Big vavle head vs High lift cam

[jeepfreak86]

First I'm sorry for posting this if it's been covered. But I haven't been able to find it yet. What are your thoughts on a big valve head vs a high lift cam for a mild stroker DD build. I don't see the point in doing both. And the horror stories about cam failures have me a bit leary about the cam route. Let me know what you think.

[Cheromaniac]

Mine fits in the category of a mild DD stroker and it has a ported head with stock-sized valves and a stock cam.

[CobraMarty]

I am leaning towards BV/Ported head and stock cam and SC. 505 has a head for $1050.

[CobraMarty]

Who sells a big valve and ported head?

[jeepfreak86]

Who sells a big valve and ported head?

505 performance has them. There are also a couple places that sell on ebay. Alabama cylinder head will also do it on one of their new heads as well.

[gonridnu]

Mine would fit the big valve ported head with smallish cam category. Fast opening cams with high lift and short durations are good in my book too. I was just cheap and didn't wanna spend a bunch of money on valvetrain. It think either approach will work out fine but a well prepared cylinder head with a mildish cam will probably out perform a lesser one with a really good cam.

[Retlaw01XJ]

In theory, for low rpm use you want smallish ports and valves to keep the velocity high. A fast moving air/fuel mixture or exhaust will fill/evacuate a cylinder more effectively. But eventually, at higher rpm's those small ports/valves will start to choke the engine and limit performance. Big valves and ports will flow MORE air at higher rpm's, but produce a slower moving mixture at low rpm's.

A mild port job will help at all rpm ranges. But significantly enlarging the ports should be done for high rpm use only.
As Gonridnu mentioned, Comp has some nice cams with more aggressive lobes, but they require additional cost to upgrade the rest of the valvetrain.

[jeepfreak86]

Thanks for all of the input so far. If anyone else has an opinion please post it and let me know what you think.

[CobraMarty]

In theory, for low rpm use you want smallish ports and valves to keep the velocity high. A fast moving air/fuel mixture or exhaust will fill/evacuate a cylinder more effectively. But eventually, at higher rpm's those small ports/valves will start to choke the engine and limit performance. Big valves and ports will flow MORE air at higher rpm's, but produce a slower moving mixture at low rpm's.

A mild port job will help at all rpm ranges. But significantly enlarging the ports should be done for high rpm use only.
As Gonridnu mentioned, Comp has some nice cams with more aggressive lobes, but they require additional cost to upgrade the rest of the valvetrain.

I agree with all this but I think that the Jeep head is still even on the small side and could withstand the porting. It would be really nice to see some flow numbers at different lifts so an analytical comparison could be made and not just a subjective comparison. If porting increases flow across all lifts, then that is good. If it only increases flow at .500"-.600" that is NG for us.

[Jeepinscott]

I have been racing Jeeps for quite a few years and have built quite a few motors and even more heads. I've put 2.02/1.6 chevy valves in a head and lost performance compared to the chevy 1.94/1.5 valves. Bottom end suffered the most but dyno results showed loss throughout the curve. The problem with bigger valves in a 4.0 head is shrouding. You can put bigger valves in them, but then 1/4 of the valve is shrouded even more than stock. The 1.94/1.5 valves fit with a valve job of the stock seats and a simple bore of the valve guide.

[amcinstaller]

heres a small amount of flow table data for a stocker 4.0 head. http://theamcforum.com/forum/40-ho-port ... c2477.html

also, youll notice a post from our very own 1bolt, who im not sure but could be the owner of said flow table. the same data is here i think but this was the first post i could find.

[CobraMarty]

Can you post that flow table, I can't access the forum. Thanks.

[amcinstaller]

nosigma wrote:
I was digging through my flow bench notes and figured this might be of help to anyone porting a 4.0 HO head. I hope it comes across clearly If you want the original power point slide pm me and I will be happy to send it.

I will put together a similar slide for the intake side of the 4.0 HO.

I have similar maps for: 1983, 1965 (big port) closed chamber, late 60's big port open chamber, Mondello ported 401 291C head and a stock 401 291C head and my best effort to beat the Mondello head for lifts under .700 inches.

The yellow boxes are flow reversals (air moving towards the valve) the larger the red number the bigger the flow reversal.


Here are the intake maps. The green areas are the areas of highest velocity. I only did .250 and .500 lift. It wasnt as interesting as the exhausts and its pretty obvious what to do.

John

sorry, photofuckit makes the picstures too small to see, ill see if attaching makes it any better

[amcinstaller]

nosigma wrote:

*snipping out the part where he compares older 258 heads to the HO head

Ported 4.0 HO Update

The dashed purple lines in the two charts to the right show the intake and exhaust flows of the ported 4.0 HO head. The flow of the intake at .500 lift is 233 cfm at 25 in H20. This is a 15 cfm improvement over the stock head at 218 cfm. Notice that the flow improved at all lift values. Almost all of this was gained by cleaning up the bowl and smoothing of the short side radius. Eventually this was raised to 246 cfm with added flow at all lift values by widening the port with most of it occurring at the roof and then tapering down to the floor. Thinning of the guide bosses and careful shaping of the ramp leading up to the guide boss increased flow at all lift values.


(Edit 1/9/08) Additional flow gains were made by widening the floor to a trapezoidal shape. The widening continued all the way into the throat. A peak of 264 CFM at .500 was achieved on the stock intake valve. I ruined one junk head (hit a bolt hole) and then hit water on the second head doing this. The port was biased towards the cylinder center side and the floor was lowered just a bit on the cylinder center side forming a very very slight trough along the wall. Biasing the valve guide boss to direct flow towards the exhaust valve side combined with the trough and port bias brought swirl way up, far more than any other head tested.


The exhaust flow of the 4.0 HO head was improved by cleaning up the terrible shelf on the short side and then widening at the roof. 148 cfm was achieved. Narrowing the guide bosses and additional widening of the port raised this 157 cfm. It would probably jump another 10 cfm if the floor could be raised as was done with clay in the earlier charts.


Larger valves were then installed on both the intake and exhaust with the exhaust valve move closer to the intake to clear the 258 blocks cylinder wall at .513 lift. 5/16 stems were retained. Unfortunately

I ran out of time and had to get the engine assembled an in the car (I was moving to California) so I never measured the flow with the larger valves. One can only guess where it ended up.



John Young