Datsun Roadster Forum-311s.org

I Decided to take Mr Green to the dyno this week to get a baseline of the engine power and validated my seat of the pants impressions before doing any fine tuning to the carbs, the car has amazing power and pulls exceptionally well in the meat of the powerband and I wanted to see how this translated in objective numbers.

Before discussing the numbers thought I think it’s important to understand the the design ideas and philosophy for this built. as we all know U20s can make a lot of power but it tends to be higher up in the revs in the 6500-8000 rpm range , that great and lots fun but not really practical for everyday street use unless you’re always pushing it . I personally prefer a car that is quick off the line and doesn’t require you to wind it I all the time to get the most out of it but that is capable and willing to reach higher up when needed.

After talking with multiple builders I selected JT at Datsun Restoration Products (DRP) for this built because I felt he understood what was looking for and adhered to a similar philosophy, we discussed many options and looked at all the pro and cons before making a decision selecting components for their functionality towards my target not output numbers ( we actually never discussed output ). Amongst uncommon choices we made for a high output U20 we decided to run Mikuni 40s as we felt that while we would sacrifice some peak HP at the higher revs we would make more usable power an torque at lower revs , ditto for the cam selection we went with a custom cam that’s somewhere between a A and a B to bring the power a little lower in the rev range .

This combined with a LW billet flywheel makes for very tractable engine with a nice spread between peak torque and peak HP. Peak numbers are 120hp and 132lbs respectively at the wheels but more important than max numbers is the way the power is delivered , of the line at 2500 rpm the engine is already making about 110lbs and increase rapidly to be passed 120lbs at 3000rpm and stay over that number all the way to 5200 where we hit peak HP.

I’m super happy with the results and again a HUGE thank you to JT and DRP this is an awesome built and love the way the car feels. Obviously, we could potentially make a lot more peak power with 44s and a bigger Cam but I seriously doubt it would be quicker to 100mph

Test details:
-U20,custom cam, 123Ignition, Mikuni PHH40 with 35mm venturi , LW flywheel , Air cleaners on , 91 octane , max timing 35 at 4K ( advancing timing further made basically no big differences)
-We initially made 116/129 leaning the pilots a slight bit gave us the final
-We tested on a Mustang dyno which as I understand it usually gives a lower number than a Dynojet because it is under load
- Since the engine has less than 1000 miles I asked the operator to stop each of the pulls a 6K rpm it was still pulling but power was tapping of anyway

thanks for reading

Very nice!

Nice curve. Should be smooth in normal driving.

Shannon

Impressive. Good ideas for keeping it fun in the lower RPM.

Nice! Came out exactly as you designed and I bet it feels Great! You're making about 8-12lb ft of torque more than my motor at 3000 rpm, and we even out at about 4000. I see your peek torque is actually around 4200, while a B or C cam is up near 4700-4900, so your comment on "between an A cam and a B" duration makes sense. I bet it is very responsive between 2500 and 5500! Congrats!

Great job. Looks like an ideal street engine.
Dan

Nice numbers. While nothing feels or sounds as good as a high revving engine, low to mid range torque makes for a much more street friendly engine.

How much loss did you have in the drivetrain and what compression ratio does your engine have.
Did you use the stock exhaust manifold?

Solex68 wrote: Wed Oct 28, 2020 9:43 pm Impressive. Good ideas for keeping it fun in the lower RPM.

sfdaugherty wrote: Wed Oct 28, 2020 8:46 pm Nice curve. Should be smooth in normal driving.

Shannon

Gregs672000 wrote: Wed Oct 28, 2020 10:09 pm I bet it is very responsive between 2500 and 5500! Congrats!

I got it to idle around 750-800 rpm so it's surprisingly quiet considering the DRP exhaust and Mikuni and super smooth on acceleration
I have a 3.9 and slightly smaller tires acceleration is impressive from 2700 rpm

Gregs672000 wrote: Wed Oct 28, 2020 10:09 pm Nice! Came out exactly as you designed and I bet it feels Great! You're making about 8-12lb ft of torque more than my motor at 3000 rpm, and we even out at about 4000. I see your peek torque is actually around 4200, while a B or C cam is up near 4700-4900, so your comment on "between an A cam and a B" duration makes sense. I bet it is very responsive between 2500 and 5500! Congrats!

It did come out as planned ,but all the credit goes to Jim he really knows what he's doing
His comments throughout the process was to build an engine that is is bulletproof and doesn't have huge overbore or compression or fancy headers etc. Jim would need to confirm but I think the block is .20 over and we're at 10.0:1 compression I run on 91 but with just regular advance with no hint of pinging even when lugging the engine.

I did try some different curves from the 123ignition tread in fact I copied one of yours on the last run with 38 final at 5K it did do much on the dyno but felt very strong on the street afterwards

richard wrote: Thu Oct 29, 2020 1:38 am How much loss did you have in the drivetrain and what compression ratio does your engine have.
Did you use the stock exhaust manifold?

Richard,
we have a regular U20 header , for compression JT can confirm but I think we're at 10:1
For the loss this is a complex question since there's so much variability between dyno types, tires differentials etc. Since we were using a load bearing type of dyno my best guest would be in the upper range of loss so between 18-20% that would convert to roughly 146-150bhp and 160-165 lb/ft of torque

"I did try some different curves from the 123ignition tread in fact I copied one of yours on the last run with 38 final at 5K it did do much on the dyno but felt very strong on the street afterwards..."

That was a pretty old map for me, and I've since discovered there's no way I could run 38 degrees advance under load. The max I let it see is 30 at WOT. Without a doubt a more reasonable compression ratio like yours is the way to go with today's gas. You can gain some fuel economy by running more advance (like 38) at cruise and low stress, but once you find near peak torque numbers, adding more advance (even if it's not pinging) is typically not helpful and puts more strain on the engine than the 1-3lb ft of difference is worth. It would be informative if you posted the cam's duration (at 50thousands) and lift (x 1.396 rocker ratio). 35 years ago I spoke to a Datsun guru who ran a local shop and autoX a 240Z. He told me about a 2liter he hated to see, as it had a torque based cam that made it a real challenge to him on the course. Not a lot of power above 5000, but below that it was a torque monster.

Pjackb wrote: Thu Oct 29, 2020 1:22 pm

richard wrote: Thu Oct 29, 2020 1:38 am How much loss did you have in the drivetrain and what compression ratio does your engine have.
Did you use the stock exhaust manifold?

roughly 146-150bhp and 160-165 lb/ft of torque

Yes, right at 10:1. Suggest you limit the final timing to 34/35. You can't hear detonation over 5k, and the risk is not worth the minimal increase in power with pump gas. If you buy AV gas, no prob. The 123 makes these adjustments so simple.

The engine was always intended to make power in the 3-6k range. It really rips through the gears.

Those are excellent #'s for a street engine. Getting that amount of torque is not common and a real testament to the selection of parts and tuning.
Likely an honest 150 HP at the crank. Shows you don't need the big carbs/venturies/headers to get really good power from these engines.
Was wondering about the variations in the A/F ratios, but, it's not that big of a range so really of little concern, and likely very little to be gained for the expense/time to smooth it out, if it was possible.
Great job, both of you.

Interesting point on A/F ratios Daryl. You see the power dip when it goes too rich in the beginning, nearing about 11.5 to 1, then comes back quickly as it gets closer to 12.5 to 1.