V Twin Hoglet Inlets

[neil_1821]

I have a quick question for you chaps.

I made a bit of a slight error on my second head for the cyclinder and I was wondering if I can get away with it or not.

The exhaust outlet is .25" and the inlet is .188". Unfortunately I managed to drill the inlet the same size as the exhaust. :wall: This could be rectified with a sleeve but does the difference in size matter that much? Would it hinder its running or will it completely fine at .25"

 

[Brian Rupnow]

Put a sleeve in it. That way if you have other troubles down the path know they aren't caused by the wrong bore.

 

[canadianhorsepower]

I have a quick question for you chaps.

I made a bit of a slight error on my second head for the cylinder and I was wondering if I can get away with it or not.

The exhaust outlet is .25" and the inlet is .188". Unfortunately I managed to drill the inlet the same size as the exhaust.

IMHO the inlet should be .250 and exaust .188:hDe:
the reasons are simple 99% of normally aspirated engine are build this way
why????????? on the intake side the air is SUCK in by mechanical design and no way to be control on the opposite exhaust is PUSH out with piston stroke. Exaust beeing blown out valve area could be smaller and have the same volumetric efficiency

that's my 2 cents

:wall:

 

[neil_1821]

Yeah I understand where you're coming from but V twins may be built differently, I don't know.

As I've always understood it, if you have a bigger inlet than the outlet then the engine's going to struggle to breath by pushing out the exhaust gases, especially in model form. This is why so many car modifiers will fit an exhaust system initially to allow the engine to breath more freely and gain a few extra horses, granted its entirely dependant on the engine.

I just didn't know if 1/16th would make much difference. Considering it's my first IC engine I will sleeve it with some phosphor bronze or brass once I've milled the angle on the intake, make sure it's a good interference fit so it doesn't decide to disappear into the cylinder.

 

[Hopper]

It probably won't make any difference. You are not going for high rev, high performance here.

Full sized V twins, Harley, Ducati, Indian etc etc all have much bigger intake valves and ports than exhaust valves and ports. Reason being, air is pushed into the cylinder by atmospheric pressure, 14.7psi, ie not much. So bigger valves and bigger ports on the intake = more air/fuel pushed into the cylinder. Exhaust gasses are pushed out of the cylinder by the pressure of combustion, about 1,000psi. So a lot of gas can be forced through a small hole at that pressure.
The reason aftermarket exhaust systems increase performance is not due directly to their allowing more gas to escape more easily but is due to their increased flow characteristics creating a low pressure area behind each pulse of exhaust gas that then creates a low pressure area in the cylinder and "sucks" more fresh air and fuel into the cylinder, through those large intake ports and valves. To take full advantage of this scavenge effect, cams with more overlap hold the intake and exhaust valves open at the same time for a longer time.

 

[neil_1821]

Ah I never realised this. Every day's a school day.

So that now leaves me with either sleeving the inlet on the one I buggered up, which could be tricky to do considering the wall thickness is rather small or drilling out the other cinder inlet to match this one at .25"

I have no expertise in building IC engines so I'm still a little unsure what to do now.

 

[Brian Rupnow]

When all else is confusion, go by the plans. Make a sleeve from brass with a 1/32" wall, coat the outside with Loctite, and slide it into place. Wait 24 hours before trying to start the engine.

 

[canadianhorsepower]

When all else is confusion, go by the plans. Make a sleeve from brass with a 1/32" wall, coat the outside with Loctite, and slide it into place. Wait 24 hours before trying to start the engine.

Brian
for someone claiming is PERSONAL DESIGN
what is wrong with an intake valve with a 1/32
more intake valve diameter.
you did your own experimentation in engine design
and no one told you not to
let him be :fan::fan:
HE might have the next Project` of the Month:thumbup::thumbup::thumbup:

 

[Brian Rupnow]

Canadian horsepower--Suck it!! Anybody building a new engine for the first time is a fool to depart from the blueprints given for that engine. If the engine doesn't run when completed, there is always the question "Did the modification I made from the original plans cause this non starting issue, or is it something else".

 

[Jasonb]

Keeping the inlet to .188 would give a smooth turbilence free flow to the gasses as it is the same diameter as the inlet manifold rather than have an increse in area and a slowing of the gas flow

The difference is actually 1/16" not 1/32" and it is the passage not the valve diameter.

