benjy_26 Posted July 27, 2014 Share Posted July 27, 2014 Hello all. As I research the Jeep 2.5 mill, I've come across a few bits of info that I find interesting. While going through a forum (I forget which), one user mentioned that the 2.5L that came in our vehicles used valve springs that are relatively weak. This is one thing that I'd like to tinker with. Anyone know of a source for stiffer springs for the 2.5? Do the 4.0 springs fit? Thanks in advance! Link to comment Share on other sites More sharing options...
Eagle Posted July 27, 2014 Share Posted July 27, 2014 If it ain't broke, don't fix it. Stiffer valve springs usually call for dual springs to avoid valve bounce and spring harmonics. The inner spring acts as a damper. Link to comment Share on other sites More sharing options...
benjy_26 Posted July 27, 2014 Author Share Posted July 27, 2014 Reason I ask is because I just picked up a spare 2.5 and am in the planning stages of a rebuild. Link to comment Share on other sites More sharing options...
Eagle Posted July 27, 2014 Share Posted July 27, 2014 The 87 - 90 Jeep parts book shows three valve springs for the 2.5L. One is unique to the 1987, the second refers to valves marked #15, 19, 20, 60 ... The third is part number 3300 4296, which is the same valve spring listed for the 4.0L. It is described as 2 inches long. Considering that 4.0Ls often run for 300,000 miles or longer, I don't think I'd be messing around with non-standard valve springs. Link to comment Share on other sites More sharing options...
dasbulliwagen Posted July 28, 2014 Share Posted July 28, 2014 I've read before on www.4bangerjp.com that people who do mods to the 2.5 internally run into valve float at higher rpms. If you're going for a stock rebuild though I would just get new stock springs. Link to comment Share on other sites More sharing options...
benjy_26 Posted July 28, 2014 Author Share Posted July 28, 2014 I'm not going for a bone stock rebuild, but I'm not going über-radical either. I'm planning on an MPFI conversion with a lumpier cam and some overbored, higher comp pistons for good measure. I don't plan on revving it a lot, but this engine will probably make most of its power ar higher rpm's; and I'm fine with that. Link to comment Share on other sites More sharing options...
benjy_26 Posted July 28, 2014 Author Share Posted July 28, 2014 Eagle, if the 4.0 valve springs fit the 2.5 head, would it be safe to assume that the hi-po springs marketed to 4.0 guys would work for us 4 banger guys too? Link to comment Share on other sites More sharing options...
Eagle Posted July 28, 2014 Share Posted July 28, 2014 Eagle, if the 4.0 valve springs fit the 2.5 head, would it be safe to assume that the hi-po springs marketed to 4.0 guys would work for us 4 banger guys too? Yes. I believe the factory cam kits for the 2.5L included new, dual springs. I would check for you, since I have one of those kits -- but I have no idea where to look for it. That said, I wouldn't look for a big increase in spring strength, just get dual springs to control coil harmonics. Standard single springs are good at least to 5,500 RPM -- I seriously doubt you'll be spending much time that high in the power band. If so -- you'd better plan on sending the whole engine out for a professional balance job. Link to comment Share on other sites More sharing options...
benjy_26 Posted July 28, 2014 Author Share Posted July 28, 2014 So... if I plan on keeping a stock redline but have a big cam, would new, OEM quality springs be sufficient? Link to comment Share on other sites More sharing options...
Eagle Posted July 28, 2014 Share Posted July 28, 2014 So... if I plan on keeping a stock redline but have a big cam, would new, OEM quality springs be sufficient? In my opinion, yes. Dual springs are for race engines that run higher than 6,000 or 7,000 RPM. Same with stronger springs -- you need stronger springs to prevent valve float at high RPM. Valve float is caused by the momentum of the rapidly moving valve overriding the spring tension. Since the stock spriings are designed to operate within the stock RPM band, they're adequate. Keep in mind that a "big" cam isn't physically bigger. The cam lobes still have to fit through the holes in the block and the cam bearings. The lobes of a "big" cam aren't any farther away from the cam centerline than those of a stock cam. Instead, the base circle is ground smaller. Your "big" can isn't going to compress the valve springs any farther than the stock cam. Link to comment Share on other sites More sharing options...
