exhaust port timings

[hendy]

I've just been reading a thread which has made me slightly nervous about the engine I've just built.

I have an Indian 190 kit matched to casings and manifolds that has had a mild tune

When i fitted it I remember noticing that the piston was about 2mm below the bottom of the exhaust port at bdc.

The thread was saying that it was critical that the piston didn't go below the exhaust port, but never mentioned why. Personally I would have thought that it was the opening time that was critical and the height of it was secondary, but I'm no tuner so my opinion isn't worth a lot!

Is this a big deal?

[hendy]

Additional information. I've just measured my port timings:

168 exhaust
131 transfer
153 intake

[tea5ive]

Additional information. I've just measured my port timings:

168 exhaust
131 transfer
153 intake

what exhaust, carb and gearbox are you using as this will have a bearing on what port timings you can use.


I have gone (after some advice from an expert)

178 ex
132 tran
150 in

will be using 125s gearbox 16/46
60/107 crank
28/30 phbh carb
and an ancillotti exhaust

don't no know how it will go as I am just building engine

[hendy]

I've got a 58/107 crank; wish I'd used a 60/107 but too late now.

25mm phbl and ancillotti. I've kept the airbox but drilled it and used a high flow bgm filter.

Got a pacemaker gearbox with 18/47 sprockets

This is the first one I've built so there's a lot of guesswork about what will work but that's part of the fun.

I think the gearing may be a little high but if it is I'll just drop the front sprocket size.

[eden]

having the piston going 2mm below the bottom of the port will just make it run not as well as it would if the port was level or slightly below the piston at bdc.
it wont rev with the timing you have as it only has 18.5 degrees blowdown. that's very short indeed!

[hendy]

having the piston going 2mm below the bottom of the port will just make it run not as well as it would if the port was level or slightly below the piston at bdc.
it wont rev with the timing you have as it only has 18.5 degrees blowdown. that's very short indeed!

Eden, you worried me so I went to double check.

Correct timings

Exhaust 168
Transfer 123
Intake 153

Furthermore my initial worry about my piston below the exhaust port was bs too! It is in fact above by approx 3mm!

At least I've learned a bit today about port timings

[shocky]

I have 148 inlet 117 trans 162 ex st4 with 110 rod 3mm packer scootrs exhast and hi flow jetex22 pulls 40mph in second 55 in 3rd top speed not known as im running in cough cough all by gps app on phone

[tarmac tickler]

if u can see 3mm of piston when it is at bottom dead centre through the exhaust port it will get nibbled to death by the heat, do not build as is it will self destruct.

[hendy]

if u can see 3mm of piston when it is at bottom dead centre through the exhaust port it will get nibbled to death by the heat, do not build as is it will self destruct.

I hope that you don't know what you're talking about but I suspect that I'm going to be disappointed for 2 reasons

1. There's a lot of info agreeing with you
2. I can't see a reason for extending the port below the piston crown as it's effectively closed below the piston

Please can anyone who knows what they're talking about give me 2nd 3rd and 4th opinions?

I think I'm about to get a new barrel!



Also, i double checked the distance and it's actually 4mm

[rosscla]

How close to the top is it at TDC?