BlownMGB-V8 Jim Blackwood 9406 Gunpowder Rd., Florence, KY 41042 (6468 posts) Registered: 10/23/2007 12:59PM Main British Car: 1971 MGB Blown,Injected,Intercooled Buick 340/AA80E/JagIRS |
Re: Opinions on this "recipe" please
Yeah, crazy ain't it? Lots of people have tried lots of different ways to define what is happening in the process, but when you're dealing with starting and stopping the flow of gas at a wide range of speeds the way we are here, good luck defining it with a mathematical formula. What is true at one speed is just going to be bass ackards at another. I think the DCR is primarily useful for figuring out if your engine is going to ping at low speeds, and as speed increases cylinder filling gets less and less effective. It's simple really, with a given orifice and a given pressure, volume is dependent on time. Less time, less volume. So as the engine speeds up cylinder filling is less effective. Tuning the system for a particular speed makes it more effective in that range, but it's the number of firing cycles, not the amount of cylinder filling that gives the increase in power.
How's that sound for a really simplified seat-of-the-pants explanation? Jim |
castlesid Kevin Jackson Sidcup UK (361 posts) Registered: 11/18/2007 10:38AM Main British Car: 1975 MGB GT Rover V8 4.35L |
Re: Opinions on this "recipe" please
Jim,
So we tune engines to run at high RPM which is less efficient, I think I'll go and have a lie down,(smile) Kevin. |
NixVegaGT Nicolas Wiederhold Minneapolis, MN (659 posts) Registered: 10/16/2007 05:30AM Main British Car: '73 Vega GT 4.9L Rover/Buick Stroker |
Re: Opinions on this "recipe" please
LOL. Sort of. It can be, technically, MORE efficient. Definitely less economical though. The thing is as air speeds up it acts more and more hydraulic. So the inertia of the gas column will want to continue to go in the direction it has accelerated to go. SO as engine speed goes up you can close the valve later in the revolution where the piston is actually starting to go back up the cylinder. As RPM goes up this effect can be used more effectively so the later the intake needs to close. This is all to counter the effect Jim is talking about, the increasing difficulty in filling a cylinder quickly. If the cam is tuned right you can actually get MORE than the volume of the cylinder into the engine (theoretically). By using the gas inertia leaving the cylinder out the exhaust port (overlap) to get the charge started in from the intake better. Then the inertia continues as the piston rises getting more volume into the cylinder. Pretty interesting.
It works awesome at the RPM it's tuned for but not as good for the RPM it's not. The closer you are to that RPM the better. That's where the problems start at low speed operation. The engine needs higher compression to offset the reduction in actual volume of the cylinder due to this modification. Let's work through an example: I've got a cam that opens the intake at 35º BTDC and closes 71º ABDC. SO at low speed the cam pushes the charge up into the intake then pulls it down into the cylinder then pushes part of the volume out the exhaust. SO since I have less volume if the CR is at, lets say 9.5:1, it would actually be around 7:1. NOT GOOD! Especially for ethanol. I set my CR to 12.5:1 which gives me a CR of about 9.2:1 at idle. That will translate to better manners at low speed operation. This is why your engine runs well at low RPM even with a "performance" cam. BUT yes this effect also pumps a bunch of unused charge out the exhaust so, yeah, less economical. No doubt. LOL. |
BlownMGB-V8 Jim Blackwood 9406 Gunpowder Rd., Florence, KY 41042 (6468 posts) Registered: 10/23/2007 12:59PM Main British Car: 1971 MGB Blown,Injected,Intercooled Buick 340/AA80E/JagIRS |
Re: Opinions on this "recipe" please
And then just to make things even more fun, wonder what happens when you throttle the intake? Obviously effective compression goes WAY down, since only a small fraction of the cylinder volume actually ever gets there. With a lot of overlap, wouldn't this effectively create an EGR effect? (So why did we ever need EGR in the first place?) Lots of exhaust gasses getting swished around in there, not all that much going out the tailpipe.
Think your head hurt before? ;-) Jim |
NixVegaGT Nicolas Wiederhold Minneapolis, MN (659 posts) Registered: 10/16/2007 05:30AM Main British Car: '73 Vega GT 4.9L Rover/Buick Stroker |
Re: Opinions on this "recipe" please
Kevin,
Something occurred to me while researching cam timing. If you are having trouble with the 8.6:1 DCR. You could retard the cam timing a couple degrees. Maybe even as little as 1º would do it. Just thinking out loud. |
castlesid Kevin Jackson Sidcup UK (361 posts) Registered: 11/18/2007 10:38AM Main British Car: 1975 MGB GT Rover V8 4.35L |
Re: Opinions on this "recipe" please
Nick,
Not so much as causing a problem as seem to have the engine running quite nicely. The main concern is the loss of torque and ultimate BHP caused by the lowish dynamic C/R I could swap the composite gaskets for tins and pick up .5/1 to give a theoretical C/R of 9.8/1 which would be about ideal for road use, but erred on the side of caution re valve to piston clearance with the extra 28 thou that the comp gaskets give. Will be able to check the clearance situation as my mechanic is getting an endoscope that will allow us to see what is actually happening inside the cylinders and if we feel there is enough we can take the heads off and double check with some plasticene and if ok swap to the tins which will also mean i can use the custom fixed length chrome moly pushrods which were made to be used with the tin gaskets. Just to clarify, the pistons i used are the chevy 305 KB flat tops with cut outs for the chevy valve positions so the Buick 300 heads would require additional relief if the tolerance starts to get a bit too tight. Question, are the Buick 300 valve centres in the same position as the Rover or Buick 215 heads? Kevin. . |
NixVegaGT Nicolas Wiederhold Minneapolis, MN (659 posts) Registered: 10/16/2007 05:30AM Main British Car: '73 Vega GT 4.9L Rover/Buick Stroker |
Re: Opinions on this "recipe" please
That's a good question. I'm pretty sure it's the same but I can't be certain. I used a junk 300 head just to be sure it worked for me. They may have the valves spaced farther from eachother in the 300 head too. Hmmm. Not much help, Sorry man.
