Eric Gorr porting

Discussion in '2-Stroke Engines' started by jaguar, Dec 6, 2010.

  1. jaguar

    jaguar Well-Known Member

    Eric Gorr is famous for being an expert in 2 stroke posting. In his book he listed these guidelines:
    top end power: 84 degree ATDC exhaust, 116* ATDC transfers, 40% of head diameter squished, 105 octane gas
    midrange power: 90degree ATDC exhaust, 118* ATDC transfers, 60% of head diameter squished, 93 octane

    So from that I could derive that porting for an engine with low rpm power such as these chinese 48cc engines would be 96 degree ATDC exhaust, 120* ATDC transfers, 80% of head diameter squished, 90 octane gas

    So I want to try that on my engine but first I need to know the connecting rod eye-to-eye length so I can calculate where the ports should begin to open in distance from piston TDC.
    Can anyone tell me this distance?

  2. Old Bob

    Old Bob Member

    That will not give enough blowdown.With your timing its only 24 degrees which is far below the recommened 30-35°. More blow down gives better power.

    That sounds too tight.There is a point of having too much squish velocity.Before trying to go to 80%, you need to measure the squish height, most of the time simply closing the distance between the combustion chamber and the piston will increase velocity enough.You have to calculate the stock squish velocity and compare to the 80% band.Velocities over 60m/s will detonate them selves to death.
    Last edited by a moderator: Dec 18, 2015
  3. jaguar

    jaguar Well-Known Member

    The 80% was just a guesstimate. I just got my HT bicimoto and see that the stock heads have zero squish band.
    Concerning the blowdown time: 32 degrees at 10,000 rpm is equal to 24 degrees at 7,500 rpm.

    Here's my conclusion about porting:
    Assuming that by "high revs" Eric Gorr meant 10,000 rpm and assuming that the blowdown time (of piston travel from exhaust to transfers) needs to stay the same with all porting then the "midrange" porting of his yields 8750 rpm max and the derived "low end" porting yields 7500 rpm max. Carrying it one step further for a max rpm of 5000 gives us 106* ATDC exhaust and 122* ATDC transfers. To achieve that you only need to raise the exhaust port .5mm and leave the transfers alone since they come at 122*. For 7500 rpm max you should raise the exhaust 4mm and the transfers .5mm. Where the intake should be is still a mystery to me since Eric only wrote about reed valved engines.

    I am porting mine now according to this derivitive calculation for 5000 rpm max and am only lowering the intake .5mm for a start. I am also shaving off .5mm from the head as well as improving the flow of intake and exhaust.
    Last edited: Dec 6, 2010
  4. jaguar

    jaguar Well-Known Member

    I found the eye-to-eye connecting rod distance. Its 85mm.
    Last edited: Dec 6, 2010
  5. jaguar

    jaguar Well-Known Member

    I just found an on-line port timing calculator which is the Jaaps Puch calculator (in Dutch) at's Puch Calculator.exe This calculator gives me more perfect calculations than the manual method I used before and so the following details supersede the ones I gave in my previous posts.

    Here's the English translations of the Dutch wording:

    * Spoel-/uitlaattiming = Transfer or Exhaust timing

    Timing: (total degrees port is open. Divide by 2 and subtract from 180 to get *ATDC)
    Poorthoogte: Port height (mm of port from top of cylinder)
    Deck: (mm from piston top to cylinder top at TDC. 0 for HT)
    Slag: Stroke (of piston=38mm for HT)
    Drijfstanglengte: Connecting rod length (85mm for HT)

    * Inlaattiming = Intake timing

    Timing: (total degrees port is open. Divide by 2 for degrees BTDC)
    Inlaathoogte: Intake height (in mm from top of cylinder to bottom of intake port)
    Zuigerlengte: Piston length (at the intake side, in mm)
    Deck: (mm from piston top to cylinder top at TDC. 0 for HT)
    Slag: Stroke (38mm for HT)
    Drijfstanglengte: Connecting rod length (85mm for HT)

    Using this calculoator I applied the measurements from my HT cylinder and piston:

    Standard HT: 112.1* exhaust (28mm), 127.9* transfers (32mm), 15.8* blowdown, 4940rpm max, 56.5*BTDC intake (55.5mm), 30km/hr 19mph

    Ideal: 106* exhaust (26.2mm), 122* transfers (30.6mm), 16* blowdown, 5000rpm, 64*BTDC intake (57.9mm), 35km/hr 22mph with stock gearing on 26" bike

    As some kind of confirmation for the exhaust and transfer timing I found from that the Metrakit 65cc (43.5mm stroke) was rated as a torquey engine that worked well at all rpm's.
    2 Transfer Ports 122º ATDC
    Exhaust port 103.5º ATDC
    Blowdown: 19.5º (6090 rpm max)

    The 64*BTDC that I am quoting as "ideal" comes from from
    "As a general rule: intake timings with openings less than 68' BTDC make good torque, Openings from 68' to 75' BTDC have good overall power and openings from 76' to 82' BTDC can have substantially better top end with a corresponding loss of bottom end."
    Last edited by a moderator: Dec 18, 2015
  6. jaguar

    jaguar Well-Known Member

    Sorry but I rechecked everything and found that there is a 1mm difference between the piston edge and the piston top which means theres a 1mm "deck height" that lowers all my figures by 1mm. So here's the new results:

    Standard HT: 108.7* exhaust (28mm), 123.6* transfers (32mm), 14.9* blowdown, 4660rpm max, 53.2*BTDC intake (55.5mm), 35km/hr 22mph, 45.5mm piston length at intake side

    Ideal: 106* exhaust (27.2mm), 122* transfers (31.6mm), 16* blowdown, 5000rpm, 64*BTDC intake (58.9mm), 35km/hr 22mph with stock gearing on 26" bike

    To get to the ideal just raise the exhaust port .8mm, raise the transfers .4mm, and lower the intake port 3.4mm
  7. GearNut

    GearNut Active Member

    Do keep in mind that different manufacturers use different connecting rod lengths and correspondingly different distances between the piston crown and center line of the wrist pin.
    Dontcha just love the subtle differences in the myriad of parts that the different factories produce?
  8. jaguar

    jaguar Well-Known Member

    By different manufacturers do you mean those other than those producing the HT?
    I just measured mine the best I could with the cylinder off and it appears to be around 85mm long center-to-center. I was under the impression that all HT's are the same in this regard and in most other ways with some little variations.
  9. GearNut

    GearNut Active Member

    As I have learned from this site and others, there are 3 different factories that produce the HT engines. Each factory has their own variations of internal parts and tolerances.
    None of the factories communicate with each other concerning parts compatibility or commonization (is that a word?). They compete against each other.
    All the engines look nearly identical on the outside. All can share magnetos, carburetors, exhausts, CDI's, side covers, and to a degree, clutch parts and cylinder heads. Maby they can share more, that's just a list off of the top of my head.
  10. jaguar

    jaguar Well-Known Member

    Yeah but in all the reading I have done on this site and others I have not read one mention of different crank lengths for the HT. So I think it's safe to assume that they are the same, being 85mm long. I think everyone should be porting their engines. It's as easy as getting a Black&Decker RTX 3 speed rotary tool and one inch grinding disks from WalMart. If the disk is too big for a small work area inside the cylinder then you can use wire cutters to trim the disk down to a smaller diameter. Just don't let it throw any metal bits into your eyes. I let that happen to me and was very unhappy for one full week. It could of been a permanent discomfort. Safety first!
    I just double checked by using the Puch Calculator; If the conrod is 5mm longer that just throws the exhaust and intake port off by .1mm which is nothing to worry about.