Compression Seems Way Too High

[tgbanshee]

I have about an hour of mild riding on a fresh rebuild. F.A.S.T. coolhead w/ 21cc domes, 4mm welded stroker crank w/ spacer plate, fresh bore and pistons .020 over, +4 degrees timing, pod filters, vforce3's, stock porting. I have 310 mains and 27.5 pilots in the stock carbs, running 93 octane with Klotz Super Techniplate at 40:1. I'm at about 180 feet elevation where I ride here in NJ. Leak down test passed, 6lbs for 6minutes needle didn't move at all. Squish was .048" in the left cylinder and .052" in the right cylinder. I've checked my compression twice since the rebuild and have kicked 180psi in each cylinder both times, and this was in 35 degree weather with a brand new Matco compression tester. Anybody have any clue why my compression is so damn high? I would think it should be around 150-160psi at my elevation with those domes. Does this mean I need to run race gas? If so what octane should I run and can I do a 50/50 blend or something along those lines? The bike runs like a raped ape but if it needs to be running race gas to keep from damaging the motor, I don't want to ride it until until I pick some up. What do you guys think?

[Handyman]

Oh yes I'd run straight race gas...they say anything over 160psi should be nothing but race gas..but some run a mix of 50/50.. I run 20cc domes at 174psi...at the same elevation and run straight race gas..... The stoker crank has a little longer stroke than a stock crank which in turn compresses the air/gas mixture a little more causing a higher compression number...

[qboronyrican]

according to the charts ive see....with 21cc domes you should be around 157 but maybe a little more cause of the stroker crank....i have 20cc domes at the same elevation and if i remember corectly i think i was high 160's when i checked it last and thats about right and i run 50/50

[Bansh-eman]

check it with another tester to see if it reads the same. if your comp is actually 180 you need to be running race gas

[4stroker]

you dont NEED race gas unless its detonating, doesnt matter what the guage says

 

pull the head and look for signs of deto in there... pitting on the top of the piston and/or combustion chamber...

 

having said all that i run 98 octane pump fuel in my bike and it has 180psi, no signs of deto whatsoever when i pull the head

[locogato11283]

call or pm jeff and see what he thinks..

[tgbanshee]

I'm wondering if maybe my domes were mislabeled and I got 20cc or 19cc domes. I can't see that having a +4mm crank would cause the compression to raise 20psi higher than it should be. I'm def going to email Jeff and see what he thinks.

Edited February 24, 2008 by tgbanshee

[BigRed350x]

Just switch over to alcohol or methanol and don't worry about the compression. More power, runs cooler, and your compression wouldn't be an issue.

 

I would definitely pull the head and check for detonation. You will see very small pitting starting out at the edges of the piston and in your domes. Detonation is an evil foe. It creates thousands of pounds of pressure inside your cylinders and will trash a crank in short time. You might be lucky and have caught it early enough to fore go any serious damage. If there's a lot of evidence of detonation I would check your crank for excessive play and check the welds on the pins for any cracks that might have started.

 

Pull your head and check everything and actually CC the head and check. You could also change over to a thicker base-gasket to help the compression a little, especially since you're running the spacer plate. Put a thicker gasket below it and above it.

[tgbanshee]

Just switch over to alcohol or methanol and don't worry about the compression. More power, runs cooler, and your compression wouldn't be an issue.

 

I would definitely pull the head and check for detonation. You will see very small pitting starting out at the edges of the piston and in your domes. Detonation is an evil foe. It creates thousands of pounds of pressure inside your cylinders and will trash a crank in short time. You might be lucky and have caught it early enough to fore go any serious damage. If there's a lot of evidence of detonation I would check your crank for excessive play and check the welds on the pins for any cracks that might have started.

 

Pull your head and check everything and actually CC the head and check. You could also change over to a thicker base-gasket to help the compression a little, especially since you're running the spacer plate. Put a thicker gasket below it and above it.

