air horn, what if i use lower amp relay?

The 12v air horn I installed in my car calls for a 40amp relay, I ended up using a 30amp relay instead, however, will this make the horn less as loud or will the volume not change?

All will be fine. If there is a problem, the only problem you will have is the relay will eventually go bad. But I really doubt that will happen.

Cool, that's what I figured. I think it may actually be a dual 30/40amp universal relay anyway. But what got me curious was, I remember reading on a forum that if you beef up the relay for your headlights you can make them brighter. So I was curious to the fact that if I did use a 30amp as opposed to a 40amp, if it would also make the air pump produce less psi.

A relay won't "limit" the current, it's simply a need to have the relay handle the required current. Changing dash headlight switches to relay switching often brightens lights due to the lower voltage drop, but I doubt OEM relays would need beefing up. If they get hot or warm, yes - that could mean over-current (assuming it isn't coil heat). But it's the shortest path with thge thickest cable that you want. Maybe the horns require a 40A relay, but many 30A relays will handle that ok - especially for intermittent use like horns. If it fails, then consider a 40A or 60A relay.

Nice thanks for the replies guys.

30/40 amp relays are rated at 30 amps on the normally closed side. The normally open side is rated at 40 amps. In your application, you will be using the 40 amp rating. The reason for the different rating is that the contacts on the normally open side are just held together via a small spring. But the contacts of the normally open side are held together via the electromagnet. The magnet holds the contacts together much better than the spring. Hence more current capacity on the normally open set of contacts.