building my 351w

[bnickel]

scat or eagle. given the choice i'd probably use the eagle crank but it's a little more expensive though it's still doable on a budget and well under $500. i don't remember off hand how much it is but it isn't just a whole lot more than the scat crank.

 

[rbohm]

if you are building a street engine, or a claimer race engine, here is a perfect crank for you. understand that it will likely need balancing, and it is foreign sourced;

http://www.competitionproducts.com/prodinfo.asp?number=C351S

however competition products does sell quality off brand parts, so this would be a good investment.

 

[ratio411]

This is sweet!
Everything 393w, less the block, and fully balanced: 740 bux!
http://www.competitionproducts.com/prodinfo.asp?number=393FF30
Just gotta find out exactly which pistons they are available with...
Gotta keep the compression streetable.

Edit: Shoulda said 'rotating assembly', because there are no gaskets or valvetrain parts.

 

[project.65]

I found this kit for $799. What do you guys think bouthttp://www.fordstrokers.com/393-windsor-street-fighter-gt-rotating-kit-p146.html it.

 

[rbohm]

fordstrokers is another good kit, BUT you WILL have to have the rotating assembly balanced before you run it.

 

[project.65]

don"t you have to get any kit balanced before you assemble it? Does anyone else have any comments on the kit? http://www.fordstrokers.com/393-windsor-street-fighter-gt-rotating-kit-p146.html

 

[ratio411]

You should get them balanced if they have not already been balanced.
The one I mentioned earlier comes balanced already for 740.

The one mentioned later, specifically says that balancing is an extra cost option.

Just gotta read what you are buying... or ask questions.

 

[Steel123]

I didn't realize rod ratio played into what type of gas you needed ? I always thought it was determined by your compression ratio and how much timing advance you run.

 

[rbohm]

I didn't realize rod ratio played into what type of gas you needed ? I always thought it was determined by your compression ratio and how much timing advance you run.

there are a few things that determine what fuel grade you need;

rod ratio
cam timing
fuel mixture
compression ratio(static)
compression ratio(dynamic)
ignition timing
engine load vs engine rpm

any of these can raise or lower the grade of fuel you need to prevent detonation. as i, and many others, have stated, you need to take a systems approach when building an engine. as part of this systems approach you should determine what fuel grade you want to run, and build accordingly. a 10.5:1 compression engine can still run nicely on 87 octane fuel, and still make good power, as long as you factor in the above factors. a long rod engine with narrow lobe center angles, and long duration, will have a lower dynamic compression ratio, which allows the engine to run a lower octane fuel.

 

[ratio411]

You forgot combustion chamber shape.

A perfect example is the Cleveland open vs closed chamber.
The closed chamber can run on premium pump gas (in stock form) up to 11:1 compression.
The same exact premium fuel is in danger of detonation at 9:1 compression with open chambers.
The open chambers are prone to excess heat and detonation, but are very clean burning. The closed chambers run cool, by design, but were very dirty emission wise. Therefore govt regs made them very short lived.

Chamber shape means alot.
Also, head material and material thickness.
Aluminum doesn't hold heat like iron, so generally speaking, aluminum allows an additional full compression point. (9:1 vs 10:1 for example)

Edit:
Forgot 2 others...
Headers (uncoated) allow the head to run cooler. Cast manifolds retain heat at the head.

Next, not only the thickness of the head material, but the thickness of the block deck and cylinder walls. Thinner material concentrates heat.
Any of this around the top of the cylinder creates hot spots for preignition.

 

[rbohm]

good points ratio. i was pointing out that you can mitigate weaknesses in one area by changing other areas to work around said weaknesses. the problem with the open chamber 351c heads is the same problem that the chrysler hemi's had, the air in the combustion chamber was "dead air", meaning no real movement of the air as the mixture was being compressed.

 

[brianj5600]

I am confused about how extra time at tdc helps reduce detonation. It seems like since it moves slower at tdc, you will get less turbulence from the quench pads and have the mixture compressed for a longer period of time. Both of those should cause an engine to be more prone to detonate. There was more than a couple Engine Masters contestants noted these things when talking about thier engine combonations a few years ago.

 

[bnickel]

I am confused about how extra time at tdc helps reduce detonation. It seems like since it moves slower at tdc, you will get less turbulence from the quench pads and have the mixture compressed for a longer period of time. Both of those should cause an engine to be more prone to detonate. There was more than a couple Engine Masters contestants noted these things when talking about thier engine combonations a few years ago.

with the piston parked at TDC longer it's not pulling air in as quickly so it promotes a more complete burn because the incoming air is not putting out the fire so to speak. it's all about flame front propagation.


also, if you have a piston dish that mirrors the combustion chambers you get even more benefit since the fuel/air combo is not getting pushed away from the flame front as much.

the best combo would be small chambers with dished piston that mirrors the combustion chambers, long rods, with the right lift/duration and LSA to match the static compression ratio and provide a good dynamic ratio and a head that with good flow numbers but small enough ports to keep the mixture velocity up. i like the valves to have a 30 degree back cut as well. also, i prefer smaller valves as well but that's just me.


we could go on and on about all this and start talking about things that nobody understands but as far as the rod ratio thing goes there are more advantages than there are disadvantages to running a longer rod from everything i can tell. longer engine life due to less side loading, better combustion, more torque and hp lower in the rpm range and still able to rev as well. they can also help in lowering emissions and increasing fuel economy as well.

that's what i've found from researching the long rod combo's for the 351w anyway.

