Porting questions

[tfaith08]

I bought a 98 Blaster for the wife that was "in great shape" and it turns out that the opposite of that was the case. A compression check revealed around 65psi so I tore it down. The piston and rings are in absolute terrible condition, but the bore is in surprisingly good shape. Neither I nor the friend/consult thought it needed a bore, but it will receive a cross-hatch.

This will be my 5th stab at porting. The bottom of the ports are all exactly how I want them, but I do have a question about the piston ports and the boost port. Why is there typically a knife edge on the material between the piston ports and the boost port with ported cylinders? I can see the piston ports having it, but once the piston opens the ports, nothing is flowing through the boost port. Wouldn't a slight radius work better here?

I'm looking at cutting boyesen ports as well and I see that most would prefer to beef the area up a bit, but I don't have anyone around that can weld aluminum. Epoxy is my solution, but some part of me knows that it's a bad idea, I just can't tell which part of me that is. I know MacDizzy filled his 250R cylinders to the brim with epoxy, so there's my "good idea" side... Any thoughts?

Lastly, I'm thinking of adding a 1mm spacer to the bottom of the cylinder to open the ports a bit more since the piston overlaps the bottoms of all the ports by at least 1mm. I know I'll have to deck the head or have it re-worked, but if I only deck it, I'll deck it 1.8mm, which is .3mm more than the spacer and anticipated gasket thickness so that compression is up a bit to accommodate the extra exhaust port height plus some.

 

[MrTwoJay]

Well, I don't like the idea of epoxy on the inside of motors, what if it comes loose.....
On the other hand, my uncle does porting and flowing for racing engines, and he claims it is perfectly acceptable to 'putty' up the ports in a cylinder head.
I would roughen op the area where the epoxi would sit, and a few angled holes would also help a great deal.

 

[tfaith08]

As far as the epoxy, I was mainly looking at putting it on the outside where the ports would likely cut through. I may add some to the inside though... Just some thoughts.

 

[tfaith08]

Bump. KOR, do you have any recommendations?

 

[tfaith08]

So slight update here as well as a few questions. After reading around a bit, I've come to the conclusion that the stock blaster base gasket is .5mm. My intent was to run a 1mm spacer and a second gasket, totaling an extra 1.5mm. With that, I'd have a total distance from the cases to the cylinder of 2mm. However, I can only find a spacer of 1.5mm and I'd have to use 2x .5mm gaskets, which would bring my total distance from cases to cylinder to 2.5mm. I'm content with that.

However, I do have a question about the head. What is the stock head volume? I can get away with decking the head for now since I have a local shop that I trust to do the work. Will decking the head 2mm and using a .018" (stock is .036") gasket present any problems? What should my static pressure be at about 150' ASL?

 

[tfaith08]

So I've been waiting for parts for quite a bit and have had the cylinder off for the entire time since I started this thread. I linked up with an old employer of mine who owns a Stihl saw and Scag lawnmower store. Though he owns the shop, his employees run it while he builds world record holding (Google Slayton stock appearing saw) hot saws. He does all of his porting, head cutting, carb mods, etc. and has been doing it for a very long time. We started talking about building and we each got pretty interested in what the other was doing.

Yesterday, I took my cylinder to him and the piston that I'm modifying and we swapped ideas (well, he swapped ideas, I just listened). He had 4 things that he recommended that I do to the cylinder that I have. To begin with, the cylinder has had the entire reed area worked, boyesen ports cut, bottoms of the transfer ports worked, and the reed cage has been ported as well. His recommendations along with those mods were to raise the area behind the boost port by about 15*, open the boyesen ports up by another 4-6mm (they're at 3mm x 5mm now with a very generous taper), reduce the radius of the corners of the piston ports to about 2mm, and to add a notch to the exhaust port.

My concerns with this are that he shoots for 5 digit rpm and super high rpm power on nitro and alky, and I'm shooting for power between 5500 and 7000 on pump gas. My thinking is that the first and the last mod would go well together since the notch would bleed the pressure in the cylinder a little earlier, reducing the need for such a high angle on the boost port.

Thoughts?

 

[Larry's Shee]

Interesting ...............
You're raising cyl so port floors are even with top of piston @ BDC?
Are you cutting top of cyl equal amount?
Have you calculated port timing with raising cyl?
More info on notch on ex port?

 

[Blaaster]

I am somewhat familiar with notching the exhaust port.

Notches were made in the port to lower compression to aid starting.

As the engine increased in speed the notch had little effect.

It was used instead of a compression relief valve.

 

[tfaith08]

Interesting ...............
You're raising cyl so port floors are even with top of piston @ BDC?

Exactly. Drew my inspiration from MacDizzy's build.

Are you cutting top of cyl equal amount?

I am from the head and then a tad bit more. The head will be the last thing that I work on. I'm shooting for 145psi with the notch. Now this is an estimate here, but I think it'll run like it's got closer to 150-155psi static once I get up in the RPM range.

Have you calculated port timing with raising cyl?

I'll get back to you on that.

More info on notch on ex port?

I'm looking at 3-5mm high and 3-5 wide up top and 5-7 wide where it meets the factory radius. There will be a nice bevel so don't worry. Now from my thinking, the notch will interact with the boost port a bit. The added blowdown at the lower rpm range SHOULD bleed pressure enough so as to allow for a more free flowing boost port roof angle. Now as Blaaster stated, the effect will be minimal as the revs increase. This is where I'm sort of at a standstill with the boost port. If I cut the boost port to play well with the notch, how will it affect the upper rpm range when the notch's effect is gone? Hmm.

