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Author Topic:   Limiting RPM
Dirt Maniac

Total posts: 180
posted July 31, 2005 08:29 PM  
Something is limiting my RPMs. The car will only turn about 6400-6500 everynight. With the gear we have in the car it should hit 7000 easy. I can hear the motor cutting out down the straights and it doesn't seem to have as much bottom end as it should.

383 with 12:1, Dart Iron Eagle Platinum 215 heads, Pro Power Hurrican intake, PRC110 cam from JR Motorsports, MSD 6Al box and blaster 2 coil, converted HEI distributor, Gaerte 750 Alky carb. All electronics are new this year except for distributor. Water temp runs about 190 with 180 thermostat.

I've run the car the entire season without it reaching more than 6500 RPMs. I ran this same combo last year exept for the MSD box and I had the regular Iron Eagles, on a motor I blew up and it was turning around 7200 with plenty of bottom end. Anyone have any thoughts as to why it won't turn any RPMs?

Dirt Full Roller

Total posts: 38
posted July 31, 2005 09:53 PM  
Check this stuff, we have had racers complain of the same problem and it turned out be a simple thing.

Had a track champion with a really fast car fight a lack of RPM at the end of the straights with a new car. Changed EVERYTHING on the motor, to no avail. One day he had the car on the lift and when the lift "clanked" into the next locking slot the negative batty cable moved. He took a real close look at it and found that the lug and cable connection was broken. As long as the car was going slow to sorta fast it was okay, but as he increased his speed and the car vibrated the connection would "bounce around". After $1500 in parts relpaced elsewhere, he bought a new $2 battery lug and fixed the problem.

Had a local sprint racer with a 410 fight it, they finally found a 3/8" wrench in the intake port covering the injector after about 3 races of frustration.

Had a modified racer who fought it for half a season, only to find out that his cam choice was WAY off. He bought what the "mail order" company told him he needed to run. This mail order company was over 1500 miles away from our track and they KNEW what cam he needed to win...

Be careful with those Pro Power intakes. We sell them to, but every once in while we find one that is "just not right". We find that ports on the head side are a bit out of whack and don't match up to some heads real well. If you cover up of 1/8" of intake port with a defective manifold your velocity goes to **** and you can't get the air into the cylinder fast enough. Good thing is these intakes have more then enough material to fix the problem, BUT you shouldn't have to fix the PROBLEM...

We see MSD box's go bad all the time. Usually they work or they don't, but we do see some sick ones. Send it back to MSD for repair if you have some off time. If it ain't broke, they will tell you and at least you will eliminate that issue.

A minimum 10 gauge wire on all ignition related wiring. No if, ands, or buts. Resistance is a tricky thing, and if you don't have a big enough or quality enough wire you can fight em till your blue in the face.

Check your OHM rating on the spark plug wires. If it is a quality brand of wire the manufacturer can tell you what the rating "should" be. If it has built more then 15 ohms of resistance in its racing life over the out of the box rating, chuck em.

How tall is your air filter? Does it filter from the top, or just the sides? We have seen 2" tall air filters on 650HP mods, it doesn't work! If it's a cleanable type check to make sure you didn't over oil it, seen this hurt a motor to.

Cam degreed in? Properly?

Hope some of this helps, or at least rules a few things out.

Dirt Maniac

Total posts: 180
posted July 31, 2005 10:03 PM  
Good thoughts.

I checked the batter connections, everything is ok

pretty sure there's not a wrench down there

The cam is as close as I could find to the one I had last year. I wouldn't think that would make the engine miss though.

It's the same intake we ran last year with no problem.

Don't know about the MSD box, may check into that.

Got the wires covered.

I will check on the plug wires even though they are new.

It's a 5" air filter.

PTM degreed in the cam.

Those were good ideas,thanks. I'm going to check what I can. I was wondering about the distributor. It's a stock HEI distributor running with all the MSD stuff. Could it be the 30 year old distributor messing with me?

Dirt Roller

Total posts: 13
posted August 01, 2005 12:22 AM  
Are you on the edge of fuel starvation here?... Read those plugs, Did you change plug brand?. Also prove out that electronic igniton (new dosen't mean squat).

Dirt Full Roller

Total posts: 38
posted August 01, 2005 07:19 PM  
From personal experience (over 200 HEI's built for racing) I can say that you might be on to something there. The most common problem I see with a "stock" HEI really isn't related to how their are built, but rather how they are used.

First and foremost check to make sure you didn't pinch the advance wieghts in between the center plate and the rotor. This doesn't explain a lack of lower end power though.... This would explain a possible misfire on the top end if the engine cannot advance itself and fire the cylinder mixture at the correct time.