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castlesid Kevin Jackson Sidcup UK (361 posts) Registered: 11/18/2007 10:38AM Main British Car: 1975 MGB GT Rover V8 4.35L |
Re: Opinions on this "recipe" please
Nick,
Not to worry, that would have been just too convenient. I didnt have any rover heads available as a comparison to the |Buick 300 oneswhen I was assembling the engine. The valve spacing in the 300 heads I believe is wider so that is why you can fit larger valves, Kevin. Edited 1 time(s). Last edit at 06/11/2009 05:03AM by castlesid. |
roverman Art Gertz Winchester, CA. (3188 posts) Registered: 04/24/2009 11:02AM Main British Car: 74' Jensen Healy, 79 Huff. GT 1, 74 MGB Lotus 907,2L |
Re: Opinions on this "recipe" please
Gentlemen, distance between valves, center/center for All 215's thru 340's should be 1.660" If your going to move valve, usually better to crowd exhaust to cyl. wall than int. Good Luck, roverman.
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roverman Art Gertz Winchester, CA. (3188 posts) Registered: 04/24/2009 11:02AM Main British Car: 74' Jensen Healy, 79 Huff. GT 1, 74 MGB Lotus 907,2L |
Re: Opinions on this "recipe" please
How to fill? We've toiled over the issues of DCR vs SCR but isn't the bottom line ECR? Some would contend that from low speed onward, it's a downhill plunge as far as cylinder filling. Point of interest, Edelbrock has made street manifolds that develop around 110% VE. in a very usable part of rpm. range. There are many others who are quiety developing "free supercharging" with ram tuning of the inlet tract. Some highly tuned racing engines are in the 130% V.E. range, normally aspirated! Lets remind ourselves that air and fuel have mass and therefore subject to inertia. When we befriend inertia, good things can happen. I hope some savy airflow types will weigh-in on this. Again my question, whats with the romance of the 4bbl? Better fuel distribution possible and ?, on Rover type heads with min. (2) carbs. Your thoughts please? roverman.
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NixVegaGT Nicolas Wiederhold Minneapolis, MN (659 posts) Registered: 10/16/2007 05:30AM Main British Car: '73 Vega GT 4.9L Rover/Buick Stroker |
Re: Opinions on this "recipe" please
You can get better throttle response across RPM range with a 4-barrel. Not so much with a 2-barrel single plenum setup. Works good at high RPM but that's it. Tunnel ram setups can be made to run at lower RPM but it's not great. From a carburated standpoint, nothing can beat individual runners and webbers IMO. Accurate fuel distribution and optimal throttle response at all RPM.
Obviously there is a trade off for serviceability, cost and simplicity. The 4-barrel is probably the best compromise in that regard. There is no doubt you can get better, more accurate fuel distribution with more expensive and tunable sources. Multiport EFI is probably the best for accurate fuel distribution. Totally tunable but expensive and complicated, not to mention more parts to fail. |
BlownMGB-V8 Jim Blackwood 9406 Gunpowder Rd., Florence, KY 41042 (6468 posts) Registered: 10/23/2007 12:59PM Main British Car: 1971 MGB Blown,Injected,Intercooled Buick 340/AA80E/JagIRS |
Re: Opinions on this "recipe" please
True, ECR is the bottom line. Biggest problem with that approach is that most of us aren't instrumented well enough or heeled well enough to really take advantage of that approach, by fully analyzing and then doing enough testing to know exactly what we're building. In short, for most of us it's a crap shoot. We can follow the established pathways to success, or take a chance that might cost us half our engine investment or more. There are just too many variables. But as we establish what works and what doesn't we at least reduce the risks. Fortunately some of us are kind of bold in that respect, trail blazers you might say. What is good for a chebby isn't always good for a BOPR. Plus we still face the traditional obstacles. Lack of space for the induction system, rpm spread, and all the usual compromises. I've gone over almost entirely to MPFI, but when you're talking simplicity, ease, compactness, performance and price it's going to be real tough to beat a good 4bbl system on any V8 engine, if one is available for that engine that is.
Jim |