First, i would like to thank the guys here.....I think i am getting a handle on these little motors now.
Now, The basic stat's that were listed, are these for a mid range motor? top end? or just a good, all around, preforming motor?
Also, If i were to raise the e-port roof more than the .5mm i need to make those numbers, will that make it a high rpm motor? or do i have to raise both the transfers and the e-port?
~Now, The basic stat's that were listed, are these for a mid range motor? top end? or just a good, all around, preforming motor?
Also, If i were to raise the e-port roof more than the .5mm i need to make those numbers, will that make it a high rpm motor? or do i have to raise both the transfers and the e-port?DK
In general the higher the ports the more peaky it will be. The spec listed may lean slightly towards favouring the top end more but it’s by no means a balls out everything or nothing setup. There’s plenty of room on that to go more extreme if you wish.
It’s important you keep the distance between ports at an expectable amount as this is your blowdown. I think with your current spec you may need to raise the transfers a small amount.
Remember that when porting you can’t add a bit back on if you take too much off! But you can always take more off if its small. If you can be bothered to just gradually open up the ports, run it, open a bit more - run it and so on until you are at a point where you reach what your after then that’s the only way really to get exactly what your after.
My setup only has 20 deg of blow down. I think i need to raise the e-port to get it up to 25 to 30 deg to start with. I just took out the gasket but, have not been able to try it yet. If it works then i will cut the transfers and e-port higher to fix the timming.
The reson i was asking is, i am thinking about getting a two piece top end. When i goto a big track i can shim up the jug to move the power band up to the top end (more duration) and when i goto the small tracks i can drop it to get the low and mid that i would need(less duration on the ports). I am sure its not that easy but, that is just a general idea. Right?
Righty, I have made myself a Excel spreadsheet using the Specific Time-area numbers and formulae as given in the ‘2 Stroke Tuner Hand Book’ (don’t worry, it’s dead easy to use).
All I do is give it the CC (40 in this case), the port durations and the RPM I desire the engine to make it’s power at and I get my area’s!
For my durations of:
Exhaust = 172 degrees
Transfer = 120 degrees (boost is 118 and mains are 122 so it’s the average of the 2)
At a desired RPM, for example, of 13,000 the mean-port-area’s needed are:
Exhaust = 263 mm2
Transfer = 234 mm2
If I go back and look at the area’s I first just guessed that were: Exhaust = 296mm2, Transfer = 256mm2, and now look at my spreadsheet, I can see that these area’s really need an RPM of 14,000. Just purely by luck they both actually relate very well to that RPM.
But, I need to bare in mind that the area’s are the Mean-port-area’s (as worked out using the methods in the 2 stroke tuning guide). So therefore the Total port area’s are larger by approx another 30 percent and need to be worked out by drawing it all up. I’m lucky in that I’m using CAD to draw up my port maps which means I can easily tweek the ports size and shape to get exactly these values.
The spreadsheet also gives the mean-port-area’s for a range or RPM’s to display how they vary, all in one place.
I am more than willing to share my spreadsheet if anyone can tell me how to post it up here I’ll do that
Here's what it looks like
Last edited by Mr_Cag : 12-20-2005 at 05:21 PM.
Reason: updated pic
If you cant post the program, can you do me a fav??? Put 40cc motor with 152 e port and 110 t port?? Would like to see how it stacks up against a stock head.
Just fill in the light blue parts (you can't actually fill in any others as their protected) and the Port Area required magically appears in the yellow boxes which correspond to RPM's
Really you don't have to pay any attention to anything not in a coloured box as it's only given for interests sake. As mentioned in the '2 stoke tuners guide' the exact port area required is dependant on almost an infinite amount of factors like exhaust design, atmospheric conditions, air temp, fuel, fuel oil ratio, etc etc. But does lie somewhere within the range given. I will be aiming for the mid point in the range, which is why I’ve chosen that as the main result of interest. Enjoy
If you cant post the program, can you do me a fav??? Put 40cc motor with 152 e port and 110 t port?? Would like to see how it stacks up against a stock head. Thanks. DK
Did you use it? Well i put your numbers in with a range of RPM from 7-11,000: see attachment for results
no, i have not had the chance too. I have all next week off soo, i will be able to put excel back into my comp and try it. Then i have to map out the ports and see where i am at.
ok, just got a quick test on the motor with the base gasket off. Yep, it made a diffrents!! A lot pepper(is that a word?) now. Seem's to have more snap threw out the power range. It accelrates(sp?) better now. Gets to top end faster but, not sure if it has the top end it did before.
I just rejeted, had to cause of the cold weather. I tried 4/66 and it was way rich on bottom(new ufo aircleaner) and a little lean on top. Plug was tan/white. I am now at 3/68 and it idles better, does not hesitate and sounds mean but, its too dark to try top end and see if its rich on top or not.
I have a whole week off and am thinking i should raise the e-port a little to see how that works. I am thinking that with ony 20 deg blowdown now, it should make a big diffrents. What do you think??
Yeah give the ex-port a couple mm height. it won't do it any harm as long as you don't go too mental with the dremel! My ports need 3.7mm of the top of the Exhaust port, and 1.8mm of the top of the tranfers. so you can use that as a guide. i wouldn't go for that much tho as your not running a base gasket and i dunno what your ports are curretly at. but 2mm off the ex and 1 off the transfer i would say is safe to do, but i must stipulate that any thing you do is at your own risk!
With the gasket, mine was good on the transfers and the ex port needed to be raised about .5 to 1 mm to be timed. Now i took the .5 gasket out, it needs the .5 off the transfers and 1 to 1.5 off ex port. Now, that is just to get to the base numbers. Not sure how high the timming can get before it starts to act like a peaky beast.
maybe i missed it in the post, but did anybody come up with a base guideline or ideal spec for these engine and will that vary for the mods done, ie.. bbk,fcsc??.....or do we have standard #'s we can apply for openings,durations,blowdown that is ideal across the board irrelevant of mods, kinda like squish.. we know .5 is ideal..
oops.. guess this is what i was looking for...i'm guessing this guideline holds true for all modifications???
A good general guide for exhaust, transfer and blow down timing is (plus or minus 5 degrees):
Exhaust= 90 degrees ATDC
transfers= 120 degerees ATDC
Blow down 25-30 degrees ADTC
So my engine standard the exhaust opens nearly 20 degrees to late! has too low a blow down time, and the transfers aren't open long enough buy approx 20 degrees (duration).
It really is quite a poor spec!
On a engine such as the cags where the stroke is only 32mm 2 degrees on the crank equates only to about 0.4mm of stroke! so if you're messing with your timing - you have to do it accurately! E.g. If you raise the roof of any port by only 0.5mm you are changing the timing by approx 3-6 degrees, which is a lot!
I'm fortuate that I am a Toolmaker for a large company so have access to cutting edge machines I can machine in the ports to the nearest micron (1000th of a millimeter!) which is nice. I'll post some pics of the process as i do it for anyone who's interested.
oops.. guess this is what i was looking for...i'm guessing this guideline holds true for all modifications???
A good general guide for exhaust, transfer and blow down timing is (plus or minus 5 degrees):
Exhaust= 90 degrees ATDC
transfers= 120 degerees ATDC
Blow down 25-30 degrees ADTC
Umm the blowdown is a typo, it's not ATDC its Duration! sorry. It should read 25-30 degrees duration. I.e. from TDC,you'll have 90 degrees of swept bore, then the exhaust opens then you'll have 25-30 degrees of the exhaust open on its own before the transfers open.
Google dominantmonkeyracing, and read the specs for the different engine he's timed, to get a feel of things.