Is your HEI modfied where the vacuum advance is locked down? If you are using a stock vac advance that is not welded it can allow the pickup coil to move around and change the phasing relation of the rotor and cap. Possible, but not very likely cause of a high end RPM problem.

Does your HEI use plastic bushings in the advance weights? If they are worn through you can have possible timing changes at high RPM that you cannot see on your timing light at 3000 RPM. We see it all the time on our SUN machine when setting up old wore out distributors.

I had a customer pinch one of the box leads off the pickup coil in between the screw and the little plastic clamp they give you use. This didn't cause an RPM issue as the car would not run at all, but something to look into.

Check the distributor gear. If you run a roller cam I hope you have a bronze gear installed. High RPM issues sometimes come to light when the gear is starting to wear excessively. What happens is you set the total timing at 3000 RPM at say 38*, but with the wore out gear it gains another 4* at 6500 RPM, and bang you have 42* total without even knowing it (until the piston comes out of the block...). If the gear has ANY type of noticable wear patter that is leaving a sharp edge, replace it ASAP. Check the pattern and shim the distributor accordingly. If you have severely decked heads and a cut intake the distributor is trying to sit to low on the cam gear and needs to be raised with a shim or to. Remember that when the motor is running the cam is trying to "spit" the distributor out of the block and is putting enormous pressure on the gear. Machining a .030 grove from the oil passage to the gear on the housing is cheap way to get extra oil on the gear.

If you think you can rule out ignition related problems, fuel starvation is the next logical step. What type of fuel pump? Block mount, rear mount, etc? Had a customer use the wrong size pulley ratio, by only 2 teeth, on his rear mount setup in a modifed and couldn't get enough fuel to the carb during racing conditions. He used a KSE tandem pump that could keep up with the power steering demand, but without the correct ratio it couldn't keep up with the carb.

Maybe the jump in head intake port size is trying to pull more fuel then the carb can support. I agree with spiirit, tell us what the plugs look like (without any idle time on them) and that may tell us alot.

Maybe I can help, maybe I am talking out my rear!


XC man
Dirt Full Roller

Total posts: 39
posted August 01, 2005 09:40 PM  
You say you put all new MSD stuff in your HEI , The last module that I got from MSD had a rev limiter built right in . It is really hard to see if you don't know its there , it is a little white dial that you can turn to limit RPM,s . Maybe yours has that and it is set to low ? But that would be too simple !

Dirt Maniac

Total posts: 180
posted August 02, 2005 06:44 PM  
I don't run any type of advance. I have MSD's lock out plate for the vacuum advance and the mechanical advance is welded up.

I run a solid lift cam and there is almost no wear at all on the cam gear. Neither the heads nor intake has been cut on at all.

I have a block mounted Edelbrock 15# pump with a C&S style bypass. Wouldn't the engine run a little warmer if it was starving for fuel?

I bought a new MSD distributor today. I'm going to put all the old HEI stuff back in the old distributor. Next time I run the car, if it still has a miss in the heat, I'm going to yank all the MSD stuff and try the old HEI for the feature. If it runs, I think I can say the 6AL box is bad. If not, I'll start looking into fuel. I just believe it's electrical the way the engine just pops and misses. I'll have to wait till I run it again to check the plugs.

Thanks again guys. I'll let you know what I find.

Dirt Maniac

Total posts: 180
posted August 07, 2005 08:15 AM  
Ok, tried the new distributor this weekend and it elimanated the miss, but the car still doesn't turn the RPM's. It gaind about 200-300 more, but I'm still getting pulled 3-4 car lengths down the straights.

Dirt Maniac

Total posts: 146
posted August 07, 2005 12:43 PM  
did you go to another track in your tracks off season and make a gear ghange that you forgot about? things happen and people forget things over the winter, not trying to be a smart but, just trying to help. that could explain the lack of bottom end and rpm limit

Dirt Freak

Total posts: 298
posted August 07, 2005 08:09 PM  
You said that the cam is close to the same correct, If the timing events are not the same it might be the differance in the rpm's.

Dirt Maniac

Total posts: 170
posted August 09, 2005 09:59 AM  
I fought a similar problem for a few weeks this year.....what I found was the guy that installed the cam might not have degreed it right in the first place .....but the biggest problem was the cloyes hex ajust timing set had moved because the hex part of the timing set was cracked....I called crower and asked if I installed this particular cam streight up with out degreeing if it would be close ....they said YES ...put in new timing gear (not hex ajust)set it streight up and now it runs strong as high as I want to turn it!....I know it is a pain to check but it might just save you some money in the long run....good luck ...JT

Dirt Maniac

Total posts: 194
posted August 09, 2005 08:50 PM  
Weak valve springs

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