- Location
- United States
Just out of curiosity how does porting and polishing the exhaust port on a 701 sj effect compression? I would think that has nothing to do with compression?
Super Jet Porting and polishing/compression
- Thread starter superjetchet21
- Start date
- Location
- United States
so would it be safe to drop in dome size to bring the compression back up?
- Location
- Pueblo, CO
As Matt_E said, raising the ex. port will give a lower cranking/indicated/static compression. However, your running (dynamic) compression will be high. This is not a measurable parameter. So IMO... NO you cannot simply put in smaller domes to raise compression. Unless you want to run race fuel and/or risk detonation. Best to ask your engine builder their recommendations for the porting that was done, pipe being used, timing, impeller pitch, etc. Too many variables to just throw domes at it. (yet people do all the time).
- Location
- United States
So my problem Is i have ada head with 33cc domes ported and polished and its only getting 155 psi with second brand new top end, is more compression better to have rather then a ported and polished cylinder and also would not running a base gasket help bring the compression up?
- Location
- Lodi
Removing the base gasket will affect port timing as well as squish.
- Location
- United States
so maybe just do a squish test first and then go from there?
- Location
- United States
I bought this thing with the motor already ported and polished, Is there a way to see if its over ported? The thing just never had as much power as it should even with a fresh top end. I mean Its got weighted fly wheel b pipe 44s delta 3s ada head and my stock sj walks away from it
- Location
- Charlotte, NC
you can still have high compression and still have a lower cranking compression... i did some porting on a 550 reed engine and raised the exhaust ports just over 1mm and cranking compression is at 125ish with a pjs head and copper gaskets..just saying, be cautious when removing gaskets.. the best way to compensate for the raised ports is to deck the top of the cylinder if i remember correctly
- Location
- Lodi
A ported 701 with a b pipe should annhilate a stock 701. im doubting the compression is the problem. Im no expert but i would look at your tuning and possibly cavitation.
- Location
- United States
yeah the things got me baffled
'Ported' cylinders and motors are often dogs for freestlye when built by inexperienced or ex-race motors. When previously ported cylinders and parts are sent in as cores for our motor builds, they are almost always rejected. Overporting the exhaust, porting in the wrong places, too much or too little, crappy epoxy work and/or wrong type of epoxy are all common.
Take matt e's advice, stop and find out what you have before dumping more time and money into what could be a lost cause.
Take matt e's advice, stop and find out what you have before dumping more time and money into what could be a lost cause.
h2odesperado
Pin it to win it
- Location
- Charlotte, NC
What pressure gauge are you using? It's common for differences to show up gauge to gauge.
There is something else wrong with your ski. A combination of high ports, low compression, and too much pump load can in extreme cases prevent the pipe from ever working. A SMALL EXHAUST LEAK WILL DO THE EXACT SAME THING! Check ignition timing and the impeller pitch, if the timing is good and the pitch is low then ride with the hood propped open to rule out an exhaust leak.
- Location
- United States
I know itll run terrible if there is an exhaust leak the b pipe that was on there had a bunch of hairline cracks i found after pressure testing it, so i fixed them and eliminated that. This may be a stupid question but how can there be to much pump load?
- Location
- United States
And looking at the cylinders it looks like only the exhaust ports were ported, sorry I didnt specify that.
Too much load on the pump. Impeller too steep, pump too big. The same thing as putting your vehicle into fourth or fifth gear and trying to take off from a stop. Overloaded.