Large rpm drop

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fishmens67 said:
Vacuum leak ? 20 degrees initial timing should be close give or take a couple degrees.
Factory Holley 750 settings should be close enough for a better generic tune than you have.
I believe you have a vacuum leak that is altering the tune.
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I'll double check that. I've done a full engine rebuild, so all gaskets are new. Vacuum to brake booster and pcv. Everything else is capped. Manifold plugs are sealed. I'll spray around the carb.

I bought the carb used from someone that didn't need to be messing with anything mechanical. 750, vacuum secondaries. Rebuilt it, set it up to factory specs, added a secondary metering block. Haven't looked at the transfer slot since that first rebuild so that's definitely a place I'll start at.
 
The idle set screw is about 1 turn for 0.20 square transfer slot.
 
Charlesvolare said:
It was just a thought, I have many of those as I'm sure you do too

Something wrong with science? Lol I'm not using it to tune my idle, just stating that's where it was landing, adding to info that may help. Not gonna completely disregard the second law of thermodynamics, but I'm not too concerned how rich it needs to be to idle decent. If I can lean it out some and good, if not oh well kinda deal.


Plugs look good, I can pull and take a picture of I need to

Some calculators put it at 10.8:1 if it makes you feel any better. I'm not sure what to say, I can send to my build sheet. Put it together myself, I'll swear on all I've got the measurements are correct.

It's an amazing cam, zero complaints at all- very streetable and plenty of power.originally I thought it was around 9:1, so we speced the cam for that. Had detonation problems running timing anything I've 32* on 93 octane. Butted rings and broke a ringland, pulled and rebuilt. Made sure to measure everything like I did putting it back together.

What cam would you have gone with?

Autolite 3923
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If it's pulling fuel from the main jets you'd see it. Fuel would be coming out of the boosters.

An O2 sensor can and will lie to you. Not all O2 sensors read the same. Different fuels require a different A/F ratio so tuning to a number is not correct. As I said, put it away until you get everything else correct and then hook it back up. Really, if you can't data log it, and overlay it with RPM, MAP and throttle position, you are still really guessing except at idle and WOT.

I'd love to see a picture of your plugs, preferable a couple of plugs with the shells off.

If you didn't actually measure the volume of the piston at say .500 down the bore, it's still a guess. How did you account for the valve notches?

You showed very little info on your cam. What I was able to see was what is most likely a slow ramp, wide LSA early intake closing cam. If you are at 11:1 and you didn't call someone and tell them what you are doing, the cam will always be wrong.

A faster ramp lobe (doesn't need to be an outrageous lobe) would allow the cam timing to be more agressive (what the cylinder actually "sees") while not giving up driveability.

IMO, you are at least 6 degrees too small on duration and I'm not sure why it's a dual pattern cam. Comp love that stuff but mostly, it's a gimmick.

You might be a range too cold on your plug. It's hard to tell without looking at an Autolite plug catalog. Getting Autolite to send me a catalog has proven futile up to this point.
 
yellow rose said:
If it's pulling fuel from the main jets you'd see it. Fuel would be coming out of the boosters.

An O2 sensor can and will lie to you. Not all O2 sensors read the same. Different fuels require a different A/F ratio so tuning to a number is not correct. As I said, put it away until you get everything else correct and then hook it back up. Really, if you can't data log it, and overlay it with RPM, MAP and throttle position, you are still really guessing except at idle and WOT.

I'd love to see a picture of your plugs, preferable a couple of plugs with the shells off.

If you didn't actually measure the volume of the piston at say .500 down the bore, it's still a guess. How did you account for the valve notches?

You showed very little info on your cam. What I was able to see was what is most likely a slow ramp, wide LSA early intake closing cam. If you are at 11:1 and you didn't call someone and tell them what you are doing, the cam will always be wrong.

A faster ramp lobe (doesn't need to be an outrageous lobe) would allow the cam timing to be more agressive (what the cylinder actually "sees") while not giving up driveability.

IMO, you are at least 6 degrees too small on duration and I'm not sure why it's a dual pattern cam. Comp love that stuff but mostly, it's a gimmick.