 

[brianj5600]

with the piston parked at TDC longer it's not pulling air in as quickly so it promotes a more complete burn because the incoming air is not putting out the fire so to speak. it's all about flame front propagation.


also, if you have a piston dish that mirrors the combustion chambers you get even more benefit since the fuel/air combo is not getting pushed away from the flame front as much.

the best combo would be small chambers with dished piston that mirrors the combustion chambers, long rods, with the right lift/duration and LSA to match the static compression ratio and provide a good dynamic ratio and a head that with good flow numbers but small enough ports to keep the mixture velocity up. i like the valves to have a 30 degree back cut as well. also, i prefer smaller valves as well but that's just me.


we could go on and on about all this and start talking about things that nobody understands but as far as the rod ratio thing goes there are more advantages than there are disadvantages to running a longer rod from everything i can tell. longer engine life due to less side loading, better combustion, more torque and hp lower in the rpm range and still able to rev as well. they can also help in lowering emissions and increasing fuel economy as well.

that's what i've found from researching the long rod combo's for the 351w anyway.

Now I am really confused. At tdc the intake valve is on the seat. So yes it is not pulling in air as quickly. It is not pulling in air at all.

Dwell time has NO effect on dcr. A longer rod does lower the piston in the bore at a given intake closing. If the piston is in the same place at intake closing it will have the same dcr whether it is a long rod or short rod.

Long rods require more octane for a given dcr because there is more time for detonation to occur. When dwell time is decreased there is less time for a wild fire to start somewhere in the chamber other than at the sparkplug. A slow moving piston will decrease the airspeed with in the chamber as well.

There are advantages to longer rods, but most engine builders will tell you it is overhyped. Reducing detonation is not an advantage, it is a disadvantage.

 

[bnickel]

sorry i meant to say that the piston stays at TDC longer but that's not necessarily saying that the cam and valve train are, i also didn't say that a long rod had any affect at all on DCR i was talking about the cam and the rest of the combo. also long rods do not require a higher octane fuel, in fact they require a lower octane fuel that is one of the advantages, you can run a higher compression ratio and use a lower octane fuel than the same engine with a shorter rod. i don't where you heard that a long rod makes an engine more prone to detonation but it's wrong, totally incorrect, an engine that is burning the fuel more efficiently is less likely to detonate. a long rod does not create a hotter combustion chamber it creates a more efficient one. also the long rod doesn't create a slower moving piston, it actually moves just as fast as a shorter rod because you're not changing the stroke, it just stays at the top and bottom of the stroke longer piston speed in the cylinder is still the same. it may be moving a tiny bit slower from full speed to a dead stop and from a dead stop to full speed but in reality that's where the main advantage comes from.

i'm not sure where you got your info that a long rod motor is prone to detonate from but it's flat out wrong. also, with piston staying parked at TDC longer the exhaust valve has more time to get the spent gases out actually make it even less prone to detonation than the same engine with a shorter stroke which is why they can run on less octane.


look at this way. in a regular 351w for instance, the piston goes down in it's bore and the intake valve opens and the piston starts drawing the mixture, on the way back up right before the piston reaches TDC the ignition lights off the mixture and the piston finishes compressing it and the exhaust valve opens and the piston starts back on it's way down. in the same 351w with a long rod when the piston reaches the top of the stroke it slows down a little more than the stock engine right at the top of the stroke and it stays at the top just a little bit longer and burns the mixture better and and then exhaust valve opens and can pull more of the spent mixture out before the piston starts going back down, this cools the mixture some, then the intake valve opens as the piston starts going back down the bore since it stayed at TDC a little longer this allows the cylinder to fill just a little more adding to the cooling effect started when the exhaust valve was able to pull a little more of the spent gases before the piston started back down.


now do you see why the long rod combo requires less octane, makes more power, produces less emissions and does NOT make the engine any more prone to detonation that it was before? if not, then i don't know a better way to explain it.

 

[brianj5600]

Do you read much on the tear down articles from the Engine Masters competition. They like shorter rods when limited on octane so that the the piston does not stay at the top of the bore. That is when detonation occurs. This is mentioned in some articles I have read on pump gas drags builds as well. People that have forgot more than both of us know.

Your valve timing events are off in the third paragraph. The intake valve opens before tdc and stays open after bdc. The exhaust valve does not open until the piston is over half way down in the bore and stays open until after tdc. If the valve events are correct for a stock rod, then there is no real advantage as far as cylinder filling goes. Todays heads are good enough to fill a cylinder with a short rod.

I am not too sure about long rods lowering emissions and having more efficient combustion either. These things are determined by the combustion space and spark timing.

 

[bnickel]

you're right my valve timing events are off, guess that's what i get for trying to remember them at 2:00 in the morning. but the point was that the long rods promote cylinder promote better cylinder filling and evacuation than the shorter rod and promote better and more complete combustion because the piston stays at the top of the stroke longer. on a long stroke engine in a short cylnder i can see where they'd be worried about detonation but we're talking about a stock stroke 351w here which is a relatively short stroke so it doesn't stay at the top of the stroke like a long stroke long rod windsor would, most of the engines in the engine masters challenge have 3.85 or longer strokes most probably with 4.00 strokes so yeah i could see that as being a problem in an engine with a relatively low deck height.

 

[rbohm]

when a long rod engine prevents detonation compared to a short rod engine is the longer rod has a more consistent acceleration and deceleration curve and reduces or eliminates pressure spikes on the compression stroke.

 

[Rusty67]

I started a new thread for this since at this point this thread has been totally hijacked. I copied most of the previous posts into the other one already.

 

[bnickel]

I started a new thread for this since at this point this thread has been totally hijacked. I copied most of the previous posts into the other one already.

i saw that, thanks. i apologize for the hi-jack as well.