Low back pressure power loss questions.

[nrschaaf]

Hello all, I have an absurd question for everyone. If I were to run a 302 with nothing but long tubes and turndowns, the intake side is a 650cfm 4bbl, what kind of low end torque loss do you think I'd experience? If we're talking like 15-20% I think it'd be manageable. But if it were talkin' 50%+ then maybe not for me. Just tryin to feel it out before I make my decision.

 

[rjcooperss396]

What size is the primary and collector on the long tubes and what 302 is it?

 

[nrschaaf]

It's an early 97 block, remanned and bored .030 over with gt-40x heads. The primaries are 1.5" and the collectors are 2.5".

 

[rjcooperss396]

I doubt you would lose any bottom end with such small headers. Are you running the stock Explorer cam?

 

[nrschaaf]

No, it's a custom cam from Comp Cams. I don't have the specs available right now to give you them. I thought the headers seamed small, but the guy that did the engine work suggested them so I figured there'd do fine. Think I should be going larger?

 

[rjcooperss396]

Depends on the specs of the cam and what RPM range you'll be running. Do you know more about the engine like what the compression is, what intake manifold your going to run, cam specs, etc.? If it's a mild build then the 1.5 primary will work well. If it's a little more on the wild side then 1.625 would work better.

 

[FIND]

An engine does not need back pressure. An engine needs flow velocity to maximize scavenging. Back pressure is a myth that came about because people realize they lose power when going to gigantic exhaust pipes, not realizing that they are actually getting more back pressure because of turbulent exhaust flow.

Not that that changes what you want or anything... I just hate seeing people ask about how much back pressure they need.

 

[nrschaaf]

Ok, so I do not have all of the info on the engine. It's really a project I'm just helping with, and the guy donating the actual engine. The cam is alittle more aggressive than the x303 can from ford racing. There was no major modifications to the rotating assy so I would assume that the rpm range is the same as the explorer. As for compression ratio I would guess higher than stock for the explorer based on the smaller combustion chambers in the gt40x heads, but then it was bored .030 over so the larger displacement would offset that I think. I'm really a novice when it comes to this kinda work, I'm really not sure. I should just ask the guy who built the thing but he's the kinda guy you spend as little time around as possible.

 

[nrschaaf]

By the way, that's good info "Find". Thanks. Now I've gotta keep that in mind for future projects.

 

[rjcooperss396]

The X303 has a much high RPM range than the stock Explorer cam does. It should pull strong to about 6500 in a 306. If I assume that the rest of the engine was built to match the X303 cam and GT40X heads I would have gone wih a 1.625 primary tube. I think the 1.5 will be a little too small.

 

[nrschaaf]

Sorry it took so long to respond. Stepped up to 1.625" primaries and 3" collectors with just turndowns. Far louder than I remember straight long tubes being. Lol. Incredible response at all RPM so I think it was a good move. Though it does start to throw flames around 5500~. I think I may need to do this work to my X. Any ideas if a tune would be all I'd need to support the gt40x heads and a custom cam like this? Obviosly retaining, and upgrading the EFI components, not going carbureted?

 

[rjcooperss396]

You lost me, what are you asking?

 

[nrschaaf]

Sorry. If I wanted to take the 5.0 in my X and do the gt40x heads, cam similar to the x grind cam, intake, and upgrade the fuel and ignition to match could the trucks ecu keep up with say a good tune/recalibration or would I be looking at a need for some elaborate management system? I've got plenty of power, but the power that this engine puts down is a night and day difference from mine and frankly I'm jealous. I want it in my X.

 

[rickybobby]

with a good tune and the right injectors the pcm can run 1000hp or more. The problem is gonna be getting headers to fit the explorer with those heads. The only headers made for an explorer are built for the GT-40P heads

 

[nrschaaf]

Oh right, the exhaust ports are taller on the "x" heads than the other heads right?

 

[rickybobby]

IIRC the port shape and size is different, and the plug angle is different. The plug angle may not be an issue though, depending on how Bob designed the TMH setup and the angle on the X heads. The key here is research. take everything with a grain of salt, even what i say lol. I am not an expert, and dont know a ton about the x-heads. I do know that TMH headers wont work on the standard 5.0 heads because of port shape, and i beleive that the x-heads use the same port type that the standard heads do

 

[nrschaaf]

Great info, thanks. I'm gonna need to do some research. I apreciate it guys.