TheSamba.com :: View topic - race 1600 engine not making power over 3800rpm

Wed Aug 01, 2018 4:02 am

Hi, im hoping someone may be able to help me. I race a formula car with a 1600 engine. Ive just had it fully rebuilt by an engine builder who builds a lot of the 1600 engines for my category of racing.

It has a counterweighted crank, new stock cam, new empi heads (standard valve springs) 1:125 rockers, new bearings throughout, new rings, lightened flywheel and all the usual stuff done to make it a race engine thats allowed.

Ignition timing was set on the dyno and is correct.

My problem is after the rebuild on the dyno it will not make any more HP after 3800rpm. Ive compared it to my other race car which keeps making HP up until about 5000rpm then drops off. The two race cars are about even until 3800rpm. My car runs a motec logger and shows im down about 8kmph on the straight compared to before the rebuild.

It has all the same ancillary bits like fan, carby, manifold ends and fuel pump. My tune is near spot on at about 0.86 across the chart.

It still revs quite happily to my shift point of about 5600rpm

Has anyone got any suggestions as to why it stops making more horsepower above 3800rpm?

Please take into account that our engines in this category are sealed and modifications are not allowed beyond minor things like valve springs, jetting and a few other little things.

Thank you if anyone can help or suggest things to check

Wed Aug 01, 2018 4:04 am

check your dist is advancing properly

Wed Aug 01, 2018 6:01 am

Ill check it again, but it was set on the dyno for max hp. I would love it to be that simple :)

Wed Aug 01, 2018 6:05 am

How does it run in the bottom end? Smooth?
I had a very similar thing happen where the engine ran great in the bottom end, try pulling some rpm and engine seemed like it dropped a cylinder around the same rpm. I believe at this Rpm is where the carbs switch over to you main jets circuit.
I pulled my emulsion tubes and found a piece of crud in one of them. Put back together and she was fine.

Wed Aug 01, 2018 6:05 am

Okay then let's look at this from a strictly straight up point of view something is holding that motor back from reaching past 35 it's either got to be carburetion or ignition if it's not overheating it's not a clearance issue and the thing isn't too tight

Wed Aug 01, 2018 6:08 am

Something restricting fuel flow?

Wed Aug 01, 2018 6:29 am

Yes perhaps something in the fuel line or the fuel pump isn't capable of pumping past 35.

Wed Aug 01, 2018 7:41 am

If it ran and pulled as supposed to on the dyno, you first need to focus on what you have touched since then.
Then check fuel supply.
Then ignition.
If all is ok, check cam lift. It may have gone flat

Wed Aug 01, 2018 9:38 am

ditto to alstrip, look for lose fuel line sucken air. or dead dying cam but that should also show up on low end, but it may take a while... jetted to much or not enough will do it as will a colapsed muffler...if your running one. also air cleaner tops too close to the stacks. air jet rong size...but probably fuel delevery issue.

Wed Aug 01, 2018 9:49 am

It's the new EMPI cam or the new EMPI heads.

Wed Aug 01, 2018 4:37 pm

dead valve sppringy thingys can also affect things like this.

Wed Aug 01, 2018 4:42 pm

First thing I'd check are dead valve springs.

Wed Aug 01, 2018 8:32 pm

mark tucker wrote: also air cleaner tops too close to the stacks. air jet rong size...but probably fuel delevery issue.

This comment just rang a bell in my head.
I have been having issues wit a set of 36's that ran great before. But once I installed a new engine (that failed do to a failed part) and reinstalled the old engine.
I have been having popping issues with correct running.

I just realized that I have put the tall stacks on the 36's. Rather than the shorter Dellorto stacks. That might be my problem.

Thank you for posting this Mark. It got me thinking.

Thu Aug 02, 2018 4:44 am

I dont think anyone that replied actually read my first post besides capt. calzone and glasser.

Cant be fuel, it has a perfect lambda reading right across the rev range.

Why would it be a dead valve spring? The heads are brand new.

Why would it be a dead cam, the cam is brand new.....

It didnt run as it should on the dyno, it stopped making more power at 3800...

capt calzone + glasser: maybe, im in the process of making a new carb up for it so ill try that soon. however my lambda reading is spot on all the way to 5500 on the dyno. I should add that i serviced the carby after the first dyno run and before the second. The problem was still there on the second run. I think its going to be something internal, so its going back to the builder after my race this weekend.

Thu Aug 02, 2018 9:55 am

Cam off one tooth?

Thu Aug 02, 2018 10:18 am

I am with John. Check the valve springs.

Stock springs are OK for 4000rpm on a stock cam. The second you add ratio rockers, ask it to rev higher, more lift, or add heavier valves, you need at least HD single springs.

Just cause they are new doesn't mean they are good or correct.

Brian

Thu Aug 02, 2018 10:26 am

because some heads are shipped with $#!^ valve springs. Check them.

The cam could have gone flat too, the most common time for it to happen is during break in, when it's "new".

Thu Aug 02, 2018 11:10 am

The cam could have been advanced or retarded 1 or 2 degrees when ground and that could make a big difference when dyno'ing a race engine. Just because any part is new DOES NOT mean it is correct. EMPI has been know for shitty quality control, and you are trusting their accuracy.

Thu Aug 02, 2018 11:24 am

If it was a " dead" spring, would not it make noise or racket, or rough running at higher rpm? Seems you should be able to hear it, if that was the case

Thu Aug 02, 2018 12:37 pm

Nope.

I built an engine that was on the edge of single or dual springs. I used what I thought were decent HD singles shimmed .100" to bind. After one summer of use it started cutting out about 4000rpm. It ran awesome and pulled hard till just over 4000rpm, then it was like someone pulled 2 plug wires.

The owner chased his tail swapping fuel pumps, complete ignition systems, adjusting valves, etc. Everything was tried. I had a hunch, and pulled a spring to find they had less pressure then new stock springs. Almost all of them were shot.

Being you have Empi springs I would be changing them regardless. A dropped valve makes a pretty big mess of nice new parts.

This would be a really good application for Dan's beehive spring kit.

Brian