Matt O.

Quick question:

Would running 5 PSI of boost through a stock 89 944 na engine with stock compression of 10.5:1 (I think it's 10.5, may be 11) damage the engine?

Thanks in advance.

-Matt

Jon-D

I've got an equation in a book at home that will show you how much boost affects your CR. I'm off work in 10 min, so I'll run home and dig it up. Check back in about an hour.

- Jon

Jon-D

Here's the equation:

FCR = ([Boost / 14.7] +1) * CR

FCR = Final (Boosted) Compression Ratio
Boost = in PSI (Pounds per Square Inch)
CR = Compression Ratio

14.7 is the constant for barometric pressure at sea level. If you lived in a higher elevation, say Denver, you'd need to adjust this appropriately by using the following equation:

CCR = FCR - ([altitude / 1000] * 0.2)

CCR = Corrected Compression Ratio

A stock 944 N/A runs a 10.9:1 CR. So, boosting it probably wouldn't be a good idea, unless you wanted to fill it with 92 octane plus octane enhancers. According to the equation your Final Compression Ratio with 5 PSI of boost would be 14.6:1. Obviously, you don't live in Denver. But for academic sake, if you did your CR would be 13.6:1. The basic rule is that for every 1000 feet above sea level you go, that's 2 tenths (0.2) off of your compression ratio.

So, to answer your question... YES running 5 PSI of boost through your stock 944 N/A engine would damage it! You'll need to destroke it and/or get dished pistons. John Anderson of SFR should know all about supercharging a 944. Or you can check out Huntley Racing.

<a href="http://www.speedforceracing.com" target="_blank">Speed Force Racing.com</a>
<a href="http://www.huntleyracing.com" target="_blank">Huntley Racing.com</a>

As an interesting side note... if you do the math for the adjusted compression ratio of a stock 944 turbo (running at an even 12 PSI of boost) the compression ratio comes out between 14.5 and 14.6:1. Thought a few of you would find that interesting...

- Jon

Robert J

JonDawg,

I think your making it sound worst then it is. 5 psi will damage your motor but not at that point unless your motor is burning 4 qts. of oil ever mile. I know lots of people that turbocharged their N/A. They usally keep a mild boost. I have seen from 3-9 psi on cars and they seem to run fine.

The main key is fuel management, avoiding detonation, the durability of your motor, and the way you drive. If you can makes those things happen, I can't see why you can't run 5 psi on your motor for a good bit with little damage.

If forced induction blew motors fast (not saying that anyone said that), then companies wouldn't be making kits for N/A cars. Keep in mind that some of these kits do come with FPR, some have intercoolors, Air-Fuel Controllers, etc., to help prevent some of the above problems.

Robert J

Also for the gas, you can run on 92 octance gas on 5 psi. Maybe 7. Like stated above depending on where you live has to do with it but I dont think any kind of enhancer (octane booster if that what you meant) is needed. According to the equation the turbo 944's are running 14.5:1 on pump 92. I'm sure 14.6:1 will do fine also.

10.9:1 ratio sounds too high for a 944 N/A. I thought they were 9.5:1?

JonDawg, thanks for posting those equations.

Matt O.

Thanks both of you. Yes, 10.9 is correct, for the later na's. I think the earlier ones were 10.5?

Thanks,
Matt

Tabor

1983-86: 9.5:1
1987-88: 10.2:1
1987-88 "S": 10.9:1
1989: 10.9:1
1989-91 "S2": 10.9:1

Note: the 968 is 11:1, but I know that Porsche sent motors out the door anywhere from 10.7:1- 11:1.

Jon-D

I guess you very well could be correct. 14.6:1 on an N/A may not be too bad. 5 PSI is pretty low boost, and considering that the 951 and 944 w/ 5 lbs. would have comparable cr's, you might be able to get away with it. I've just heard (from magazines & other threads) that there's a lot more to adding a blower to a car than just bolting one on. Every kit is different and I hope there is a viable solution out there for ya. I think that every N/A would like an easy 5 PSI of boost for their car

Best of luck to ya,
- Jon