Cam LSA of 100 with big ports feedback

[lc-gtr-1969]

Hi everyone,

I've been lurking here for a while but have never posted- generally I have read through many engine tech posts as there are some extremely knowledgable members here who make for fantastic technical conversation...

So basically I have a question about lobe separation angle effect when used with heads with relatively big, good flowing ports.

I am currently running a single pattern race cam with duration of 266@50thou, on 100 lobe centres. This cam works quite well in my current setup, with a relatively restrictive head, but big high flow intake and carbs. the cam sweet spot starts at 4000rpm and pulls strong until 6800, at which point power starts to drop off. It has a very flat torque curve between 4500 to 6500. Intake is 51'/ 74' and exhaust is 55'/ 70' as advertised. Lift is .500" gross.

The engine is a 3400cc straight six running 3 45DCOE webers, with very big chokes/ venturis at 40mm- most run 36mm but my setup seems to tolerate the bigger venturis.

My question relates to the fact that I have been offered a very high flowing, good race head with bigger valves- this is an aftermarket alloy head which has proven performance, however I am interested to know whether my current cam will work with the bigger, better flowing head? My current head has 1.7" intake valves and flows 185cfm, the new head has 1.9" intake valves and flows 250cfm at 28".

I know Vizard believes that tight LSA works well with small valve diameter heads, as the high overlap helps pull the intake charge through the (restrictive) valve curtain area... However I have also read conflicting information, which argues that the tight LSA cams work well with big, oversized and normally slow velocity heads, as again the overlap helps kickstart the intake flow at lower RPM, helping negate the poor effects of slow intake velocity at lower RPMs.

So, what are thoughts on this? Any input much appreciated!

[bigjoe1]

How many cubic inches ?What is it used for ?


JOE SHERMAN RACING

[PackardV8]

How many cubic inches ?

3400 cc = 207 cubic inches.

[bigjoe1]

I did not know that Jack! Based on that along-- The very big, better flowing head will probably kill the performance below the torque peak--


JOE SHERMAN RACING

[MadBill]

First, is 100° a misprint? I'm sure even David Vizard, a proponent of tight LCAs, would raise an eyebrow! (although an individual runner set up like yours might require a tighter spec than a typical single 4 bbl. one)

Some 'big heads' give up low lift flow for the big top end numbers, but a 0.20" larger valve would surely have better LLF regardless. (it's usually fairly proportional to diameter) Re LCA recommendations, among other factors he takes into account valve circumference Vs. cylinder displacement, which should be pretty universal for 2 valve engines. For example, with a 1.7" valve and 34.8 c.I. per cylinder, the CID/inch of diameter is 20.3 and by his reckoning for an engine in the 9.5 -10.5 CR range the LCA should be ~ 109°. Bumping the diameter up to 1.9" gives 18.2 and a 2°wider LCA.

Bottom line: compare the LLF of the old and new head and find someone who can, with head flow and a dozen other variables, spec out an appropriate cam. Camking here on S/T and Vizard's own COS selection program come to mind; there are certainly others...

[lc-gtr-1969]

Thanks for the replies...

I can certify that the cam is 100LSA. Also, the engine is 208cubic inches.

I am aware that the current cam may not be ideal, but all the maths aside, the cam works well with the current head- the torque curve is very good once above 4000rpm. Albeit, the idle is demonic and the performance below 3500rpm is already poor due to the overly big carbs. I am basically curious as to what the cam would be like with bigger ports, but keeping the carb size better matched to the cam and engine capacity.

I agree that bottom end performance will suffer with the bigger ports- I figure that I would keep the venturi size down on the carbs to try to 'crutch' the poor air speed.

The car is occasional street driven and occasional drag raced. The midrange is extremely strong, but the challenge is launching the car at the track- below the sweet spot and the car bogs, in the sweet spot and grip is an issue.

[MadBill]

"Works well" is pretty imprecise. If you have the interest and provide the necessary data (all of which it appears you have) I could run it through the Dynomation engine simulation program, which has always proved to be quite close to reality. I wouldn't be surprised if the 'right' cam picked up 20 HP or more and had better manners to boot...

[pdq67]

What engine?

pdq67

[lc-gtr-1969]

Hi MadBill...

By works well, I mean the engine meets my expectations for what it is. You are correct, however, as I know the 'most' appropriate cam for my setup may gain 15-20hp. My peak HP is 255hp, which is about 15hp below some of the more modern setup engines running similar duration cams but on 106 centres. Albeit, my peak HP is about 700rpm lower than these engines.

The cam in this engine was in it when I got it, and to be honest I am generally interested in the setup as its outside of convention...

[lc-gtr-1969]

pdq67, The engine is an Australian engine from GM Holden, originally 202 cubic inch. Very similar- actually extremely similar to the 250 chev straight six engine. The inlet ports are shared between the 2 adjacent cylinders.

[greenhj]

Is the head you're looking at using the J-Zed recreation of the old Duggan thing or a Yella Terra?

Tony Knight in adelaide (knightengines on here I think) has a bit to do with iron headed versions of this engine so maybe he will chime in when he gets a chance.

[PackardV8]

Off topic, but given the Aussie predilection for building OHC heads for old US six-banger short blocks, I wonder why Holden didn't ask GM to send down the tooling when they discontinued the Pontiac 3.5 liter OHC version?

[lc-gtr-1969]

Is the head you're looking at using the J-Zed recreation of the old Duggan thing or a Yella Terra?

Tony Knight in adelaide (knightengines on here I think) has a bit to do with iron headed versions of this engine so maybe he will chime in when he gets a chance.

I have been looking at the JZED, yes- they are a lovely unit.

In reality I will probably end up putting a cam with duration of about 255' @50 thou on 106 or 108 centres- centerlines will probably depend on the CC of the chambers- if the comp is really high I will run the wider LSA to bleed a little comp off with the later Intake valve closing. However, I am still interested in these tight LSA cams and what conditions they can work in...

[MadBill]

Even though it's obviously not targeted at your engine, the section in David Vizard's How to Build Max-Performace Big Blocks on a Budget (it's in some of his other books also) would be worth the cost to you for the LCA discussion alone.

In my opinion, cam specs are the single biggest factor re whether or not an engine performs up to its potential and without a TON of experience, selecting one out of a cam company's catalog for example is like looking up prescription medications on the net and obtaining the one(s) that seem to suit your needs, rather than letting the Doctor do that thinking.

[BrazilianZ28Camaro]

In reality I will probably end up putting a cam with duration of about 255' @50 thou on 106 or 108 centres- centerlines will probably depend on the CC of the chambers- if the comp is really high I will run the wider LSA to bleed a little comp off with the later Intake valve closing. .

Wise move IMHO.
The new bigger valves will increase the overlap flow area and may overscavenge the cylinder, so wider LSA=better to your aplication. The duration above is good to 7200 shift point with adequate headers.

The CR should be 10.5 to 1 or better for this cam above.

The Chevy 250 L6 we have here luv 106 LCA cams with the siamesed head on applications like yours.