During a recent chassis-dyno session, my car kinda told me it didn't like last years upgrade from castiron manifolds, 2-1/4" dual seperate exhaust, Turbo mufflers.

Last year's setup:
- mid '80s 318ci, 8.6:1cr,
- stock flattappet cam
- 4-bbl IMPCO Propane carb, Edelbrock Intake.
- Stock castiron exhaust manifolds
- Dual 2-1/4″ diameter seperated exhaust tubes (no crossover), ‘pushed’ exhaust bends, with Turbo mufflers and tailpipes.
- Stock mechanical distributor and vacuum adv, curved ad nauseam for best overal performance.

During the previous (2016) dyno-session it put out a ground, belly-shaking 169 RWHP to the rear wheels.

Current setup and changes:
- 318ci, 8.6:1cr, 4-bbl IMPCO Propane carb, Edelbrock Intake.
- same stock cam
- Added cold air intake w/ late '70s 400-engine airfilterhousing.
- Full length Doug’s Headers
- Dual 2.5″ diameter fully mandrell-bent exhaust system, SummitRacing X-pipe, Borla ProXS mufflers and tailpipes.
- MegaSquirt digital ignition ECU, controlling ignition timing through a 7-pin GM HEI-module and MSD Blaster-SS e-core coil.


Last weeks dyno-session showed the current setup having 8 HP less. Lower range torque was down too.


Now,
I knew the current exhaust system was (way) too large in diameter for this little smogger 318, but still, going from;
- Castiron manifolds to full length headers
- Seperated dual exhaust to dual w/X-pipe, all Mandrell bends including tailpipes.
- Restrictive Turbo mufflers to good flowing Borla ProXS mufflers

I would have expected to see at least some gain.

Could the added tube-diameter slow down the exhaut gasses enough that it would hinder power-output, even with the other mods being done?

AFR's during the dyno-runs where similar.
One difference I can think of was the outisde/dyno-cell temperature which was about 16°C / 60°F last year and 26°C / 78°F now.

Those pipes are BIG for a 318.. what is
the cam in at(measured)

I did replace the timingchain set a few years ago as the old stock one had a good amount of slack, but I didn't degree the cam for being a stock daily used engine and thinking an engine swap would have followed soon at the time.

The cam's stock specs are supposed to be this:
318 - 2 bbl, through '88 - Hydraulic - 373/400 lift - 240/248 dur.

Factory cam timing events:
Intake : 10 - 50 - 240 dur.
Exhaust: 52 - 16 - 248 dur.

Here's a dyno-run from last years (2016) session:




Here's the chart of the most recent run;

Peak torque was varying around 2800-3000rpm mostly.
HP tops at around 4250rpm.

As a guess I would think your cam is in retarted
I haven seen a engine that low if it was degreed
JMO... plus I would have advanced it to make more
low end power

At the time I installed a normal stock timing set and unfortunatly didn't go with one with a 3-way crank sprocket.

Engine vacuum while running is nice and high though.
23" vacuum, idling in Neutral @ 660rpm.
20+" vacuum, idling in Drive @ 510rpm.
17-18" during steady highway driving.

the difference in temperature is worth 3-4% in power, which probably explains most of the power loss.

Are you running the car on Propane or some other non gasoline fuel?
I think my old motorhome on propane had that same carb

Cab, the car runs on propane indeed.
Low CR engines like this 318 only want a max 32° of advance on propane.
We tried 34°, 28° and 30° degrees but it liked 32 best.

It also liked some timing to be pulled to 30° after peak HP.
I had some timing pull already entered in the ignition table, but it liked about a 2° pull to 30° after peak HP.

Was fun to see this confirmed on a dyno.


7e5dartsport,
I could go with that, if the setup was the same on both dyno-sessions.
But not quite with all the mods done in between.

Mr P body,
Got to thinking last night, the timing chain set was installed before the 2nd dyno-session (where the car made more power).
So that variable should be the same across both dyno-sessions only a year apart.
I also believe a retarded cam should show more power at higher rpms.

Exhaust is way too big. What is the primary tube diameter. Might be too big also.

Headers are Doug's Headers (D453) with 1 5/8" primaries.
The headers (and exhaust system) were installed in preparation for a high cr 360ci engine swap in the near future.

Its possible. I think some of the other variables are more likely to be culprits.


If you can get the data points and expand that, there may be more there than is first apparent. I've observed the WBO2 output on a Dynojet needs to be expanded to see the trend better. I was able to compare it to an Innovate WBO2 which was quicker responding and showed the trends more clearly, but the trending was basically the same. Also if possible, looking at Lambda instead of the AFR sometimes is better.