You might be a range too cold on your plug. It's hard to tell without looking at an Autolite plug catalog. Getting Autolite to send me a catalog has proven futile up to this point.
Click to expand...

I'm using fresh, dual O2 sensors, calibrated to gasoline, data logged with rpm.

I work in r&d at comp, went and talked personally with our one of our senior cam designers, who has since retired. It's a roller cam so ramps are different than tappet. Not trying to argue cams but it's matched better than you probably think. What all do you want info wise? I'll try to grab it for ya

Valve notches are right at 5cc's. Not trying to argue engine compression either.

I'll try to pull some plugs today.
 
Charlesvolare said:
Something wrong with science? Lol I'm not using it to tune my idle, just stating that's where it was landing, adding to info that may help. Not gonna completely disregard the second law of thermodynamics, but I'm not too concerned how rich it needs to be to idle decent. If I can lean it out some and good, if not oh well kinda deal.
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More like there are things wrong with the marketing, much of it further perpetuated by the magazine writers and internet.

Lets start with the AFR meter. It doesn't read AFR. It is an interpretation of AFR based on oxygen in the exhaust. In any engine, but particularly a hopped up engine, the products of combustion at idle can fall outside that interpretation. A CO meter is better at idle. If you are interested I can post up some links which get a lot more into this. Both some on why what is in the exhaust pipe may not be what happened inside the cylinder, and how the products of combustion themselves can be different than expected because the components did not burn in a 'normal' manner.

Rich or lean at idle. The whole 'lean idle' thing is to reduce amount of unburned hydrocarbons and CO. You don't have to beleive me, read the Chrysler Master tech booklets about carburetion from mid 60s to '70. They, as do the shop manuals, explicitly explain that to reduce emissions at idle the idle mixtures will be run leaner. That instead of using the old methods, adjustment of the idle mixtures must be tuned to as lean as 14.2:1.

Further, the idea of running somewhat richer mixtures at idle for best economy is not just a 'it worked for me - it must work for you' theory. It comes from the scientific and engineering community (academic, industrial and military). There has been extensive testing of engines in laboratories and in the field which can be read in several textbooks and published papers.
 
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This is a cut and past but worth reposting -

If you're just going for engine effiency (in terms of making good power for amount of fuel used) and not dealing with emissions requirements, then the general trend of AFR can be described as follows:

"The [relationship] is rich idle, lean cruise, leaner (leanest, actually) part-throttle acceleration and rich WOT. The leanest is at mid-load, half-throttle or" somewhat more. Cruising, especially at lower speeds, requires little throttle and puts relatively little load on the engine. Half throttle or mid-load would be going up hills or part throttle acceleration. "The thing to understand is most engines respond to being leaner...at part-throttle..."

In summary "at moderate to mid load, engines will run lean and like it, and burn much less gas while doing so. They must be rich at idle and very low load, lean in the middle, and rich at WOT." The load where richer is needed varies with engine, gearing and vehicle. It may be 60-70% as shown here, or as high as 90%. Load relates to manifold vacuum, and therefore vacuum is used to signal when enrichment is needed.
blob-jpg-jpg-jpg-jpg.jpg

F is Fuel/Air ratio for gasoline. Invert the numbers to convert to Air/Fuel ratio.
0.08 = 12.5 AFR
0.667 = 14.7 AFR
0.06 = 16.6 AFR
Constant power is any steady throttle condition.
Steady 15% might be something like driving 40 mph on a flat.
Steady 25% might be cruising 60 mph on flat or steady 40 mph up a long grade.
Steady 100 % might be something like towing or dragging max load up up a really steep long grade, foot to the floor, without losing or gaining speed.

The acceleration loop shows that maximum acceleration (full throttle) has about the the same fuel-air needs as constant 100 % power.

