intake reversion puzzle ?

[guest]

hello all , i know there maybe no easy answer to this but this is my delema . i run a dirt late model car . 358 ci , cast iron (ported pro action 220 cc ) heads . 47 cc chambers . we have to run a two barrel carb . i have a tremendous amount of exhaust deposits in the intake runners ! the deposits are from seat to intake flange . the cam is a roller , specs @ .405 lobe lift int&ex duration @.050 =258 int&ex ,,,intake centerline is @ 104 , cam card wants 102 but i didnt have enough p.v.c . NOW TO THE PUZZLE last year i saw some reversion only in the intake bowl area , but the cam was centerlined at (106) so naturally i thaught the intake valve was closing to late so i advanced the cam to 104 BUT the reversion is much worse ! 106=bowl reversion / 104= complete intake port reversion ? could this be a by product of extreme intake vacuum because of the restricted carb ? i know you gurus might need more info to answer so fire away !!! .....thanks ,gary

[bill jones]

-what rpm range do you run, what 's the low and the high rpm during the race?
-how much vacuum are you seeing at WFO during a race condition?
-how big are the headers? how long are they? what do you have for mufflers? how long is the secondary exhaust system?
-What manifold is on the engine? what carb spacer? what sort of air filter assembly?
-The cam is what lobe spread?
-what is the seat and open valve spring pressures? after a season?
-What do you have for pushrods? diameter and wall thickness or weight?
-What do you have for rocker arms? stud mounted or shafts? rocker ratios?
-What has been done to establish some sort of decent rocker arm geometry?
-how high does the intake valve valve sit in relation to the exhaust valve?
-How competitive is the engine? has it ever been better and then gotten worse with time?
-Does the engine run the same thruout the race or does it lose some of it's edge late in the race?
-What type of track do you race on? dry slick? moist and sticky?
-How big and sticky are the rear tires?
-how bad are the valve seats and valves and guides for wear, runouts etc.
-Do all 8 intake ports exhibit the same length of reversion or do they vary?
------------------------------------------------
-That manifold vacuum crosstalks across the valves and right to the primary header tubes so you have to think about that.
-I would think the intake valve springs could be too low on pressure and the pushrods need to be stiffer and larger to insure that the intake valves don't bounce during the first stages of intake valve closure which is the first stage of the compression stroke.
-If the intake valves are bouncing, the pressure from the compression blows right back up and out the intake.

[guest]

thanks bill , ill try to post all info bye mid week , but here is what i have so far.... intake is victor jr #2996 , lobe sep on cam is 106 (102 int c/l x110 ex c/l ) rocker ratio =1.6 on both .this engine ran great ! we won alot of races i just want to optimize it as much as possible. we have to run a very hard d55 hossier tire on a slick dirt track . rpm is 3800 lowest to 7800 highest . i sunk the exhaust seats (valve jod i mean ) .035 trying to go for the z flow effect , but because of intake p.v.c i also had to sink them .030 , (didnt have time to flycut intakes , long story ) anyway, headers are 1x3/4 by 34 , have tried to get the driver to look ot the vacuum guage while racing and he almost killed himself . carb is a 650 cfm 2 barrel . .... my thaughts are i might need to go to at least 108 lobe sep , one last thing , intake centerline on cam card is 102 as mentioned above but cam was raced @ 104.5 thanks ,gary

[bill jones]

-How about installing the vacuum gauge with an electric solenoid that only allows the guage to read vacuum at WOT and maybe then use an MSD rpm switch so that you can turn the solenoid on at a particular rpm.
--------------------------------------------
-Thinking about this reversion a little more I'd wonder what the valve lash is SUPPOSED to be set at, and what you actually run at, checking it when it's too HOT to check without getting burned?
-I would think the looser the lash the greater the chance of valve bounce.
-AND I strongly feel that valve bounce is the problem here.
----------------------------------------------
-Do you run a rev limiter? have an rpm recall? what numbers do you see on the recall?
--------------------------------------------
-One thing I always saw on slick dirt tracks is the max rpm always came in about 1500 rpm higher than what the car was geared for because of the tires spinning real bad especially when the track gets bumpy or gets wet slick spots.
-----------------------------------------
-The 650 cfm carb may not be so bad for WOT vacuum if the air entry and the carb spacer has been done right.

[dbusch]

Bill, i wanted to bring this thread back because i have reversion problems too on my customer's engine. looking at the list of possible problems, i can see how valve bounce could cause some issues. The valve seats all looked great , though. intake keepers were tough to get loose, a prime telltale sign of valve float/bounce. this engine used 2.02" hollow stem SS intakes and std exhausts. 200 lbs seat and 450 open, 5/16 .080 pushrods, shaft mt. Harland Sharp, titanium retainers and keepers. this is a .500" roller cam too.

I also thought it could be related to the valve job vs. the exhaust system vs. overlap in the cam. it used to have a 50* intake seat and now it has a 45* seat. much better low lift flow, but maybe that was a mistake. the blackness of the intake ports is much worse now. we must run a 2.5" exhaust on this car to within 12" of the axle.

