Inlet Reversion At Low Rpm 3K Bigport

[Andy43]

I'm getting inlet reversion.

Interesting thing to see the air / fuel mix being pushed upwards out of the carby.

 

The set up is 3k with a bigport, 3k BP inlet manafold, 60 thou of the head, stock exhaust manafold, cam grind is wade 446b.

 

Compression is 150 psi per pot all are within 5 psi.

I have opened up the vavle lash to 20 thou hot and advanced ignition timing to 12 seems a bit better.

 

Cam timing is running as per Wades recommedation.

 

Should I be looking at dropping a small port head on ?

Or changing cam with a increase of phase angle ?

 

Has anyone else had a simlar problem ?

 

Thanks

Andy

[Evan G]

Compression seams very low imo

[ke70dave]

150psi +-5% seems good to me.

 

how aggressive is that cam? that will decrease the compression when you measure it.

 

as for the inlet reversion, what is the rest of your exhaust like? standard? that might be your trouble, air just can't get out. (thats my understanding what you mean by inlet reversion)

 

i googled it and on the rollaclub FAQ it sais something like "Make sure you have extractors before fitting twincarbs or you will get inlet reversion"

 

i assume, more/bigger carbys = more air, stock exaust manifold = air can't get out, thus air goes back up carby when valve opens?

[ijg033]

Ignition or Valve Timing is out to cause this sort of a Problem as Obviously the Inlet Valve isn't closed when the Piston is coming up on Compression Stroke or Could have BURNT valves or seat wear , but with even comp I'd probably Degree the cam in correctly using a Degree wheel to Make sure there are any over laps causing the prob ,

 

Cheers

 

Dave

[Felix]

Put a small port inlet manifold on it. That will help with reversion.

[ijg033]

How can a Smaller Inlet Manifold stop this , If its getting pressure out of the Cylinders into the carby How will this make any difference ,

 

I'm a Mechanic & have been for knocking on 20 years & what you have written Doesn't make sense to Change the Inlet , Inlet manifold change WON'T change anything apart from Performance as the Smaller Runners with take Power away which is totally useless if your building a engine & fitting/fitted a Performance cam , Might aswell just bolt a Stock engine back in there so theres no probs .

 

Most People actually Increase Manifold Port size not decrease it when building performance engines as it makes them breath better , Geez I think I'll have to advise everyone to Put Small manifolds on & take the extractors off & fit a Big Cam :bash:

 

As Said Timing either Cam or Ignition is where I'd be looking 1st , Not Manifold size as this is just a BIG Hole flowing to the head

Edited January 21, 2011 by ijg033

[altezzaclub]

I don't think a leak in a valve causes this, its a function of the cam and inlet manifold shape. You get standing waves set up when the valves close at certain throttle openings/revs and the last waves exist outside the carb throat. So although air is screaming into the carb there is a fuel mist sitting in one place outside the carb in that airstream. We used to get it with the twin 45DCOEs, but didn't worry about it.

 

What rev range is it at Andy, and is it at full throttle in that range?

 

Are you allowed to change the stock exhaust manifold? That would be the first thing to try. Then try changing the cam timing if you have a degree wheel type cam cog. Otherwise a different cam would be one answer, a change in inlet size or length another answer.

 

150psi seems fine with that cam, as that is measured at cranking speeds and the cylinders will fill better at high revs.

[Felix]

ijg033, Haven't you ever heard of an anti-reversion step???? I'd suggest you read some books by David Vizard and expand your mind. :)

 

By the way the small port manifolds have the exact same sized runners, they are just flared near the head/manifold face on the bigport versions.

[ke70dave]

this is interesting:

 

http://www.circletrack.com/enginetech/ctrp_1008_intake_reversion/index.html

 

and after reading that, I'm still going for needing to free up the exhaust. (pending rules in the class he drives in). also they mention "increasing low lift exhaust flow" as well. to get the exhaust going out your exhaust port, not your intake port.

Edited January 21, 2011 by ke70dave

[Andy43]

I don't think a leak in a valve causes this, its a function of the cam and inlet manifold shape. You get standing waves set up when the valves close at certain throttle openings/revs and the last waves exist outside the carb throat. So although air is screaming into the carb there is a fuel mist sitting in one place outside the carb in that airstream. We used to get it with the twin 45DCOEs, but didn't worry about it.

 

What rev range is it at Andy, and is it at full throttle in that range?

 

Are you allowed to change the stock exhaust manifold? That would be the first thing to try. Then try changing the cam timing if you have a degree wheel type cam cog. Otherwise a different cam would be one answer, a change in inlet size or length another answer.

 

150psi seems fine with that cam, as that is measured at cranking speeds and the cylinders will fill better at high revs.

 

altezza

Ok

What I have done, Is built two motors basiclly the same.

 

One motor for the speedway car that my 12 year old drives, and One for My daily driver as a test platform and also a know set of running spares.

 

The speedway motor is yet to be started so where talking about my daily driver.

 

The daily driver cam is dialed in at inlet lobe 8 btdc at 50 thou, noting a servicable timing chain was used, The speedway on is at 10 btdc at 50 thou with a new chain, I figure chain streach will retard the cam a bit.

 

Speedway rules state stock inlet and exhaust manfolds, The speedway car does run a open exhaust, The Daily currently has stock KE30 all the way, I have a set of extractors 2 into1 / 2 into1 that I will be fitting the next few days, I'm hoping this will solve my problem and hopefully have both cars with simlar performace figures.

 

The mid range to top end power is feels good, The engine seems to run out of puff around 7500.

The reversion problem seems to be just off idle, bliping the throttle at idle you can see the effect, driving it only seems to happen at a full throttle launch of idle, This wont effect the speedway car as idle performace is not a issues with rolling starts.

 

So I think I'm getting a better understanding.

Reversion can happen at both just as the inlet valve open or just as it closes ?

Or could be due to valve overlap ?

 

So my current plan of attack is.

Change the exhaust as I think the theory about the exhaust backing up seems right.

( noting old cam had 2 deg overlap the new has 10 deg overlap at 50thou)

Next then reset cam timing advance or retard ?

Finally change cam,( noting phase angle change is a must ?)

Also the speedway engine gets sealed up Monday so drop engine in speedway car and see if it has same issues.

 

 

Cam spec's are

 

............... Cam lift / Duration adv / Duration 50thou / phase / timing adv open / timing adv close / timing 50thou open / timing 50thou close

Intake........295____290_________228__________109_____34 btdc_______76 abdc________8 btdc_________40 abdc

Exhaust....295____290_________ 228_________... ______ 72 bbdc______38 atdc_________46 bbdc________2 atdc

 

Over lap adv 72

Over lap 50th 10

 

Thanks To all for feedback so far.

Andy

Edited January 21, 2011 by Andy43