skep419
5.9 Magnum 4 speed afficionado
When I read AJ's posts I think of the mechanic from two lane blacktop hahaha. 
Dissapointing performance
- Thread starter Dusterguy
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I farted.
Ironmike
Well-Known Member
It's like it came outta my own mouth. Dead on advice..........".078 piston to head distance". Bet he checked that, too.Well. It seems like you don't really want help. But I'm a sucker for punishment. So here it goes.
Yes, the exhaust is all wrong. And YES exhaust contributes A LOT to your power loss.
Congrats. You've put an exhaust on there that can support a whopping 289 horse. What did you say you thought you ran at the track again? Oh right, about 300 horse. Put those headers to work and put on at least a 2-1/2 inch system.
Carb is wayyyy to small. 750 double pumper fits the bill nicely on something like this. Don't use a vacuum secondary on a car that has greater than 230@50 duration cam.
Piston to head clearance. How did you measure that? If your quench is actually .078, I would try and get it down. the .040 area is great.
Once you do all that. Re-tune and re-time. It will need it.
BTW this is the definition of mismatched combo. Hence why it's a dog.
Oh and try to be decent to other people on here that are offering help and such. We are here to help but not when you give us **** attitude in response.
I was on weekend trip and was not able to use internet and was not here to respond. What I realised is that my car can make burnout - when it rains!
Yes I checked that too. What I mean by piston to head distange I mean piston to head clearance, head gasket thickness plus deck height. quench distange is too far to take any effect. When I build this engine I didnt care about it.
I dont have compression gauge yet but I assume my compression numbers are good. Im at sea level and my dynamic compressionratio is about 7.9. David Vizard had said in his articles that anything above 8.2 is dangerous in premium fuel. (there is good article somewhere in internet about this subject written by David) So that means my cam and static compression ratio make very good pair therefore my compression test numbers will be good. I understand that measuring it can possibly reveal other problems.
Car is street car but not daily driver.
When I rebuilded my engine I used old timing chain and it had somewhat slack.
Yes I checked that too. What I mean by piston to head distange I mean piston to head clearance, head gasket thickness plus deck height. quench distange is too far to take any effect. When I build this engine I didnt care about it.
I dont have compression gauge yet but I assume my compression numbers are good. Im at sea level and my dynamic compressionratio is about 7.9. David Vizard had said in his articles that anything above 8.2 is dangerous in premium fuel. (there is good article somewhere in internet about this subject written by David) So that means my cam and static compression ratio make very good pair therefore my compression test numbers will be good. I understand that measuring it can possibly reveal other problems.
Car is street car but not daily driver.
When I rebuilded my engine I used old timing chain and it had somewhat slack.
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racerdan80
Well-Known Member
Could try thinner head gaskets. Try for 0.020 compressed. That would put the piston about 0.060" from the head, better than 0.080". When you have the time and before you dive into tearing down the motort, try removing the exhaust and run it with open header. The loss of back pressure from running a 2" system, to an open header you should see a noticeable difference. Do any of your friends have a larger carburetor they could loan you?
Compression Ratio above 8.2 is dangerous on 93 octane? You mean DCR? not static?
I have homemade shorties with 2 inch outlet. I need to work with them to change 2.5" output. (if not bigger)
I have 750 holley but it needs new base plate to work.
Yes I mean DCR.
racerdan80
Well-Known Member
I think 2.5" would work. Know a few guys running in the 11's quarter mile with that size
Yeah its fine for exhaust pipe. But all long tube headers tend to have bigger collector than that, I dont know if there reason to be so.
So i take it that since we have been rambling for a couple pages without you! You didn't read this on the previous page?
IF YOU LIKE "DAVID VIZARD" STUFF YOU SHOULD LOVE READING THIS!?!?!
VVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVV
"AJ/FormS Wrote"
Dusterguy
You probably understand the 4-stroke cycle as Intake/Compression/Power/Exhaust.What you may not understand is overlap/scavenging. George (post 110) has a good handle on this. But with bigger cams there is a 5th cycle called scavenging that is supposed to occur during the overlap period when both valves are open. That is the whole idea of having tuned headers and overlap period in the first place. This scavenge-cycle does more than just help exhausting; it actually helps to pull in a larger fuel/air charge than the engine can physically pull in by itself. So if the engine could pull in 10% more charge with scavenging, it will feel 10% bigger than without.If you prevent scavenging from working, by forcing the exhaust into positive pressure with too small an exhaust system, then scavenging cannot work.
