Stumped on the dyno

[Robliepse]

Prior to installing the 440 in my Roadrunner I decided to bring it to a local shop to have it broke in and tested on a dyno. It's the same shop that did the machining and some assembly last year and they are a busy shop and appear to know what they are doing. The engine started just fine, they dialed in the timing and set the carb and without a load the engine sounds great and revs as high as you would want to go. Then they add the load, the engine barely makes to 3,000 RPMs and it sounds like a rev limiter kicks in. They have tried a known good test carb and a couple of different stock type distributors new plugs, new wires with the same results. The motor has good compression and no leaks. They suspect it's ignition and suggested we could try a new style all in one type of distributor but I'm not ready to drop $400+ without understanding what is going on.
Any ideas on why the motor would totally drop out with any type of load?

 

[Robliepse]

One more comment, the shop typically works with Chevy engines and the Mopar stumped them

 

[YY1]

What ignition are you running?

If it's OEM Mopar, try a different brain, coil, etc.

Could it be valve float? Way too early for that. cam installed correctly?

 

[Robliepse]

I think the cam was installed correctly. is it possible to install them incorrectly? We have tried 2 different Mopar brains, one OEM and one Orange, a few different coils.

 

[Robliepse]

the shop is recommending a MSD Ready to Run distributor 8387.

 

[440+6]

Most dyno shops have a few different ignition systems to use, mine wanted to use their 6al but I made them use my FBO box.
The cam would have to be a mile off to stop at 3000, sounds like ignition related to me.

 

[Hemirunner]

Regardless of what make they specialize in they should have had enough engines on the dyno to have some sort of idea as to what the issue is. The igntion system should be part of the dyno and the only thing in question should be wires and the distributor. As to those RTR distributors, I've never had any luck with them on a performance build. Posting some info on the combination would really help you get some suggestions.

 

[Robliepse]

The block is casting number 4006630-440-7 which I believe is a 1978 RB block. The heads are 906 castings. The engine has Speed Pro L2355F pistons, with stock 906 heads , Holley 750 carb, Edelbrock Performer RPM dual plane intake and a Comp Cam 21-402-4 cam. also trying to use a stock oem type ignition system.

 

[DLevel]

I don't see were you stated what type of fuel delivery is being used. Does it have enough fuel pressure when this problem occurs ? In my experience the engine doesn't care if it only has a couple pounds of pressure,until you put it under a load then bad things can happen. Peak fuel demand will occur as it reaches its peak torque number wherever that rpm is then it will demand less and less fuel. Can't tell you how many times I found a weak or bad fuel pump of a customers on my Dyno. Have also run into low voltage to the ignition system on the dyno before, due to low batteries. That will cause problems with even the best MSD systems. Don't know if this will help but when these type of problems arise they just seem to be time and money eaters. Good luck

 

[Robliepse]

They required the ignition and replaced the plugs and it's working. Thanks for the input

 

[Robliepse]

This was before the final tuning