I have found great difficulty in tuning an engine with a blower installed. I have received excellent advice that has gotten me to this point and the advice is Greatly appreciated! Now, I am experiencing some problems that I cannot seem to resolve. Let me give you some history.

ENGINE
’76 L-48, bored .030” over, turned .010/.010 on the crank, stock length rods, forged pistons. Static comp ratio = 8.35:1.
Cam is a Comp # CS 268AH-14 / .464 lift for int. & exh., 222* and 226 dur @ .050 respectively, 114* lobe separation. Hyd. flat tappet lifters
Gear drive degreed @ 0*
+.100 hardened push rods
1.5 roller rockers
64cc aluminum heads w/2.02 and 1.6, dbl valve springs, 200 cc runners (Brodix IK 200)
Wiend 2 lobe rotor 6-71 blower 6 psi max. and matching intake.
2-Holley 4150 600 cfm mech. Secondary carbs w/manifold referenced power valves.
Small base electronic ign. w/6al box 18* initial timing – 32* total timing
No A/C, no power steering.

This engine is bolted to a 4 speed in my 76. My problems are:
1. There is no torque at low rpms.
2. commonly misses and backfire at low rpms

An example of “no torque” is, at anytime under 2800 rpm, there is not enough power to break the tires loose, no exceptions. If taking off from a stop sign and I accelerate very heavy, my engine feels like the 180 screaming horses that it came factory with (or less). Once 2800 or so rpms are achieved, there is enough power to take over the world (ok, maybe not the world, but you get the idea). I have read on the forum of “too large of runner volume” causing a loss of torque. So I called Brodix and asked about the runner size affecting the low end torque. Their response was “yea, it can affect it but it would not be THAT bad”.

Backfiring – When the engine is cold, it will have a “miss” until the engine backfires thru the carbs. Once the engine backfires, it will idle HORRIBLY for 5 – 15 seconds, then smooths out and runs what seems to be “Good” with the exception of no low end power. This is also true if the engine is run for a very short time (5 minutes or less) and has already backfired and running smooth, then shut off and restarted within a few minutes of shutoff. The engine will miss until it backfires again.

I have replaced and rechecked gaskets thinking I may have leaks and this has helped nothing. I have adjusted the timing in so many ways and this has not helped. I have adjusted the carbs as described on the forum for a single carb and this has not helped. I have purchased other ignitions and installed just to “test” and no other ignition has helped. I have tried “vacuum secondary / non-manifold referenced power valve” type carbs, this has not helped.
All parts are new parts and testing with “other” new parts indicates the parts are fine.
Any ideas?
Thanks.
KC

 

PM norvalwilhelm. He runs a blower, it's on a BB, but I bet he can help.

 

Jet it richer! Your getting a lean poping back through the blower. Of course it would run bad after a pop. All the oxygen is burned out.

6 psi is nothing.

You actually have small heads and a wimpy cam.

When somebody tells me that they use an underground crank............. My first opinion is they didn't do their home work!

Of course it will run like a 8.35 compression motor until the boost comes up

 

Gkull .... your thoughts on a possible similar build. Since I have a extra 010 block and friends with a shop. I've always wanted to try to get something like a vortec supercharger with the carb enclosure under my hood. I am a machinist also. My concern is I will not be able to clear the hood OR if I machine or find a manifold short enough It will at best not be ideal for a forced induction application. What would be a good choice for a 350 not 383 good bottom end and what at a minimum would a head choice be??

Thanks Gordon.

 

Well first off don't listen to people on a forum telling you to jet the engine without even seeing the car run or know the A/F.

Take the car to a dyno. Periord. If you want to tune a car properly get it on the dyno. Try and find a local tuner that knows what they are doing. If you were in the chicago area I'd be able to point you to the right place. Maybe someone on the board knows one in you area.

 

Summit racing here has some display motors with a centrifical SC with the enclosed carb box. It appears to be a very low profile system. the main concern to me would be having to make some custom SC mounts to clear the A-Arms.

The head cc volume is only a small part of the total intake track. The dead air space that has to start moving also includes the intake manifold runner. that's why some of the big single plane manifolds give 4500 - 8500 rpm operating ranges.

Blowers change the whole dynamics on the intake because boost fights intake reversion caused by cam overlap.

The blower intake manifold will be the determining factor on the max head size. If the intake has Felpro 1205 port size you don't want to intall it on heads with a larger port size like Felpro 1206

I have not seen one of the new generation AFR heads yet. so i can't comment on them. I have only used 4 types of after market aluminum heads. In the past i had several iron versions.

Dart 215, Dart 227, Brodix -11, and AFR 210. Each one of these heads have pro's and cons. My advice to you would be to get the biggest CFM flow for the gasket size and consider heads with a spread exhaust port so you can install headers larger than 1 3/4

 

Thanks Gordon.

