Topic: Engien building question. Ok to use shorter rods to drop Compresion Ratio?

[cafebob]

So in a modern engine this is a big no-no.  Quench distance needs to stay right around .040".

BUT... I have read a number of posts (see http://forums.sohc4.net/index.php?topic=10171.0) on the forum about how our heads use don't really use Quench distance to aid in cylinder cooling. And it is more the shape our head shape to mix and clear our cylinders.

So the question is it it ok to use shorter rods (say from a 750 DOHC) to drop the CR of pistons?  The whole reason for this is that i am building a turbo motor, and shooting for a CR of around 7.5-8.0 and would like to use a set of 836cc pistons I already have, as opposed to trying to source a set of turbo ones. Keep in mind the whole reason for dropping the CR is so I can avoid detonation/running around 10psi of boost. I would also like to take advantage of the slightly stronger DOHC rods.



Here are some hypotheticals

Stock K motor

Deck height is 0.0"
Head Gasket is 0.045" (compressed)
CR is 9.0:1
Clearance Volume is 23cc
Quench is 0.045"


836 motor, arias 10.5:1 CR pistons (domed)

Deck height is 0.038"
Head gasket is 0.045"
Clearance Volume is 22cc
Quench is  0.083"
CR 10.5:1

And now my big questions ...

836 motor, arias 10.5:1 pistons, DOHC rods (.0590 shorter than stock)

Deck height 0.097"
Head Gasket is 0.045"
Clearance Volume is 26.98cc
Quench is 0.142" (!!!!!)
CR 8.74:1

Now the real stretch...If I do not use a base gasket, and use a F2/3 head and get the following

Deck height 0.077"
Head Gasket 0.045"
Clearance Volume is 30.395cc
Quench is 0.122"
CR is 7.9:1

which is about right where I want it, using a stock F cam i get a dynamic CR of 7.4:1 which is perfect for my application.  The only problem is if this was a modern motor this level of quench (0.122") would probably cause more detonation problems than the drop in CR would protect against. Also, even with the drop of 1.5 mm the dome of the pistons still stand proud of the deck by about 0.100".

Anyway, if anyone has any input about high quench of 0.122" is ok for these motors, I would like to hear about it.

bob

[HondaMan]

I haven't tried the wider quench. But, I have modified the profile of the quench band, through the hemi-head approach. I added a little over 1.5cc to my heads last summer, then milled them back to about 9:1 again (carbs only). No detonation appeared, and the engine has a wider powerband as a result. The quench area is slanted at a much wider angle than stock, essentially making it almost quench-band-less.

Food for thought.... 

[JLeather]

Bob,
  I am very interested in this as well, because I would like to build a mild turbo engine for my daily rider.  ~836 with DOHC rods for keeping the price down.  Any ideas on the amount of boost you're going for?  What are you using for timing control?

My biggest concern with riding a turbo engine long distance is fuel consumption...

[754]

Am I missing something?? does everyone have a set of DOHC rods laying around? Are the bolts in them strong enough??

I think we should perhaps be looking for some reasonable cost stonger bolts and get a few rod sets done up for medium strength rods.

I also see on here people wanting to do turbo motors 1/2 way. instead of  starting off with a bulletproof lower end with any boost you want.. I am seeing, "Lets use weaker rods and only rurn the boost up part way" Half building an engine may come back to bite you..

[HondaMan]

Am I missing something?? does everyone have a set of DOHC rods laying around? Are the bolts in them strong enough??

I think we should perhaps be looking for some reasonable cost stonger bolts and get a few rod sets done up for medium strength rods.

I also see on here people wanting to do turbo motors 1/2 way. instead of  starting off with a bulletproof lower end with any boost you want.. I am seeing, "Lets use weaker rods and only rurn the boost up part way" Half building an engine may come back to bite you..

I'm with you there, 754...turbos have a nsaty habit of sticking a rod out of a crankcase, sort of like the 3rd finger. 

Myself, I'd like to find a set of stronger rods bolts for the stock rods for my next "upbuild". I may wish to reach back up into the 12s on occasion... But, I don't have the kind of $$$ lying about for the ones from R/C today. Any other ideas?