"This ... graph is from Walter B. Larew, Carburetors and Carburetion. At the time he wrote his book on carburetors he was a retired Brigadier General who taught Military Science at Cornell, among his other accomplishments. He published this carb book in 1967. He didn't specify an engine type for this graph but his information is in the context of engines in general. His sources were most likely military aviation research. The math in his book is from NACA TR-49 and similar publications.
This graph is representative of a richer part-throttle that may be necessary to tolerate with an engine that has radical valve timing and perhaps not so good A/F distribution at part-throttle."

quotes and graph from Tuner on Innovate Motorsports Forum and full text now reposted at RFS
edit. If you follow the link to RFS and tapatalk requests you join, just click the x, and ignore the request. You do not have to join to read the forum.
 
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Mattax said:
More like there are things wrong with the marketing, much of it further perpetuated by the magazine writers and internet.

Lets start the AFR meter. It doesn't read AFR. It is an interpretation of AFR based on oxygen in the exhaust. In any engine, but particularly a hopped up engine, the products of combustion at idle can fall outside that interpretation. A CO meter is better at idle. If you are interested I can post up some links which get a lot more into this. Both some on why what is in the exhaust pipe may not be what happened inside the cylinder, and how the products of combustion themselves can be different than expected because the components did not burn in a 'normal' manner.

Rich or lean at idle. The whole 'lean idle' thing is to reduce amount of unburned hydrocarbons and CO. You don't have to beleive me, read the Chrysler Master tech booklets about carburetion from mid 60s to '70. They, as do the shop manuals, explicitly explain that to reduce emissions at idle the idle mixtures will be run leaner. That instead of using the old methods, adjustment of the idle mixtures must be tuned to as lean as 14.2:1.

Further, the idea of running somewhat richer mixtures at idle for best economy is not just a 'it worked for me - it must work for you' theory. It comes from the scientific and engineering community (academic, industrial and military). There has been extensive testing of engines in laboratories and in the field which can be read in several textbooks and published papers.
Click to expand...

I guess I should've just left all of that out. Lol I just wanted help fixing the idle drop and thought maybe that info would help. I wasn't trying to say I was using that to say I was using that to set my idle, rather that's the result of how it is now. It's mostly used to monitor things, make sure I'm not too late or too rich when driving around.

Again, everyone can just forget that I even have that, just know it's already a little rich at idle:thumbsup:
 
Mattax said:
This is a cut and past but worth reposting -

If you're just going for engine effiency (in terms of making good power for amount of fuel used) and not dealing with emissions requirements, then the general trend of AFR can be described as follows:

"The [relationship] is rich idle, lean cruise, leaner (leanest, actually) part-throttle acceleration and rich WOT. The leanest is at mid-load, half-throttle or" somewhat more. Cruising, especially at lower speeds, requires little throttle and puts relatively little load on the engine. Half throttle or mid-load would be going up hills or part throttle acceleration. "The thing to understand is most engines respond to being leaner...at part-throttle..."

In summary "at moderate to mid load, engines will run lean and like it, and burn much less gas while doing so. They must be rich at idle and very low load, lean in the middle, and rich at WOT." The load where richer is needed varies with engine, gearing and vehicle. It may be 60-70% as shown here, or as high as 90%. Load relates to manifold vacuum, and therefore vacuum is used to signal when enrichment is needed.
View attachment 1715457913
F is Fuel/Air ratio for gasoline. Invert the numbers to convert to Air/Fuel ratio.
0.08 = 12.5 AFR
0.667 = 14.7 AFR
0.06 = 16.6 AFR
Constant power is any steady throttle condition.
Steady 15% might be something like driving 40 mph on a flat.
Steady 25% might be cruising 60 mph on flat or steady 40 mph up a long grade.
Steady 100 % might be something like towing or dragging max load up up a really steep long grade, foot to the floor, without losing or gaining speed.

The acceleration loop shows that maximum acceleration (full throttle) has about the the same fuel-air needs as constant 100 % power.