-But it is worse than that.If you force the exhaust into positive pressure, the pressure backs up, and when the exhaust is supposed to be leaving the engine, it is possible, during overlap, for the exhaust to back up and enter the intake manifold. It can get so bad that the exhaust will try to exit the intake through the carb.Guess what that does to power.Your cam has a lot of overlap, making this phenomenon a very real possibility.If the exhaust ends up in the intake, it reduces the pressure differential across the venturies, which means a much smaller A/F charge can enter.
-So now your engine is under a tripple whammy. Loss of scavenging, loss of induction, and intake charge dilution.
-The proof is very easy;just drop the pipes and watch the trap speed.You can also see the proof in the intake manifold; pop the carb off, and look towards the ports.You will see the head ports filled with soot, and the worse it is, the further into the intake,the soot will extend.Alternatively, you can braze a length of brake tubing into an exhaust pipe (leave at least a foot sticking out), and measure the back-pressure. The closer to zero, obviously, the better and more than about 2 or 3psi will seriously affect the scavenge-cycle. By 4 psi your high-performace engine, isn't anymore. This test has to be performed at full throttle,full load, and the bigger the gauge the better,cuz the engine is very sensitive to back-pressure and the more accuracy in the gauge the better.Your 10 psi vacuum/fuel pump tester won't cut it, except to ballpark it, or to point out an obviously bad condition.Neutral blips will never find it! Get a helper......drive safe, I ain't paying your speeding ticket.....................
-Chrysler knew all about this EGR back in the 70s and used it to good effect in that it delayed the use of EGR valves for a couple of years after every body else had them.
IF YOU LIKE "DAVID VIZARD" STUFF YOU SHOULD LOVE READING THIS!?!?!
VVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVV
"AJ/FormS Wrote"
Dusterguy
You probably understand the 4-stroke cycle as Intake/Compression/Power/Exhaust.What you may not understand is overlap/scavenging. George (post 110) has a good handle on this. But with bigger cams there is a 5th cycle called scavenging that is supposed to occur during the overlap period when both valves are open. That is the whole idea of having tuned headers and overlap period in the first place. This scavenge-cycle does more than just help exhausting; it actually helps to pull in a larger fuel/air charge than the engine can physically pull in by itself. So if the engine could pull in 10% more charge with scavenging, it will feel 10% bigger than without.If you prevent scavenging from working, by forcing the exhaust into positive pressure with too small an exhaust system, then scavenging cannot work.
-But it is worse than that.If you force the exhaust into positive pressure, the pressure backs up, and when the exhaust is supposed to be leaving the engine, it is possible, during overlap, for the exhaust to back up and enter the intake manifold. It can get so bad that the exhaust will try to exit the intake through the carb.Guess what that does to power.Your cam has a lot of overlap, making this phenomenon a very real possibility.If the exhaust ends up in the intake, it reduces the pressure differential across the venturies, which means a much smaller A/F charge can enter.
-So now your engine is under a tripple whammy. Loss of scavenging, loss of induction, and intake charge dilution.
-The proof is very easy;just drop the pipes and watch the trap speed.You can also see the proof in the intake manifold; pop the carb off, and look towards the ports.You will see the head ports filled with soot, and the worse it is, the further into the intake,the soot will extend.Alternatively, you can braze a length of brake tubing into an exhaust pipe (leave at least a foot sticking out), and measure the back-pressure. The closer to zero, obviously, the better and more than about 2 or 3psi will seriously affect the scavenge-cycle. By 4 psi your high-performace engine, isn't anymore. This test has to be performed at full throttle,full load, and the bigger the gauge the better,cuz the engine is very sensitive to back-pressure and the more accuracy in the gauge the better.Your 10 psi vacuum/fuel pump tester won't cut it, except to ballpark it, or to point out an obviously bad condition.Neutral blips will never find it! Get a helper......drive safe, I ain't paying your speeding ticket.....................
-Chrysler knew all about this EGR back in the 70s and used it to good effect in that it delayed the use of EGR valves for a couple of years after every body else had them.