[cafebob]

i want to run 10 to 12 PSI on pump gas.  Keep in mind I also live in Denver (5800 feet above sea level)

Ignition control is a dyna 2000.

Carbs are my big discussion right now.  I have a TM 40 mikuni if I go draw through. But if I can find that a set of 650 CV carbs cheep I may try to get them to work, and go blow through.

I got a set of perfect DOHC rods for 10$ on ebay.  DOHC's run 78-80 HP stock.  SOHC are like 68.  I am running a 78F bottom end.

bob

[nippon]

Why you don't use a second bottom cylinder gasket?

[754]

I think it would be really good if we could find somegood bolts and reworek some rods, a group of them to make it worthwhile. I guess DOHC rods are probably cheasp on ebay, I did not check. The bad part I see about them is they are probably heavier than stock.

The problem with a turbo is its hard to go 1/2 way.. sooner or later it may get cranked up!! A bit like building a 900 engine and saying I will keep RPM under 9K or I will run a milder cam so I dont make to much HP!!

Although a good set of rods will cost 1000 to 1350 they will last a while and  are your insurance, and you can do virtually anything to your motor after that.

 To put in perspective, I saw an underseat Wazooo exhaust canister pair the other day for 1400.00 and change!! Makes rods look cheap to me, by todays pricing..

[cafebob]

Yeah it all ways comes down to money. "Speed is just a matter of money.  How fast do you want to go?"

For me, if I wanted to drop 1000's of dollars on a bike I'd go with a nice new ducati. 

Part of the allure of these older bikes is that I can build them on a decent budget.  The cost of my turbo kit so far is under 500$, and that is including the cost of pistons and and bored out cylinder jugs.  If I can build it out for under a grand I'll be real happy.  Spare parts for the 750's are everywhere, and real real cheap if you keep you eyes open.
bob

[754]

Grenades are cheaper than tanks!

I ran an 836cc with stock rods on and off over a 25 year period, in the back of my mind there was always the possibility of the big bang!! And I ran it really hard.

So I am now convinced I wont get away with it twice, considering I know better I surely wont.

So next build gets rods, it has to!

I can fully appreciate buiding things on a budget, and yes there are lots of deals to be had, but some things are cheap for a reason.. maybe they need work?? maybe it is a turbo.. but not THE turbo that you really need..  Big learning curve  building turbo or blown engines over naturally aspirated.. there are reasons not everyone is doing it.

In short, you can take your chances, good luck hope you pull it off.. if you dont.. well there is always other 750 motors around.

[cafebob]

I cut my mechanical teeth building turbo RX7 drag cars....I know all about motors blowing up.  The old school rotary motors where some of the most temperamental motors under boost.   Plus rebuilding one is extremely difficult to do right, not to mention expensive. Compared to my 7 days, motorcycles in general are a walk in the park.

Plus its a project, I'm building it for me, not clients, so if things aren't perfect I can live with them.

bob

[HondaMan]

CafeBob;
Where do you live? I'm in Denver area, too.

[cafebob]

im right down town, curtis park...near coors field.  PM your info we should meet up.


bob

[Tower]

@nippon, your suggestion was too low tech and cost effective...thats why no one noticed it. Where's the challange in merely adding a thicker gasket..sheesh!

[Soos]

i want to run 10 to 12 PSI on pump gas.  Keep in mind I also live in Denver (5800 feet above sea level)

Ignition control is a dyna 2000.

Carbs are my big discussion right now.  I have a TM 40 mikuni if I go draw through. But if I can find that a set of 650 CV carbs cheep I may try to get them to work, and go blow through.

I got a set of perfect DOHC rods for 10$ on ebay.  DOHC's run 78-80 HP stock.  SOHC are like 68.  I am running a 78F bottom end.

bob

Uhhh..... you WILL have a problem with cb650 CV carbs.
the spacing is WAY off from 750 heads.
'79/some '80 piston typr(non-cv) carbs space the same as a 750 head though.

