Stroker build question

[1991MJPioneer]

So I'm new here and thought I'd post somthing to get myself started. I've been building a 4.5L stroker for my MJ for the past few months in my spare time. I'm running stock pistons and stock cam from a 98 4.0, 7120 head, 4cwt crank and 707 rods, and pretty much stock everything else. According to JeepStrokers.com its a bad idea to use stock pistons and cam with a stroker because I'll end up with 9.7 to 1 compression and bad quench and so on..

 

My plan is to run the coolest spark plugs possible (champion RC9YC) plugs and to get a bigger radiator to reduce the chance of detonation...

 

Anyone have any input on what might happen with this motor and what I'm gonna have to do to keep detonation under control other than premium fuel? :dunno:

[wallisek]

I've been planning a stroker build for my 2001 XJ when it reaches around 200k. Mine also involved the same setup as yours with stock parts except for .030 over pistons. You could have your pistons dished to yield a lower static compression ratio. As for the dreaded quench everyone talks about over at jeepstrokers, you'll have to have the block decked to lower it.

[Eagle]

According to JeepStrokers.com its a bad idea to use stock pistons and cam with a stroker because I'll end up with 9.7 to 1 compression and bad quench and so on..

 

My plan is to run the coolest spark plugs possible (champion RC9YC) plugs and to get a bigger radiator to reduce the chance of detonation...

 

Anyone have any input on what might happen with this motor and what I'm gonna have to do to keep detonation under control other than premium fuel? :dunno:

Neither a cooler plug nor a larger radiator will prevent detonation. A colder plug might help a little, the radiator will do nothing.

 

IMHO you need to use a different cam, with enough overlap to bleed off some of the compression pressure.

[1991MJPioneer]

Neither a cooler plug nor a larger radiator will prevent detonation. A colder plug might help a little, the radiator will do nothing.

 

IMHO you need to use a different cam, with enough overlap to bleed off some of the compression pressure.

 

Thanks for the info...unfortunately I already have the short block assembled with the stock cam in it... :wall:...oh well I'll just take it out I suppose. Any ideas on what cam I should run? I know comp cams makes a .476/.484 lift cam thats popular.

 

I have a rebuilt 4.0 head...7120 casting sitting in my garage...it has stock springs...how bad will my valve float be with a cam? I'm on a pretty tight budget. Can I still use stock springs at all?

[wallisek]

Discussion about Comp Cams and stock springs here.

 

http://www.jeepstrokers.com/forum/viewt ... ?f=5&t=848

 

Looks like you'll probably end up with valve float. Seems a lot of people are happy with the mopar camshaft and springs. Seems like that would be a better option or just button everything up as-is. You would have to run higher octane fuel though to avoid ping.

[Eagle]

Discussion about Comp Cams and stock springs here.

 

http://www.jeepstrokers.com/forum/viewt ... ?f=5&t=848

 

Looks like you'll probably end up with valve float. Seems a lot of people are happy with the mopar camshaft and springs. Seems like that would be a better option or just button everything up as-is. You would have to run higher octane fuel though to avoid ping.

From that thread in the link:

 

for cam break-in i used the stock 4.0L valve springs, and after break-in i had a serious valve float after 4200RPM.

then i installed the mopar performance springs, and now my stroker is happy till the rev-limiter.

Seriously, with an engine geared to cruise at around 2,000 to 2,500 RPM, who CARES if the valves float at 4,200 RPM? I have 287,000+ miles on my '88 Cherokee, plus probably another 100,000 to 150,000 on various other XJs and MJs, and I am 100 percent certain that NONE of them have ever seen 4,000 RPM. The "sweet spot" for shifting with a stock engine is around 2200 RPM for the Renix and 2500 to 2700 for the second generation HO. With a stroker producing gobs MORE torque, there's simply no need or reason (unless you're racing) to be running the engine up to 4,000+ RPM.

[1991MJPioneer]

Well I'm in no hurry to get the engine done so I should do it at least semi-right with a cam. Yeah I hardly ever get near 4,000 RPM with a 4.0. I have an 01 XJ with 3.55's and an auto and if I punch it at 65 on the highway..then maybe it'll get close to 4,000...but I don't do that often. So, I will just keep the stock springs and go with a good cam.

 

It kinda sucks that I already have everything together...but I'd like to have a decent running engine...so its worh it.

 

Thanks for the links!

[jmderyke]

Whats wrong with 9.7:1 compression on a inline six that never gets passed 6k rpm?.....Nothing

There can be some other factor influencing engine behavior not addressed such as fuel injection tuning. Most fuel injection systems in late 80s+ run closed loop injection systems, this is when the car is constantly adjusting air fuel mixture based on exhaust,throttle position, altitude, manifold pressure, engine temp, combustion, etc etc. Then the Engine control unit chooses a map (air fuel mixture, timing etc. etc.) The problem that most people run into is that they change engine parts in your case and the computer has no maps that really match what air/fuel mixture, timing etc etc you really need to run properly. So you may need to get new program for your e.c.u. to over write your existing maps, like a dyno programmer for example. Think about this... sport bikes run 12.5:1 on 91 octane fuel with rpms at 12,000. You just need the right tuning for you application I hope this helps. p.s. tuning is one of the best power adders!