On line Twin Cam porting article

[garyeanderson]

The following link was posted on one of the Cortina Mail lists. it seems to be a well thought out and with an eye on a nice street engine with some extra low end torque.

http://www.flowspeed.com/brady-tc.htm

p.s. - the head that was being modified was a stromberg converted for Webers.

[TeeJay]

Gary, thanks for the link.

How timely, I have just removed the valve seats and guides on my +2 total rebuild project.

[mikealdren]

Gary,

A great article, many thanks for posting it. It shows what can be achieved when you know what you are doing (and what can happen when you don't!). I wonder how long it took and what it cost.

Mike

[PWTrusty]

My 72 Stromberg head probably needs to go..if olny to be ride of the federal carbs. For some reason the CU conversion was not satisfactory.
I am looking for a sound weber ( 40 dcoe probably) head that could be built for road driving..anyone worked with Flowspeed before and any idea of the cost of rebuilding a head. Suggestiosn and horror stories appreciated.

Phil In Tennessee

[garyeanderson]

Hi PW

John McCoy does a weber conversion to the Stromberg head. the link to his site is

http://www.omnitech-engineering.com/auto/strom.asp

John is located in

Omnitech/SCC Inc.
3313 McAlpine Road, Suite C
Bellingham, WA 98225 - 8437
U.S.A.

Phone: 360-671-1472
Fax: 309-210-7620
Email: [email protected]

[garyeanderson]

I like to give the folks credit for the information they offer, I thought that I read about it in an email from the Cortina list. I went to look for the message and couldn't find it so I did some searching and found the site. I don't know Steve Simmons but he deserves the credit for the original posting. Thanks Steve

http://www.socalbritishmotoring.com/vie ... b6ee8115e3

[elancoupe]

Nice find, Gary. Just goes to show that bigger isn't always better.

[rgh0]

The porting work done to get it flowing right is what i would expect - you never want a port that contracts and expands like it originally did. You want to aim for a continuous port size reduction smoothly down to a minimum at the valve throat normally.

The actual flow limit to a head is due to sonic flow at the minimum flow area. if your port it right this minimum flow area is the valve curtain or valve throat at maximum opening and thus the rest of the port is not limiting flow and its all driven by the cam and valve lift. and to a lesser degree the seat shape

Pity the article did not discuss the 2 types of cams used - I did not think Isky made any good twink cam profiles


Only 2 people have done Stromberg to Weber conversions in my knowledge - QED did it about 20 years ago and McCoy does it now.
The porting shown is not McCoys CNC controlled porting - I have moulded his ports and they are very different.

1.7 / 1.45 valves have only been ever somewhat successful in extreme high rev racing engines and even in these I think they are overkill. !.625/ 1.4 are all you need for a 180 hp plus 1600 racing engine and beyond that I think your into dimishing returns as you have a whole lot of problems getting clearance between the valves on overlap as the article observes which leads to other trades offs.

Overall though a great article - would have like to have seen some of the flow bench data also.

cheers
Rohan
Rohan

[PWTrusty]

Hello Gary, Thanks for the reference. I may have found 1 or possible two ZS heads that are sound enough to machine. I will contact OmniTech after I have these boat anchors magnafluxed or dyepen checked.
Phil in Tenn

[msd1107]

Re: On line Twin Cam porting article.

Very interesting article concerning valve sizes and porting strategies.

Valve sizes in relation to engine size, rpm, etc has seen research interest. There are some sophisticated formulas in the literature. A simple engineering rule of thumb is to size the intake valve head so that the intake gas speed, when calculated on the valve head size, is around 200 ft/sec at peak power rpm.

If we take the original TC engine with 95 net (105 gross) hp at 5,500 rpm, this produces an intake gas speed of 205.6 ft/sec. The BMEP (a figure related to torque but independent of displacement) was 144psi/9.92bar net, 159psi/10.96bar gross. These were good figures for a street engine for the day. Harry Mundy was aware of these design guide lines, so this is hardly any coincidence.

Extended cam timings can increase the peak power intake gas speed to around 220 ft/sec, with still reasonable drivability for the street. The Sprint TC with 126 hp at 6,500 rpm translates to a intake gas speed of 223.6 ft/sec and a BMEP of 161psi/11.13bar.