"This ... graph is from Walter B. Larew, Carburetors and Carburetion. At the time he wrote his book on carburetors he was a retired Brigadier General who taught Military Science at Cornell, among his other accomplishments. He published this carb book in 1967. He didn't specify an engine type for this graph but his information is in the context of engines in general. His sources were most likely military aviation research. The math in his book is from NACA TR-49 and similar publications.
This graph is representative of a richer part-throttle that may be necessary to tolerate with an engine that has radical valve timing and perhaps not so good A/F distribution at part-throttle."

quotes and graph from Tuner on Innovate Motorsports Forum and full text now reposted at RFS
Click to expand...
That's a pretty cool read, I'll check out the whole link when I get a chance
 
I ran your numbers and got 10.7 static CR, so you are in the right ballpark for that. Your DCR is 8.4-8.6 so that explains the detonation issues. A larger cam by 1-2 steps would lower that and would make tuning easier.

Go back to post #20 and re-read what AJ has explained. You have too much initial timing and that is messing you up. The old idea of adjusting idle timing for maximum idle RPM goes out the window when you get to the higher dynamic compression ratios like you have. The cylinder's torque pulses get much 'peakier' than for a low compression engine, and that spins the crank faster when it is unloaded. But when you load it, the average crank torque is not all that much higher and the RPM's load way down.

And with your 'all-in' being so low (1800-2000 RPM), you may be getting more advance pulled in at that idle of 1100-1200 RPM. Having your mechanical advance come in so fast is also contributing to you detonation issues.

Our 340 at 10.0 SCR and 8.0 DCR has 15 degrees initial timing. No idle drop problems and all was good for the transition once the secondary transfer slot was closed down a bit.

And as said, if drawing off the mains at idle, you will see fuel dripping down from the boosters (and even hear it if your ears are still good).
 
Charlesvolare said:
I just wanted help fixing the idle drop and thought maybe that info would help. I wasn't trying to say I was using that to say I was using that to set my idle, rather that's the result of how it is now. It's mostly used to monitor things, make sure I'm not too late or too rich when driving around.
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thumbs_up-gif.gif
All's good.
I thought you were chasing AFR numbers from the comment of going leaner.
 
FWIW With an auto transmission I always tune in gear. Wheel chocks work wonders for the self tuner.
 
So my plan for the weekend so far is:
  • Check transition slots, adjust if needed
  • Small adjustments on carb, dial back timing
  • If idle gets better, change mechanical advance and possibly stiffer springs
If this doesn't affect the issue, start to look at possible torque converter/transmission issues.

Anything else I should look at?
 
Edit Do not blip throttle when in gear while using wheel chocks. :D it can get very exciting. BTDT
 
Charlesvolare said:
Anything else I should look at?
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The 10.5 PV will do wonders for your PartThrottle torque delivery, and you can probably take out some MJ.
When you match the Vacuum advance to it, you might never have to floor it again ........ lol.

Kill the power timing curve, to something like 30* at 3600; then work back into it little by little, say dropping 150rpm atta time, or 2 or 3 degrees at a time, but not both together. One change atta time, don't be in a hurry, and Stop! when you hit the detonation wall. This wall is a moving target. It is highly dependent on the chamber pressure and temperature, which varies in multiple ways, so it isn't always throttle related.
Remember; you can't always hear detonation, and sometimes when you do, it is already severe. And if you don't sneak up on it, you may not feel the associated power loss.
Finally, you can be 4 degrees and nearly 400rpm on the conservative side and not notice the powerloss ........ cuz as a streeter..... you are rarely blasting thru there without tirespin, and if the tires are spinning you already have more power than you need.
My 367 combo runs 32/34* at 3400rpm, and in the zero to 65mph arena, spins the 295s all the way...... so do I care if I'm 7 hp shy? No I don't lol. I wouldn't care if I was 50 short or a hundred, cuz the chassis cannot handle what it already has. ChitI shouldda thought of that when the 223 cam died,lol. Imadope
However, I can run 87E10 at 185psi (aluminum heads) at WOT with this curve, and that makes the lousy fuel-economy not hurt so much,lol.
 
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fishmens67 said:
Edit Do not blip throttle when in gear while using wheel chocks. :D it can get very exciting. BTDT
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laugh2-gif.gif

That's why most of the time I work in Park and then shift it into D to see how it went...
Always some exceptions...

Charlesvolare said:
So my plan for the weekend so far is:
  • Check transition slots, adjust if needed
  • Small adjustments on carb, dial back timing
  • If idle gets better, change mechanical advance and possibly stiffer springs
If this doesn't affect the issue, start to look at possible torque converter/transmission issues.

