Triumphrat

RockyRaccoon

ok , ive gotten tired of lurking around this site reading every forum as if it were a novel so i decided i'd join up finally. i dont currently own a triumph but have had the pleasure before , in fact i just sold my bike (honda shadow) a few months ago. My next bike will most definitely be a thruxton, but i cant buy it yet as im in the service and will be deploying for 6 months shortly (bummer). the problem is i started looking at triumphs and ive got the itch but no time to scratch so to speak. so untill then ill just have to continue to find satisfaction in reading about yall's bikes. now here's my question.

ive been reading a few very old posts about porting and polishing heads and they all seem to say the same thing (do not polish the intake runners for fuel atomization reasons) ive been p&p'ing my automotive(cage) heads for a while with great success and i believe strongly in leaving the factory cast minus burs and heavy casting marks in the intake runners for street applications . this is where i get conflicted, for street apps in a car engine you should leave this factory cast surface but in nonstreet applications intakes can be polished because of the higher rpms that the heads will see thus imperfections are no longer needed to create turbulence in the mixture . now with that being said most street driving accurs between 1500 and 2500 rpm's track driving occurs between 3000 and 8 or 9000 rpms. based on the fact that head polishing is ideal for a 3000 - 9000 rpm engine and these bikes cruise around at 4000 rpm's and dont spend much time in the lower rpm range (unless you lugg the engine every where you go) i dont see any problems with polishing intake runners to achieve better flow. any thoughts?

Gob-ny-geay

RR,

I give this a shot, but I've got a feeling it's gonna be a long, fun and interesting debate.

I apologise way far in advance if you already know all of the following, but you're having posted and asked the question, I'll assume you are seeking advice.

a.) The main purpose of P&P ing cylinder heads is to increase real and potential CFM flow. Likening the engine to an air pump - more air in/compress it/ignite it/get it out quick.
b.) Most flow in head modification work is gained by straightening out any turns within the intake/exhaust ports, and smoothing / polishing around protruding valve stem areas to reduce mass and increase potential CFM flow around these obstacles.
c.) Additional flow potential is gained by multi-angled valve and seat grinding as well.
d.) None of this should ever be attempted without a high quality flow-bench. The days of "seat-of-the pants" porting and polishing are long gone.
e.) Todays combustion chambers are often factory produced via a CNC machine, ions more accurate than the "Old" days. That said, there may not be huge gains to be had. Some, but not huge. If you are a cronic "Modder" the fun of obtaining that last tiny bit of performance is very likely achievable.
f.) Air flow within the combustion chamber, and getting the drawn in air to compress and ignite evenly - Squish, and terms of that nature, are also huge considerations to undertake. Again, impossible without a high grade flow bench.
g,) Since with Triumph Twins we have 2 cylinders to deal with, equal chamber volume, and exact match "squish" factors are extremely important as well.
h.) Matching ports (carb intake to head, exhaust port to exhaust header inlet) is a good thing as well, to prevent potential blockage/ unwanted turbulance.

Once you've done all this, the question of whether or not the standard carbs can match (allow) the potential flow increase arises, and then re-jetting, and then perhaps considering aftermarket higher flow exhaust piping. All of which call for the aforementioned flow bench.

Now, finally to your original question: Noting the above, polish/non polished intake walls has been a debatable subject for years. More important is to NOT reduce the velocity of the intake rush, compromised when we polish, and in doing so remove material from the inner walls. It LOOKS nice to polish these, but I would be more worried about removing material and decreasing velocity potential. The I.D. of the intake port determines the Velocity of which the charge enters the chamber, we do NOT want this to be less than the stock offering. De-burr, yes, polish? I suggest No. Focus on the other items a through h above.

__________________
"I found myself homesick for a place I've never been to."

marc hanna

Porting, I believe, is still a valuable upgrade because it also gives you the opportunity to fix inherent flaws in the casting such as porosity and voids; modern problems that have arisen from "lowest bidder" production lines and unqualified inspection staff - this is a rampant problem in manufacturing today.

The polishing, I agree with Gob-ny-geay, its value is debatable. However, dimpling seems to be a worthwhile course of action. This is where you create a golf-ball type surface in the intake, it doesn't restrict flow but does create a little turbulence which seems to be of value after the fuel and air are mixed.

PieMan

The inlet ports should be smooth as in removing the casting bumps and no sharp angles, but polishing does not help with atomisation of the fuel. On a polished inlet it tends to run along the walls and hit the valves in globes, as water does running down a window. Good combustion is helped by good fuel atomisation.

__________________
Mike (UK)

04 Bonnie Black

http://www.triumphtwinpower.com

RockyRaccoon

all good points. let me rephrase though, im speaking in regards to the fact that pooling of fuel is not an issue in engines that run at higher rpms (such as these vert twins) neither is atomization and yes flow testing is needed if you're changing the shape of the runner (such as straitening the short side radius or any removal of metal that significantly changes volume or shape ,polishing removes a very small uniform layer so flow matching isnt an issue so long as you dont dig in ridiculously hard with your abrasive. just enough to remove casting bumps. really im challenging the idea that polishing these heads would result in pooled fuel and poor atomization with the belief that these engines regularly operate at an rpm that will prevent this from occurring. and am open to debate and opinons about it.

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In the sixties we took acid to make the world look weird, now the world is weird and we take paxil to make it look normal.

BorgCymru

Years ago I was talking to a famous engine tuner. I asked about porting and he said

The biggest mistake ppl make is polishing the intake to much, you need an 'amount' of roughness to make the air/petrol mix swirl together and mix well, if it's polished to much the fuel and air will separate.

Now I have heard this mentioned from quite a few tuners since then.