7.5:1
1973.5 "T" w/CIS (per Spec book) = 8.0:1

[tobluforu]

All cars are different, put it on a dyno.

[RWebb]

aha - mine is/was a 1973.5 "T" w/CIS

[That Guy]

Originally Posted by Steve W

Generally each additional point of compression increases the power output of a motor by 6%. So assuming you had 130hp to begin with at a CR of 7.5:1, going to 8.5:1 increases the output to 130*1.06 = 137.8 hp. Going from 8.5 to 9.5 CR is another 6% or 137.8*1.06 = 146 hp. Going from 9.5:1 to 9.8 is about another 2% power increase, or 146*1.02 = 149 hp.

Or you can calculate it by the equation for Otto cycle efficiency:

where is the compression ratio, for air fuel combustion is approximately 1.3.

Thanks for this - I reckon your 6% method seems about right - 150hp is what my seatofthepants-ometer reads...I tried the Otto cycle formula and clearly after years away from school I still suck at maths because I kept getting 0.97

thanks

[regency]

Ok, so I take my stock 73 911 MFI T and come back with E cams, E stacks, E Throttle Bodies, MFI pump re-built to E spec, 85mm cylinders & 85mm J&E 9:5.1 pistons.

Stock T = 130hp
Stock E = 165hp

Do the engine pro's think this engine is making over 165hp?

Steve

73 911 T MFI Coupe, Aubergine

[kwikt 911]

Gosh I hope so. Porsche lists the 72 911E with 165HP stock with 8:1 compression ratio.

[Porboynz]

Originally Posted by regency

Ok, so I take my stock 73 911 MFI T and come back with E cams, E stacks, E Throttle Bodies, MFI pump re-built to E spec, 85mm cylinders & 85mm J&E 9:5.1 pistons.

Stock T = 130hp
Stock E = 165hp

Do the engine pro's think this engine is making over 165hp?

Steve

73 911 T MFI Coupe, Aubergine

I have a similar spec but with lower compression 2.4 S pistons (8.5:1?) and it makes 175 on the dyno easily with a two pipe RS style exhaust and still runs fine on 91 octane. Its not the flat out horsepower that makes me smile, its just so much more driveable with the E cams. OK, I do smile when the engines at 6000+ in 3rd gear, can't help it.

[tobluforu]

Originally Posted by regency

Ok, so I take my stock 73 911 MFI T and come back with E cams, E stacks, E Throttle Bodies, MFI pump re-built to E spec, 85mm cylinders & 85mm J&E 9:5.1 pistons.

Stock T = 130hp
Stock E = 165hp

Do the engine pro's think this engine is making over 165hp?

Steve

73 911 T MFI Coupe, Aubergine

Maybe, maybe not. If you want to find out look for a dyno in your area, costs about 100 bucks for three pulls, and in your area I bet there are few. Until you do, its just pure speculation. On two other cars I have it took over 10 times to tune and get the numbers I want. On my saab it started with 236 whp, now after about 5 years i'm at 289 whp, just couldn't break the 290 mark, safely.
Now if it makes you feel good that someone states your car has 165 based on calculations, then feel good.