Backcut Intake Valves

[dzertracer]

I did some reading on backcut/undercut valves. It seems that this improves low lift flow but hurts low end torque and h.p. while increasing top end torque and horsepower. The backcut valve acts like a camshaft with more duration and increases overlap.

I planned on having the intake valves backcut to improve low end flow. Now it seems like it will hurt the performance of my daily driver.

I have a 1987 EFI 300 that will have a Clifford 264 cam and slightly higher CR. The cam specs are .474 valve lift and 206 duration at .050.

The motor will never be revved higher than 4,000 rpm.

Should I just run a stock size stainless steel (Manley) valve with no backcut?

 

[Thad]

Letting my logic run free. Can't quite follow the info in the article.

IIRC, Highest torque is at the rpm that has the most efficient cylinder filling.

Backcutting valve increases low end flow then logically it would increase low end cylinder filling causing a higher torque peak beginning at a lower rpm. And because of better flow overall it would make the torque peak be wider.

If it were to move the torque event higher in rpm than desired then advance the cam 4 degrees to cause an earlier torque event.

 

[Thad]

UPDATE

OK, went by Procell's and talked with their resident genius and he explained a little.
Backcutting valves increases flow and at low rates decreases velocity of air flow. That is the rub. When an engine has been built to be as efficient as possible it will have an effect on bottom end. Then he said as poor as the flow is at the 300 valve there would not be that effect; the port, valve pocket and valve are so restrictive it would be almost impossible to effect velocity with just backcutting the valves. Backcutting the valves will only help.

 

[Lazy JW]

My first thought was "nonsense". But I can see it hurting low end power if you have a cam with lots of overlap, this would tend to increase the amount of reversion or back flow at lower speeds, resulting in lower cylinder filling at those speeds.
Joe

 

[dzertracer]

Thanks guys for your thoughts. After speaking with a cylinder head guru I have decided to have my new valves backcut. A good bang for the buck compared to everything else.

 

[xctasy]

Simple, uniform back cutting reduces 'mixture motion too'. The old Chev OHV engines run in modern engine labs produce a virtually static air fuel mix flow in the combustion chamber. If you use a flow wheel, the actual mixture motion is barely 50 paddle revs per minute at full flow in the combustion chamber.

On an LS-1, get this. It's 200 revs per minute. That's why it can run 10.6:1 compresion, brillant mpg and also get 400 hp without detonating on an ohv head. The air is tumbling in at four times the rate as an earlier small block head.

Back cutting is the answer on a modern high swirl engine, but the best bet is getting swirl polished intakes, which induce mixture motion. They show nothing on a flow bench, but if mixture motion is imroved, you can run a higher degree of compression before detonation sets in. If compression is set up at the stock level, and you back cut a baised swirl into the intake valve that pusheds the air fuel mixt towards the sprking plug, mixture motion will go up, and part throttle torque should improve if the engine is tuned to suit.

This mixture motion, along with intake port roughness, is the secrete to why some engines get great economy at part throttle, and others get grat power with lousy low end performance.

 

[6bangerbill]

Swirl is an interesting and often misunderstood subject. My understanding is that swirl is the motion of the inlet air-fuel charge spinning in a circular motion inside the cylinder bore. I have never heard the spin rates you quoted and am interested in their source for my own use. I have read everything available on the ls1 head development before welding my own ls1 xflow hibred head. If you examine the ls1 head you will notice that all the ports are at an angle and apparently flow directly into the face of the cylinder wall and logically should not flow well. But if you look carefully. the ports actally go past the cylinder wall and turn back , thus causing the flow to follow the tangent line into the swirl pattern arround the cylinder wall in a smooth motion.After porting and flow testing, I found better flow rates and better swirl seem to result from an increase in this port bias idea. Another interesting item is that the ls1 open chamber on the sparkplug side is laid back to the cylinder bore edge to allow unempeded swirl motion. What is confusing is the fact that the heads on 400 hp version of the ls1 has a shelf added directly below the spark plug to add compression and some squish area to this side of the chamber. I suppose this is a trade off between swirl and turbulance to develop the additional hp. It is also interesting that these 400 hp engines required additional pollution equipment on the exhaust system to meet epa requirements because of this change. It would be interesting to compare the results of milling an open chamber head to get the same 11 to 1 compression yet retaining the ideal swirl without the additional turbulance and polution.

 

[xctasy]

Source was from the 2001 summary issue of Pontiac Engine Guide Page 61, Pete Incaudo of CNC Cylinder Heads says
190 to 200 paddle wheel rpm at 0.450 " lift,
for 215 cc runner LS1 heads.

In the August 1998 issue of US Hot Rod, Page 78 to 86.
Jim Mc Farland interviewed Ron Sperry ( a member of the tight knit and extreamely valuable GM Powertrain team. These guys are such hot proprty, GM keeps them on a consultancy retainer to ensure their info doesn't leak out to too many Ford guys!). The info was that Ron did an assesment of mixture motion with his brother Ken Sperry, with a huge background in airflow management and an airflow lab technican at GM's Tech Center).


"We came to the realisation you can have too much of both (swirl and tumble)......lost power....unburned hydrocarbons (result)....So we had to find a compromise between good mixture motion for both power and emissions".

There was some other source for the stock casting 350 or 400 head. I can't remember where, but I think it was a Pontiac article, and it was 50 paddle wheel. There was some very valuable information from Wayne Jones in Australia back in 1990 regards poor 350 Chev mixture motion, but it wasn't backed up by paddle wheel rpm.

 

[6bangerbill]

My original flow testing involved the 200 cc ls1 port with the original 2" valves. I had no means to test swirl, tumble or turbulance. These heads were really responsive to our porting efforts. The maximum piston speed for the ford 300 occures at about 75 degrees atdc With the cam intake lobe center set at 104 degrees, lift at max piston speed lift is .574 inches, and max lift occured of course at 104 degrees for .617 inches . This lift point was our target of our efforts. All testing was performed at B J's Head service in Broken Arrow , Oklahoma.

Removing the rocker stud protrusion, increasing port bias by .080 in. and increasing the throat under the valve to 1.825 in increased .600 flow by 23.7 cfm with 28 in of test pressure. Trueing up the short turn radius gave us another 11 cfm. Max total air flow produced 297 cfm at .600 lift on a 2" valve. Flow at .800 in increased to 307 cfm, but was of little interest since our cam did not lift to this height( this is a hydraulic flat tappet street cam). Cutting the valve seat with a radius actually reduced flow at this valve lift. Huh! Go figure.

Im looking farward to testing with a 2.05 dia valve on my new ls6 hibred head, it is originally produced with a 210 cc port and it will eventually be enlarged to about 230 cc, hopefully resulting in close to 320 cfm at .600 lift. Port biase will be inceased even more than the previous ls1 head to see if that helps. the valve throat will increase to about 1.885 dia to see if the valve is restricting flow at this point.

The effect on swirl of these modifications is unknown but air flow is definitely effected.