Frustration
Thread Starter
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Mountain Pass
Joined: Oct 2020
Posts: 118
Likes: 6
Frustration
I just realized when checking part numbers that I replaced my pistons with the same lousy 1.66 dish pistons that it originally had instead of flat tops. This is really disappointing.
So now I am assuming my compression is crap.
Here are my engine specs-
040 1.66 390/410 pistons p1529
390 rods
comp 268h cam
hei dist
d2teaa heads
performer intake
headers
600 carb
Before I had-
Same pistons & rods
stock cam
duraspark with spark module
cast intake
log manifolds
same heads
same carb
Before even with 4.56 gears it was a dog. 8mpg with no power. It was horrible.
I am hoping I will see a decent power increase even with the crappy pistons I used.
So now I am assuming my compression is crap.
Here are my engine specs-
040 1.66 390/410 pistons p1529
390 rods
comp 268h cam
hei dist
d2teaa heads
performer intake
headers
600 carb
Before I had-
Same pistons & rods
stock cam
duraspark with spark module
cast intake
log manifolds
same heads
same carb
Before even with 4.56 gears it was a dog. 8mpg with no power. It was horrible.
I am hoping I will see a decent power increase even with the crappy pistons I used.
Logistics Pro
Joined: Jul 2005
Posts: 4,217
Likes: 154
What engine are you working with? FE it looks like? One thing I didn't see mentioned was your timing set. Not sure if the FEs are the same, but I know that the 335 engines, (400 and 351M) which replaced FEs, will run a lot better with a straight-up timing set.
Again I don't know if later FEs were built with the retarded valve timing, but if they were, then that stock timing set would be a good thing to replace even if it's brand new.
If everything else is the same, then the heads, cam, and intake, will still surely gain you some more power.
Again I don't know if later FEs were built with the retarded valve timing, but if they were, then that stock timing set would be a good thing to replace even if it's brand new.
If everything else is the same, then the heads, cam, and intake, will still surely gain you some more power.
Lead Driver
Joined: Oct 2018
Posts: 7,893
Likes: 2,371
You still align the bosses it's a matter of which key way you use. this would be on your timing set not the cam. if you used stock settings that's fine.
Compression is a give and take. flat tops most likely would have run you up to where it would require premium gas at the least. you'd get more power but about the same fuel economy . hard to say if the power is worth the expense if it's a daily driver.
Compression is a give and take. flat tops most likely would have run you up to where it would require premium gas at the least. you'd get more power but about the same fuel economy . hard to say if the power is worth the expense if it's a daily driver.
Thread Starter
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Mountain Pass
Joined: Oct 2020
Posts: 118
Likes: 6
It wont be a daily driver and I dont care about mpg. Just want more power. Hopefully the loss of compression doesn't lessen the power too much. Before it ran out of steam at 3000rpm and was pretty lousy under 3000rpm to be honest. It had way less power than my 89 bronco with stock 302. Not that I need to or ever will, but neither this truck or the bronco could spin the tires if I even tried.
I dont think the timing set had more than one key way.
I dont think the timing set had more than one key way.
Lead Driver
Joined: Oct 2018
Posts: 7,893
Likes: 2,371
The D2 heads are stock pickup heads with 72 cc chambers , so with stock pistons assuming they're 4v ratio you shouldn't be that low. nothing about your build is going to be a tire fryer but it should do a decent job if you get it tuned in right.
The 268H cam isn't going to help you a bunch with the build you have, it will actually bleed off a little more compression than your stock cam did. you might notice a little gain at higher rpm though.
The 268H cam isn't going to help you a bunch with the build you have, it will actually bleed off a little more compression than your stock cam did. you might notice a little gain at higher rpm though.
Thread Starter
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Mountain Pass
Joined: Oct 2020
Posts: 118
Likes: 6
Eventually I am going to upgrade heads. Just a little pricey right now. The motor runs real smooth. Just need as much fun as possible out of it in the mean time.
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Lead Driver
Joined: Oct 2018
Posts: 7,893
Likes: 2,371
Peak your timing out if you haven't already, that will make sure you're getting everything it has to give.
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Lead Driver
Joined: Oct 2018
Posts: 7,893
Likes: 2,371
It all has to start with compression, but it's easy to take it too far and have a race gas motor . by heads, if you mean aftermarket aluminum ones that alone might get your compression where you want to be, they CC less and flow much better. you'd have to do some research and math on them.
Laughing Gas
Joined: Oct 2015
Posts: 991
Likes: 114
From: Dallas, OR
I had the same issue when I recently rebuilt my 390. You pretty much have the choice between the dish pistons you have which end up close to stock compression even with higher compression height and the flat tops which are generally considered too high compression for pump gas. Well that or $$$ for custom pistons. I rolled the dice and went with the higher compression. Guess who is now putting premium in his gas guzzler? Yup, this guy. Really the only choice that I found to hit the middle ground is to spend the extra money and have the heads and block milled using the dish pistons until you hit the ratio you want (in hindsight I wish I had done this). Aluminum heads are supposed to help keep the detonation down some too, but I haven't had the budget to try that yet.
Laughing Gas
Joined: Dec 2020
Posts: 869
Likes: 186
From: Near my truck
Unfortunately that would be a yes.
I have no idea if a duraspark required a resistor wire, but if it did, that would rob you of power with an HEI, so you'd see an improvement with a new unresisted wire.
In another thread you said that you didn't have the vac advance connected. That will rob you of mpg's.
You also said in another thread that your base timing was 10 degrees.
Without looking at the cam specs, I assume from 440's comment that the cylinder pressure has been reduced, and add in the fact that the heads are open chamber, you'll need more ignition timing to counter the inefficient burn and lower pressure.
Start at 16 degrees base timing and take it from there.
Measure your timing curve at 500rpm intervals and adjust accordingly. (A crap curve will rob you of both power and mpg's)
Hmm, dying at 3,000 rpms, check that your throttle plates open all the way.
I have no idea if a duraspark required a resistor wire, but if it did, that would rob you of power with an HEI, so you'd see an improvement with a new unresisted wire.
In another thread you said that you didn't have the vac advance connected. That will rob you of mpg's.
You also said in another thread that your base timing was 10 degrees.
Without looking at the cam specs, I assume from 440's comment that the cylinder pressure has been reduced, and add in the fact that the heads are open chamber, you'll need more ignition timing to counter the inefficient burn and lower pressure.
Start at 16 degrees base timing and take it from there.
Measure your timing curve at 500rpm intervals and adjust accordingly. (A crap curve will rob you of both power and mpg's)
Hmm, dying at 3,000 rpms, check that your throttle plates open all the way.
Thread Starter
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Mountain Pass
Joined: Oct 2020
Posts: 118
Likes: 6
Well took it around the block and has a definate improvement from before the rebuild. You can be rolling in 2nd (np435 & 4.56) and hit the gas and it will break traction with 28575r16 mud tires. Runs better with the vacuum advance hooked up and carb adjusted. No resistor wire. I am waiting on wiring harness so run on toggle right now.
Lead Driver
Joined: Oct 2018
Posts: 7,893
Likes: 2,371
Keep advancing your timing a little at a time until you hear pinging under load , then back it of until you don't. your stock recommended settings don't mean crap at this point.
in the future for compression there are two options, custom pistons or take the flat tops and have whatever dish you calculated them to need and get them milled . I've done this more than once and it works great. obviously this would be for a stocker type build not something on the bottle.
in the future for compression there are two options, custom pistons or take the flat tops and have whatever dish you calculated them to need and get them milled . I've done this more than once and it works great. obviously this would be for a stocker type build not something on the bottle.