johnj92131 Posted July 28, 2014 Share Posted July 28, 2014 I'm not going for a bone stock rebuild, but I'm not going über-radical either. I'm planning on an MPFI conversion with a lumpier cam and some overbored, higher comp pistons for good measure. I don't plan on revving it a lot, but this engine will probably make most of its power ar higher rpm's; and I'm fine with that. Why do you want a lumpier cam? I like the MPFI conversion. I understand a bit more compression, but I am not a big fan of using premium gas. Over bore is a no brainer. The factory spent a whole lot of time and money on the cam. Read the Stroker forum and you will see lots of problems with after market cams. Yes, those are 6 cylinder engines, not 4. But it shows problems going with aftermarket cams. If you really want more power from that 2.5, then look at a low pressure supercharger. Superchargers tend to deliver a bunch more power/torque at the low end. And the low end is where a truck needs torque. Other than a supercharger, gears are your best friend as they multiply torque. Turbochargers deliver boost and power at mid and high range. Low end respose needs a small turbo, But there are anynumber of small turbos in the junk yards to chose from. The other thing with turbos is they need some tuning. Superchargers are a bit simpler in that respect. Link to comment Share on other sites More sharing options...
benjy_26 Posted July 28, 2014 Author Share Posted July 28, 2014 While the 2.5 is loosely based on the L6, it makes all of its power above 3000rpm, torque too. A cam with more lift and duration will give it more of what the engine does naturally, as will the higher compression. While it's not a Japanese DOHC mill, it's still a revver, compared to the 4.0, at least Link to comment Share on other sites More sharing options...
mnkyboy Posted July 28, 2014 Share Posted July 28, 2014 I was gonna run 4.0l springs in my 2.5l but when I was crossing part numbers I kept coming up with the same numbers for both engines. I have a set of 4.0l HO springs I was gonna use but I couldnt find anything different about them. Also my Mopar stage 3 cam said it was fine with the stock springs so I just left the springs alone for now. Link to comment Share on other sites More sharing options...
benjy_26 Posted July 28, 2014 Author Share Posted July 28, 2014 Cool. Where'd you get a Mopar cam? Link to comment Share on other sites More sharing options...
mnkyboy Posted July 28, 2014 Share Posted July 28, 2014 The NOS fairy gave it to me, actually a buddy stumbled across it somewhere still new in the box. Link to comment Share on other sites More sharing options...
benjy_26 Posted July 28, 2014 Author Share Posted July 28, 2014 Damn... That's a great find! Link to comment Share on other sites More sharing options...
Eagle Posted July 28, 2014 Share Posted July 28, 2014 While the 2.5 is loosely based on the L6, it makes all of its power above 3000rpm, torque too. A cam with more lift and duration will give it more of what the engine does naturally, as will the higher compression. While it's not a Japanese DOHC mill, it's still a revver, compared to the 4.0, at least Not really. Look at specs for so-called "RV" cams. Typically, they are higher lift than factory cams but shorter duration and less overlap. Their purpose is to increase torque at the low end rather than horsepower at the top end. When I built the first engine for my Javelin race car "back in the day," the cam maker completely misunderstood the performance characteristic I had given him and he gave me ... an RV cam. Gobs and gobs of low end, but where the factory engine would rev to 5500 and stop there due to valve float, the "race" engine was all through at 4500 RPM. Literally ... in top gear that was as fast as it would go, because aerodynamic drag exceeded the available power. Needless to say, I swapped out that cam post-haste. See if you can dig up the specs for the factory Stage 2 cam kit. That would probably be a good target for you. If you might be interested in a factory Stage 3 kit, one of my friends from NAXJA had one. I don't think he has sold it, but he no longer has the 4-banger MJ so he would probably be willing to part with it. If you're interested, send me a PM. Link to comment Share on other sites More sharing options...
benjy_26 Posted July 29, 2014 Author Share Posted July 29, 2014 While the 2.5 is loosely based on the L6, it makes all of its power above 3000rpm, torque too. A cam with more lift and duration will give it more of what the engine does naturally, as will the higher compression. While it's not a Japanese DOHC mill, it's still a revver, compared to the 4.0, at leastNot really. Look at specs for so-called "RV" cams. Typically, they are higher lift than factory cams but shorter duration and less overlap. Their purpose is to increase torque at the low end rather than horsepower at the top end. When I built the first engine for my Javelin race car "back in the day," the cam maker completely misunderstood the performance characteristic I had given him and he gave me ... an RV cam. Gobs and gobs of low end, but where the factory engine would rev to 5500 and stop there due to valve float, the "race" engine was all through at 4500 RPM. Literally ... in top gear that was as fast as it would go, because aerodynamic drag exceeded the available power. Needless to say, I swapped out that cam post-haste. See if you can dig up the specs for the factory Stage 2 cam kit. That would probably be a good target for you. If you might be interested in a factory Stage 3 kit, one of my friends from NAXJA had one. I don't think he has sold it, but he no longer has the 4-banger MJ so he would probably be willing to part with it. If you're interested, send me a PM. Thanks a bunch! :D Link to comment Share on other sites More sharing options...
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