Anything else I should look at?
Click to expand...
If you have the carb off, measure the opening per turn, and using drill bit or gage you can also get a sense of the t-slot widths and lengths.
On an older Holley carb, the widths and lengths are usually what works so not a major concern.

I hesitate to say whether the base timing ought to be advanced or retarded. See where you need to put it so the engine will idle with the t-slot exposure in the working range. It will be an iterative process. Start where it is now, and after the engine fires up, decide what you need to do. As it warms up you should be able to back things down. Then you can begin your in-gear testing, whether its going back and forth or leaving it in D is up to you.

I would not touch the distributor springs at this point. There's no vacuum advance, right? If there is, don't use it for now. Once the idle in D is better - assuming it is better - then do some gentle accelerations from idle.
If there is no hesitation and that works well, try a gentle uphill.

The only reason to mess with the springs at idle will be if the timing is not stabile at the idle rpms. In other words if the timing in P or N is 18* at 800 rpm and in gear it is 16* at 700 rpm, then increasing the spring tension on the light spring will solve that.
 
Mattax said:
View attachment 1715457944
I would not touch the distributor springs at this point. There's no vacuum advance, right? If there is, don't use it for now. Once the idle in D is better - assuming it is better - then do some gentle accelerations from idle.
If there is no hesitation and that works well, try a gentle uphill.

The only reason to mess with the springs at idle will be if the timing is not stabile at the idle rpms. In other words if the timing in P or N is 18* at 800 rpm and in gear it is 16* at 700 rpm, then increasing the spring tension on the light spring will solve that.
Click to expand...
Of course, only touch springs if obviously needed.


I also have a Fast E6 ignition box and a dial timing retard unit under my dash, so I can set the distributor and dial back timing while I'm in the car. Then check to see where it's at, dial back to zero, clock distributor to match and go from there. Experimenting with timing is no big deal so I'll see what works best. Thanks for all the input!
 
Since it came up earlier, here's a graphic of 'idle' system vs. main system.
Idle is in quotes because the Holley 'idle' system really is a low throttle position system. It's not just idle so the name is deceiving.
The lower pressure (vacuum) under the throttle at the transfer and idle port is what moves the fuel.
upload_2020-1-24_12-28-14.png


The mains start when the air velocity through the boosters creates a low pressure zone.
upload_2020-1-24_13-0-2.png

The idle and transfer lot may continue to be subject to low pressure and contribute some fuel, or as throttle opens further, they may not.

My experience has been the mains fully take over under steady state cruising somewhere around 65 mph, 3000 rpm.
 
OMM asked about converter brake stall.
That shouldn't effect idle or off idle much, because there's not enough to get it even close to stall speed.
BUT. I totally agree that a med stall speed converter is needed forthis engine. 2800- 3200 is a med stall range.
I was going to say that you should be able to run very quick advance since vacuum advance is not being used.
But with a stock stall speed I have doubts as to whether that would be true.
Anyway one step at a time. Get the idle and low speed, low throttle stuff figured out first. That's the foundation. Otherwise the engine is trying to clear up when you try to open up the throttle quickly.
 
Charlesvolare said:
I'm using fresh, dual O2 sensors, calibrated to gasoline, data logged with rpm.

I work in r&d at comp, went and talked personally with our one of our senior cam designers, who has since retired. It's a roller cam so ramps are different than tappet. Not trying to argue cams but it's matched better than you probably think. What all do you want info wise? I'll try to grab it for ya

Valve notches are right at 5cc's. Not trying to argue engine compression either.

I'll try to pull some plugs today.
Click to expand...


I'd like to see some plugs. That would be a start.

I'm sticking with my observation that your cam is wrong. I'm betting it's on at least a 110 LSA and maybe even a 112. Doing that changes the opening and closing events on a lobe package that may be correct. You don't have the induction system to run cam timing with that LSA.

Again, I'm guessing because you haven't posted all the cam numbers yet.

