Good Day to All, As my title says, I'm having trouble (always have had) getting my 91 DB51T to go over 90 KPH. It pulls like a Banshee up to about 75KPH then I have to work and wait for it to climb to 90 KPH. I've checked the timing. I've also heard that a plugged Catalytic Converter will take away my top end. However I can't find one on my truck. Just the muffler midway along the exhaust system. I hear other mimi truck owners say that they have to hold their trucks back from doing more than 100 -110 kph. Mine is totally floored and as I said I have to wait several minutes to get to 90. Only if I can go down a hill will I top 100 - 110. Can anyone give me a suggestion as to what to check to get more top end? Thanks. G
a lot of times poor top end performance is a lack of fuel and that can be as simple as a clogged fuel filter or an incorrectly float in the carb. if you have a tach find out if it's only in your road gear or if it has weak pull in all gears at the same rpm. fuel filters are cheap enough that it's a good place to start if you haven't got a tach.
What about brakes? As in are they all releasing ok? no drag? Park brake cable not jammed? Sometimes the Cat can be hidden up in the exhaust manifold so it's not as obvious. I also concur with Milt on the fuel filter/ carb float.
do you have shift marks on your speedo?...if you do, are you holding it in gear all the way up to the shift marks before you shift into the next gear? these engine don't have a lot of HP or torque so you have to make sure the rev's are up to the shift marks so it will pull better into the next gear. when i got my mitsu i didn't like the sound of the high rev's so i was shifting early and it did seem to have the power i thought it should have for a small truck, and when i started shifting at the shift points marked on my speedo it was way faster up to speed i have a tach and shift rpm is about 4000 to 4200rpm and i can get to 100k's 6500rpm in about 20 sec., its no dragster, but it does the job
does your ride have a 4sp. tranny? my 98 with a 5sp. with stock wheels and tires would do 10mph for every 1000rpm in 5th. so at 65mph i was doing 6500rpm. you might be running out of revs.
Yeah, I'm having the same issue with getting up to speed. 80-85 km/hr is about all I can get (which is around 50-55mph following a friend in a jeep commander). And going up a hill slows me down to a good 40mph. I left my truck at the shop to be looked at. I'll hear back from them tomorrow. They'll look it all over. Maybe fuel filter. I was advised by a fellow minitrucktalk-er to check out the air intake. They'll look at that as well. The dealer said it could be the larger wheels/tires that are heavier. I have 14" Konig wheels (14x6) and 195/65 R14 Dunlop SP60s. Do you guys think it could really just be the wheels tires being too heavy? I have the 3-speed auto and know the gear ratio is different. 1st gear: 2.659 2nd gear: 1.530 3rd gear: 1.000 Even so, I would think the truck could still hold 85-90 km/hr without so much trouble.
i noticed a lag when i went to my 14" wheels... hills are a little more work to get up in 5th gear, but not like what you guys are talking about. i just keep it floored and around town on the freeway a hill might slow me down to 55 or 60, but never worse than that. i didn't really gain any top end power from the taller tires but it keeps the motor from breathing so heavy on the big end.
Do you have access to stock rims & tires? If you do take it for a drive for comparison. BTW the 195/65R14's work out to 13.47 % bigger than stock so maybe with auto tranny it is a issue. If the speedo reads 80 km then actual speed is 90 km/hr and yes I realize that you had outside source for your speed reading just more FYI After my run on the dyno I spoke to the operator on tire size affecting the vehicle [had stock tires on then], he said that larger tire size would not affect the HP but the torque takes a direct hit. So it depends somewhat on if the auto tranny in the Sambar has a lockup torque convertor or not. If there is a lockup on the torque convertor then you should be simlilar to the 4sp manuals and just run out of rpm's. However if there is no lockup then maybe with the tire combo at higher speeds the torque convertor is slipping (?) or ?
Interesting info guys, I will post tomorrow with anything I find out. And you are correct, I would have thought the larger size would have affected the speedo more, but it still seems to be fairly accurate. I'll keep you all posted on what comes up. I don't really want to give up the wheels/tires. Perhaps I won't have to.
