Squish Clearance

[Fastdadtsmith]

I have been playing with a new motor, cub cylinders, 34 pj's, Noss head, 16cc domes,FMF pipes. The guy I bought it from said it was a long rod motor, but I have my doubts. Compression was low at 112 lbs with 18 cc domes. I have not checked yet with new domes. Squish clearance with 18's was .054 and .057. With 16 cc domes squish is .053 in both cylinders. When it ran with 18's, it lacked bottom end, but when it hit, HOLD ON!! I have been talking with DL Noss about domes, but I am a little confused on how to reduce squish clearance to hopefully raise compression. I would like to see about 160-175 lbs. Also, how to check stroke? Thanks for the help..... :beer:

[badassbanshee479]

thinner base gasket will reduce squish but I think you are in a good spot with yours, you reduce the squish you increase the chances of detonation. if you want more bottom end I would say turn the timeing up! what elevation are you at? sounds real low on the compression. are you holding it wide open when you check it?

[Fastdadtsmith]

Timing is +6, elevation is 2500 ft. I have other quads that require good gas, so I run straight C-12 with Super M @ 40-1. Have never lost a piston yet (after getting jetting correct). I haven't heard any detonation yet but my ears could be bad from all the 70's and 80's hard rock. Is there a better way to LOOK for signs of detonation before I see and feel it?

[badassbanshee479]

listening for it is about the best way. I would check your compression again maybe with someone elses gauge or something cause with 18cc domes at 2500 ft you should be in the 170psi range. are you sure the guy that put the engine together used the right pistions? cause if he put long rod pistions on a standard rod motor you are gonna get low cranking comp. can you see the numbers on the top of the pistions still? if so what are they? should be able to check the stroke by measureing the pistion at tdc then measureing it at bdc stock stroke is 54mm

[dajogejr]

You won't gain much (lose much, make that) squish clearance by going to a smaller cc dome, if the squish angle on the dome is cut the same as the previous dome....

 

I agree, at that elevation the comp sounds kinda low, unless someone got their hands on that motor and gave it a helluva drag port.

 

I run 185PSI in mine, on gas...with a great igntion system and a TON of timing, and I still wouldn't write home to mom about the bottom end. Just not in the nature of this beast...plus, Shearer style inframes don't help bottom....

 

Here's the tricky part. You're gonna have to mock up that motor with different thickness base gaskets to get the desired squish. I believe mine is at .045....

 

I agree with JT...you're gonna have to listen to the motor. It will tell you...but not for very long.

Once you start to see the evidence of it on your spark plugs (spotting of aluminum) it's too late.

[Fastdadtsmith]

[Would it be such a bad thing to run squish where it is now? I'm just lacking the compression and not getting all out of it? I'll pull the head off again and yes I can still see the numbers on the pistons, I'll post them up asap. Would a cylinder gasket that is .010 thinner build up compression by 60 lbs, or should I be looking for another problem even though it runs great after about 4500 rpm? :shrug:

[dajogejr]

You're not going to get 60lbs more out of a thinner base gasket. You'd be lucky to get 10 to 15, and by then...the squish would be too small and risk causing damage.

 

If it's making power at 4500RPM you're probably doing fine. Mine doesn't start making real power until 6K or so, and I've got more compression, more timing AND a tighter squish....

[Fastdadtsmith]

Thanks dajogejr for all the input. So to clarify the low compression numbers I need to look at piston number for correct model? Any other areas that could be giving me low compression, and still run OK? I will be testing with gauge #3 tomorrow to verify. :shoothead:

[dajogejr]

I think your squish is ok....or within acceptable, anyways. I'd check the piston ad JT said.

I'm not sure what size domes I bought, but...I think they're 19cc or so.

 

You didn't get that thing drag ported, did you?