I also have noticed a rich condition at high rpm (7000-8000 rpm) on the chassis dyno that wasnt there when it had a cam with less overlap (about 18* less at .050").

should i go back to the 50* intake seat to kill some low lift flow? BTW, we are switching to some 3/8 .120 wall pushrods...

[bill jones]

-what are you using for fuel? specifically the evaporation range numbers?
-What is the vacuum you are seeing at WFO max rpm?
------------------------------------
-If you have ever worked on a 4 cylinder with individual stacks you might know of some steps that taken to reduce reversion.
-----------------------------------------
-This is the way I approach the problem
-Everything needs to have anti-reversion steps.
-This means the intakes ports should be smaller by something like 1/8" (1/16" mismatch around the full port exit) smaller at the manifold exit or the heads can be flared out larger.
-The intake valve margin needs to be about .030" thicker than normal, have an absolutely straight and flat margin and the break between the margin to the chamber needs to be sharp enough to cut your thumb.
-Absolutely do not want to ever touch that margin with scotchbrite or sandpaper or a wire brush when cleaning the used valves.
-The margin can have and actually probably would like to have real precise machining lines left so that you can feel those edges, sort of like a real nice fine thread.
------------------------------------
-I would use a very flat top cut for the intake in the chamber, never steeper than 15 degrees and no narrow 30 degree break to the 45.
-Or better yet if you have multi angle seat cutter machine like a Serdi or a Sunnen and you can stand to sink the intake valve anything, I'd set the top angle cutter at flat and about .080" diameter larger than the valve margin diameter and drop the seat leaving that sharp ugly looking recess there.
-What this does is it seriously kills low lift flow.
-Ford did that on production 390's that had the 30 degree intake seats, had that recess almost the same depth as the total margin thickness or about .070" deep.
-------------------------
-The exhaust needs the port throat smaller than you can get, meaning all aftermarket exhaust ports (that I've seen) are already too large especially if the heads have been prepared with a valve job or CNC'd.
-I found that the high vacuum engines like the choke pretty small and the exhaust velocity thru that throat needs to be way too high for what we are used to.
-The headers should be stepped with the inside tubes having sharp edges but not being reduced like when you use a tubing cutter, and all the exhaust joints need steps, and maybe adding a few extra joints wouldn't be a bad idea.

[ozrace]

hello all , i know there maybe no easy answer to this but this is my delema . i run a dirt late model car . 358 ci , cast iron (ported pro action 220 cc ) heads . 47 cc chambers . we have to run a two barrel carb . i have a tremendous amount of exhaust deposits in the intake runners ! the deposits are from seat to intake flange . the cam is a roller , specs @ .405 lobe lift int&ex duration @.050 =258 int&ex ,,,intake centerline is @ 104 , cam card wants 102 but i didnt have enough p.v.c . NOW TO THE PUZZLE last year i saw some reversion only in the intake bowl area , but the cam was centerlined at (106) so naturally i thaught the intake valve was closing to late so i advanced the cam to 104 BUT the reversion is much worse ! 106=bowl reversion / 104= complete intake port reversion ? could this be a by product of extreme intake vacuum because of the restricted carb ? i know you gurus might need more info to answer so fire away !!! .....thanks ,gary

The exhaust reversion you are seeing occurs on overlap, not at valve closing, so advancing the cam will make it worse (as you saw).
Remember, you are opening the Intake onto a piston moving up and pumping exhaust gases. Too much cam advance will always show up in this way from my experience, especially if the engine sees a fair amount of low RPM load.
You will notice better response and power when you are off the cam if you fix it, but you may not be concerned about the engine in that region.
Open the intake later, and use anti-reversion as per Bill's suggestions should help.

[dbusch]

still havent checked WFO manifold vacuum. fuel is VP 100 octane unleaded. this stuff makes the chambers and exhaust ports look extremely rich. doesnt burn and look light grey like leaded fuel does. the intake ports have almost an oily look to them. somebody suggested it might be oil going into the intake ports from the cylinder. we cant run a vac pump and the rings are .043/3mm type. maybe the rings are shot and there is blowby forcing oil into the chamber and the ports. engine has about 100 runs since new. if it is blowby, then maybe that is making the engine A/F ratio go rich at rpms above 7000....

[Joe Mendelis]

Duane, what type of heads and manifold are you using? What duration is the cam @ .050 and seat? Lobe seperation?

