zguitar71 Posted September 15, 2013 Share Posted September 15, 2013 When I built the motor in my car I whent with OER 45s with 36mm ventures. I settled with 36 after talking with local people who said that would be almost too big while Rebello (who built the head) said 36 is the minimum I should run. So I took the safe ground. The max power of the motor should be around 7100-7200 rpm, the 36 Venturi starts to run out of steam around 6600. Currently I do not have any problems driving at lower rpms and can open the throttle fully at 2500 in second without stumbles. A 37 would put max power at 6950 and a 38 would be 7350. Would I sacrifice too much on the low end moving to a 37 or 38. Much of what I was told by my local buds was not correct so that Leeds me to believe I can run the lager sizes. Having to roll into the throttle at 2500 as apposed to flooring it is fine with me at lower rpms to gain more on the top end. Any one running big venturies on the street? This is on a 2.8 with a zx 5 speed and a 4.44 diff with a 279/540 lift cam 10.7:1 comp and a ported Rebello head. Thanks for any advice Quote Link to comment Share on other sites More sharing options...
Tony D Posted September 16, 2013 Share Posted September 16, 2013 (edited) Do they make them in 1mm increments??? Get another set of 36's and chuck them in a lathe, take them up 0.5mm at a time and chart your progress...stop when you think you have what you like....or you will be cutting he set you currently have or he size you just turned upsize 0,5mm... Edited September 16, 2013 by Tony D Quote Link to comment Share on other sites More sharing options...
zguitar71 Posted September 16, 2013 Author Share Posted September 16, 2013 They do make them in 1mm I increments. http://www.piercemanifolds.com/product_p/72116.370.htm Unfortunately I do not have a lathe or access to one. You suggest .5mm steps, is that because changes in Venturi size are very sensitive? In other words would I be foolish to spend money on the 37s much less the 38s plus the jetting that would accompany them. Quote Link to comment Share on other sites More sharing options...
JMortensen Posted September 16, 2013 Share Posted September 16, 2013 I did have a friend who polished the venturis with jeweler's rouge. You might try that before you spend $120. Your cam has a lot of lift and not so much duration. Maybe look at that? I'm just thinking that 1mm in choke size might not even register on a dyno. Quote Link to comment Share on other sites More sharing options...
Tony D Posted September 16, 2013 Share Posted September 16, 2013 (edited) I knew someone who hoarded the 32 and 34mm venturis everybody took out.... He turned them in a lathe to the sizes he needed. Even at a machine shop, it's likely cheaper to do that than buy another set every dyno pull! I suggested 0.5 mm increments because if you go to far, you can't go back! With a dyno pull to quantify EXACTLY the rpm shift versus diameter change trend will appear early on and you can estimate what you will need after one or two pulls to give the rpm range you want. Sizing up,can be done with a flapper wheel and a drill press using a calliper. It's not rocket science! Edited September 16, 2013 by Tony D Quote Link to comment Share on other sites More sharing options...
zguitar71 Posted September 16, 2013 Author Share Posted September 16, 2013 Ok Tony D, I do have an extra set of venturies so I think the flapper wheel will be in my future. So just a guess here but removing aluminum probably calls for about a 60 grit to start then move to finer grits to smoothe it out a bit. What do you think? JMortensen, are you thinking that the lower duration of the cam could be the reason for not making power to 7200 verses the size of the Venturi? In the end if I increase the Venturi size and the top end still stays the same then I will know its not venturi size. Quote Link to comment Share on other sites More sharing options...
JMortensen Posted September 16, 2013 Share Posted September 16, 2013 JMortensen, are you thinking that the lower duration of the cam could be the reason for not making power to 7200 verses the size of the Venturi? In the end if I increase the Venturi size and the top end still stays the same then I will know its not venturi size. Yes. Another way to say it is that I think that you have to worry about more than just venturi size when it comes to determining where the peak power will be. Runner length and diameter, carb size, air horn length, timing, cam, how the porting was done, etc. Seems way too simplistic to say 36mm = 6800 rpm and 37mm = 7000 or whatever. Quote Link to comment Share on other sites More sharing options...