As some already posted, check to see if the dyno data is corrected for Temp & Humidity. Also if there has been any changes in the Dyno software, or how the load was ramped up. What I see in the graphic is a very different shape in the Torque curve.

I agree with your initial thought there are there are at least two other variables.

Let's consider them all:
1. - Added cold air intake w/ late '70s 400-engine airfilterhousing.
This should not have a major effect on a carburetor with unless the air filter is way different or causes strange pressure changes on the bowl vents. (lid to close to vent tops).

2 - MegaSquirt digital ignition ECU, controlling ignition timing through a 7-pin GM HEI-module and MSD Blaster-SS e-core coil.
This is a big variable or actually variables. The HEI modules can vary in slew rate and performance. The Megasquirt timing may not be as ideal as your highly worked out distributor. Possibly the Blaster SS is not a great match for the HEI and your fuel. My WAG is that propane isn't going to care much about the ignition energy after the kernel is burning. So my suspicion is that the timing seen at the spark plug is the big change.
-> If possible, swap back the old ignition system and retest. Based on those results, decide if this is the avenue to explore.

Exhaust
- Stock cast iron exhaust manifolds ->- Full length Doug’s Headers
This should have helped higher rpm and not hurt under 2500 very much because its a stock engine. My reasoning here is that maintain velocity at the exhaust port is less important on an engine with a stock cam than one with longer valve overlap resulting in reversion.
As far as ideal match goes, having run Pipemax for a stock 340, 1.5" primary is the best match.
If you want to read more about exhaust port velocity, I think Calvin Elston has been generous in sharing his insight. see What is Blowdown Length & What Size tube off the Head

- Dual 2-1/4″ diameter seperated exhaust tubes (no crossover), ‘pushed’ exhaust bends, with Turbo mufflers ->
- Dual 2.5″ diameter fully mandrell-bent exhaust system, SummitRacing X-pipe, Borla ProXS mufflers.
Two thoughts here:
1. It's just possible that the exhaust is simply seeing more pressure (atmosphere) and that's hurting the begining of the torque curve.
2. X pipe placement ended up in the worst possible position for harmonics. Whether its an open collector, an H-pipe, an X or a set of mufflers, the distance from the end of the primary (or secondary) tubes can be important.

Mattax, thanks for the interest and links.

The Dyno's output is DIN-corrected from I can gather.

Don't have data-points at this moment. Maybe I can still get them.
From what I could get from the videos, I notice AFR at WOT & Peak HP was around 12.3.

I manually wrote down the AFR numbers from a few videoclips of the runs:

- First run (Too lean):
AFR - RPM - (5803mov)
13.49 - 2020
13.39 - 2250
13.31 - 2500
13.13 - 2750
13.24 - 3000
13.90 - 3250
13.99 - 3500
13.90 - 3750
13.94 - 4000
13.92 - 4250
13.86 - 4500
(152 HP)

- Other runs:
AFR - RPM - (5804mov)
13.05 - 2000
12.97 - 2250
12.79 - 2500
12.23 - 2750
12.06 - 3000
12.53 - 3250
12.54 - 3500
12.49 - 3750
12.47 - 4000
12.37 - 4250
12.35 - 4500
(158 HP)

====================
AFR - RPM - (5807mov)
13.00 - 2020
12.92 - 2250
12.72 - 2500
12.20 - 2750
12.06 - 3000
12.49 - 3250
12.53 - 3500
12.45 - 3750
12.38 - 4000
12.31 - 4250
12.32 - 4500
(158 HP)

====================
AFR - RPM - (5808mov)
13.00 - 2030
12.94 - 2250
12.73 - 2500
12.21 - 2750
12.05 - 3000
12.49 - 3250
12.48 - 3500
12.39 - 3750
12.36 - 4000
12.32 - 4250
12.32 - 4500
(161 HP)

The Dyno-load was ramped as same as the same Dyno-sessions from what I could tell;
Engine is kept at 2000rpm by the Dyno while throttle is opened to WOT, then the run was started.

Interesting reads about timing retards because of the longer 'electronical chain' in the system.
There's a function in MegaSquirt that can compensate for timing retard at higher rpms. I've looked at it before. Might be worth looking into again.

I did make sure not to buy a Chinese HEI module, but I have 2 spares from a different brand as well.


The 400ci airfilterhousing, I highly doubt this is a restriction to the small 318, but I did notice a slight load increase in MAP-values during the runs.

