SO, I was wondering about slightly enhanced cams instead of "performance" and "full race" options. Designing a new cam is a little more involved than just adjusting the Duration, Overlap, Lift etc. Would it be easier (less expensive) to design and produce a higher ratio rocker? I don't know the ratio of the RE rockers but say something modest like going from 1.5 to 1.6? And if someone was were going to do this, how about a roller?
Hi Jon,
Well, you may be interested to know that I've been discussing this with some other engine builders and Bullet owners that I know, on some private technical forums, for about 3 years. We have worked on trying to do design improvements in these areas, and every way you look, there are limiting factors. These people in this group I've worked with are some of the most knowledgeable people you are going to come across, and have worked with people like Yoshimura and Jerry Branch.
Let's take a look at the option of increasing the rocker ratio. It's possible to get just a little bit of change, before you angle the pushrods too much. We've looked at that, and at an entire re-design of the rocker system. Even gone so far as to CAD/CAM some designs, and run them thru computer modeling. It's not impossible, but very difficult and costly to get anywhere near where we want to go.
And even if we could get some benefit, the lift amounts needed to get appropriate lift to achieve a true hi-lift valve system that can achieve the cone swirl around the valve head in this hemi chamber is on the order of .650" lift, which is never going to happen in the Bullet. We are stuck with a low-lift system, and even the max attainable is something around .425" lift, which is hell on the valve train and still doesn't get into hi-lift territory.
So, basically I can tell you that this territory has all been covered with a fine tooth comb. Certain things "could" be done, but nobody is about to fork over the amount of money necessary to do them, because there's a very small number of people interested in purchasing such items. and the costs to have them tooled up and made are very high. These ambitious projects are just DOA because of that.
Now, that is not to say that some things can't be done, and I have some answers for you.
The answers are primarily involved with knowing how to maximize what we have to work with, and making the best of it.
These answers revolve around making the best of what we can fit into the Bullet package without heroic efforts that cost beyond what is recoverable, and work in the ranges that a normal Bullet bottom-end can withstand, or even a bit higher with the billet crank and steel rod and alpha bearing big end.
We need to maximize our low-lift package, and we have duration room to work with.
First of all, we need to be realistic about our goals. We are not going to be a Molnar short-stroke Manx with 150mph capability on the track. We want good streetable performance.
For cams, the best option for street use, while retaining good low and midrange torque is a re-phase of the stock cams, like I mentioned on the other thread. This activity is not for the casual user. This is for somebody who knows how to work an engine.
The stock cam timing, as fitted, are not the same as the original Redditch cams. They have been adjusted to suit the larger port size in the Indian-made head, and give larger burnt-gas-fraction(BGF) so as to be better suited for emission control. They are more suited for TDC to BDC cylinder fill, and thus have reduced power levels compared to the original Redditch engine, and the hp figures bear this out.
It turns out that the Indian-made intake cam is further advanced in it's timing than the original Redditch cam. It's advanced enough that it is on the order of about one tooth difference, about 18 degrees. So, if we retard just the intake cam by one tooth, and leave everything else alone, we can approximately restore the old 1950s Redditch cam timing, and get back up to the Redditch 25hp figure of the old British Bullets.
Now, we have to be careful, because the Indian-made Bullets have cams which seem to vary in timing from one to the next. So, we have to use a degree wheel and dial indicator to check the cam timing with what we've got, so that we can be sure what we are doing is going to work out as we intend. "Assuming" that our cam timing is as expected, we can then retard the intake cam by one tooth, and leave the exhaust cam alone, and restore the original cam timing for more power. BUT, we don't have the small ports of the Redditch Bullet in our head, so we need to install a 8.5:1 hi-compression piston to make this work right with the bigger ports in the Indian head, due to lower port speeds in the larger head, so we can recover the lost early compression building involved with the later cam timing.
Ok, that sounds pretty good, but we can go further.
If we use the CMW adjustable cam timing pinion, we can then advance both the intake and exhaust cams by 4.5 degrees, and make the intake cam closing timing a little bit better for the big Indian-made head port, and also close the exhaust cam a little earlier to limit the overlap on the exhaust side for better cylinder trap, since we can't really use all that overlap with our oversize ports.
These mods can get us up into the 30hp and 30ft/lbs torque area at the rear wheel, assuming we have already done the freeflow intake and exhaust system changes and re-jetted.
In fact, this cam timing change can get us within 1hp of the Performance Cams sold by CMW and Hitchcocks, without the serious midrange losses that those cams give.
