What year Triumph 650/750 stock cylinder heads have the best ports for horse power? Was the EPA 750 parallel ports better than the previous splayed for horsepower? I heard they gave better gas mileage? Thanks joepenoso
Triumph Racing in America book states in the back the best 750 heads were the 1973-76 model years. They didn't say why and I guess that leaves out the parallel ports model. I wonder what was wrong with the 1977-78 heads? joepenoso
The splayed port head has the better flow characteristics. If you look down the the parallel port head you will see a sleeve for a head bolt in the port disrupting flow. The splayed port head is unobstructed and a straighter shot to the valve. The parallel port head, however, runs cooler. HTH
Joe Seems I remember a discussion here a while ago on this topic. In a nutshell, The early (don't know exact years) 9 bolt heads had smaller valves and actually made better HP than the later big valve versions. Try a search - it was an interesting read. Mike
Steve, I thought that the EPA forced the parallel port mods and if it improves emissions, how can it be less good flow-wise?
AIUI, the EPA-mandated changes were related to the snorkels on the airfilter box and the breather hose routed to the airbox and the exhaust rocker box....
I was under the impression that the parallel ports were designed to allow the twin carbs to have a commom choke lever....ditching the mechanical airslides and ticklers allowed for a leaner burn and met the levels mandated by the EPA.....
Steve (the other Steve :))
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Jubee My understanding as well. A cooler head along with the EPA carb and breathing met the standards - I guess. But the splayed port makes better HP. The MKII chokes, however, can be cable actuated from the bars or the common lever. I prefer the lever which is what I have. HTH
blapper - its new valves and guides ive got, the seats look ok to me but not having a known good head to compare to i dont know if the seats are sunken or not, if that makes any sense? guess the best way to find out is run it?
If the top of the seat on the valve head is below the floor of the chamber (holding the head upside down in your hand,) you have valve seat recession. But a little bit is tolerable. If the entire valve head is flush or below the floor of the chamber, you've definitely got issues and should either fit larger valves or replace the seats.
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#225799 - 11/28/086:26 pmRe: Stock cylinder head porting.
Joined: Aug 2007 Posts: 4,542Blapper
BritBike Forum member
Right. The flow characteristics depend on the chamber, shape and obstructions. In the old days we used to polish ports. Then we learned that a polished port may restrict the flow/mixture. This is because there is no flow at the wall. Hard to believe but true. The flow speeds up - flows in a more laminar pattern - at the center of the venturi or port. This flow speed increases exponentially as measured from the wall to the center. If the wall is left rough it increases the swirl, and so improves the mixture, as the mixture heads towards the valve. The parallel port head has a sleeve in the intake tract the head bolt goes through. The sleeve seals the port. But it disrupts the flow. So the flow becomes laminar to the bolt, gets disrupted and the smooths again. It's a matter of creating the "swirl" and keeping it going straight to the valve. Anytime the flow is slowed and reaccelerated it takes power. anybody yet? Sorry. OK, Steve, quiet now...
That may be offset by the better combustion chamber which naturally follows from a parallel port. In addition the sparkplug benefits from better cooling from the incoming charge. Racers of days past spent a lot of energy de-splaying their splayed port heads or just using single carb heads with twin carb adaptor. A little down-draft would have helped also. Bill
Last edited by HawaiianTiger; 11/29/082:36 am. Reason: No can spell!
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A more homogeneous air/fuel mixture, with finely atomized fuel clusters will burn more uniformly and more completely when the spark occurs; providing both lower combustion temperatures via "controlled burn" vs "detonation"; and more power since the available fuel in the charge burns more completely. Strong swirl is vitally important in a hemispherical combustion chamber, especially as compression ratios rise since the hemi shape is less than optimum for non-swirl induced charge dispersion. See also "squish" as a means toward better atomization and improved burn.
i'll bite on John's question. swirl better atomizes the air/fuel mixture...a side effect of which is that it draws heat from the surface of the head (something to do with surface area)...then it (the mixture) ignites better because it's hotter (cuz it just pulled all that heat from the head). (that's just me guessin', i have no idea really) mitsubishi used to add a jet into the combustion chamber of their cars (starion/conquest, anyone?) this jet injected a very lean air/fuel mixture into the combustion chamber (aimed right at the spark plug) right before the spark. it was also angled so that it would cause a swirl. it was supposed to cause a better burn, what it really did was crack cylinder heads by causing a super hot area right between the valve seats.
And HT... parallel ports do not a better combustion chamber make... they may contribute to better fuel atomization or some other tangent factor; they alone do nothing to enhance the efficiency of the combustion chamber shape itself. Packing more charge in the space allotted is only effective if you can burn all; or most of each charge each time. I submit that a second spark source in a hemi chamber with pop-up pistons might give more reliable power gains than porting or roof shape changes in most cases.
while we're on the topic, how does one extract the most HP out of single carb head? my bonnie does alright with two carbs. my arms are actually longer after a few good pulls on the throttle however, it looks like my next motor might only get one carb....having trouble selling off my single carb head so i might as well use it!
Thanks John... I try and pay attention. For MarkB... I am also a Mark B... see the above and try and find a balance between the ability to fill your cylinder and the ability to burn; not explode each charge. Better cylinder filling can come from increased valve lift, overlap and/or open duration; increased CFM via bigger carbs/ports, and smaller increases via exhaust port/ pipe design. You must then juggle piston/head dome shape, cam timing, ignition timing, multiple spark sources, and various other variables such as the fuel you can expect to run in your area to get maximum power from any combination you choose. There is no set answer since a change in, say compression ratio from 9/1 to 10/1 is related to each other variable mentioned... tuning is truly a matter of attempted, failed and retried experiments to get good result. Thewre are probably no user serviceable parts inside your strong "tuner" engine... find a local hotshoe and hope to get what you pay for.