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MG MGB GT V8 Factory Originals Technical - Loosing power around 5500 rpm's
|I'm starting to work out a few little problems on my motor and thought I'd run this one up the flagpole and see who salutes. At about 5500 rpm's my motor begins to really stutter. I have a buick 300 ci distributor with pertronix II electronic ignition. Lucas hotwire fuel injection system with brand new throttle position sensor, new coolant temp sensor, new fuel pressure regulator, and brand new 19lb injectors. My ecu is now the famed "14cux" ecu too. I've had this problem even with my old ecu (14cu) and old injectors, and everything else, so I'm thinking maybe there's too much advance around 5500 rpm's?|
What do you guys think?
|What type of valve springs? You may be floating the valves at 5500 rpm.|
|Do those ECUs have built in rpm limiters? Some types of ECMs will limit rpm, by momentarly cutting power to the injectors.|
|Valve springs are from comp cams, and were chosen to match the cam so there wouldn't be valve float. I had to have the valve pockets machined to allow them to fit properly. The cam was designed for my fuel injected B with 5 speed gearbox, and good low/midrange torque with a power band from about 1500 rpm's up to 6500 rpm's, so I'm not really suspecting the cam right now. But I'm open to looking at that as a possibility.|
From what I understand, the rev limiter is only enacted when the speedo cable is electroncially tied into the injection system, which right now it isn't.
|Isn't the speedo connection for limiting top speed not rpm ?|
|Does it do it in all gears? Accelerator floored or steadily pushing it down? Do you know that your tach is correct? Are there any restrictions in the intake air tubes, etc? Fuel filter clean? Kink in a fuel line? Do you know what pressure the regulator is set at and can you increase your fuel pressure?|
|Wayne has an idea there. It would be great if you could get a WideBand O2 sensor to monitor (preffeably by a passenger) while you make a few runs. If you get a spike or jump rich or lean you will know something is up air/fuel wise..|
My 302 used to give way around 5200rpm. At this point I believe it was either bad vapve springs or possibly running out of air/fuel on top end.
Hmmmm 5500rpm in top.....
That would be very quick :)
|I haven't hit 5500 rpm's in 5th gear yet (I know, I'm a baby) But it stutters in 1-4 gear every time I mash the pedal and take off from a light. I'm pretty sure the tach is correct, it's a VDO unit that feeds right from the coil. Only 1 wire involved.. The only restrictions in air might be inherently built into the system, the air pipes are all pretty open. No known kinks in the fuel system, all the pipes are new, and are 5/16" i.d. The fuel pressure regulator is a 2.5 bar unit, and I'm wondering if I shouldn't have a rising rate fuel pressure regulator? The fuel pump is designed for fuel injection systems, and pumps 43 gallons per hour at 40 psi.|
I'm still leaning towards maybe too much advance, but maybe it is just running out of juice on the top end?
It might be worth a quick look to see if you are getting full WOT with the butterfly. On my set-up, similar to yours, I found that I still had about 15-20 degrees to go when I had the pedal to the limit. Had to modify the cable/linkage geometry somewhat to get a full WOT.
Has your manifold/trumpet/plenum been cut down? Mine has, and I don't get anything much above ~5200 rpm, but I attribute that to having valve springs, although new, about the same rating as the OEM.
I would assume that your advance is full in about 3000 rpm, easy to experiment with however.
5500 in 1st hey, just about gets me across the intersection...
|5500's pretty high. You'll recall that the factory V8s put the redline at 5200. I've never seen a normal Rover V8 spin up beyond around 6K unless there's some exotic stuff on board. And even 6K may need a little stronger valve springs.|
|I thought the standard Rover hydraulic tappets (lifters) were designed to pump up and reduce valve lift beyond 5200 rpm.|
Over here they sell solid or modified tappets for high revvers.
One question......why would you want 5500rpm...The Rover engine's real attraction is its low speed torque , not HP I seldom run mine beyond 4000rpm..it's plenty enough to see off all but the hottest hatches.
|I guess for some reason I thought redline was around 7000, maybe it's a little lower. But hey the more the merrier right? It is fun getting it up that high then shifting gears because the thing pulls all the way to that mark. =)|
Off topic: Think the motor would be ok spinning about 4,000 rpm's (80 mph) for the trip from detroit to townsend? I won't have time to get my IRS in before I go, but I still want to go...
|No problem to engine - (just wallet - Just be thankful you're not in UK and paying 80p/litre) ;¬)|
|Gosh, you guys run low gearing!|
My V8 has a 3.08 diff with 15" wheels with 65 section tyres, and 80mph is barely 3000rpm. I did try a 2.84 diff., but it was just too high around town.
Mind you ,as Stuart says, to fill your tank over here would cost you the equivalent of $60!!!
On this gearing I can get 30(UK)mpg (Just) on a motorway run, which just about gets me to the south coast non-stop.
At 5500 you are close to the limit of the pushrods but if the power loss is very noticeable, you might focus on fuel starvation - either your 40psi pump not doing what it should or the lines constraining output or the regulator at its limit.
If you like the sound of your own right foot - you might do better with a double overhead cam V6 !
Michael - I can get 30 mpg at 56 mph too if everyone else stays off the road !
|I had similar problems especially when stop starting on track days. Discovered alternator not giving enough amps. Using high volt coil and 8 injectors drawing a lot of current at those revs... it was useful to do the sums. Now have 75amp alternator with steady 14.5volts and not a miss until rev limiter at 6800rpm. I hope this helps|
|Just re-read some of the postings and would like to comment that I recently took part in a testing day on a race track followed the next day with a mornings racing then participated in a 'Le Mans' style 2.04 hr relay race. Over the two days the car clocked up 240 miles of full on race speeds. Top speed on this track was 140mph for me (I start to loose my nerve) and mostly drove up to the rev limit of 6800rpm. The engine used about 2 litres of oil ( 1 litre went into the catch tank). I use the car on road also. |
These cars are lovely to drive at lower revs but there is a whole new world up there in the higher revs.
What basic mods have you done to the engine to allow it to rev so freely. The V8 I've bought has an ISKY cam and antipump lifters. It's an ex drag race engine with 20 miles on it total (about 1 days racing) but the heads are now standard SD1.....Above all I have to get something drivable out of this for least money....and it will be nice if it goes well too...Any ideas?
The only work to make the engine rev sweetly is balancing - clutch to front pulley. I use hydralic lifters and standard rocker gear. The heads have had a lot of work and the extractors are precisely made, then two 2 1/2 inch pipes into single 3 inch straight through, develops 3 lbs vacumn just after extractor joins 21/2inch once it gets over 4000 rpm. Your cam profiles will determine drivablity. I have Autronic fuel injection which is very tuneable and therefore can compensate to some degree for a lumpy cam. I have about 350 bhp at flywheel and is still excellent in traffic, although extractors heat up the inside of front tyres and brake pipes at low speed.
I think I'm only going to find out when I actually fire her up. At the moment it's a complete mismatch. Cam I think I was told was a TR8 works rally car spec, bottom end all balanced, SD1 standard heads, Twin SUs and worse of all, cast iron Landrover Exhaust Manifolds. Unfortunately the manifolds are the only ones that will fit my application (Unless I make them myself) as space is REALLY tight. I'll see how it goes I suppose.
This thread was discussed between 23/04/2003 and 29/04/2003
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