I'm 100% all for your 11:1 compression ratio. I hope that's what you actually have, because if you do, and you are married to that cam, I'd put the cam back to straight up, or I may even retard the cam 2 degrees past the LSA to make the intake closing a bit later. With the CR that high, you won't lose a thing down low. In fact, you may gain some if you can get to 33-34 total.

Also, 180 is as hot as I'd want it to run. Any hotter and it makes it hard to control detonation and you pull timing to help with that and you kill power.

If you have exhaust heat in the intake manifold BLOCK IT OFF. There is no need to heat the fuel like that.

Mattax has posted some incredible links in another thread (maybe he can put them in this thread because some may not see it in a different thread) about fuel vaporization and combust tendencies. It's very very good reading.

I hope your CR is that high. I'm also a bit leery of using a claimed number for valve notch volume. I've never measure them where they weren't at least 2 CC's bigger than advertised.
 
yellow rose said:
I hope your CR is that high. I'm also a bit leery of using a claimed number for valve notch volume. I've never measure them where they weren't at least 2 CC's bigger than advertised.
Click to expand...
That's interesting info, YR.... Curious: Is that from measuring the actual eyebrows or some total volume and assuming the difference is in the eyebrows? I measured some eyebrows but not on a KB or Icon. And I always add in 1 cc anyway for the volume down by the top of the piston. If Charles piston eyebrows are 2 cc larger (than the 5 cc published number), then he should be right around 10.5 for SCR... so still pretty good.

(FWIW... I assumed 110LSA and 106 ICL in my DCR number.... just because they are pretty common factory grind numbers.)
 
yellow rose said:
I'd like to see some plugs. That would be a start.

I'm sticking with my observation that your cam is wrong. I'm betting it's on at least a 110 LSA and maybe even a 112. Doing that changes the opening and closing events on a lobe package that may be correct. You don't have the induction system to run cam timing with that LSA.

Again, I'm guessing because you haven't posted all the cam numbers yet.

I'm 100% all for your 11:1 compression ratio. I hope that's what you actually have, because if you do, and you are married to that cam, I'd put the cam back to straight up, or I may even retard the cam 2 degrees past the LSA to make the intake closing a bit later. With the CR that high, you won't lose a thing down low. In fact, you may gain some if you can get to 33-34 total.

Also, 180 is as hot as I'd want it to run. Any hotter and it makes it hard to control detonation and you pull timing to help with that and you kill power.

If you have exhaust heat in the intake manifold BLOCK IT OFF. There is no need to heat the fuel like that.

Mattax has posted some incredible links in another thread (maybe he can put them in this thread because some may not see it in a different thread) about fuel vaporization and combust tendencies. It's very very good reading.

I hope your CR is that high. I'm also a bit leery of using a claimed number for valve notch volume. I've never measure them where they weren't at least 2 CC's bigger than advertised.
Click to expand...
It was a busy day at work and didn't have time to pull the cam lobe info. But it is on a 110 lsa with 4* advance ground in it. I'll look for the cam card and see if there's anything else I missed.

I do have a 180* thermostat, engine sees close to 200* in traffic on hot summer days sometimes. Magnum heads, dual-plane air-gap, no exhaust heat anything.

I'll get to pulling plugs tomorrow when I get to work on it, hopefully it won't be raining.

Lol you're pretty set on not liking my cam, not knowing anything about the car, usage, gearing. There might be a better cam for this engine in a 2000 lb racecar set up strictly for the strip, or a better cam for a truck used to tow that racecar. Or a highway geared land yacht. To each their own though. If you know what cam I need without knowing anything else other than my compression, which you don't believe supposedly, I'll be impressed. It may not be perfect, but my cam was specced by someone who's done it thousands of times for most of his professional career, knowing most of all there is to know and actually seeing , touching the car the engine is in, backed by years of r&d research. Again not saying it's perfect, but it's probably pretty good given the situation of it all I think.

As far as my initial issue, how much of a factor would engine compression, total timing, cam choice be at idle? It's not a very radical cam.
 
nm9stheham said:
That's interesting info, YR.... Curious: Is that from measuring the actual eyebrows or some total volume and assuming the difference is in the eyebrows? I measured some eyebrows but not on a KB or Icon. And I always add in 1 cc anyway for the volume down by the top of the piston. If Charles piston eyebrows are 2 cc larger (than the 5 cc published number), then he should be right around 10.5 for SCR... so still pretty good.

