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Sun Aug 09, 2009 11:03 am

When it comes to vacuum hoses, does size really matter?

I'm just talking about the small hoses that connect the fpr to the plenum to the decel, and the vac can to the throttle body to the air box.... I think that's how they connect.

Should all of these hoses have the same inside diameter?

Thanks, and have a nice Sunday.
Patrick

Sun Aug 09, 2009 11:04 am

Check Ratwell
http://www.ratwell.com/technical/VacuumHoses.html

Sun Aug 09, 2009 11:09 am

Yes, I have checked Ratwell. He doesn't say much about the sizes or difference in sizes. Or, maybe I'm just missing it.
I have also done a search but have not been able to find the answer to my question.

Sun Aug 09, 2009 11:16 am

scroll down on the link i just sent. There is a chart with inside and outside diameters of the various hoses.

Sun Aug 09, 2009 11:30 am

Thanks. I've seen the chart.

I know there are slight differences in the i.d. of some of them.

I'm just wondering how much difference a millimeter makes in the overall performance of the engine.

Sun Aug 09, 2009 11:52 am

Someone will be able to elaborate more, but I believe the vacuum hose diameters are important in that they dictate how much vacuum is used by the different vacuum-attached engine components (oil breather, distributor, decel valve, etc.)

Sun Aug 09, 2009 4:34 pm

I replaced all my vacuum hoses according to Ratwell's chart linked above. Most of the hoses I got from German Supply. Worked great.

Sun Aug 09, 2009 6:12 pm

Patrick199 wrote: Thanks. I've seen the chart.

I know there are slight differences in the i.d. of some of them.

I'm just wondering how much difference a millimeter makes in the overall performance of the engine.
Think of it this way, Say your engine requires a gallon of water moving thru a 1 inch inner diameter pipe at one gallon per minute. now , reduce the inner diameter of the pipe to half that .there will be a difference in flow. Either you will need more pressure to move the required liquid in one minute or a larger inner diameter. likewise for the enlarged version, you would have to reduce the flow capacity when the I.D. is increased.

Sun Aug 09, 2009 7:55 pm

Quote:
Think of it this way, Say your engine requires a gallon of water moving thru a 1 inch inner diameter pipe at one gallon per minute. now , reduce the inner diameter of the pipe to half that .there will be a difference in flow. Either you will need more pressure to move the required liquid in one minute or a larger inner diameter. likewise for the enlarged version, you would have to reduce the flow capacity when the I.D. is increased.

Thank you 78Kombi. I guess it's more important than I thought. I've always just sorta used what I could make fit. I even made a rigid setup once with copper tubing. It was leak free and looked great, but probably the wrong size. It's hard to say how performance was affected. I'm going to try to do it to spec and see what happens.

Thu Mar 04, 2010 2:29 pm

Hi all,

Did anybody find an answer to the i/d size of the vacuum lines?
Ratwell's chart lists an o/d size of 5mm for the decel to pressure regulator line and 4mm o/d for the distributor to plenum/ air filter line, but no i/d sizes and i don't have any originals to measure.

Mine currently has 3mm i/d silicone vac line for both.
Is this OK? I would prefer it to be right.

Thanks

Tony

Thu Mar 04, 2010 2:46 pm

the original hoses are made from short lengths of cloth hose connecting longer runs of hard plastic hose.

on the decel/FPR run the cloth covered hose is 4.5mm ID and the plastic hose is 5mm OD. on the vac advance/EEC run the cloth hose is 3.5mm ID and the plastic hose is 4mm OD.

the sizes of the hose have more to do with the nipples they fit onto than managing vacuum with a particular size of hose. so the silicone is fine if it fits. if you're worried about originality then keep it original and use the combination of hard plastic and cloth hose. if you don't care about originality use the silicone. i don't think the engine cares too much, but it's not usually a bad idea to make things like the original specs.

