More exh duration

[dbusch]

i do almost exclusively N/A stuff, and i never "try" to get more than 70% ratio ex. to intake.

I would like to see a test where someone had a 70% ratio with around 14* split and then went to an 80% with only 4* and see which combo did better. I personally think the smaller exhaust port with more ex. duration will be better. You also have to think about how the larger port is going to need a larger header tube or it will be hindered. Now the rpm tuning range of the header is out of wack with the rest of the engine.

[MadBill]

As I understand it, it all starts with compression ratio. If it's sky high, most of the power is made in the upper part of the stroke, since the expansion ratio is equally high.
This means that the early EVO required to compensate when sacrificing exhaust valve diameter (and thus flow) to free up real estate for bigger intakes is a good trade-off. Hence High CR = small ex. valve/flow = big duration split.

[Windsor377]

I would like to see a test where someone had a 70% ratio with around 14* split and then went to an 80% with only 4* and see which combo did better. I personally think the smaller exhaust port with more ex. duration will be better. You also have to think about how the larger port is going to need a larger header tube or it will be hindered. Now the rpm tuning range of the header is out of wack with the rest of the engine.

I would like to see that too. Should be pretty interesting. Haven't had lot of opportunity to work with 4v DOHC designs, but just thinking off the top of my head, I bet they can move the exhaust valves quick enough in a short duration window to make it work...just thinking out loud...?

[Windsor377]

As I understand it, it all starts with compression ratio. If it's sky high, most of the power is made in the upper part of the stroke, since the expansion ratio is equally high.

I guess that's part of it. I think UDHarold's comment about the exhaust being the beginning of the 4 stroke cycle is true too. I think they both set the stage...?...and everything needs to work together.

This means that the early EVO required to compensate when sacrificing exhaust valve diameter (and thus flow) to free up real estate for bigger intakes is a good trade-off. Hence High CR = small ex. valve/flow = big duration split.

It's funny, the tq numbers coming out of these small blocks are the biggest numbers I've ever seen for na, gas burning engines of their size. On top of that, the exhaust is opening prior to 90* atdc when considerd at .050 duration reading. This is with exhaust side of low to mid 60s%. Those BBs I'm guessing begin opening 5 ~ 7* sooner.

There was a time I would have thought someone would have to be on drugs to say something like that.

[CamKing]

If your exhaust is flowing 80% of your intake would you then use 8* less duration on the exhaust? What about lift. I see where they use less lift on the exhaust side as well.

same duration, and less lift.

[CamKing]

A prostock engine has about a 50 percent ratio,

And an almost 30 degree longer exhaust to intake duration.
The average 8-12 degree spread wouldn't be optimum on the 50% exhaust to intake ports on a pro-stock, just like they wouldn't be on a 80% Chapman Racing heads 18 degree smallblock.

BTW, I'm looking at the average over the whole flow curve, not just the peak numbers.

[CamKing]

i do almost exclusively N/A stuff, and i never "try" to get more than 70% ratio ex. to intake.

You should run a 4-8 degree split between intake and exhaust.

I like to see an average ratio of 74-76%.

[andyf]

Mike, I'm trying to triangulate between your responses and it seems like you're saying that if the exhaust port average flow is 75% of the intake then the exhaust duration would be about equal to the intake duration? I'm assuming this is for an open exhaust race engine.

I've had good luck with some street engines using single pattern cams. These are usually 550 hp type pump gas motors with full exhaust. What I've found in those applications is that extra exhaust duration just causes more problems than it solves. My guess is the back pressure from the exhaust system causes more problems on the bottom end of the torque curve than the extra duration solves on the top end.

I did some dyno testing a few years back and the best cam was a 40 year old single pattern design from Mopar Performance. It worked better than the whole box full of "new" cam designs that Comp sent me. All of the Comp designs were split pattern cams but none worked better than that old school Mopar cam.

[randy331]

I like to see an average ratio of 74-76%.

Does it make a difference at what lift, or piston position/speed the average EX flow is achieved at?
If I gained 10cfm of flow at .300" lift, which happens to be at BDC, will that change the cam requirement, the same as if I gained 10cfm of flow at .600" lift, at well past BDC, and at a much higher piston speed?

Randy

[bigjoe1]

I have had just the opposite results in the 383 Chevys I build for the street. One I did last year with the new Comp Cams muther thumper grinds. They have about 15 degrees more on the exhaust and tigher lobe centers ( 107) It made more HP than the usuall extreme energy I use in these engines(571 vs 550)These cams have a radical drive in idle that many people want to hear, and everybody loves the way they sound. I feel the crappy street exhaust system would need MORE duration to make up for the fact they are not as good as open headers.

JOE SHERMAN RACING ENGINES

[sanfordandson]

we've found the same results as camking. we've gone to straight pattern cams on 2bbl circle track stuff with a less aggressive exhaust lobe and less lift.
some of the 4bbl stuff is going the same direction but not to that extreme.

[andyf]

I haven't tried one of the new Comp mother thumper grinds yet so I'll have to do that. My experience in the past tells me it won't work but I am wrong a lot of the time, just ask my wife!

The one combo that I thrashed pretty hard on a chassis dyno led me away from dual pattern cams and a lot of exhaust duration. The extra exhaust duration just seemed to always kill the bottom end torque and the engine would end up with less area under the curve. The top end numbers were often a little better, but the car wouldn't go down the track as fast since the torque just wasn't there to get it moving out of the hole or at shift recovery.

Of course, those results could be highly dependent on the exact parts used in that test and not representative. Who knows, maybe a slight change to the muffler or collector diameter or something like that can chang the whole equation.

[PFC1]

we've found the same results as camking. we've gone to straight pattern cams on 2bbl circle track stuff with a less aggressive exhaust lobe and less lift.
some of the 4bbl stuff is going the same direction but not to that extreme.

That's a whole new ballgame. Your 2 barrel motor is telling you "I can't breath... leave that intake valve open longer so I can get a breath". (well it may not actually be talking). Anyway since you have a very much restricted intake it's needing more duration on the intake as opposed to needing a shorter exhaust duration.

Bret

[Warpspeed]

It really comes down to the package you have, and the application.

Are you after power regardless, or is the fuel specific important ?

It might well be, for long distance circuit racing, where distance between fuel stops is an important factor in winning. Over scavenging the engine with excessive exhaust duration is a great fuel waster. This may be significant in a dual purpose road driven car too.

For drag racing, who cares about the fuel specific ? Power wins hands down, so you may be able to afford to over scavenge if it gains you a little bit more top end.

For most applications, excessively shortening the length of the power stroke is not a great idea, unless the exhaust is particularly restrictive. It probably will be if you run an exhaust turbine <grin>.

[CamKing]

we've found the same results as camking. we've gone to straight pattern cams on 2bbl circle track stuff with a less aggressive exhaust lobe and less lift.
some of the 4bbl stuff is going the same direction but not to that extreme.

That's a whole new ballgame. Your 2 barrel motor is telling you "I can't breath... leave that intake valve open longer so I can get a breath". (well it may not actually be talking). Anyway since you have a very much restricted intake it's needing more duration on the intake as opposed to needing a shorter exhaust duration.

Bret

That's not the case.
On a 2bbl engine that turns 7,200rpm, you don't run any more intake duration then a 4bbl engine that turns 7,200. You do run shorter exhaust duration though.