[bill jones]

-I would expect that you'd have at least one inch of vacuum for each 1000 rpm so that'd be 7" at 7000rpm.
-When you have this sort of vacuum at 7000rpm WOT the carburetor not only disperses the fuel thru the jets and boosters, it also comes thru the transfer slots---the idle discharge ports----and the squirters are usually dripping----AND the air flow can in some circumstances siphon fuel right out of the bowl vent.
-------------------------------------------
-This vacuum at WOT closes off the power valve if the power valve is a lesser number than the manifold vacuum like if you have 5.0 power valve and the engine sucks 5" vacuum at 5000rpm the power circuit is closed off from 5000 to the end of the straightaway.
-So to compensate for that most people use too large a jet so the engine is too rich everywhere except maybe for the last little bit of the straightaway but then the rest of the carburetor circuits are dumping fuel because of the high vacuum so it's still too rich.
-------------------------------------------
-One other thing is how much total timing are you running?
-with all that vacuum we can run a ton of timing here at 4200 elevation like typically we run near 50 degrees.
-You can't make the engine ping when the vacuum is diluting your effective compression ratio down 30% from what it is static.
-So if the vacuum wants 10 or 15 degrees more timing and you aren't giving it, then the engine runs likes the timing's retarded that 10 or 15 degrees and we all know what that does to a really good unrestricted engine.
----------------------------------------------
-Ozraces' statement about overlap is right, particularly when the engine has NO positive help to combat the reversion.
-But when you get the reversion tactics in place and you start to see how those work, the reversion fixes then allow you to advance the cam some without seeing such a drastic and immediate change from having no evidence of reversion to serious evidence with such small changes.
---------------------------------------------------
-When you have freedom of the exhaust system you can virtually eliminate reversion by fabricating a set of headers that are as close to being 8 into 1 as you can fit on the engine and into the car.
----------------------------------------------
-The fuel you are using is basically Ok as far as the evaporation or distallation curve is concerned.
-I believe the oil you are seeing is fuel residuals of the smoke that has been filling up the insides of the intake tract, where those oils have condensed onto the manifold walls.
-One thing people never seem to pay attention to is where they park their car in the pits relation to the sunshine and the hot asphalt.
-If the sun is getting to the rear of the car and heating the asphalt or the ground below the fuel cell the fuel tank becomes super heated and that is bad news for a restricted intake engine.
-That fuel needs to be kept in an ice box until just before the race then dump that cold fuel in the tank.
-Measure the temperature of the fuel in the tank and you'll see what I mean.

[ou812]

I tried the 15 top angle, and knife edged valve margin on a 350 chevy using dart 215 iron eagles...this engine was mild, with a hyd. cam and 10:1 compression. Throttle response is excellent and the car went 114mph in the 1320! Very mild street car. Your not kidding about killing low lift flow either! This was not a restricted intake engine either.
Brian

[SWB]

Every time I flow check heads or develop a new combination, I always check the reverse flow. Doing this (especially playing with valve jobs and combustion chamber shapes) allows you to find out what "shapes" generally work with either exhaust or intake flow.

Example:

I was getting "trick" and figured I was going to set the world on fire with a hot new inlet seat idea. I went to work grinding and hand blending, etc. After two or three renditions of this idea (and it not working worth a darn) I decided to reverse flow test it and see how it did that way. The end result was that my hot new inlet profile actually worked much better as an exhaust. I had a similar revelation with a "hot new" exhaust profile one time and between those two experiences, I came to some conclusions that were not at first intuitive.

With the exhaust side you want the valve to be as high on the seat as possible. The radius (or top cut/cuts) into the seat have to be proportioned correctly and not too big or the valve gets sunk in too far and the flow goes down in the positive direction and up in the negative direction.

The inlet seems to be somewhat the opposite.

If you really look at the Cd numbers, you will see this play out.

Another spectacular way not yet mentioned to kill reversion on the exhaust side is to ditch cut the underside of the exhaust valve (where you're back cut would be). I make no guarantees on exhaust valve life, but it's not as scary as it sounds and you will notice a huge difference immediately. (Do take care in making the ditch round at the base and not too deep or you might cause the valve to break in two. If it's done right though, my experience has show that the valves last as long as you dare run them.)

Inlet throats should be relatively larger for less reversion and as Don mentions in his book, the back cut on the inlet valve should be small and about 28 - 32 degrees. On the exhaust I use a large back cut and usually use 35 or 38 degrees depending, though it's definitely combination specific.

I have used the 15 or 0 deg. top cut and it works pretty good, but it depends on the chamber shape as far as I'm concerned. If you're chamber is pretty flat that's a good idea, but if it's fairly straight walled or convex then it's time to experiment with better ideas.

SWB

[dbusch]

SWB, i have seen stock exhaust valves that have the "ditch" that you speak of. what would be the best way to get this done? i figured you could put the valve in a lathe and trim the backangle area until you had a decent edge next to the seat on the valve...

[SWB]

Use the tailstock to support the stem and cuck the head in the lathe with a stop of some kind to hold it. Then you can cut the ditch into the back of the head. You can cut it fairly deeply before it becomes a liability. 1/3 the depth from the seat ID to the face is probably enough, though I've seen it pretty darn deep before.

I should clearify also that I have not used a back cut with the ditch method and vice-versa. The only real danger I see with the ditch is possible overheating providing the depth is OK.

I'll see if I can dig up a picture.

SWB

[SWB]

This should drop some jaws!

The following picture was shared with me a number of years ago by a notable cylinder head specialist. I consider this valve to have been cut too deeply and with too sharp an edge inside the cut.



Scary looking, but I have found it's actually much stronger than you think.

SWB