 

try calling some marine places. around here they seem to know blowers. half the fast lake boats use them.

 

One way to kill a good roots blower is to backfire it. A good starting proceedure to keep out of a lean startup is to spin the blower for a few seconds first with the starter and then hit the ignition switch. Should light right up if installed correctly.

 

Thanks for all of the responces.
I have been in contact with Norval and he has provided me with an enormous amount of help. Thanks Norval!
Taking the enigine to a dyno is certainly a good idea, but I am a little reluctant to pull it back out and I am not sure if we have a dyno anywhere around. I will check with some of the machine shops and see if they know of one. I may still do this if I can not resolve my issues.

I am not so sure jetting richer will help either. I have been using an air/fuel reader and it is telling me I am fine or a little rich as far as mixture. I have noticed that if I richen the air screws a little more, I get black smoke out of the exhaust.

You are correct Gkull, this is a wimpy cam and somewhat small heads, but certainly much greater than a stock 76 350. Perhaps I had my expectations too high, I was expecting more responce/power from this engine with the mods than something comparable (or less) to the stock 76 at low RPMs, other wise there is no performance benefit by installing the cam and heads. The 200 CC runner head is a fair breathing head along with the .464" lift 222* dur. cam as this is just a street car, no racing is intended. Again, I cant see why there is performance loss compared to the stock 350 at low RPMS.

When somebody tells me that they use an underground crank............. My first opinion is they didn't do their home work!

I dont understand this statement.

That is an interesting recommendation Ironcross. I had not thought of this but it is worthy of keeping in mind. I am not sure but I do not believe I am experiencing a lean starting condition as the engine fires and runs nice in the driveway. Infact, I am not sure I am experiencing a lean condition at all.
I have never setup a blower before so my thoughts are irrelevant.

I do know at any speed/RPM, as I try to accelerate, the engine seems to hesitate before taking the fuel. If I accerate too aggressive at a low RPM, it backfires.
Another observation is, I have to change plugs about every 1000 miles or less. I have about 2000 miles or so on this engine I think and I am on my 3rd set of plugs. I dont know if a lean condition can cause this or if it is something else.

Again, Thanks!
KC

 

OK, I dont know if anyone is keeping up with this or not. If I am boring you to tears, sorry.

I did make a change in the configuration today and had noticable improvement.
I have been giving all of the comments allot of thought and decided to try a few things. One of which was increasing the total boost PSI.
After counting the teeth on my pulleys, I learned my lower was 34 tooth and my upper was a 38 tooth. So, I switched them.
I was about 10% underdriven and now I am about 11% overdriven. This did make a VERY noticable difference in power!
I have no idea what my max PSI is now, I have not re-mounted my boost gauge since installing the new carbs awhile back.

I also had a someone take a look at my plugs and he said (I hope he knows what he is talking about) that I am running a little rich and recommended leaning it out a little or leave it where it is. I have not done anything to the carbs yet, running out of time for the day.

KC

 

The overdrive was a good move. Helps if rich. Another trick is to give it some lead. 36+ total all in before 3000 is not unheard of. Too many people dont run enough lead. Helps clean the engine up also if you understand what I mean.

 

Question for all the roots blower guys. When using a B/M,Holley,Weiand blower of that kind on a SBC would it be best if total timing is more near 30deg v.s. 36deg? Im going on the rule of -1deg timing for every 1psi of boost. Is that logical,practial for a 8.1-9.1 comp. SBC with good matching components/parts to the blowers specs?

Thanks,Mike

 

I dont know Mike. This is a learning curve for me as well. I am running a total of 30* on the timing and had 6 psi max boost, but not sure what my max psi is right now since I changed my pulleys. I have the headers off right now wrapping them.... (and this is a PITA!!). Once I reinstall I will see where my max boost is and start playing with the timing.

KC

 

I have completd a few tasks with the Vette this week and thought I would post the results I have.

First - I removed, wrapped, sprayed and reinstalled my headers. The engine compartment was so horrible with heat, I think I was experiencing vaporlock when setting idleing for longer periods (ie.stop lights). This is a real PITA! I would not want to do this project again. I used COOL IT/Thermo Tec - 1" wrap from Summit. I figured 1" would allow contouring around bends better than 2". For future reference, it took 4 rolls of the 1" X 50' (2-rolls per header) and 2 cans of spray (1-can per header). Perhaps I could have been a little more efficient, but not enuff to salvage an entire roll. This is a fiberglass wrap and buddy, you will know by the time you are finished!

