Increasing Looper Compression

[doogall]

For the V6 looper (1991 225 in this case) is it better to cut the head to increase compression or block the idle relief holes?
Do you get more power from the smaller combustion chamber or is it from the increase in compression?
I have the 331490 heads and presently have 110 lbs at cranking with a good battery. I'm looking to bring that up to 125 - 130 lbs. But which way is best?

Thanks in advance
Craig

[baja200merk]

Use the 490 heads. If you havent already, use thin head gaskets from 93-up engines and plug the idle reliefs. Should get you some more squeeze, just remember to run good fuel 93 octane.

[phil blomquist]

Plugging the reliefs will bring the compression up 10 psi or there abouts, so i would do that first before cutting the heads. Those heads are coming to be to hard to find hate to see them get cut up too much.

[doug vaughn]

how do U and where do U plug the reliefs

[Matts94z28]

how do U and where do U plug the reliefs

I found a thread with the explanation a while back, but can't find it now. I may do this as well.

[Lockjaw]

Most people use marine tex epoxy. Pull the heads, spray some carb cleaner or brake clean in the holes, and then plug with epoxy. Make sure you have some acetone or something to get the residual off, its kind of a pita to stuff it in there. Then let it dry, and you should be good to put the heads back on.

I did my 91 150 block that way. The nice thing about epoxy is you could drill it out easier then trying to thread it and run a pipe plug down the hole, which most agree distorts the bore.

Be sure you get some jetting advice, any change to a looper requires re-jetting. I'd probably be looking at going up at least a couple sizes on the main jet and then working back down after some good plug reading.

Always better to be rich then lean when you are messing around.

[doogall]

Thanks to those that have replied. But as yet nobody has answered the main question, which was about which method creates more power. Or are they both the same?
thanks Craig

[JohnR]

I would guess that if you raised it 10 psi both ways you'd be better off with 10psi from plugging the reliefs because cutting the heads will change combustion chamber shape, probably for the worse and that is pretty important in a 2 stroke.

[phil blomquist]

More compression more power!. Pick your poison or both dont forget to jet up on the mains you will pick up about 10.psi either way or possibly more on the heads depending how much is cut.Did you ever bolt on the heads that were cut and test the compression? i would do that to see where your at Before doing anything .Cheers

[Lockjaw]

Thanks to those that have replied. But as yet nobody has answered the main question, which was about which method creates more power. Or are they both the same?
thanks Craig

I would say properly done, the head cutting would offer more power potential, since you can raise the compression more then with idle reliefs, however, you are risking powerhead damage if done improperly or you get a batch of bad gas. You will have to rejet, which adds to the cost. I think main jets are like 7 or 8 bucks a piece now.

[racer]

Plugging the idle relief gives more low end because as rpm goes up less escapes out the relief hole.

[doogall]

Plugging the idle relief gives more low end because as rpm goes up less escapes out the relief hole.

Racer, thanks for explaining the difference. I hadn't considered the total effect of the constant bleed off of compression through the relief. Has anyone used J B Weld high temp expoxy for this as we don't have your popular marine tex in New Zealand?

[Lockjaw]

I bet that would be ok to use. Marine Tex isn't high temp that I am aware of.

The main thing you got to worry about is it hardening in the bore. When I plugged my idle reliefs with marine tex, it was tough getting the excess off the bore. So have plenty of shop towls handy.

[davemvegas]

Racer, thanks for explaining the difference. I hadn't considered the total effect of the constant bleed off of compression through the relief. Has anyone used J B Weld high temp expoxy for this as we don't have your popular marine tex in New Zealand?

it is not constant bleed off. after about 4000 rpm engine speed. no compression really bleeds off. I do agree that plugging idle reliefs gives a much better hole shot and midrange. but I do notice that it sounds like a barrel of marbles when you back off the throttle. it would be nice to have a ratcheting lower unit like a mercury has. I plug mine and cut the heads to get 130 psi. I forget what side. but port or starboard head needs to be cut .025 more to even the compression.

[wanagofass]

it is not constant bleed off. after about 4000 rpm engine speed. no compression really bleeds off. I do agree that plugging idle reliefs gives a much better hole shot and midrange. but I do notice that it sounds like a barrel of marbles when you back off the throttle. it would be nice to have a ratcheting lower unit like a mercury has. I plug mine and cut the heads to get 130 psi. I forget what side. but port or starboard head needs to be cut .025 more to even the compression.

Why the difference in compression?