My next concern is with the heat around the notch area. I know that I'm going to have to bevel it a bit, but I'm worried about the piston. Thoughts?

Before I tore the bike down, I rode it down the driveway and then showed the wife how to use a clutch and shift, etc. She takes off down the half-mile long driveway with the throttle pinned in 1st the whole way. I sprinted over to my Banshee and caught up with her about 100ft. from the end of the driveway. Yep. After that ordeal, I expect this top end to last a month AT MOST.

 

[tfaith08]

So there's been a significant change of plans. The area where we typically ride has been cut off. This leaves nothing but mostly tight trails and a few fast sections. Fortunately, we have over 400 acres to ride between all of us and I have a personal 20 acres that I'm cutting a 3/4 mile track into. Should be done by Friday.

Raising the cylinder is a no-go for now. However, a few of the topics still stand.

1. Exhaust port notch - need details
2. Boost port - mainly concerned with how it will act with the notch
3. To radius or to knife-edge between the piston ports and the boost port...
4. How big should I cut my boyesen ports?
5. How should I direct them for a trail port? Why?

Thanks for any help or ideas in advance.

 

[Blaaster]

The benefits of a notch may be interfered with by the kinetics of the exhaust header, especially at low revs.

One would need to research what type of header would be suited to notching the exhaust port.

 

[tfaith08]

FMF Fatty. It came with it and it doesn't have any damage, so I'm sticking with it until the wife gets good enough to develop preferences.

 

[rich1]

Instead of raising the cylinder why don't you just cut 1 MM off the intake side of the piston to change port timing without effecting the exhaust kinetics , old CR Honda trick.

 

[Larry's Shee]

IMO, the notch isn't needed. Not like you're trying to kick over a YZ490/500.

 

[tfaith08]

I mainly wanted to throw the notch in for the wife, but wanted to see if it made any difference in power (small, I know). She has trouble kicking my Banshee over and it's easier than the Blaster was.

As for taking 1mm off of the intake side of the piston, how will that affect the squish? I put some thought into it before you mentioned it, but I was a little leery. Now that you mention it, I'm rethinking it. I know it'll cut compression a tad, but from what I understand, the dome is horrible from the factory. Wouldn't that make it worse? Or would it?

 

[best]

I have shaved the top off the exhaust side to see the effect of a raised exhaust port without grinding an expensive cylinder. It does mess up quench and is a test measure only. I have filled the intake port with epoxy and strongly recommend it (pictures on this site somewhere). Boosts midrange and brings it on pipe earlier and longer and helps jetting consistency. Lifting the cylinder 1mm is good and it works and raises rpm, and you can shave the head to match, but a 3mm stroker may be another answer for the lower port mismatch, which doesn't matter much anyway. A modified head is THE place to start on these Blasters. Hemi for high rpm, torroidal for mid-range guts. RPM peak with these is typically around 8000.

Steve

 

[jus_me]

@ Best, I have seen the few mentions and pics in the past of filling the intake side with epoxy. What epoxy have you used with good results?

 

[best]

@ Best, I have seen the few mentions and pics in the past of filling the intake side with epoxy. What epoxy have you used with good results?

I've used several Devcon products but JBWeld and several "putty-steel" have worked well. For the pictures below we sandblasted the port to get good adhesion. Open the port windows before you add the putty.

Note the side plates to take up volume. This is the rough lay of epoxy.
It will be finished up with the porting tools.

The whole point it to keep consistent cross sectional area, just like a supersonic fighter.
Notice it is still a straight shot into the cylinder even with 3oz of epoxy filling it.

The "dimples are for the reed screws. No reed stops used now, the upper and lower surfaces are shaped to do that now.

 

[jus_me]

@ Best. I am sure you have tested the epoxy filled cylinder. Good results I assume? It certainly looks like the air would have a much easier time flowing through the ports with a strait clean shot than having voids and lets say "stagnant" areas. Seems like it would also kelp keep the fuel/air mix better atomized.

If anything i would expect filling with epoxy would give you much better throttle response. Time for me to look for another spare cylinder.

@ tfaith08 srry for kind of jacking your thread, it all pertains though..

 

[best]

@ Best. I am sure you have tested the epoxy filled cylinder. Good results I assume? It certainly looks like the air would have a much easier time flowing through the ports with a strait clean shot than having voids and lets say "stagnant" areas. Seems like it would also kelp keep the fuel/air mix better atomized.

If anything i would expect filling with epoxy would give you much better throttle response. Time for me to look for another spare cylinder.

@ tfaith08 srry for kind of jacking your thread, it all pertains though..

You are exactly right. Better throttle response at all rpm, easier jetting reads, and surprisingly a broader powerband, kicking in lower than before and reving out higher. This cylinder has been on a couple Blasters now and the epoxy has stood up well. No need to drill anchor holes, the sandblasting was enough. The intake windows were opened up very slightly, the bridge was left alone for piston strength, and the exhaust was raised 0.030" or so, were the only changes to this cylinder. It was run in a before epoxy and after test for power effect. Overall HP increased only slightly but the broadening of the powerband was awesome.

Steve