Leon Posted September 16, 2013 Share Posted September 16, 2013 It's the cam. Changing venturis will only help if they're beginning to choke your engine, i.e. airspeed through venturi is approaching ~0.4 Mach. This can be calculated. The other method would be empirical testing, by iterating choke sizes, as has been mentioned. Quote Link to comment Share on other sites More sharing options...
Tony D Posted September 16, 2013 Share Posted September 16, 2013 I would say 60 is too rough, a good flapper wheel with 120 or even 240 will take 0.5 mm off and leave a reasonably smooth surface...if it's to your liking then you can get the felt buff and make them mirror-chrome finish later. If you have a spare set, you can take those then, "too far" in stages till you see a diminishing return and decide what to do with the ones you have. and what final size you want based on the testing of the set prior. Quote Link to comment Share on other sites More sharing options...
zguitar71 Posted September 16, 2013 Author Share Posted September 16, 2013 Yes. Another way to say it is that I think that you have to worry about more than just venturi size when it comes to determining where the peak power will be. Runner length and diameter, carb size, air horn length, timing, cam, how the porting was done, etc. Seems way too simplistic to say 36mm = 6800 rpm and 37mm = 7000 or whatever. Yea I am still figuring all this out and still have a long way to go as far as education when it comes to high performance motors. When Rebello made the head they also ported the intake to match the carbs and ports on the head and have the right amount of taper ect.... When I talked with them they said that my head work, carbs intake and header are the same that would be used on a stroker that they advertise as 300+ hp. I was told to set the cam on the second hole of the stock sprocket and the timing at 34 and the max hp should be around 7200 and that I should go with a minimum of 36 for the venturies (I also bought the carbs and header from them). So my initial assumption for the venturies being too small was based on a that info. Quote Link to comment Share on other sites More sharing options...
zguitar71 Posted September 16, 2013 Author Share Posted September 16, 2013 I would say 60 is too rough, a good flapper wheel with 120 or even 240 will take 0.5 mm off and leave a reasonably smooth surface...if it's to your liking then you can get the felt buff and make them mirror-chrome finish later. If you have a spare set, you can take those then, "too far" in stages till you see a diminishing return and decide what to do with the ones you have. and what final size you want based on the testing of the set prior. Great, thanks for the advice. I will start messing with it and try to figure thing out before winter is here and I have to shut the garage doors for 4 months. Quote Link to comment Share on other sites More sharing options...
Tony D Posted September 17, 2013 Share Posted September 17, 2013 Hint: eBay "Atlas 109 Lathe" Once you get one...you find all sorts of uses for it! Quote Link to comment Share on other sites More sharing options...
Xnke Posted September 17, 2013 Share Posted September 17, 2013 And you'll want a bigger one! (I have a Sieg 7x12, and then bought a 1947 Atlas-Craftsman 12x36!) Quote Link to comment Share on other sites More sharing options...
zguitar71 Posted September 27, 2013 Author Share Posted September 27, 2013 OK Tony D, I bought a flapper wheel and so far have the venturies at 37 mm and upped the main from145 to 150 and I like the way it runs. I have not lost any bottom end and it seems to be better now on the low end with response than before. I know it could be wishful thinking but power is available to near 7k but no loss on the lower rpms. Before it was going down after 6500. This is with my butt dyno, so..... Anyway the flapper wheel worked great and I would have never thought of it so thanks for the advice. Quote Link to comment Share on other sites More sharing options...
zguitar71 Posted September 27, 2013 Author Share Posted September 27, 2013 OK Tony D, I bought a flapper wheel and so far have the venturies at 37 mm and upped the main from145 to 150 and I like the way it runs. I have not lost any bottom end and it seems to be better now on the low end with response than before. I know it could be wishful thinking but power is available to near 7k but no loss on the lower rpms. Before it was going down after 6500. This is with my butt dyno, so..... Anyway the flapper wheel worked great and I would have never thought of it so thanks for the advice. Quote Link to comment Share on other sites More sharing options...
Tony D Posted September 28, 2013 Share Posted September 28, 2013 (edited) I got a 17x72 Milacron as well. I would have settled for the 12" Atlas with all the attachments... Good to hear the manual resizing went well... No need to buy something you can make cheaply yourself, right! (My 6" 109 is driven by an electric motor "Dec 24, 1925" listed as production date... Edited September 28, 2013 by Tony D Quote Link to comment Share on other sites More sharing options...
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