5.0MX5

John, I've truly enjoyed following your build and the advise you've shared with others as they plan build Ford engines. The engine for my car is currently bolted to the engine stand but I plan to have it back in the car in the next few weeks. I built it about 6 years and 10,000 miles ago and it's been fine till I started hearing a very slight noise discussed here. Thought it could be a wrist pin but after pulling all rod caps and checking was unable to detect any wristpin slop.

But the above is another issue. My hope is that you can run my combination on your dyno software and share the findings.

Engine details; 92 TBird 302 bored .060
'89 HO (stock roller cam)
Federal Mogul H273CP60 Hypereutectic flat top pistons, app.9.2 CR,
AFR 165 1402 heads (58cc), with Scorpion 1.6 roller rockers,
Edelbrock Performer 4 barrel intake with FiTech Street 400 Throttle Body FI.
Headers are the Hedman mismatch 1.5" primary shorty 4 into 1 block
I'm expecting mid 200s at the flywheel.

I contacted CompCams and they recommended cam kit k35-440-8

220 \ 220 @050

512 \ 512 lift

110lsa

2000 to 5500 rpm range

I actually like the stock '89 HO roller cam, it has very good street manners, sounds Ok and gets good mileage. From your advise to others I'm thinking I'd benefit from a change though. Looking forward to your advise.

RonR




.

engineer

Hi Ron,

Thanks for the kind words and the info on your build:

I did model your build in my software but there are a few assumptions that I had to make as I don't have all the info. But it doesn't matter too much as its the relative change we are looking for on the cam.
Firstly I would recommend a custom cam with a billet steel core from Ed Curtis at FTI. He will ensure that you get the most out of the combo, the OTS cams are not always great in all applications.

Thirdly the cam that comp recommended is OK however if you like the manners of the OE cam then the Comp cam at 220@50 will be lumpier at idle and will give soggy part throttle performance due to your compression ratio. Your static compression ratio is quite low at 9.2:1 for aluminium heads, for iron heads 9,2:1 is OK but Aluminium heads can take a full point higher on the same fuel. What that means is when you add in a longer duration cam the cam will BLEED off more cylinder pressure at certain speeds (normal driving speeds). This gives you a soggy engine because your "dynamic" compression ratio has actually fallen because you have a later inlet valve closing event. In addition that Comp Cam has less lift than is ideal to get the max flow from your heads, needs to be around 0.575-0.6in, that way it spends more time in the higher lift part of the valve lift curve...thereby giving you more flow from the AFR head and power.

With any step up in cam size you can expect about a 1.5-2 hp for every 1 degree of duration @50. FOr example if you go from a 210@50 duration cam to a 220@50 it is worth about 15-20HP. Jump to a 230@50 cam its about 30-40 FWHP. You can keep stepping up in these sorts of increments and keep adding top end power. There is an inflexion point where further increases in top end power will compromise the lower end. This is around 222-224@50 deg intake duration...it will start to sound lumpier after this point.

So this is what I think can happen for you based on where you are at:

1) Put in a bigger cam from Ed Curtis (he will spec somewhere around 214-220deg@50 depending on what parameters you give him. His cams will likely hit 0.6in lift to make sure you get every last cfm from the head.
2) You need to increase your compression ratio for better throttle response (run a thinner Cometic MLS head gasket) OR you can deck the block as I assumed its close to 15-20 thou in the bore.
3) Your exhaust is restrictive..but packaging is tricky in the V8 miata without a custom subframe, larger diameter and better length custom headers will yield some more power..but it will cost

With a modest cam from Ed you can expect an increase of 30 FWHP over what you have now.

Cheers

5.0MX5

MX-Brad

5.0MX5

engineer

Hi Ron,

Ok I have run up some numbers for you in Dynomation 6. OK according tot he simulation which at this point is not super accurate as it needs some very detailed parameters, so I am sort of guessing and improvising a little. So here is your baseline config:




It is sitting at 322 FWHP or 258 RWHP. My old 5.0L with TFS heads and an HO cam made exactly that so I don't think that you are too far off this:

From the graph you can see that the cam is too small as the power flatlines after about 5000 rpm.

So I added in an Ed Curtis cam that I have used previously...its 214@50 thou. So its pretty mild from a duration perspective but it makes solid power...You now have an extra 35FWP or 28 RWHP




Putting them on the same graph yields the following comparison:





So you can see that the Ed Curtis cam gives you uplift everywhere. You now have a much larger area under both the Power and Torque curves. It looks healthy everywhere.

If you wanted to get a bit more (say an extra 15 FWHP and 10-15 Ftlb of torque, then you will need to up the compression ratio. From the information you provided 9.2:1 is low and it really needs to be about 10:1 but at least 9.5:1. Your engine will be much crisper at 10:1 on the throttle, you will notice it will be very responsive.

The easiest thing to do is run a thinner head gasket and maybe shave the heads a little . These two things should get you around the 9.6:1 mark without much effort or cost and will help once you cam the engine up.

I guessed that your deck height is 15 thou down the bore (factory level deck height) but it may be different . Did you measure it on install?

Ideally deck height should be zero...and the quench area with the head gasket will then be around 35-40 thou which is spot on.

Hope that is useful and let me know if you have any more questions

Cheers
John

p.s I love your miata engine bay it looks super clean...wish mine looked like that

5.0MX5

John, THANKS again for the work and information! I really like how the suggested changes help power in the driving range (2k to 4K rpm). I'll give Ed a call and visit for sure. This is definitely a fun little car and a little extra just adds to the fun factor. I don't think I'll be increasing compression at this point, however if the need for another engine happens, then 10 to 1 sure sounds about right. Current set up was built with reg. gas (87 octane) in mind and it's pretty happy running it.