Thanks to all for the input. I'd love to hear more. To answer some of your inquires. Mighty Milt: I don't have a Tach. Great suggestion about the fuel filter. I'll look into that. As for the Carb float...I got to that last as I don't want to do a O/H of a carb. Timetripper: The Brakes are releasing fine. No issues there. And I'll have a better look for the Cat behind the manifold. Oldsnowman: I have shift marks on the Speedo for 1 -2 - and 3. I do use them. Its got tons of "Get up and Go". Its the Top End that is lacking. b_eastep: I have a 5 speed Tranny with Stock wheels. The wee beast has lots more revs left in her, it just won't give it up easisly or quickly without the help of Gravity (hill). Any more ideas, I'm all ears! Thanks.
do you have the stock tires still on the truck? like you have read in the other posts too big of tire will greatly affect power to pull highway speeds. you should keep with 3% of stock. i have worked in the tire business a long time and one of the problems with guys going to over size tires on auto's or even standard with out axle gear changes is loss of power to pull on the highway. look at it this way...stock tires you take off in first over size tires you take off in second or third in your case HTH
Alright guys, dealer called today and checked through it all. Had a slight oil leak that is fixed now. Changed oil, changed fuel filter, and put in new spark plugs. Checked air intake as well. So all mechanically seems to be well. So the top speed power must be affected by the heavier wheels/tires. It is super windy here today, so I'm waiting till thursday or friday to drive it back home. I will know then whether I can tell much difference, but I think the top speed power will probably be around the same. So now I'll have to make a decision on if I want to keep my larger wheels/tires (great for stability and snow) or go with 12" wheels with the nice rugged stock tires. Tough decision because I like what I have and paid $660 for wheels/tires. We'll see.
Oldsnowman, I do have stock tires on the stock wheels. They are a winter tread. They came on the truck from Japan. By the way, are you the fellah I met at the PetroCan in North St. Albert about 3 weeks ago? G
Don't you have that backwards on the 1st, 2nd gear thing? The bigger the tire the less torque you will have at the road. On the full size P/U's [3/4 ton,1 ton] from factory they come with 4.11~4.56 rear ends, if they are putting on oversize tires [like 39~44"] then they will be dropping the diff to 5+ something. No? The bigger the tire the more distance travelled per 1 revolution and the more torque it takes to get it going from start.
i think he meant it's LIKE taking off in second gear. the larger tire makes the gearing taller and you have to slip the clutch or lug the motor AS IF you were taking off in a higher gear even though you are in 1st
Hey again guys, I just looked it up and each of my wheels weighs 16lbs, and each tire weighs 16lbs, so that makes each wheel/tire weigh 32lbs. How does this compare weight-wise to the stock?
i thought you might be that guy...yep that was me at the gas station. right how i am driving on my stock wheels and new 145/80r12 all seasons from CT until my 165/60r14's come in. the total difference in hight is .64" so its well with in the 3%. below are the wheels i bought for my truck should have had my tires by now...i put up some picture when everything is on.
your right milt...when you go to 2" taller or more, from these little tires stock size, it like putting taller gears in your diff
like milt said...its like taking off in second gear...you are right about the revs per mile, a taller tire will travel more per rev changing the axle gearing (ie; from 4:11 to 3:50 with tall tires...example only ) needing more torque to get going and to maintain highway speed
it's been my experience that increasing the rotating mass typically affects only drastic changes. like taking off from a stop light, or slamming on the brakes. it may take you longer to get to your hiway speed and a little longer distance to stop, but should not prevent you from achieving your previous speed. my truck accelerates a little bit slower now & i can't get my speedo needle past 135 with my new 14" tires but when you do the math on it actually i am going about 150kph or 92mph.
i have also done a lot of first hand research on the effects of weight & rotating mass when building drag engines. adding counterweights to a crankshaft to get the motor to live longer makes it too heavy to rev quickly, then lightening the flywheel to compensate... oops that was too much, now i can't get the car off the line. taking weight off the reciprocating mass to aleviate loads on the rotating mass... oops! too much off the piston skirt = forceful removal of wristpin = grown man crying in the garage. i paid more for my education in physics in my garage than i would have at an Ivy league school