I'll probably pull the head and check for signs of detonation next weekend. I really hope nothing is damaged too bad, everything is brand new in this motor except for the trans, and I def don't have money for another damn crank right now and won't until the summer. Not to mention the crank that is in there is brand new and it would really piss me off if I had to chuck $500. :mad:

 

Is there a proper way to CC the domes? I would assume you block off the plug hole at the bottom of the face of the dome and just fill it with water up to the top and measure it with a graduated cylinder.

[yamaha04]

yup just put your spark plug in all the way tighten it down like normal then measure the water you get in

[tgbanshee]

yup just put your spark plug in all the way tighten it down like normal then measure the water you get in

Does it matter that a small amount of the water is going to flow into insulator of the plug and possibly throw off the measurement? Or would it be better to put some electrical tape over the hole?

[inthemail]

your insulator wont even hold 1cc of fluid. round your findings.

[FASTOYS]

LOL,wow lots of opinions on here and some of them could cost you alot of money in repairs. :sad: Ok at 180 compression you WILL need at least 50/50 mix or straight 108 race gas depending on timing etc..With the squish you have you do NOT want to run more base gasket thickness. Your high compression, IF your guage is accurate is because of your almost sea level elevation, 4mm stroker plate and crank, extra crankcase volume, and NO porting. You may need to run 23cc or get the cyls. ported to lower the compression. Oh and in time yes it will show signs of detonation w/180 compression if running pump gas and advanced timing. Please call me if you have any questions...

[BigRed350x]

Jump over to planetsand and do some reading up over there. Lots of helpful information about your squish, dome size, compression, and detonation information.

 

Above all, your engine builder should have checked and corrected/suggested what to do before you ever ran it. If you built it yourself, you should have checked your compression before ever riding it.

 

The spacer plate is not generally the best way to do a 4mil. There is more of a chance of a leak with the spacer plate. Call some of the big-name builders around the country and they can set you up with some good porting. A&S, K&T, Nate McCoy, Dave Moore, Louie at Twister, LC Performance, Packard. There's also some of the smaller guys here on the site who can help you out. FAST, HerrJugs, Flotek.

 

Use the search button and do some research on 4mil porting, and go read up on planetsand in some of their 2-stroke technical posts.

[tgbanshee]

Jump over to planetsand and do some reading up over there. Lots of helpful information about your squish, dome size, compression, and detonation information.

 

Above all, your engine builder should have checked and corrected/suggested what to do before you ever ran it. If you built it yourself, you should have checked your compression before ever riding it.

 

The spacer plate is not generally the best way to do a 4mil. There is more of a chance of a leak with the spacer plate. Call some of the big-name builders around the country and they can set you up with some good porting. A&S, K&T, Nate McCoy, Dave Moore, Louie at Twister, LC Performance, Packard. There's also some of the smaller guys here on the site who can help you out. FAST, HerrJugs, Flotek.

 

Use the search button and do some research on 4mil porting, and go read up on planetsand in some of their 2-stroke technical posts.

I built the motor myself and did check the compression before I started it but from what the general consensus was as far as what I should be getting in compression from everyone (150-160psi) , I thought maybe it would lower after it broke in which was def not the case. This is my first banshee engine build so it has been a good learning experience. The hour of riding time that it has on it wasn't very hard riding and 45min of that were very easy riding with the break in mix.

 

As far as the spacer plate goes, I know it's good spot for an air leak to develop but unfortunately I did not have enough money at the time to have the jugs ported for the 4mil and was steered away from getting cut domes because I was told that port timing would be off with them and no porting. I also used a very thin layer of Threebond 1211 on each side of both base gaskets as added insurance.

 

Next winter I'm going to send my jugs to Jeff and get a modified trail port for the 4mil and ditch the spacer plate. For the time being I think I'll run race gas and probably switch to larger domes when it gets warm and I start riding it more so I can run pump gas again.