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Old 04-09-2022, 03:32 AM
Echo1975 Echo1975 is offline
Join Date: Dec 2021
Location: Finland
Vehicle: Volvo 240
Posts: 33
Smile Increasing max rpm for NA d24 84 model engine.

I finally got a tach and calibrated it and the max rpm seems to be 4800 rpm. Max power is 82 hp at 4800 rpm as well so would increasing the rpm along with fueling increase hp?

I have seen a video of D24TIC going max speed at 5500 rpm.

This would have the benefit of being able to go faster on a lower gear and possibly even accelerate faster?

Can the naturally aspirated engine valve train handle this and how would I increase the pumps rpm limit?
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Old 04-09-2022, 08:07 AM
v8volvo v8volvo is offline
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Join Date: Jun 2009
Location: Montana, USA
Vehicle: '86 745, '83 764
Posts: 1,627

Max governor RPM (aka "high idle") should be something like 5000 rpm to the best of my memory. So, a little more than you are seeing now, but not a big difference. Maybe a small enough difference to be within the margin of error of your tachometer.

Is the engine CAPABLE of spinning faster than 4800, 5000, 5200 rpm....? Probably yes, if everything is in perfect condition and adjustment (timing belts, valve clearances, etc etc). But there is not really any point other than making life harder for your engine IMHO. The torque curve is already falling off very steeply by the time you are turning that speed, as intake and exhaust restriction are on a steep upslope. So even though you are correct that peak HP is rated at 4800 RPM, in reality you will want to be changing gears right then because there is nothing to be gained from pushing beyond the peak. Most of us change gears well before then anyway since the flat torque curve of the diesel engine means that better "seat of the pants" real world acceleration often comes from staying closer to peak torque (~2400 rpm IIRC) in a higher gear than screaming near at the HP peak in a low gear.

If you were talking about a D24TIC engine with MANY supporting upgrades to assist better breathing (like improved exhaust, intake, turbo, intercooling, etc, maybe cylinder head porting) then you might be able to carefully take advantage of higher engine speeds. But not on an airflow-limited NA engine. And also, just because someone did something in a video on YouTube doesn't mean it was a good idea.....

If you are looking for more power, your time would be better spent swapping in a turbo engine or adapting turbo parts to your NA motor.
86 745 D24T/ZF 345k lifted 2.5"
83 764 D24T/M46 155k
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Old 04-09-2022, 03:00 PM
Echo1975 Echo1975 is offline
Join Date: Dec 2021
Location: Finland
Vehicle: Volvo 240
Posts: 33

Thanks for the great answer! I have taken this as a project to get the most out of the NA diesel engine. I made a new header for the car already which should help with exhaust flow.

I also turned it up today, maybe too much because of the smoke but I can feel it pulling noticeably harder above 2000 rpm.

Because of your answer, I decided to leave the max rpm alone.

I have driven the car extremely hard for over 6000 miles now and it seems to be taking it.

I'm trying to get some more miles out of the engine because I'm planning on porting the head and don't want to waste the new head gasket by swapping it whenever there is no need for it.

I'm also pretty sure that at least 1 of my valves aren't sealing properly so I am going to be fixing that at the same time.
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