I just finished a rebuild on my S5 N/A. During the rebuild I went the cheap route and installed used rotor housings. It fired right up and I have had it idling in the driveway for about 20 minutes. After 20 minutes I did a compression test and I got 72psi on all 6 faces. Question: How does this rate for a 20 minute motor with used housings? I expect it to go up but any guess as to how much?

Also, I did a streetport on the intake ports for the front, intermediate, and rear housings. I'm not sure if that would affect compression or not.

Thanks for your help.

Brian

 

It should build up, but that seems awfully low.

 

I think mine were 90-100 when I built it up last time, but the more I ran it the higher they got.

 

I understand that 72 is low but I thought for only running the motor for 20 minutes that would be normal. These engines usually have to be pull started because the compression is so low after a rebuild.


Does streetport affect compression? I imagine it would because by enlarging the ports you are shorting the compression "stroke".

Would love to have some more experts chime in here...

 

I was just reading Rotary Resurrections website. He says with the valve out, you should get a reading of 30-35psi on each face. With the valve in, you should be looking for above 100psi. My 72 is the with valve out watching the bounces on each face, I didn't bother to leave the valve in and get an overall compression.

Can someone help me sort this out?

 

We have never had to pull start any of our rebuilds. Sounds like you have something out of spec.......... housings, side seals etc.
Porting will not lower compression, it does lower the amount of vacuum the engine pulls at idle. For example I built a raceport for our FD, I compression tested before even firing it up to establish a baseline for seal break-in and then wear over time. It was 122 or above on all faces with our digital compression tester. This will increase as the engine breaks in (20miles on it currently).

Your compression will increase with time, but you will be lucky to see 90 psi if it is starting out that low

 

Thanks for your input guys. Anyone else? I would love to hear from Rotary Resurrection or Aaron Cake on this one...

RR: Why does it say 30-35psi with the valve open on the compression check section of your page?

 

I think your methodology is bad, so your numbers are bad.

It started too easily for the compression to be that low under the circumstances you describe.

Redo the entire test, carefully following all steps from the RR site. Write down the numbers, then post back. It might look a lot better.

Good luck.

 

Yeah, I agree. When I started it for the first time, I turned the key and after 3 rotor faces it fired right up with a perfect idle. With 72 psi, I'm surprised it even runs. Sorry to beat this topic to death, I just spent a lot of time and money and I am a little worried my engine is worthless. I feel like such a noob.

 

It says to look for 30-35 because you are only looking for the needle to move with that test, not get any type of numbers.

http://www.banzai-racing.com/compression_test.htm