Headers.....holding motor back?

[SactoJim]

Got my motor out of my truck due to a couple bad cylinders (heavy leakdown numbers) and will take the opportunity to replace the flat tappet cam with a hydraulic roller (more aggressive). Without hard dyno numbers, I'm guessing the motor makes in the neighborhood of 500hp. Truck is only occasionally street driven and I have no plans of taking it to the track, so I want decent street manners with adequate vacuum for power brakes.

Specs....383, Weiand 142 blower, ELdy RPM heads, about 9:1 static comp,800 Eldy carb, all forged Eagle rotating assembly and SRP pistons.

current cam specs:

Isky flat tappet hyd,
Int. lift/dur 490/274
exh. lift/dur 510/284
Int. @ .050 - 226
exh. @ .050 - 236
112 deg lobe centers

Right now I'm running a set of Doug Thorley tri-y headers (5/8" primarys/2.5" collectors) that I've had for a long time, fit very well, good ground clearance, and have never given me a problem....nicely made header. My main question is if I'm giving up much hp with these and is it worth spending the money for some 3/4" primary headers or not?....and if so, the challenge is finding some that fit well and give me decent ground clearance. Current exhaust is 2.5" all the way back with an H pipe.

Also.....is it worth it to do some port work on the heads why I have it apart?

I should probably leave the whole thing alone because it ran very well, but it's just about impossible to not want some more out of it!!!

Any other advice for improvements is appreciated.

Sorry for the long read.....just trying to "paint a picture"!

Thanks....Jim

[derrickmanx1]

I'd jump up to 1 7/8" minimum. 2" if you can.

[BigDan3131]

Up to 500HP the 1 5/8" headers should be fine but unless you think your going to be making 600+.

[vin63]

Originally Posted by SactoJim

...Right now I'm running a set of Doug Thorley tri-y headers (5/8" primarys/2.5" collectors) that I've had for a long time, fit very well, good ground clearance, and have never given me a problem....nicely made header. My main question is if I'm giving up much hp with these and is it worth spending the money for some 3/4" primary headers or not?....and if so, the challenge is finding some that fit well and give me decent ground clearance. Current exhaust is 2.5" all the way back with an H pipe.

Thanks....Jim

I'm assuming you mean 1.625 primaries, which is plenty (I'm currently making 528 bhp @ 5200 RPM with 1.625 primary headers). If there is any restriction it would be the H-pipe. I tried just about every combination of headers on my Super Gas car over a period of 20+ years (650+ bhp @ 7900 RPM). The only improvement I found was a 2-3% increase from a 1.750 to a 2.00 primary (same configured headers) at the 7600-7800 RPM range where the exhaust velocity could take advantage of the larger area...but, I lost about 2% torque at the lower RPMs, so it was a push for me. Unless you plan on/presently operating your engine at WOT frequently, you'll probably not realize any improvement. I would use the money someplace else.

[SactoJim]

Yes, primaries are 1.625. That's good news if that's the case....not real interested and spending a bunch of money for new headers if the improvements would only by marginal. You do confuse me on the "H" pipe maybe being a restriction.....I have dual 2.5" exhaust all the way to the back with a 2.5" crossover pipe (H pipe) about 12" beyond header collectors.....don't understand how that could be any kind of restriction. Appreciate the info......Jim.

[vin63]

Originally Posted by SactoJim

You do confuse me on the "H" pipe maybe being a restriction.....I have dual 2.5" exhaust all the way to the back with a 2.5" crossover pipe (H pipe) about 12" beyond header collectors.....don't understand how that could be any kind of restriction. Appreciate the info......Jim.

I used to work closely with the engineering group that was responsible for the exhaust system designs and testing at Gale Banks Engineering - we did both CFD modeling, flowbench testing, and chassis dyno flow analysis on every exhaust system imaginable. Basically, H-pipes create turbulence in the flow, which creates backpressure. This is OK and in certain applications purposely leveraged on a street car system that operates at lower RPMs the vast majority of the time...it reduces a certain frequency of exhaust noise. At the upper RPMs it will probably restrict more flow than what a larger set of primary headers will provide in flow gain.

The best way I can describe it is...it is commonally accepted that if you are driving at high speeds, a vehicle with its side windows down/open is less aerodynamically efficient than with its side windows up/closed. It's the same principle. Which is why you might see more van trailers on the road with deflectors between the kigpin and the rear wheels to reduce the wind turbulence and help improve fuel mileage for the tractor pulling it.

[Marv D]

Not to hijack things,, but Vin peaked my curiousity on something.
Vin the exhaust side of a blower motor is obviously somewhat more tolerant of a poor flowing exhaust than is a turbo motor, but I'm curious in the R&D you were involved with just how bad does a undersized / poor flowing exhaust effect the turbo motors? With the hot side of the compressor being a restriction it'self,, does downstream restriction make more of an impact than with power than with .. say n/a
Turbo is kind of it's own world in being more tolerant of the street and crap fuels of today,,, and still making BIG power when you get after it.

Here's my random foolishness on where that is going. Do you (or anyone) know of any builds using small turbos on a pump gas TOWING motor??? What a turbo does for diesel is pretty impressive. Thinking along the same lines for a gas motor, any R&D that you know of? I've got this smallblock 383 TBI motor in my 1/2 ton I tow with. With tracks in AZ closing the tow now is 100 miles uphill to Tucson. 1700' of elevation over 100 miles isn't huge,, but the ol truck sure feels it on the overpasses and the few grades. Just toying with the idea and curious if gas and turbos mix as well towing as they do at the track.

[vin63]

Yep, my annual beginning of April race at Speedworld has been moved to Firebird, but in May...a little worried about the heat. I'll PM Marv D about the gas turbocharging so as not to hijack the thread.

If I was the OP, I would keep the same exhaust system, and not worry about porting the heads. If I were going to save or spend money on anything in the future for better streetability (and fuel mileage), is to eventually go to an electronic fuel injection system.

[derrickmanx1]

Originally Posted by derrickmanx1

I'd jump up to 1 7/8" minimum. 2" if you can.

Since I said this I dynoed my 408 bbc normally aspirated and my headers were not availible yet and we used a set of 2 1/8" headers. My motor made 496 hp @ 5600 479 ft/lbs @4700. In saying that the headers were too big for my motor. I lost more torque than anything on the dyno. I am running a set of 2"x3" headers from hedman. I did that so I could run nitrous and feel a little better about it in the end. I believe the increase from 1 5/8" to 1 7/8" would be good for a few horsepower like 2-5% but if you are in the 500hp range now that is 10-25hp. The only time you really would see a benefit would be above 4-5000 rpm. We have about the same size cams in our engines mine made 1.21 hp per cu/inch. Your's should be slightly better than that since you are running a blower. 2" might be too big but 1 7/8" even 1 3/4" should be slightly better.