[bansheeseat$$]

I have been playing with a new motor, cub cylinders, 34 pj's, Noss head, 16cc domes,FMF pipes. The guy I bought it from said it was a long rod motor, but I have my doubts. Compression was low at 112 lbs with 18 cc domes. I have not checked yet with new domes. Squish clearance with 18's was .054 and .057. With 16 cc domes squish is .053 in both cylinders. When it ran with 18's, it lacked bottom end, but when it hit, HOLD ON!! I have been talking with DL Noss about domes, but I am a little confused on how to reduce squish clearance to hopefully raise compression. I would like to see about 160-175 lbs. Also, how to check stroke? Thanks for the help..... :beer:

Your compression readings sound like you have a cylinder desighned for a different stroke crank.I went through that with my P.V engine.Had 115 psi with 17 cc domes.once I relized the crank was the wrong stroke and installed the correct stroke mine went to 195 psi with the same 17 cc domes and I'm at 4500 feet.

[No Limit Powersports]

First thing we need to know is what motor it is exactly there are a lot of combinations of cub motors. Mid .50 squish imo is way to much i run all my motors between .036 and .040. You will gain some compression by tightening down the combustion chamber, but what motor you have depends on how you do it. Each motor is design to have a paticular base gasket in it to keep the port timings right even know that every builder has a opinion on that. If it is a stock stroke stock bore cub with a long rod you should be using a .022 base gasket which will leave the pistion .018 in the hole so then you have your dome cut with the correct step in it for the squish you want. If the motor is a stock stroke 68 bore with a long rod it should have a .012 base gasket which will leave the piston .018 in the hole... the reason there different is because a 65x54 long rod uses a 795 series piston and 68x54 long rod uses a 573 series piston.

[Ducman]

Theres 2 routes you can go.

 

1. Mesure your squish (you already have) and send the domes to Noss, He can then cut you domes with the correct squish clearance. He can also cut the dish of the dome a little higher with a wider squish band or flatter with a narrower squish band to give you a little more low end or top end power.

 

2. If you have a stock .030" base gasket, get a .010" base gasket.

 

If you have a .030" gasket then I would reccomend route#2. By going with a thinner base gasket you will lower all the ports and tighten up the squish at the same time which should get you more static compression than just getting new domes cut. I had the same exact problem with my cubs and I got new domes. My compression now is around 135 with 19cc domes (.040 squish) and it was around 115 with 20cc domes (.065" squish if I remember correctly). I should have just went the .010" base gasket route and new domes but I didn't want to pull the cylinders since it was a new motor. Another thing that I had all jacked up was that I used "Off the shelf" prodesin big bore domes and they were cut for banshee pistons, I have blaster pistons which have less of a dome = less degree of an angle to the squish. So my squish clearance was around .065" near the edge and off even more once you went towards the center of the cylinder.

 

 

Also on the detonation, you wont always hear it. I killed a set of pistons on a +4mm stock cylinder dune port motor with 19cc domes running 100 octane because I went a little too lean. I was only like 135 psi due to high exhaust ports. My pistons were covered with voids around the edges like little pin pricks and I never heard any obvious detonation noise and trust me I can ususally hear it from a mile away.

[Fastdadtsmith]

Thanks for the help guys. I bought a new compression tester yesterday, 175 lbs both cylinders. Still have idle trouble, but response is there, everything sounds good. I am going to ride next week and play with jets, any recommendations? I am currently at 48 pilot, CEK mid clip, 155 main. Sure sounds and feels good down the street, Dumont next week for testing and 4 stroke munching!!!!

[dajogejr]

I would say by your jetting, you're damn close....a little fine tuning one way or the other.

175 is more like it. Can't tell you how many times I've seen el cheapo compression testers show off the charts...

[Fastdadtsmith]

Thanks for all the advice on this thread. Banshee ran GREAT. A little lean after first plug chop, went up on the main and it ran great all week. Never hot, started within three kicks even when cold, def need a longer swingarm now. Again, thanks for the advice, much easier typing then throwing wrenches!!!!! :beer: