FAQ: Can I run a 3.25?

This is a very commonly asked question. Its a complex answer because every car will be different, but there are also many similarities between cars that have the supporting mods for a 3.25, from them we can draw the answer you need.

There are several reasons why a 3.25 can't be run right off the bat but can with complimentary mods.

Firstly, a 3.25 will produce a lot of boost between the supercharger and the cylinders on applications that don't flow enough air through the engine. An excess in pressure will cause a lot of heat in this air, which will increase combustion temperatures to the point of preignition. While it is often talked about in reverse context, you are infact aiming for a certain boost level with your pulley size choice, usually around 10-13psi. Without a good exhaust system, you will see the high end of that range and possibley more with a 3.25 on an otherwise stockish car, since a 3.4 pulley on a stock car already pushes this boost limit itself. A good exhaust for a 3.25 can be considered a 3" downpipe, a high flow catalytic, and a Ubend and resonator removal; in reality the more the better.

On the theme of heat, you must also take measures to decrease it in all its forms. Being a non-intercooled engine, preventing excess heat is key to preventing preignition. Firstly you should draw in the coolest air possible from your air filter, that means NO OPEN CONES! A manufactured or self made cold air intake can be quite effective, my personal favorite however it still a fenderwell intake! Secondly, reduce the cylinder temperatures with a colder range spark plug. Please see my spark plug writeup for reasons for this and recommendations but 2 range colder plugs with a 3.25 works very well at reducing cylinder temps and I'd highly recommend Autolite 104s to just about every 3.25 user. Thirdly, reduce heat in the engine overall, because a cooler engine means cooler cylinder temperatures. A 160 or at most a 180 thermostat is a good idea for reducing engine temps, don't forget to accompany this with a fan switch to manually turn the fans on and off if you don't have a DHP reprogrammed computer.

Lastly on the topic of DHPs, this is in reality a must have mod for those running 3.25s. With the excess boost, the stock fuel mapping will be strained to keep up. There's a good chance of running lean atop 2nd gear and especially in 3rd at WOT, and fuel enrichment at part throttle will be late coming in for the best performance. A DHP battles these tuning problems quite well, as well as being a good mod in terms of shift point alteration and lower fan turn on points.

Considering all these factors, a good mod list for a 3.25 would look something like:

• Some form of fenderwell or other cold air intake
• A good aftermarket exhaust system consisting of a bare minimum of a 3" downpipe and Ubend removal, a high flow catalytic converter and larger diameter cat-back will be of great additional benefit. A ported front exhaust manifold or headers will also be of great help however are the last thing I would look at taking cost into consideration.
• A Digital Horsepower reprogrammed computer!
• Lower temperature thermostat
• 2 range colder plugs (AC Delco 41-919, Autolite 104, NGK TR6)

So can you run a 3.25? Its of course at your discresion, but hopefully some light has been shed on the topic for you.