3.8 MFI slide valve build

[MBruns]

There are fresh calibrated sensors in each collector, the exhaust is a GT3 muffler, dyno is a DTS with a very good room, we have seen peak tq/ hp at many different AFR’s, as far as loading the engine down low at full throttle before the pull is asking for trouble, he’s going to the chassis dyno to confirm the real world loads, these old MFI pumps are tough to get results like a Motec system can in every cell in the map
Mike Bruns

[MST0118]

Can you post that "12.9" run as it should have picked up more Tqe and hP than the sheet shows under the same conditions. Perhaps that was a different day?

I wasn't able to upload the photo, so I dumped data in a spreadsheet. The run was done the same day and here is the data for slide valve with the richer pump setting that I'll be using:

RPM torque power AFR

5000 295.6 281.4 11.9
5100 294.9 286.3 11.77
5200 290.6 287.7 11.65
5300 287.6 290.3 11.57
5400 289.5 297.6 11.63
5500 291.7 305.4 11.77
5600 296.8 316.4 11.91
5700 302.3 328.1 12.16
5800 307 339 12.4
5900 311.7 350.1 12.8
6000 312.7 357.2 12.97
6100 312.5 362.9 13.08
6200 311.5 367.8 13.08
6300 309 370.7 12.99
6400 306.3 373.3 12.9
6500 303.5 375.7 12.77
6600 300.3 377.3 12.68
6700 297.1 379.1 12.67
6800 295.4 382.4 12.73
6900 294.8 387.3 12.6
7000 293.1 390.6 12.74
7100 292 394.8 12.81
7200 292.2 400.6 12.78
7300 291.4 405 12.87
7400 289.3 407.6 12.94
7500 285.3 407.5 12.9



Just for comparison, I am showing the peak torque and power for the same engine with PMO carbs which was the first run of the day:

50 PMOs
5900 317.7 356.9 13.76 peak torque
7100 292.5 395.5 13.24 peak power.

[JoeMag]

Mike -- Based on your experience, how much add'l power could the motor make running 110?

[MBruns]

Joe, it made less with 110, little better with 100, then 100/93 and 93 were about the same and 26-28 total timing
Mike

[MST0118]

Suppose if this were pure racing engine with 12.5 or higher compression, you’d make more power on 110, but I don’t know how much more. Maybe 10+?

[JoeMag]

Mike -- Thanks for the info... So you're saying that with the higher octane fuels, the motor could not take more timing to drive more power? Though 26-28 is getting up there, right?

[MBruns]

Joe, the compression ratio has more to do with what it wants for fuel, all of the twin plug 911 two valve non boosted like 26 - 30 degrees total, we test it all over to see what it likes and on the analog MSD we had timing drift to account for but the digital is much better
Mike Bruns

[Oldskl60]

Thanks to everyone who has contributed here, lots of good information and a beautiful build!
Will look forward to hearing of it in the car!!

[MoreGAS]

Hi MST and Mike
Since this is a streetcar and it will do most of it's running below 5K RPM(say spending 80-90% or greater time there) ; why only focus on it's WOT running above 5K? The mixture looked pretty rich where you began at 5K w MFI , and we often get WOT at 3K to 3500 RPM on the street. You can program the RPM gain per second to be more suited to replicate street use in a lower gear instead of replicating a 4th or 5th gear pull at say 400-450 RPM per second, maybe use say 600- RPM per second? Do you use some form of audio knock detection as I'd hazard a guess you'd be knock limited on 93? This would make the programmable digital MSD a good option to take out timing where needed but put it back in where it wants it and is safe. We've used those a few times w good results.
This drift you speak of means the ignition advance is bouncing around at high RPM?

This extra loading and lower RPM starting point can really show you how safe you are on this 93 octane fuel. I presume you also get the head temps really hot to do these runs say 350 F as it will see these temps frequently, just as we do running on the track? We've experienced a streeter w mild compression ratio say 10.3:1 will get in to this range idling at a stop light for a minute or two, after a spirited pull through 3 gears on only an 80 F degree day.
As someone who has a not so great(yet-pump has been off 3 X for internal mods -not lol.) tuned MFI hotrod 3.55L in one of my street cars, I know how frustrating it can be to have something that looks so Retro cool, but hate how it drives in some regions. It can be alot of work particularly for a streetcar, to get it running well at all temperatures, and all the various load levels. This is much more of a task than to tune it for the track where we mostly run WOT or are transitioning to WOT(corner exits) and say using only 4700 rpm and up. This can make one really really miss having the EFI controls.

It will be very interesting to see the comparison runs on a Dynojet, which are often quite rosy on power (but repeatable) compared to the engine dynos we have experience with. If I can suggest use SAE spec for displaying the data, and record A/F w the dyno's lambda sensor for more info. This is what is used for all our Hp/WT GT classes out here in The West and NASA nationally also.

Kevin
GAS Motorsport

[MST0118]

Kevin, thanks for your comments. I realize you have a lot of racing and engine building experience so appreciate the input.

First, want to respond that this is not really a street car in the sense that I don’t plan to drive around town with traffic lights and stop signs, go shopping, etc. Thus, the rpm will not be in the lower range most of the time. Instead, I plan to drive it no more than a few times per month on weekends, generally on secluded roads, highways and even some track days (non racing for now). This is similar as to how I drive my race bike which goes up to 13,200 rpm and I do take it there from time to time. With this engine, I will take it to a max of about 7,500, but with my short ratio gears, I expect to get over 5k quite often.

I realize these MFI engines can be “rough” for a street car, especially with slide valves but these type of custom MFI builds always take time to dial in, and I knew that up front when planning for it. I have had two other custom MFI builds and in both those cases it took awhile to dial in the AFRs and they ran way too rich at low end of RPM and a bit lean on top. In both cases the injection pump had to go back for recalibrations. It would be interesting to see AFRs at lower range so will look at that. Mike told me that engine idles good and runs well to 5,000 rpm with no flat spots, so have no reason to doubt that. Mark from MFI works really helped get this very close from the start.

As far as the issue of compression, ignition timing, octane, knock control, etc., I’ve talked to Mike at length about that and I’m comfortable with the 93 octane and he gave me other tips which I promised to keep confidential. I realize this is a very controversial topic but all I can say is that I have run twin plug 2.8 motors on 91 octane with higher compression using crankfire ignition with no issues and no evidence of detonation on inspection.

On those motors, ignition advance was about 26 but had much higher domed pistons). I actually know a local Porsche mechanic that runs over 12 to 1 on pump gas using those peanut chamber shaped heads that are similar to what Henry has posted on other threads but designed by another heads builder that passed away several years ago.

This engine is more old school, so it doesn’t have cpu or knock control or other advanced settings that an EFI system can use. I’m sure Mike can chime in to answer other technical questions. Thanks for questions.

[Mahler9th]

Nice!!!

When do you think the engine will be in the car?

When do you think you will try an AX or track day?

Perhaps you and Winders and I can have lunch sometime.

[MST0118]

Good to hear from you and would be great to meet up with you and Winders for lunch sometime. I’ll definitely reach out to coordinate.

Plan is to get the engine installed later this summer. I’ll probably get engine later this month so likely installed late summer at the earliest.

A/x or track day events would be great after I get car settled. Thanks!