(FWIW... I assumed 110LSA and 106 ICL in my DCR number.... just because they are pretty common factory grind numbers.)
Click to expand...


It's easier to figure out something is wrong when using a flat top piston.

When you do the down fill you can easily calculate the valve notch volume doing that, and almost all the time they are bigger than advertised.

It a bit more difficult with a domed piston. But you can do it. It's also quite funny that dome volumes are usually not what the piston manufacturers claim. They are usually LESS that advertised.

That's why I don't like taking the word of the guy making the Pistons. They are wrong often.
 
Charlesvolare said:
It was a busy day at work and didn't have time to pull the cam lobe info. But it is on a 110 lsa with 4* advance ground in it. I'll look for the cam card and see if there's anything else I missed.

I do have a 180* thermostat, engine sees close to 200* in traffic on hot summer days sometimes. Magnum heads, dual-plane air-gap, no exhaust heat anything.

I'll get to pulling plugs tomorrow when I get to work on it, hopefully it won't be raining.

Lol you're pretty set on not liking my cam, not knowing anything about the car, usage, gearing. There might be a better cam for this engine in a 2000 lb racecar set up strictly for the strip, or a better cam for a truck used to tow that racecar. Or a highway geared land yacht. To each their own though. If you know what cam I need without knowing anything else other than my compression, which you don't believe supposedly, I'll be impressed. It may not be perfect, but my cam was specced by someone who's done it thousands of times for most of his professional career, knowing most of all there is to know and actually seeing , touching the car the engine is in, backed by years of r&d research. Again not saying it's perfect, but it's probably pretty good given the situation of it all I think.

As far as my initial issue, how much of a factor would engine compression, total timing, cam choice be at idle? It's not a very radical cam.
Click to expand...


You need to control the heat better. If it's just creeping up at stop lights or in slow traffic that's not too big a deal. But if on hot days the engine is running at 200 I'd be finding a way to get it back down to 180.

Yep. Not liking the cam. I could tell by the few numbers you posted that it was on the Comp de facto 110 LSA. Comp loves that, but for monetary reasons not because it's the best.

When any can grinder calls out something like that LSA for virtually everything, I question that. On top of that, everybody and their mother loves to roll the cam ahead 4 degrees just for ***** and giggles I'd guess.

Or, they are crutching something else. I've spent enough time testing ICL, and almost always, if the cam is correct, the ICL should be at the very most, 2 degrees advanced, and that's because timing chains retard the cam timing a couple of degrees.

So now, you already have a very early closing intake valve, and you move the cam ahead 4 more degrees and make it worse. The theory is it will make more "bottom end" but in reality, you make detonation much more likely. Then you retard the timing, which kills power, right off the bottom where everyone thinks they want more power and it kills the middle power too.

So yes, I'm saying I don't like your cam. I love the CR, not a real big fan of the intake manifold, but it's better than a manifold with heat in it.

Like I said, put the cam in on a 110 and help it out a bit.

Or better yet, get a cam with the correct timing events.
 
Make sure the power valve is not hitting the screw boss inside the power valve cavity when the metering block is installed.
 
yellow rose said:
It's easier to figure out something is wrong when using a flat top piston.

When you do the down fill you can easily calculate the valve notch volume doing that, and almost all the time they are bigger than advertised.

It a bit more difficult with a domed piston. But you can do it. It's also quite funny that dome volumes are usually not what the piston manufacturers claim. They are usually LESS that advertised.

That's why I don't like taking the word of the guy making the Pistons. They are wrong often.
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OK, well I'd expect that if the eyebrow/notch volume is being deduced from the down-fill method, then some of that volume is actually from the crevice volume down by the top of the piston. That usually works out to 1-1.5 cc in the size range of these bores. I add that to the published eyebrow volume (or reduce the net dome-eyebrow volume for a domed piston) to account for that. Pretty much the same end result either way.
 
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