Also as only one head has been drilled wrong I would rather have both heads the same rather than one with almost 50% larger cross section than the other to keep both cylinders working as near as possible the same.

So like Brian, pop a sleeve in for peace of mind, once it is running then if you want to play about with port sizes, timing etc.

 

[Charles Lamont]

canadianhorsepower - if you remember, the OP was asking about an error, not trying something different.

BTW, the more you scatter your posts with emoticons, the less informed, reasoned, balanced or helpful they seem to the rest of us.

 

[neil_1821]

Yes, my original question was what difference it would make. Since there is no definitive answer without experimenting I will try sleeving it tonight to keep in check with the plans and the first correct cylinder head. Heaven knows I will most likely struggle starting it once completed but it will narrow down the error list of things to check.

Yeah I realise the difference is 1/16th but if its being sleeved each wall will only be 1/32 thick, which makes things all the more interesting.

Got plenty of brass at home, will try and get a nice tight interference fit. Already had to plug one hole on it, which thankfully you can hardly see!

Lesson I've learnt is to triple check the plans and that engineering is not specifically getting it right every single time but the compromises you are able to make in order to rectify the fault. Thankfully this error can be corrected!

Thanks for all the advise

 

[Jasonb]

Neil, it was Canadian HP who I was correcting about the 1/16" difference as he said 1/32" difference in diameter

 

[canadianhorsepower]

Canadian horsepower--Suck it!! "Did the modification I made from the original plans cause this non starting issue, or is it something else".

That print out about your motor really got to your head to bad :fan:
but mechanical math didn't come with it
you would realize How small in volume/ area going from .188 to .250
on an intake time sheet mean's Nothing:wall::wall:

 

[canadianhorsepower]

When all else is confusion, go by the plans. Make a sleeve from brass with a 1/32" wall, coat the outside with Loctite, and slide it into place. Wait 24 hours before trying to start the engine.

JasonB
ir was Brian's quote
SORRY

 

[Cogsy]

what is wrong with an intake valve with a 1/32
more intake valve diameter.

JasonB
ir was Brian's quote
SORRY

Nope. Sorry but Brian referenced 1/32 wall thickness of the sleeve, whereas you referenced 1/32 increase in valve diameter.

 

[Brian Rupnow]

Be careful of the 'interference fit' if you are going to put in a brass sleeve with 1/32" wall. It will deform very easily with much pressure. I haven't seen the plans, but I assume that something, either a carburetor or an exhaust pipe is going to get bolted on to "trap" the sleeve in place. That being the case, I would try for a good slip fit and a coating of Loctite rather than an interference fit.---and yes, Luc, having my engine on the front cover of an international machining magazine has "Gone to my head"---and I'm loving it!!---Brian

 

[neil_1821]

Be careful of the 'interference fit' if you are going to put in a brass sleeve with 1/32" wall. It will deform very easily with much pressure. I haven't seen the plans, but I assume that something, either a carburetor or an exhaust pipe is going to get bolted on to "trap" the sleeve in place. That being the case, I would try for a good slip fit and a coating of Loctite rather than an interference fit.---and yes, Luc, having my engine on the front cover of an international machining magazine has "Gone to my head"---and I'm loving it!!---Brian

Thanks Brian

I've been considering how to do it successfully, its where the inlet pipe slides in.

I'm considering using solid bar and the re-drilling the correct size once it's in place, failing that using some phosphor bronze drawn bar machined down to size.

It's probably going to be this weekend before I get chance to do it, so i'll let you know how I get on with it.

Little side diversion, you had an engine on the cover of international machining magazine? Anymore details?

 

[johnny1320]

That print out about your motor really got to your head to bad :fan:
but mechanical math didn't come with it
you would realize How small in volume/ area going from .188 to .250
on an intake time sheet mean's Nothing:wall::wall:

If you go from a .188" valve opening to a .25" that's over a 50% gain in surface area so that .0625" IS a big difference, could be a big problem on over lap of the cam. I would stick to the plans for valve size and the sleeve with such a small wall would be risky in my opinion, maybe drill it larger so it has a thicker wall or bite the bullet and start over. Just my two cents.

 

[Brian Rupnow]

neil_1821--see post #314 in this thread
http://www.homemodelenginemachinist.com/showthread.php?t=23315&page=32