This is your answer to streetable cams for the Bullet that are better than stock, but not as "lumpy" as the Performance Cams or Race Cams.
What we are doing here is re-phasing the stock cams to move the intake valve closing timing of the intake cam later in the cycle, where we can use it for better power, while moving the opening timing of the intake valve later, because our overlap performance is poor enough that it isn't doing us much good at all. We can sacrifice early opening to get later closing, and that effectively extends our useful duration by 14.5 degrees where it will do us some good. We don't really lose anything by opening the intake valve 14.5 degrees later. In fact we gain something in power, and the reduced overlap makes it so we don't lose our bottom end torque. The earlier closing timing on the exhaust also helps reduce overlap, and helps keep bottom end torque. Since we are not revving up real high, the shortened overlap period is not hurting us.
It's the answer to hot road cams for the Bullet.
Now, you mentioned roller cams. Well, we really can't design and have these roller cams made for any affordable sum. But we can get some of their effects in another way.
This is pretty complex, and I hope I'm not losing anybody.
The hemi head in the Bullet is very good at low lift breathing capability. The hemi chamber is not shrouded by chamber walls like bathtub chambers are. This is a big breathing advantage over most chamber types. And we have low lift cams, and an engine design which mechanically limits us from getting into the hi-lift neighborhood.
So, we can try to maximize the low-lift flow performance of our system, to get more air in there in the low-lift area that we can access.
If you look at the valve and valve seat, they use a 45 degree seat angle, like most engines do. The valve needs to lift off the seat at least .040" before the valve is high enough to unshroud the valve curtain and allow relatively unimpeded flow into the cylinder, because the seat angle creates this condition. Normally, in bathtub chamber heads, this is not an issue because the chamber walls are partially shrouding the valve anyway, and the 45 degree seat directs the incoming air up along the chamber wall. But, in a hemi head, we don't have chamber wall shrouding, so we can work with the seat angle.
David Vizard, head porting guru, has done alot of work with Mopar Hemi heads, and has found that they can benefit from shallower seat angles. A hemi is a hemi, so the Bullet can also benefit from shallower seat angles for the same reasons.
If we can't get alot higher lift, we can start our flow earlier and get more out of our low-lift area, where most engine builders just ignore.
By cutting in a 30 degree valve seat angle on the seat and the valve, we can unshroud the valve curtain earlier, and begin flow at lower lift than the 45 degree seat would allow.
In practice, this does the same effect as a roller cam. It has been shown possible to get 15-25% more power out of a hemi head by using a 30 degree seat angle on the intake valve.
Now, with the Bullet, we have to go to the bigger intake valve to get the 30 degree seat angle, and in the process we get more valve area to flow, but we also lose a bit of mixture speed at the valve transition area. However, this would seem to be a good trade, as the benefits can outweigh the losses. We have to be very careful to flow the area around the valve seat to have minimum impedance to the airflow, because we've expanded the area there, and we can't afford to slow the flow any more with bad flow shape. We want to keep as much flow speed as we can, so we can use that late valve closing timing to best effect.
If we do it right, we can expect about 15-25% greater performance than using a 45 degree valve seat angle.
This could get us into the 35hp neighborhood at the rear wheel, if all goes well.
And it will all run on pump gas.
This stuff is highly critical in nature, to make sure that it all comes together right. There is not alot of room for making mistakes. It's for the experienced builder. But, it provides a way to get good power and torque in the Bullet, without much or any sacrifices, and doesn't cost a huge sum of money, but takes alot of time and care.
Perhaps even if you don't get everything exact, you can still get a good fraction of the performance, and wind up with 30hp or maybe 32hp.
Since we're operating now with higher compression and better breathing, we need to retard the ignition timing to suit. About 30 degrees of max advance would be about right, or maybe even a bit less. Failure to address the ignition timing issues will cost you a burnt piston.
Truthfully, this is about on the ragged edge that a stock bottom end can take, and it could push some "less than perfect" stock bottom ends over the edge, and take out the big end or the main bearings, or even the rod. And the stock clutch won't handle this either.
Best bet would be to use the billet crank and steel rod setup, and the expensive racing clutch.
With that setup, you'll be able to wind it to 6000rpm without concern of failure, and it will really kick ass for a street Bullet.
Again, this is not for just "throwing together a bunch of parts". This is to be done with the same care as if you were building a racing engine, which ispretty much what you are doing.