The Path to Modification

[190mech]

Bell's book shows 184-186 exhaust and 126-130 transfer durations for a 9000+-rpm setup,those are reasonable numbers..Getting the transfers adjusted first(hardest),then exhaust port(easiest).and on to squish adjustment..

[Kenho21]

Oh okay. I see the numbers in the charts in figure 3.1 and whatever the other one is now. I thought those numbers varied, missed where it said those measurements were constant.

I’ll probably go in the low/middle range of those numbers to play it safe.

When I’m actually removing the material, is it best to just raise the ports? Should I aim for something like 90GTVert does in his video?

[Kenho21]

And in regard to squish, I generally just want to be within 0.7-0.9 mm, correct?

[190mech]

Its difficult to modify transfer port windows so the whole cylinder is raised or lowered to get correct numbers,then the ex port is cut to the proper size,widening to 68% chord width,then height set..The .7mm squish distance is a pretty common dimension,could go less if all the parts are high quality..

[Kenho21]

Okay, so let me go over this and make sure I'm getting the gist.

The transfer port will only be adjusted on the Y axis and via moving the cylinder up or down, then the exhaust port will get cut to 68% of chord width (going to lookup what that is haha), then the desired height is cut into the material and it's this cut that determines how many degrees duration is, correct? Then hone with the honing tool I bought.

After that, it's just dealing with squish and then tuning. Am I missing any broad strokes as far as porting goes?

[Kenho21]

This chord width?

infouse.com/planemath/activities/pmenterprises/wingshape/wingshape54.html

[Kenho21]

Nope... it's not that haha

[Kenho21]

I think I've got most of my own questions answered now, but I'm unclear as to whether I should be looking at removing material from the transfer ports or just making adjustments with gaskets?

[90GTVert]

Chord Width :
LINK


Transfers are adjusted by raising or lowering the cylinder or by filing the piston crown in some cases. You don't want to mess with the transfers with a grinder. First, it takes a small right angle tool that most don't have. Second, you can screw up the way that they are aimed and flow into the cylinder. So you'll prob use a degree wheel (or there are calculator programs, but I have always used a degree wheel) and see where the cylinder starts out with the supplied base gasket. If those are really close to what you want, you may not do much to the porting/spacing, though you may still want to widen the exhaust port and work it over a bit. If the durations are high (not usually the case with sport stuff), then you can consider using no base gasket or a thinner one. If durations are low, then you may want to raise the cylinder with a thicker base gasket or spacer. Ideally, you'll find transfer durations that are basically what you want and the transfer floors will be flush/even with the piston crown's edge at bottom dead center. Then all you'd have to do is work on the exhaust port. Just kinda have to check and see what you've got before you know much.

In addition to all of that, you have to keep your squish clearance in mind. Let's say your durations are all low and you want to raise the cylinder, but raising the cylinder would give you 2mm squish. Then you have to decide if you are capable or desire to try to mod the head to knock that down to where you'd like to see the squish or not. If it's only 0.5mm off or so, that may be covered by just not using a head gasket. Getting the ports setup just right won't do much if you dramatically lower compression, which is going to happen if you have a large squish clearance most likely. Plus it's good to have a tighter squish.

[Kenho21]

Great tutorial of a rebuild there. I was reading over all that info probably as you were typing that response haha. I was thinking of saving squish checks until the end, but I'm glad you mentioned keeping compression in mind and measuring squish beforehand. I'm hoping since everything is still stock as far the the cylinder is concerned, I won't be dealing with extreme numbers. Lot to consider...

If I end up having to go without a head gasket, should I still use some RTV?

Should I get going on some of this today with the existing piston or wait for the new piston to come in tomorrow? Just a stock replacement piston from PFS.

[90GTVert]

If you don't use a head gasket, most of us use copper spray.
www.permatex.com/products/gasketing/gasket-sealants/permatex-copper-spray-a-gasket-hi-temp-sealant/


It wouldn't be a bad idea to at least get setup with the wheel and make sure everything is in order. Make sure you don't need anything else for that and collect whatever else you think you may be missing. Most likely the replacement will be close, but sometimes the compression height of pistons is a little off and that would change basically everything that you're trying to check. If you get the wheel setup, then you could take a look. Again, it will likely be close and could even be spot on. I wouldn't trust it enough to mod too much till I checked with the piston I'd be using though.

[Kenho21]

Okay, sounds good.I'm going to get all the stuff together tonight to make a leak tester and piston stop and get as far along as I can.

That was kinda my plan already, just wasn't sure how far I could go without all the necessary pieces though. In all likelihood, the parts will be waiting on me and not me waiting on the parts to arrive though lol. I doubt this will all go as smoothly as it always seems to in the movies haha.

[Kenho21]

I'm assuming a rejet will be in order after this?

[90GTVert]

You will at least want to do some test runs and find out if it likes a different jet.

[Kenho21]