Getting close to doing the assembly on my BB Rajo headed engine. I have a Stipe OHV special 340 cam and also a adjustable cam gear. I suspect that I will have at least 10-1 CR. I'm trying to decide whether to use this gear here or save it for another engine. For ignition I'm using a Bosch front plate converted for dual plug. I would appreciate the benefits of the experience of others. I plan to do some engine Dyno testing in February
Les, FWIW.. Are there, or do you have the specs for that particular cam? You would be able to try any settings with the adjustable gear? Since you're heading to the Dyno, I think that would be ideal time to experiment. Thoughts?
Les, This former post covers what I was thinking.
The Stipe gear referenced is what I have and I have installed one on a Flathead I am just finishing. It will go to the Dyno first. As it is about all "Schubert " I'm resigned to doing a bunch of "tuning ". On the BB perhaps there is more experience out there to benefit from
Les, Sounds like "quality shop time" Wish I was there! Keep us posted.
I believe retarding the cam gives better performance at higher RPM. Since that is what overheads are best at, I would certainly give it a try. It will be interesting to see the dyno results under different cam settings. You must have some wild pistons to get 10:1 on a BB Rajo. Something by Ross?
If you are estimating the CR to be 10:1, that would imply you are going to be running pretty tall custom pistons. Couple that with you .340 lift cam, you may slamming the piston into the exhaust valves on the exhaust stroke. With the 1.5:1 ratio rockers you will have complete valve lift at .460. I believe the RAJO chamber depth is 1.125. With copper head gasket installed (.090) about 1.215. You may want to check your clearance during assembly.
Yes I certainly plan to check for piston valve contact/clearance. "Eyebrows " may be called for!! Preliminary measurements are indicating about 1/8-3/16" clearance at TDC with the valves closed.
With 10:1 compression you are going to start having to worry about BMEP. Too mild a cam with too much compression and it will be hard to find gas with enough octane to stop pinging.
You kind of need to decide what RPM the engine would be most likely to operate at and work backward from there.
Like Les I would consider the valves hitting the piston something to fix prior to engine assembly. You don't want to limit camshaft design by piston design. Rather, you want to make the piston work with the other choices.
Contrary to what was said in another post, an overhead valve engine is a more efficient way to fill a cylinder, and it naturally has a more efficient combustion chamber too. With all other variables being equal an overhead will make more torque and horsepower at any RPM.
I plan to use premium fuel, so detonation should be manageable.
This engine will be sitting on a "Fronty" aluminum crankcase that accepts a model A transmission. It will all be connected to a Halibrand QC rear axle. Totally "off the wall"!!
So the drive line will be altered to fit the days "event"!! Too many years in the planning, now time to execute
Since the Fronty pan or Szumoski copy thereof is made for a model A tranny, it can be used uses with anything the A guys are upgrading with. We built a T engine for a guy using a Szumoski pan and a T-5 trans. With that kind of gearing you wouldn't have to change the QC gears so often. Off the wall is OK with us at Wild Cherry Racing. We have found our Winters QC rear end perfect for tuning the car for the event at hand.
I, too, thought a .340" lift cam a little extreme for a ratio rocker arm OHV. You are going to have to notch to bejezus out of the cam bores to stuff it in there. We are using something something like .300" lift on the Gemsa engine and worry about the stress on the whole valve train at the high revs this thing lives and dies for. Just some thoughts. Race on!
The engine is my one and only aluminum 5 main block. The cam also has 5 bearings. So putting in the cam was a small part of the job!! I m using 292-312 lifters.
Transmission. I'm using the A at the moment for simplicity. Perhaps next year a T5 (which I have one of).