MopaR&D
Nerd Member
He's coming around to it he just said he's going to modify his headers for a 2.5" outlet, don't bust his chops too hard lol.
Dusterguy, as far as I know long-tube headers almost always have at least a 3" diam. collector because that's the optimal size for the collector to 'taper down' to once the primary tubes have come together at the front of the collector. If you hook up a 2.5" exhaust you just use a section of pipe right after the collector that gradually goes down from 3" to 2.5". It's also kind of an "industry standard" so that different header models/sizes don't have different size collectors too, and in cases where you're making 500+ HP a dual 3" system will just bolt right up.
I'm guessing your primary tube diameter is 1-5/8"? You would also probably get a bit more power with 1-3/4" tubes, 1-5/8" is kind of the "entry level" size for smaller/milder V8s. Compared to making the rest of your exhaust bigger though it's really not important right now just something to think about.
Regards,
Khalid
Dusterguy, as far as I know long-tube headers almost always have at least a 3" diam. collector because that's the optimal size for the collector to 'taper down' to once the primary tubes have come together at the front of the collector. If you hook up a 2.5" exhaust you just use a section of pipe right after the collector that gradually goes down from 3" to 2.5". It's also kind of an "industry standard" so that different header models/sizes don't have different size collectors too, and in cases where you're making 500+ HP a dual 3" system will just bolt right up.
I'm guessing your primary tube diameter is 1-5/8"? You would also probably get a bit more power with 1-3/4" tubes, 1-5/8" is kind of the "entry level" size for smaller/milder V8s. Compared to making the rest of your exhaust bigger though it's really not important right now just something to think about.
Regards,
Khalid
MopaR&D
Nerd Member
When I read AJ's posts I think of the mechanic from two lane blacktop hahaha.
I love how there are like 4 instances in the movie where the mechanic says "it's not running right, gotta change out the jets" while they're driving down the road cross-country.
Yes I have read it and I already knew what scavenging effect is but I newer thinked that exhaust backpressure and big cam together will make it worse. Believe or not but I agree everything what AJ wrote.
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That might be the reason for bigger collector. Yes 1-5/8" is my tube size. Best option would surely be long tube headers but my car is lowered and I think not even TTI headers can make good enough ground clearance to this car.
racerdan80
Well-Known Member
1-5/8" tube should be plenty for your headers, even in a shorty configuration. As said, a 2.5" system should suffice, a 3" will work quite well and support a stroker like a 408 should you choose to upgrade in the future.
I did a test and backed timing to 15 degrees at idle (from 20). Car started to idle more poorly, but it accelerates better in lower rpm and started to make burnouts! Looks like I need to recurve my distributor and possibly add some vacuum advance to make it run better in part thorttle. Now car doesnt have vacuum advance at all. It gets full mechanical advance in at about 2000 rpm.
racerdan80
Well-Known Member
What is your total mechanical advance? More initial should help your idle and low rpm torque.
I know it sounds strange but when I did try less initial advance I got better low rpm torque and it idled more poorly. I dont know what is total now, but when i made performance tests earlier I set it to 34
racerdan80
Well-Known Member
You'll have to re adjust your idle air bleeds to bring back up your idle. Do you have a vacuum gauge?
AJ/FormS
68 Formua-S fastback clone 367/A833/GVod/3.55s
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fratzog lover
Well-Known Member
Listen to this man. He knows engines.
racerdan80
Well-Known Member
Aj/forms, a pretty good guideline to your timing. Every motor combo is a little unique.
AJ/FormS
68 Formua-S fastback clone 367/A833/GVod/3.55s
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This seems to indicate that your combo is running too much centrifugal advance. You have two choices, well really only one, and that is to slow it down and peak later.
I run 14/34@3400plus 22 ported. Works for me.With 87E10.
But it may also speak to a restricted exhaust.
AJ's guide to Transfer Port Synchronization
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68gtxman
I used to reMember
Sorry Dusterguy, but I don,t have time to read 6 pages of posts. But from what I did read, I didn't hear what setting you degreed your cam to. It is a 110 degree cam but it is supposed to be installed at 106 degrees. I believe that some folks even set them to 104. What did you set it to?
Cam experts can insert their advice here:
Cam experts can insert their advice here:
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