In a previous dyno-session we tested with and without a airfilter (in smaller housing) and found no results.

I have a few other coils to try out, like a MSD TFI e-core coil and a stock e-core coil, but they have different connectors. Maybe I can unify the connections upfront for quick reconnecting and coil-swapping.

I'm not sure how great the GM HEI module will work. The Chevy powered Mustang I built, I swapped out the module for a chrome box, and recurved the distributor. Worked very well.

No wonder the motor is not making any power, propane an all the other liquid gas like that have no latent heat in them compared to gas or diesel fuel
I had two propane powered vechicles, one 1 ton car hauler truck and a big Class A motor home 1.5 ton chassis, they both ran better on gasoline

Engines not optimized for propane can indeed be dogs.
That doesn't mean they should loose more power the moment you install headers

The next engine that I'm currently building (360ci) will have 11:1 cr. That's more what propane likes.


The 7-pin HEI module has a few benefits when using it with a digital ignition;

- No matching ballast resistor needed for the coil. Everything runs on 12v. Simplifying the system.

- The engine will always start with a fixed 10 degrees advance, controlled by the HEI module. After 3 seconds of runtime, if the module senses a control unit present, it will release control of timing and let the ECU take over.
Great backup to have in situations where the ECU has failed or connection has been lost for instance.

About 1990 I built a little 318 for something to do.
I was working at a machine shop and there was a core short block there that had been turned in for some reason I can't recall, but it was already .040 over and what appeared to be very little use on it.
I pulled apart the short block, made sure all the clearances were good, polished the crank, honed the bores, cleaned everything and put it back together with new rings and bearings, and a Sealed Power stock replacement cam for a 340.
I took a set of mid-70's open chamber 318 heads and installed new guides, ex seats, did a nice valve job, and did a mild port job on them. As near as I can remember they flowed around 180, and I used a stiffer set of springs than std 318 springs.
I don't remember exactly what, but more than likely it would have been some 340 replacements or some Comp 901's.
Put the heads on the short block and buttoned it up with a regular performer intake.

I dynoed it with some 1-5/8" headers and a Holley 650dp that we had as a shop carb.

Made just about 275hp, at a pretty high rpm(for what it was) as I recall.
I'll have to see if I still have any of those dyno sheets.

I remember thinking it was pretty darn good for the sum of the parts used.

I think my current 318 would now have some 200+ (?) crank hp perhaps?
(A518 transmission / 8-3/4" axle.)

If I didn't have a 360 on the stand waiting to take the 318's place, I would be more than happy to work on the current engine a little more.

But the 360 has most of the goodies already I would want to install on the 318;
- 302-heads, CompCams XE256H cam, thin gaskets, KB-pistons, 1.6 rockers.

Just for kicks, I could try the 1.6 rockers on the 318 perhaps. Wouldn't be much work to try out.

But before I do the engine swap, I really like to find out why power is less with the current setup.

Well I don't know how much more I can help other than the suggestions on eliminating the possibilities. I still suspect new timing curve as a possibility - which may or may not be related to the slew rate. Apparently the good GM module is only 1* per 1000 rpm so that wouldn't be alot, but real enough.

Interesting the fuel goes rich around 3000 rpm on all the runs, then stable for the rest.

On the AFR my assumption is that the device was converting using a ratio for petrol. In other words it was programmed to convert based on a stoich ratio of 14.7:1
Propane's stoich is around 15.7:1 and best power should be around 13.2
Using Lamda avoids any question mark about fuel density and stoich. Anyway, thats an aside. I'm sure both before and after were done the same way so that's all that matters.

I don't know what the time - money - access situation is. My inclination would be to test out the changes that can isolated. Header swap to manifold is difficult so I wouldn't do that. Collector back tests with a couple of different length extensions, pretty easy. Ignition box and distributor swap might not be too hard. Coil not too hard if they don't require special connectors.

Pipemax suggests the X pipe will be best on a multiple of 24" or so, and worst if its around 36 or 72" from the end of the primaries. Real effect or not, I don't know. If you want to test use open collector extensions that end 20-25" after the end of the primaries.

The carb kept the AFR curve mostly the same over the runs.
3000rpm being max. torque area. This might mean something.

I did change the IMPCO propane carbs since last year I remember.
But looking the Dyno-charts from last year's runs, I'm seeing the same rich 'dip' in the AFR curves.

Been thinking if I want to install the old exhaust pipes to the current header collectors or not.

I'm currently uploading a video showing all the dyno-runs and dyno-charts.
Will post the link when it's online.