New camshafts with lower exh lift?

[Steve.k]

The last while I've noticed cam grinders are making cam grinds with less exh lift than intake. It has been for many years exh lift was the same or slightly more on most the small and bbf engines I've been around. Now i see this trend more and more. Is it because the new heads are flowing better on exh than previous ford designs?Most still show the split in duration mind you! Or does this offer less contamination during overlap period and reversion?

[GARY C]

Most test have shown that the exhaust does not benefit from more lift like the intake does but it usually responds to duration, I am sure head design plays a part, many aftermarket heads now have a sightly raised ex runner.

I have not seen the less ex lift cams but many will run a higher ration intake rocker to create the same effect.

[Steve.k]

Thanks Gary was just wondering as i have seen this in past but never understood why. A buddy just specd cam with a 790 int lift and 735 exh. Sbf deal so was curious on reasoning. Still has a 4* duration split.

[bobalattie]

I think with stock heads it was needed because ford exhaust ports were grossly inefficient, but now that everyone is getting aftermarket heads and diy porting advice is all over the internet, it isn’t needed as much..

After reading some posts from well respected members on this site, I pulled the trigger and went with less exhaust lift .600 intake and .525 exhaust on my 408 with N heads, and it doesn’t seem to lack anything against my competition. My build wasn’t max effort, but best useable power from 4-7k circle track application.

Having to run 2 different valve springs was a little inconvenient, but now I have a 2nd set when it’s time to swap them after the season.

[Steve.k]

Did you have a different style grind before and what kind of difference did u see.

[1972ho]

I have 4 SVO V-6 roller cans and all 4 of the are spec with less exhaust lift.

[MadBill]

I believe it frequently was and for some cam companies still is the case that the same lobe family's used for both intake and exhaust and typically lift went up with duration, so any split duration cam got more exhaust lift too.
Better simulation programs such as Dynomation show diminishing gains for ever more intake lift, but a peak value for exhaust and a drop off above it. For example, simulations for my 495" BBC with ZL1 heads ported circa 1970 show power still increasing slightly with lifts of over 0.800", but a peak at ~ 0.680" exhaust lift.

[mag2555]

The thing is though that you need to know that certain current aftermarket BBC heads flow nice out of the box on the Exh side , but really level off and peak out by .500" to .550" lift.
A bowl blend really does not pick you up what it normally does and your looking at a full scale rework of the short turn .

[Steve.k]

I've noticed this a couple times on sbf with a nascar style head. This last engine sees 270cfm at .800 and high 3's at same on intake. The owner is upping cubes by 20 and bolting his same parts on. The cam grinder spec'd this offset but he never knew why. I said the same it's typically the other way around. However I thought maybe it was a reversion issue.

[CamKing]

The last while I've noticed cam grinders are making cam grinds with less exh lift than intake.

We've been doing it, since the early 80's.

The only reason the old "off-the-shelf" cams had more exhaust lift, was solely fore the reason MadBill gave. Cam companies would make a family of lobes with the same velocity/acceleration rates. This meant, the longer the duration, the higher the lift. They would pick the intake lobe they wanted to use for an application, and then put a 4.8, or 12 degree longer lobe from the same family on the exhaust. Most professional engine builders would just put less rocker ratio on the exhaust. top make the larger lift lobe, more efficient. This was common practice for longer than I can remember.

[Steve.k]

Mike does the extra exh lift make for a diluted charge or is it simply not required as the charge is under pressure and also being chased out by piston. I understand stand the simplicity of picking a standard grind. However ive had numerous custom grinds with the same lift as intake and anywhere from 5-10 more degrees duration at .050.Maybe haven't noticed as much as i do now, primary big and sb fords I've worked on.

[Steve.k]

I believe it frequently was and for some cam companies still is the case that the same lobe family's used for both intake and exhaust and typically lift went up with duration, so any split duration cam got more exhaust lift too.
Better simulation programs such as Dynomation show diminishing gains for ever more intake lift, but a peak value for exhaust and a drop off above it. For example, simulations for my 495" BBC with ZL1 heads ported circa 1970 show power still increasing slightly with lifts of over 0.800", but a peak at ~ 0.680" exhaust lift.

So Bill what gains would a guy typically see by shortening exh lift? Or would you see any?

[MadBill]

In Dynomation sims on a number of engines in the 600 HP range, 0.040", too much or too little EVL from the optimum would drop the predicted peak power by 6-10 HP; +/- 0.100" occasionally showed as much as 20.
Tests I ran on a showroom stock 305" TBI Camaro (low 400s lift) picked up 10-12 HP going from 1.5 to 1.6 intake rockers but no net gain for 1.6 exhausts.

[Steve.k]

All very interesting for sure, I've done rocker changes on both before (sbf) and seen gains as you did. However never thought about leaving the exh.

[CamKing]

Mike does the extra exh lift make for a diluted charge or is it simply not required as the charge is under pressure and also being chased out by piston. I understand stand the simplicity of picking a standard grind. However ive had numerous custom grinds with the same lift as intake and anywhere from 5-10 more degrees duration at .050.Maybe haven't noticed as much as i do now, primary big and sb fords I've worked on.

If the duration is correct, but the lift is too high, you have too much area. Too much area will evacuate the cylinder too soon, and lessen the pull on the intake, during overlap.