You,me GT & Rally
 

What's the difference between On-Road,GT & Rally engines ?

Ok,that talks about off-road & on-road engines..... What is a rally engine considered as ?

Somewhere in between.....

From what I understand a rally engine is closer to an on road engine (usually based on an onroad engine usually 7-9 ports) and a GT engine is closer to an off road engine (usually based on an off road design 5 ports).

So can a rally engine be used in a GT car without no type of alterations,and still be competitive ?

Let me remind you the GT spec engine ROAR has decided, it is a 5 port engine. It is very wrong to say 7 and 9 port engines are onroad engines because you never know the timings which are actually making the engine purpose.
I never had a GT engine in my hands to measure the timings so I can not say if it is closer to an offroad or onroad..

I am basing my facts on having an example of each of the engine types in question:

ISON GT 5-Port
Ielasi Tuned GT 5-Port
359 Plus2-Rally 9-Port
Various Novarossi Ielasi buggy and truggy engines.

I run GT to ROAR rules and the engines are definitely closer to buggy engines. The 359 Rally 9-Port is basically the onroad version with a tall buggy head on it.

Of course there are variations however in my experience of all the types mentioned I believe it to be the case.

To answer the OP, when I first started running the 1/8 GT car I ran it with the 359 Rally engine and it was very quick. Once I started racing I had to switch to a proper 5-port GT engine so yes you can run a rally engine in the GT without issues however it may not be legal to race depending on local rules in force.

Yes, a Rally engine can be (very) competitive in a GT, you just have to check if your track follows IFMAR/ROAR/EFRA rules, in which case only 5 ports would be allowed (per current rules)

to comply with the ROAR GT class rules, most drivers started with Offroad based engines, which where good on torque, but not that good on the topspeed side. Most competitive racers started to modify engines especially on the exhaust side, but now with the known brands either having or bringing specially designed GT class engines (OS GT, Novarossi Mito GT, WERKS GT) on the market, it is very difficult to remain competitive with an Offroad engine (without modifications) in this class.