Second - After conversing with Mike (Yellow72) on another post http://forums.corvetteforum.com/show....php?t=1529458, and Ironcross, I decided to re-adjust my timing. I was running 18* initial and around 32* total timing. I adjusted this for 28* initial and around 40* total. This in itself made as much difference as changing pulleys as stated earlier! The engine fired right up and was able to drive with excellent responce and very good low rpm power. Also, no backfiring what-so-ever! I do have a little rpm "roll" but maybe I can adjust this out of the carbs. With the change in timing and readjusting the air/fuel screws, I am maintaining 15-17 Hg vacuum under the carbs and 12-13 Hg vacuum under the blower at idle. Not sure what my boost is, my gauge only registers 10 psi. and the hand went past the 10. I only drove it a few miles today (5 miles maybe), maybe with a little more seat time I will know if this is a good setting.
If anyone is interested, I will keep you posted as time goes. It will be next weekend before I can do much else. Work has me traveling this week.

KC

 

A couple of things to consider...get an MSD boost retard box. This way you can run 32 or 34 degrees of timing and as the boost comes on you can retard the timing through the box. This will keep you more safe as your overdriven boost comes on. Check your spark plug gap and make sure that your gap is tight like 28 to 30 total and your ignition box is strong...you could be blowing our your spark when the boost comes on (doubtful but certainly possible if the gap is wide). Also, your black smoke coming out the back leads me to believe that your carbs are not specifically set up for a roots blower but you could be WAY right instead. I have run many blower engines in the past and I have some experience. How about your fuel pump...is there any chance you have a really small fuel pump and it is simply running out of fuel as the engine comes into boost and that is why it pops? I am just asking questions to try to find your issue...also is your idle pulsing or is it flat and smooth?

NitrousSam

 

I would highly recommend installing a stoichmeter (shows A/F ratio, there are some nice ones out now0, not one of the cheesy ones. Think "02" sensor and you get the idea. You have to know whats going on with that thing before you start throwing more boost at it.

As posted above, some dyno time will tell a bunch, also. Not just at WOT either, spend some time on it with the sniffer in your pipe.

 

kd: Keep it coming. Interesting to watch the process of improvement!

The comment on the dyno..I'm sure folks mean take it to a chassis dyno...not remove the engine for a engine dyno-not too many people do that anymore. A chassis dyno may fix all your issues in one visit! I've found worth the $90-400 a bargain for lost power and preserving engine parts...

Good Luck!

 

Sorry about not posting on this for awhile, I have been obtaining knowledge about this the hard way.
I have made several changes that has made a HUGE difference in the way this thing runs.
There is some very good advice in these posts and I Thank all of you for your recommendations.
I do think the boost timing retard is a good thing to acquire and install as I believe my timing is/was coming in too high too fast.
Moving the timing up and bumping up the boost made an astronomical diffence in the performance. It was unreal!
Another change I made was the fuel lines. Initially, I had fabricated some aluminum -6 fuel lines and there was alot of this line above the engine. With the exhaust getting as hot as it was, the fuel lines were getting hot enuff that you could not hold your hand on them at all.

I have since re-routed with -8 stainless steel braided over rubber. I think this was a very good move. My fuel pump is a manual 110 gpm Holley. I also purchased a A/F meter and welded in the bung, but have not got to "try it out" yet.

Also to help aid the cooling, I purchased a Dewitts direct fit aluminum radiator and installed. We will see how that does.

I now had a good smooth idle, no "up and down" roll, no backfiring of any kind, and hopefully much less heat in the engine compartment.

With these changes comes confidence. This is something I do not need much of and can be dangerous if pooled in large amounts.

Immediately after installing the radiator and fuel lines, I bumped the timing up just a little more trying to find "the right spot". I drove the car to the gas station just for no reason other than I wanted to. During the short trip (about a mile one way) I did get in it pretty hard once I was out of the neighborhood and on our local highway.
This in itself was a Rush! I dont have the words to describe the feeling I felt with the immediate responce and immediate power.
I never exceeded the speed limit (I dont think) but I did leave plenty of rubber sealant for the road surface behind.
Once the engine idled down and the smoke cleared, the "ear to ear" smile on my face turned into concern as I head a knock.

Detonation???

She did backfire once while I was around 5500 rpm. That is when I let off of it.

Today I have my engine out and dis-assembled. I have 2 bent pushrods and a bent connecting rod.




Well, she (my financial controller who sleeps on the other half of the bed) has taken all of the plastic from my wallet and says "it will wait". I am not sure how long I will have to set in time out, but I hope it is not too long.

I was able to get a set of rods ordered before I was sent to the corner.

Happy Holidays my brothers. Be safe!
KC

 

Backfire is the death of blowers. Inspect the rotors while the blower is off. See if there is any tightness or even scratches in the case and on the rotors. There is nothing magical about a blower except it can produce tremendous power if your not scared of it. You will have to use racing gas if you want serious power as the timing can be advanced to the limits that way. Bent rods are not a good sign. Go back to my first answer as to starting the engine it will help prevent backfires. By the way tuning is no different than without one. Go for the max.