Mabey using a set of carbs off a '77/78 F motor will work better for what you need.
I would suggest those over the 650 carbs due to the limited bore on the 650's.
I believe i remember the 750's are 2mm bigger bore?

Am I wrong, or isn't drawing through the carbs better than forcing the air through them?
If so, I would go with that 40mm mukini if it were mine to do.


l8r

[cafebob]

actually i have been reading a lot recently about using CV carbs fro blow through.  Lots of good reason to do it on a street bike....stock idle, good throttle response, good transition.  Plus converting CV's to blow through is strait forward.  Newer CV's are well sealed and just need boost signal added to them below the membrane, and in the float bowls....

Drawn through can be real hard to tune to get good idle and transition.  Plus NO inter cooler or blow off valves.

I have some feelers out right now to a couple people running CV's on 750's to find out what bikes theirs came off of.  we will see how it goes.

Actually he just got back to me 83-85 CB650SC carbs bolt on and work very well of our bikes...the hunt is on, any one have a set the want to sell (cheap

bob

[JLeather]

I'm going draw-through on my setup.  The original kit actually ran a shovelhead Bendix carb with an accelerator pump and I'm thinking of rebuilding it and running with it.  It's a pretty sound carb, and it's got an externally adjustable main jet.  If I ran an A/F monitor I could adjust the mixture while riding if the temps change a lot or I go up in elevation.

As for the turbo, I have a copy of the original Mr Turbo manual for my kit.  It was designed to run up to 4-5 pounds of boost on a stock 750 with stock points and no timing retard.  I would think that dropping the CR to 8.XX:1 and running forged pistons would allow you to run atleast 7 pounds without problems.

I'm still pondering the results on fuel consumption.

Part or me is thinking of rebuilding the kit and putting it on a bone-stock engine with 4 pounds just like it was designed for.

[754]

Cafebob, I am apologizing, you seem to have thoroughly thought it out and realize the imnplications and probable results of what you choose to do.

I cant see any drawthru carb being compatible with blowthru, it is just not built for that. JMO

 Lots of turbos used a Bendix as a start point and many used an S&S to do a lot of the work on more hopped up setups. I know a local used S&S, not sure if he still is but he runs 190+ in the 1/4!!.. (2cam Motor)..again JMO..

[Soos]

I have some feelers out right now to a couple people running CV's on 750's to find out what bikes theirs came off of.  we will see how it goes.

Actually he just got back to me 83-85 CB650SC carbs bolt on and work very well of our bikes...the hunt is on, any one have a set the want to sell (cheap

bob

Off of a DOHC cb650 then?
I would love to see pics!! And how he hooked 'em up?
stock carb tubes?
Were they spaced as the 750 carbs are? Or close enough to work?

And quite a ride you are making there.
I miss Colorado.



l8r

[cafebob]

So....I have some more information on the cb650sc carbs.  Apparently they are about 1/4" off for the stock rubber mounts, a little work with a heat gun and they fit real nice.  Still working on rejetting, but I bet it only needs a couple of sizes up main jet. 

As for using CV's and blow through Check out this link...

http://www.turbo-bike.com/index.html under the "pressurizing carburetor R1" He is doing everything by the book.  For me the main strengths of using a newer CV carb is that they are sealed well, and I probably won't have to modify them too much.  Our older CR carbs take a lot more work to seal them up...the throttle shafts are the real problem. Plus, and this is a big plus, you can tune blow through carb fairly well with out the turbo hooked...at least to the point the the motor will run and you can drive the bike.

As for the other blow through hassles (high fuel pressure management, rising rate pressure regulator).  I have experience setting these systems up so they don't worry me too much.

A lot of newer gixers run the stock CV carbs in blow through mod.  Some run with the same jetting!  I have been reading the book "Motorcycle Turbo charging, Supercharging, and Nitrous Oxide"  it was recommend to me by a list member and it is a very good resource and probably the most up to date one I have read.  More timely than Huge McInnes book, and a little better than Maximum Boost IMO.  Anyone turboing there bike should have it on their shelf.  The book relates a number of success stories with CV carbs...


I am going to try to source a set of CV's and see what I can do on a stock motor...

bob