I appreciate your comments about valve train stress. Definitely something I need to be careful about. In your opinion/experience what would be a reasonable red line for the valve train I have? You have a lot more experience with this than I do
A five main cam will solve the cam stress issue, I think. The Y-block 292 lifters work great for overheads. We have been shifting the Wild Cherry at around 5000 revs but there have been many oh-my-god, my hair's on fire moments when it has seen 6 or 7 grand in the heat of battle. This on three main crank and cam. Babbited mains at the moment. Planned top end upgrades dictate five mains for the next engine. We race with a club that requires a Ford block, so we will have to use a girdle for the other two. I would be interested in what you are using for a crank, perhaps we could utilize something like it. I believe you said it was a Crower product. Something you researched and found available, or a custom build?
I probably have something to help you. I have cast from ductile iron a conversion girdle to give a stock T block 5 mains. And no actual significant block modifications required. I made 3 of them and had 3cranks made by Crower. It replaces all three existing main caps. Maybe this suits you
Les, we will be in touch by PM. Thanks. Erik
I haven't had a conversation involving BMEP (brake mean effective pressure) in almost 20 years. A bit of a "blast from the past ". Thank you for the input
Cylinder pressure may or may not be a factor. If the valvetrain integrity is the limiting factor for engine output then you will be forced to use a pretty mild camshaft profile. Likewise the size of the cam lobe is a physical limitation that will guide your choices. To some degree you can overcome the cam lobe limitation with rocker arm geometry modifications.
If this is a 100 horsepower engine then the cam is going to be say .300" lift at the valve, with a duration of say 180 degrees at .050" and a lobe separation angle of 118 degrees. You could tighten the LSA to 114 to reduce the BMEP if necessary, it all kind of depends on how well the combustion chamber design works.
The block and crank ought to be capable of 250 horsepower, maybe more. Figure the cylinder head might flow 100 CFM on the intake side? A flow bench session would be interesting.
I have some of the same choices coming for my own overhead T engine, but mine will be limited by a stock block and a 4 valve RAJO head.
Less Iam using a stipe 335 with 1 1/2 to one rocker arms which gives net valve lift of .4875 lots of duration 244 int degrees at .050 lift 250 exh it is running straight up or evenly split at t.d.c. very lopey idle but boy does it rev. comp. ratio is 10.1-1 no valve notches needed as long as cam and crank relationship stay were they are. also using y-block tappets 170 lbs. open pressure has proved to work up to 6k tried to keep valve train as light as possible, pistons are .545 above deck gasket .065 SR fronty depth .890 or 176cc have fun
I am intrigued by the 5 bearing cam shaft (same as I'm using in my 5 main block)
At this point I'm just using ordinary T "high dome" pistons. I put snap ring grooves in them. I guess I'll see how I like it.
On my cast iron 5 main I'm doing it as a flathead with special Ross pistons and a special home brew cylinder head.
I appreciate all the input
Les if you are talking about egge high dome iam pretty sure you won't get to 10-1 comp. all rajo's ive seen are at least 1 inch deep BBR unless milled alot, I ran a twin spark Sherman for years at 7 3/4 -1 it ran great as long as I mixed couple gallons of 110 octane with pump premium (crappy L head combustion chamber) my old SR fronty had 8-1 and would run on anything without pinging as long as not lugging it by the way the sherman and the fronty ran same e.t.in 1/8 mile fronty had 8 more m.p.h. my new SR with 10.1-1 runs fine on pump premium though I upped the octane when racing at the drags
You definitely don't get 10:1 running Egge domed pistons. I have a setup sheet at the shop with that head/piston combination and I recall that the static CR is around 8:1 or so. I also have air flow #'s for a B Rajo and will look that up also.
Sent you an email
I am fascinated and very well pleased by the amount of thought provoking information that has been put forward. This is precisely what I was looking for. Thank you Mike B as well
A carb option I'm considering for my flathead T engine that has 4 intake ports is a pair of 40 DCOE Webbers. Any one else try this?
I like the look of the Weber 40's but have you considered four individual Mikuni or Kehin motorcycle carbs? They are cheap and easy to tune, with great throttle response.
Thank you for the idea. At the moment I already own the Webbers. But I'm not committed to them
hey Les anything new?