[ENGINE] do I need ported heads for stock revs???

[Forsh]

1641cc, twin carbs DRLAs of ICTs not decided yet, Engle 110, stock crank, stock heads, heat exchangers and 4-1 tuckaway exhaust

question is where is the weak link that limits performance?

I want to replace the current 34 pict Solex with twin carbs and self port the heads IF it's worth it.

as I see it if it's only safe to rev to 4500 rpm is there any point in making it flow better as it will be capable of sucking in enough mixture upto the stock red line anyway?

or will it make a difference in the quickness that it get's to the red line before changing up to the next gear?

[Thin white duke]

Ported heads won't increase the speed in which your engine will rev, only lightened engine parts will help that, although the bigger carbs may make your engine a little more responsive. Replacing one single engine part rarely makes a huge performance jump unless you improve another part relating to that and so on and so on! There are several very good vw engine building books around, consult one or more and find out how much a standard head will actually flow, my betting is that it will be efficient enough to cope with a little more than a standard carb.

[psimitar]

Basically an engine becomes more efficient and powerful the better it can breathe. Fueling and ignition optimise the power available from the air being drawn into the engine.

So yes you will see more power higher up the rev range than a std crank can rev but if the cam is designed for max power for std revs then flowing the heads will increase the air the cam is capable to pulling in.

A basic home port can give between 5-10% more BHP and torque. You don't need to go crazy but just clean up the casting flash and smooth the seats into the ports plus match port the inlet mani to the head mounting flange. A coupla nites work but worth it I think

[allubo]

as mentioned above a stock crank which hasnt been balanced to the flywheel and pulley along with balanced conrods and cc'd heads wont rev higher, weak valve springs will also limit higher rev power and they begin to bounce thus not working perfectly.
also as above porting heads is free HP.
but like i said get your lower end balanced it will allow your engine to rev higher (stateside tuning)------->
will balance your crank to your flywheel and pulley for £75


alex

[Forsh]

Thanks for the input guys!

As it stands the engine is built and running so won't be getting a counterweighted crank or balancing any time soon - it's built by Rich at Airspeed with about 4000 miles on it so the case will remain bolted up but to whip the heads off is do-able if it's worth while - sounds like a nice little weekend job with the battery drill/Dremel/burr tools the next time the engine is out

I just want to make the most of what is there which means sticking to 4500rpm rev limit without embarking on a money pit exercise

[Mr.Whippy]

The engine will still rev nice & high with a non counterweighted or unbalanced crank (assuming the rest of the engine spec will allow it) the only difference is that it will trash your light alloy engine case within a few thousand miles as it hammers the bearing shells into the soft metal. The crank does not limit the engine's actual RPM, it limits the maximum SAFE RPM.

Well balanced stock cranks are good up to around 5000 rpm for very occasional use, any higher you need a counterweighted crank if the engine is to last, but it still needs to be balanced.

The engine is an air pump. To make more power you need to burn more air and fuel. To do this you need to flow more air through the engine, think of it as a complete system all the way from the air filter to the tailpipe. It's all got to flow as one.

Changing one or two things on their own won't achieve a lot because there will still be other restrictions, but changing several things can really add up the HP gains.

Roughly in order of preference -
Ditch single carb in favour of twins or dual twins. (not one carb in the middle)
Free flowing exhaust.
Increase CC's by piston size, stroke, or both.
Head porting job (including a 3 angle cut on the valve seats - important)
Higher lift / duration camshaft and / or higher ratio rockers.
Forced induction.

Dave.

[psimitar]

and if you're gonna do a 3 angle seat then backcut the inlet valve and chamfer the outer edge of the exhaust valve. Small things but makes the most of the porting by a few cfm's

[justbuggin]

Originally Posted by Mr.Whippy

Head porting job (including a 3 angle cut on the valve seats - important)

Originally Posted by psimitar

and if you're gonna do a 3 angle seat then backcut the inlet valve and chamfer the outer edge of the exhaust valve

Any pics of these mods anyone?

Sorry to hijack