Even more extended cam timings can extend the peak power intake gas speed to 240 ft/sec and above. However, low rpm torque usually suffers, making the engine suitable only for racing. If the Sprint engine had not used larger valves, the intake gas velocity would have been 243 ft/sec. The larger intake valve was necessary to get max power up to 6,500 rpm, and also warrants its largely marketing oriented name of ?big valve?. It would have been even better, with more development potential, with 1 5/8? intakes (207 ft/sec) or larger.

For more than a generation, larger valve sizes have been tried on the TC head, with uniformly mixed results. The geometrical limits of the original cylinder head design (fitting it on a Ford short block with stock pistons) precluded installing large enough valves to take the engine power curve efficiently above 7,000 rpm. Of course, determined tuners have gotten the engine to rev higher, but with compromises to the torque band and amount and in ways that are not amenable for street engines.

The engine that was the subject of the article developed a peak of 150 hp @ 6,800 rpm. This is an intake gas speed of 216.6 ft/sec and a BMEP of 184psi/12.66bar. The increased intake valve size of 1.625? kept the intake gas speed to a reasonable figure, while the cam profile produced 184psi/12.66bar at max rpm. Unfortunately, in order to produce a wider torque band, the cam timing was changed so that the torque at the peak power rpm was reduced, so the previous results are not so stellar.

Guidelines for sizing exhaust valves suggest a velocity based on the exhaust valve size of up to 50% greater than the intake, or even more, than the intake gas velocity or around 300 ft/sec. The exhaust valve size may not be as sensitive as the intake valve size. The original TC had an exhaust gas speed of 296.1 ft/sec, well within these guide lines. The modified engine had an exhaust gas velocity of 302.5 ft/sec. There is anecdotal evidence that the TC likes larger exhaust port sizes than standard.

Experimentation from the 1930s on lead to intake ports having reduced minimum sizes compared with the corresponding intake valves. Here again, a rough guide line is around 300 ft/sec, although engines have been successful with velocities below 300 ft/sec and up to 350 ft/sec. A recent article in a motorcycle publication proposed even higher velocities. It is difficult to determine the minimum port sizes on a TC engine, and the article did not enlighten us much in that regard.

Reading between the lines of the article, the author declared the starting point of the proffered head pretty much a waste, and what ever results he got were better than none. It is interesting to read the so called details of the porting modifications. I am sure that do one is going to divulge their innermost secrets, and the pictures published do not seem to correlate well with published results.

As is usual, speculation is useless without mathematical backup. Enclosed as an attachment is a downloadable spreadsheet that allows the interested and somewhat sophisticated user to verify these results and do their own modeling. Although the spreadsheet was not written for this purpose, the results are illuminating.

David
1968 36/7988

[SJ Lambert]

Do Isky do valve springs for T/C engines?

I understand that they do offer them for the 105E/120E pushrod motors, maybe their Super Competition (pushrod) Cam and matched Silicon Chrome springs would be worth considering in a pushrod motor?

[lotocone]

There are Isky valve springs in my twin cam, but I don't have any details about them. They were installed by the engine builder Barry Sale at PHP Racengines, Wauconda, Illinois 60084, USA. Phone 847-526-9393. Fax 847-526-4050.

[cabc26b]

Call John McCoy over at Omnitech - I have some of his springs and am running a cam with some crazy lift with no failure .

[SJ Lambert]

I'll use John's cams and springs in a twin cam race motor.

I'm still casting about for a cam & springs for a pushrod short stroke (53.3mm) iron head (downdrafted) 1150cc 120E engine. Will use rods on 5.63" centres.

One thought that I've had is that as the Silicone Chrome springs by Isky are reputedly pretty good, then if their AN-88 super competition cam is matched to their VWE-005 SP-116 spring set, then they might work well together. It has .507" lift, I don't know, but I wonder whether a small capacity engine could utilize a cam with such high lift....

A mate of mine thinks that Tony Ingram may use Isky springs for some applications, though I've not asked him. Tony certainly seems to know what he's doing with the drivetrain stuff for Twin Cams!


Another thought that I've not researched fully yet is Richard Coles in the UK with his Coltec concern. He offers a 658 grind that works from 4500 to 10800 rpm, has 66/88 88/66 timing & .421" lift. I think his springs are called "Rocket" springs & I think the Australian Formula Junior Association engine men thought they were pretty good......

[sevenhead]

The owners of Flowspeed, Sean Brown and his father Mike are very fine people. They rebuilt my Lotus Seven and are very much into the details.