Raise Tri Z exahust port 1.5 mm Big difference???

[1972mercury]

I got a 85 tri z and I'm rebuilding the engine and was looking at the Yahama wrench report I did shave the head 0.5mm and ported and polished the cylinder would raising the exhaust port that little bit make a difference or is it necessary???

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[christph]

I'm wondering this myself. I did the entire wrench report except for raising the exhaust port. I decided against it because I figured it would reduce the bottom end in favor of the top end, and for me it has enough top end. I have an 86 and with a 6 speed tranny I rarely top it out.

[pcs]

that is bc you need more top end =)

[badass350x]

Made a big improvement on mine..

[MTS]

The biggest improvment your going to see out the "wrech report" is raising that exaust port...

[christph]

The biggest improvment your going to see out the "wrech report" is raising that exaust port...

Describe the improvement if you would. Did you lose any bottom end?

[barnett468]

Hello pcs


Engine perf is a subjective opinion. Some people say top end is best and they don’t care about low end performance. Others say the opposite. I suggest you run it as it is and if you want more top end and less bottom end then go back and raise the ex port a bit. You can ALWAYS raise it but once it is raised you can NEVER lower it if you didn’t like the change.


PERFORMANCE – If you thought it had more than enough bottom end and it was noticeably slow but not horribly slow on top then raise the ex .75-1 mm. If the top was horrible than raise it 1.5 mm – 1.25 mm. Raising the ex port also lowers compression.


EFFECTS OF RAISING EX PORT – In general it increases top end power/rpm and reduces bottom end power. It basically raises the operating rpm range of the engine. You can’t have both, something must be sacrificed.


EX PORT HEIGHT - You do not need to raise it the full recommended 1.5 mm. You can start with half that and see if you like it.


COMPRESSION – Machining the head to increase compression is designed to compensate for raising the exhaust port and both should be done in conjunction with each other otherwise your compression may be so high that you now need race gas to keep it from detonating/pinging whereas you didn’t need it before.


DETONATION- If it detonates/pings much and you ignore it and don’t correct the problem it may quickly remind you it is there by either melting, seizing and/or exploding your piston into tiny bits of shrapnel which will be very hard to glue back together.




Keep it clean and on topic!!
Howdy

[pcs]

i go for top end then just gear the atv accordingly to my riding conditions. when i start messing with a motor its usually for the sake of increasing hp of the motor. if the motor loses some low rpm hp and i need a snappier throttle response then i will just gear it accordingly since sprockets are inexpensive. when you ride do you lug your engine and hit the throttle from low rpm or do you down shift and get the motor in a higher rpm and then hit it? most of the time im riding im around the middle to the top end of the rpm range of the engine so that is why i build towards that rpm range. IMO 250 engines are to small to lug around, if you want to lug around get a 500 or a 4 stroke, how often do you hear a 2 stroke lugging around in general? i always hear them reving out, RING DING DING =) . if i didnt want to make more hp then i would just leave the motor as it came from the factory and just ride it as is. you are 100% correct in stating its a subjective opinion based on the rider.

[El Camexican]

You did say you shaved the head, right? If so you could just put another base gasket under the cylinder to offset the shave and it will give you an effect similar to raising the exhaust port with the option to return if you want. This would be the cheapest, easiest and least permanent option, just don’t overdo it, 3 base gaskets max. and check the torque again after it has an hours or so of run time on it.

[Rigaman]

OH thats pretty smart... That raises the exhaust port also. Pretty cool.

[El Camexican]

OH thats pretty smart... That raises the exhaust port also. Pretty cool.

It's an old poor man's top end RPM trick. Common as putting on taller gearing for high speed tracks and such.

[greenhuman]

Widen the exhaust port 1mm per side. Same effect as raising without the loss of bottom end.

[El Camexican]

Widen the exhaust port 1mm per side. Same effect as raising without the loss of bottom end.

Have you done that on a Tri-Z by any chance? Just wondering as my cylinder was worked on before I got it and the exhaust port seems awful wide. I’m surprised it doesn’t have a bridge. Does anyone know how wide a stock ext. port is on a Z?

BTW I don't think widening the port will lower the compression which is part of what is needed to get additional RPM if he's shaved his head.