Thu Mar 04, 2010 3:07 pm

Patrick199 wrote: Quote:
Think of it this way, Say your engine requires a gallon of water moving thru a 1 inch inner diameter pipe at one gallon per minute. now , reduce the inner diameter of the pipe to half that .there will be a difference in flow. Either you will need more pressure to move the required liquid in one minute or a larger inner diameter. likewise for the enlarged version, you would have to reduce the flow capacity when the I.D. is increased.

Thank you 78Kombi. I guess it's more important than I thought. I've always just sorta used what I could make fit. I even made a rigid setup once with copper tubing. It was leak free and looked great, but probably the wrong size. It's hard to say how performance was affected. I'm going to try to do it to spec and see what happens.

Vacuum is different. There is no flow to speak of. There is a minor bit of flow as vacuum is established, but once in place, the pressure is soley dependent upon the surface area of that which is under vacuum. So a brake booster can actually develop full power with a 4mm vacuum advance hose, it just can't recharge as quickly. Most items in the engine compartment have very little need for any sort of recharge.
Colin

Thu Mar 04, 2010 4:15 pm

Amskeptic wrote: Patrick199 wrote: Quote:
Think of it this way, Say your engine requires a gallon of water moving thru a 1 inch inner diameter pipe at one gallon per minute. now , reduce the inner diameter of the pipe to half that .there will be a difference in flow. Either you will need more pressure to move the required liquid in one minute or a larger inner diameter. likewise for the enlarged version, you would have to reduce the flow capacity when the I.D. is increased.

Thank you 78Kombi. I guess it's more important than I thought. I've always just sorta used what I could make fit. I even made a rigid setup once with copper tubing. It was leak free and looked great, but probably the wrong size. It's hard to say how performance was affected. I'm going to try to do it to spec and see what happens.

Vacuum is different. There is no flow to speak of. There is a minor bit of flow as vacuum is established, but once in place, the pressure is soley dependent upon the surface area of that which is under vacuum. So a brake booster can actually develop full power with a 4mm vacuum advance hose, it just can't recharge as quickly. Most items in the engine compartment have very little need for any sort of recharge.
Colin

Again, but in English this time please.

Thu Mar 04, 2010 4:24 pm

Put on a vacuum hose that fits as well as you can find and make sure it does not leak. Mr. "Zip Tie" is your friend here.

Thu Mar 04, 2010 5:36 pm

In general the only vacuum hose where size is very important is the vacuum advance on the distributor. You want a distributor that has a quick response to changes in manifold vacuum. If the distributor responds too slowly you will get momentary knock when you punch it. Either a hose that is too large or too small will slow response. A large hose has a high internal vacuum which acts as a reservoir and too small a hose will not release the stored vacuum fast enough.

Most of the time an automaker will size the nipple on the particular appliance to the size hose they feel best to run. If you have a 5mm OD nipple then a 4 or 4.5mm ID hose is probably going to work just fine. You occasionally may see where the factory has forced something like a 3mm hose onto a 5mm nipple, never really liked seeing this, but it is done.

Thu Mar 04, 2010 6:00 pm

Thanks for clearing that up.
I will sleep well now.
:)

Wed May 05, 2010 5:54 pm

I found this thread doing a search. I have a '71 bus with the stock dual vacuum advance/retard distributor. The retard hose nipple is a larger diameter than the advance nipple and I remember reading somewhere that the blue(or green) cloth vacuum hose is a larger ID and it is supposed to be used for this fitting. the reason I did a search on this subject was to find the correct hose because my engine takes a few seconds to idle down to the 7.5 atdc timing mark. I'm thinking that there is an insufficient amount of vacuum to do this as soon as it goes to idle. Also I have to have my idle speed set very low or it wont draw back at all. Any thoughts?

Wed May 05, 2010 10:07 pm

the retard run is green cloth covered hose, this is correct, and it is 4.5mm ID.

however the size of this hose has nothing to do with your problem. it could be a few things but it isn't related to using the wrong size or type of hose.

make sure the throttle butterfly is correctly adjusted, for one thing. if the throttle adjust screw is too far in it will induce the wrong vacuum signal and this can upset the advance and retard. your issue could also be poor operation on the advance mechanisms inside the distributor, or failed vacuum canister.