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Mazda BP-4W Rod to Stroke ratio

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Hello everyone. I am currently rebuilding a Mazda BP-4W engine to fit in my Lotus 7 replica.

While doing the engine building course I was highly intrigued with the rod to stroke ratio topic. This happens to be lowish on the engine I am building, 1.56.

Since I am building a naturally aspirated engine project. Is it worth looking into looking for different rods and pistons?

Traditionally, a lower ratio has been considered good for low rpm torque, and there have been many engines that used this - the 400CID Chev', for one.

If you're looking at replacing pistons and rods, anyway, I'd be looking at a high pin piston/long rod setup as the additional cost of custom (if not available off the shelf) parts would be proportially small over the whole build. However, that said, Mazda would have spent a lot of development money on the design and if doing a simple rebuild with OEM cam's, etc, rather than chasing a high power, high rpm build, I wouldn't worry about it.

Thanks for the reply. My idea is to build an engine that's happier revving close to it's upper rev range, when eventually I change to some upgraded cams.

The reasoning being that since I am getting forged rods and pistons, I might as well look into the possibility of increasing the rod to stroke ratio while I am at it.

Where would be a good source to start looking?

I don't have a specific suggestion, but I would check out companies offering pistons for the 'stock' engine and phone, or e-mail, their support/tech' dept' and see what they can do. It's normal for the ring pack to be moved up a little for NA high rpm engines and this gives more room to raise the pin position. You're going to need that, and some careful measuring of block height, to establish the deck height of the piston and hence the length of connecting rod required for the stroke to position the piston correctly. If you think about it, it's quite simple.

Seems to be a few variations of that engine, with the preferred being MBSP - Main Bearing Support Plate - equiped. Basically a bottom end girdle - might not be needed on a N/A engine, though, as the loadings will be a lot less than for a forcedinduction engine - might be worth keeping in mind for later, though?

Correct, I've looked into using a MBSP ... Since I am sticking to N/A and not really inclined to rev above 7k, I sort of abandoned that idea. Good news is I can fit it to my present block, if I wanted to.

Thanks for your help.

With NA, the only effective way to get power is with rpm, as torque will reach a point where it is maximised and you'll need to raise the rpm it is reached at.

At this point, you're still in the build phase - I'd recommend you have a really good think about where you want to end up. By that I mean something like a 3 column list for "What I want", "What I don't want", "What I can live with either way", as any engine is a compromise, and plan your build around that.

For example, what sort of power/torque do you want, can you live with a peakier engine, are you wanting something that will work with the stock gearbox and/or rear eng gearing or are you willing to change it/them, what fuel are you going to be using - you may have access to something good locally but if you're goint to be travelling where fuel is less good..., what about induction (can be VERY loud) and/or exhaust noise, etc.

When you've a good idea of the final product, you can start building towards it - in this case you're starting with the short block but are you planning to use the OEM bolt on parts initially and upgrade over time, if so you may be compromised regarding the compression ratio you can run with stock camshafts? If the engine is currently in the vehicle, you may prefer to start with bolt-ons, like a good exhaust and intake (and fuel) system that will work with the stock engine but designed to work with the final long block and cam's.

What I recommend for anyone doing a full engine build for a vehicle they don't want off the road for long, is to purchase a second hand long engine (complete block and head) the same as they have, and build that up separately as there will always be little problems with part supply, machining delays, cash shortfalls (budget?), etc. that will extend the build time.

further to what has already been said.

Start from your use case - a Locost/Cater-field/Lotus 7. This should be a 500KG car so no need for masses of torque and mid-range.

Then look at your engine - the Mazda B-series is a cast iron block with a lower rod ratio which make sense given the rage of vehicles its used in, from 1600kg pick-ups and hatchbacks to big saloon cars with the MX5 being the only anomaly. Forced induction would be a good option to get the most from this engine.

So to answer you question going for a longer rod will allow you rev higher or rather make the engine more reliable at higher rpm due to a number of factors chief amongst them being a reduction in thrust loading and friction. keeping the standard ratio will increase torque and somewhat inhibit its ability to rev both of which are contrary to your use case.

For an NA build do your research and aim for a rev limit of around 8000rpm (the factory limit for the BP is 7000rpm) and depending on what parts are available go for a higher rod ratio. However you will have to replace the cams and the valve train in addition to the rotating assembly to achieve this reliably.

As an aside are you stuck with the Mazda BP? An aluminium blocked engine such as the 1.8VVC rover K series is around 40kg lighter and has much more potential as an NA engine together with a strong aftermarket, this would save 8% on total vehicle weight which will have a noticeable impact. The Ford Zetec 1.7, and Duratec 2.0 would be another option.

Thanks to both of you.

At this point it is only fair that I provide a little more information on the project. Where it is and where I would like it to go.

In a nutshell, yes I am stuck with the BP engine, mainly because of the registration laws in my country. I CAN if I wanted to go for a K20 for example or a Duratec but being an amateur built vehicle they will start asking questions. The kit I built was based on the MX5 being the donor vehicle.

The engine in the car is absolutely stock, bar a DIY ITB setup canibalised from a GSXR1000 bike, running the stock injectors, MS2 ECU and a 2.5" exhaust system that picks up from the stock 4-2-1 collector. With the present setup the car is 540kg dry and I would say rather lively to drive.

Attached is the best dyno run we achieved, I do have my reservations with the calibration of the Dyno, I am seeing those figures a bit too high.

What I want? ... Responsiveness rather than power.

I have a second long block, that I have dismantled and started cleaning. This will be my project engine.

For the time being, the only things I know for sure that I will be doing is a Lighter flywheel, Fluidamper, and billet oil pump gears. Anything from the crankshaft upwards is still open for debate.

Attached Files

Cool, and I see why you're focussing on what you have. From that graph, the camshafts are already biased towards top end, and for what you're looking for, I'd be inclined to keep them until you KNOW that you need more - I would definitely have the pistons made with deeper valve notches, though, just in case.

Regarding camshaft selection, if you do change them, don't got too carried away (I don't think you will, though) as too many people select for peak power at peak rpm - this can mean a lot less actual 'under the curve' power that can be effectively used and, personally, I'd suggest aiming for a peak 500-1000 rpm below peak rpm - with the latter biased towards a road car.

You may wonder why I'm making a point of that - many years ago (more than I care to remember) I had a friend who built a rather powerful (for the time) engine that was quite 'peaky', with a 'full race' camshaft, and he couldn't afford the close ratio gearbox it required. This meant that even changing at maximum rpm in 3rd, it dropped too far down the power/torque curve to pull 4th against the air and friction drag of the vehicle and it would lose speed. Sure, if he was on a downhill the gravity assist would suffice and it would then hold and accelerate in 4th if it was high enough on the curve all the way to red-line - but how often do you get suitably placed down-slopes. He ended up fitting a milder, 'fast road', camshaft and was much the happier for it.

I am not too keen on getting wilder cams, one reason being that the car needs to pass emissions testing, however I know that more power on N/A ultimately requires more RPM, more air and more fuel. So if I get better throttle responsiveness, then I am already a happy man.